U.S. patent application number 15/137965 was filed with the patent office on 2016-11-03 for environmentally friendly aluminum coatings as sacrificial coatings for high strength steel alloys.
The applicant listed for this patent is THE BOEING COMPANY. Invention is credited to Usoa Izagirre Etxeberria, Nieves Lapena Rey, Laura Sanchez Cupido, Ainhoa UNZURRUNZAGA.
Application Number | 20160319449 15/137965 |
Document ID | / |
Family ID | 53177319 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160319449 |
Kind Code |
A1 |
Izagirre Etxeberria; Usoa ;
et al. |
November 3, 2016 |
ENVIRONMENTALLY FRIENDLY ALUMINUM COATINGS AS SACRIFICIAL COATINGS
FOR HIGH STRENGTH STEEL ALLOYS
Abstract
Electroplating process is described for coating a ferrous alloy
steel cathode substrate with an aluminum coating, the process
comprises: a) immersing an aluminum anode substrate in a plating
bath formulation comprising: a source of aluminum, an ionic liquid,
a brightening agent, and a metal-salt compound; b) etching the
cathode substrate by immersing it into the aluminum plating bath
and conducting an anodic polarization step; c) electroplating the
etched cathode substrate with the aluminum plating bath
formulation; and d) rinsing with alcohol and water, and drying.
Preferably, the process further comprises a heat treatment applied
to the aluminum coated ferrous steel alloy obtained in step d).
Inventors: |
Izagirre Etxeberria; Usoa;
(San Sebastian, ES) ; Sanchez Cupido; Laura; (San
Sebastian, ES) ; Lapena Rey; Nieves; (Madrid, ES)
; UNZURRUNZAGA; Ainhoa; (San Sebastian, ES) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THE BOEING COMPANY |
Chicago |
IL |
US |
|
|
Family ID: |
53177319 |
Appl. No.: |
15/137965 |
Filed: |
April 25, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 11/38 20130101;
C25D 7/00 20130101; C25D 5/003 20130101; C25D 5/36 20130101; C25D
5/52 20130101; C25D 3/44 20130101; C25D 17/10 20130101; C25D 3/665
20130101; C25D 5/50 20130101 |
International
Class: |
C25D 3/44 20060101
C25D003/44; C25D 7/00 20060101 C25D007/00; C25D 5/00 20060101
C25D005/00; C25D 5/36 20060101 C25D005/36; C25D 5/52 20060101
C25D005/52 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 28, 2015 |
EP |
15382212.7 |
Claims
1. An electroplating process for coating a ferrous alloy steel
cathode substrate with an aluminum coating, characterised in that
the process comprises: immersing an aluminum anode substrate in an
aluminum plating bath formulation comprising: a source of aluminum,
an ionic liquid, a brightening agent, and a metal salt; etching a
ferrous steel alloy cathode substrate by immersing it into the
aluminum plating bath formulation and performing an anodic
polarization; electroplating the etched ferrous alloy steel cathode
substrate with the aluminum plating bath formulation, to form an
aluminum coated ferrous steel alloy, wherein electroplating is
carried out with a current density ranging from 1 mA/cm.sup.2 to
100 mA/cm.sup.2, at a temperature ranging from 20.degree. C. to
100.degree. C. and under a dry inert gas; and rinsing the aluminum
coated ferrous steel alloy.
2. The electroplating process of claim 1, wherein the ferrous alloy
steel is a high strength steel alloy.
3. The electroplating process of claim 1, wherein the source of
aluminum is an aluminum halide.
4. The electroplating process of claim 1, wherein the ionic liquid
is a nitrogen-containing compound selected from N-alkyl-pyridinium
salts, N-alkyl-N'-alkyl' imidazolium salts, N-alkyl-N-alkyl'
pyrrolidinium salts, N-alkyl-N-alkyl' piperidinium salts,
quaternary ammonium salts and combinations thereof.
5. The electroplating process of claim 4, wherein the counter-anion
of the nitrogen-containing compound is a halide, and the cation is
selected from N-alkyl-N'-alkyl' imidazolium (I) and
N-alkyl-N-alkyl' pyrrolidinium (II), wherein the substituents R and
R' independently represent an alkyl group. ##STR00002##
6. The electroplating process of claim 1, wherein the brightening
agent is selected from the group consisting of 1,10-phenanthroline,
phthalazine, saccharin, isoniazid, coumarin, isonicotinic acid,
nicotinic acid, 3,4-(methylenedioxy)toluene, 1,4-butynediol,
2-aminothiazole, 2-mercaptothiazoline, 1-methylimidazole and
combinations thereof.
7. The electroplating process of claim 1, wherein the metal salt is
an alkali-metal halide.
8. The electroplating process of claim 1, wherein the aluminum
plating bath formulation consists of: from 95.30 to 99.95 wt % of a
mixture of aluminum trichloride and 1-ethyl-3-methylimidazolium
chloride, wherein both components are present in the mixture in a
molar ratio ranging from 80:40 to 60:40; from 0.01 to 1.0 wt % of
1,10-phenanthroline; and from 0.04 to 3.7 wt % of KCl.
9. The electroplating process of claim 1, wherein the
electroplating step is carried out with a current density ranging
from 5 mA/cm.sup.2 to 25 mA/cm.sup.2, a temperature ranging from
40.degree. C. to 75.degree. C. and stirring the electrolyte at a
range from 500 rpm to 1000 rpm.
10. The electroplating process of claim 1, wherein an aluminum
anode substrate is subject to a pre-treatment step comprising:
mechanical polishing an aluminum anode substrate; alkaline cleaning
the polished aluminum substrate followed by water rinsing;
deoxidizing the cleaned aluminum substrate followed by water
rinsing; and drying the deoxidized aluminum substrate to obtain a
polished, clean, deoxidized and dry aluminum anode substrate.
11. The electroplating process of claim 1, wherein the ferrous
alloy steel cathode is subject to a pre-treatment comprising:
degreasing an steel alloy substrate, and dry-blasting the degreased
steel alloy, followed by removing any powder remaining in the
surface of the stripped steel alloy to obtain a degreased and
blasted ferrous steel alloy cathode substrate.
12. The electroplating process of claim 1, further comprising heat
treating the aluminum coated ferrous steel alloy.
13. The electroplating process of claim 12, further comprising
applying a conversion coating to the aluminum coated ferrous steel
alloy obtained by the heat treating or the rinsing, wherein the
conversion coating is selected from a hexavalent chromium
conversion coating or a chromium free conversion coating.
14. An aluminum coated ferrous steel alloy obtained by: immersing
an aluminum anode substrate in an aluminum plating bath formulation
comprising: a source of aluminum, an ionic liquid, a brightening
agent, and a metal salt; etching a ferrous steel alloy cathode
substrate by immersing it into the aluminum plating bath
formulation and performing an anodic polarization; electroplating
the etched ferrous alloy steel cathode substrate with the aluminum
plating bath formulation, to form an aluminum coated ferrous steel
alloy, wherein electroplating is carried out with a current density
ranging from 1 mA/cm.sup.2 to 100 mA/cm.sup.2, at a temperature
ranging from 20.degree. C. to 100.degree. C. and under a dry inert
gas; and rinsing the aluminum coated ferrous steel alloy.
15. Use of the aluminum coated ferrous steel alloy of claim 14 in
aeronautical, automotive, marine, construction, industrial and
household applications.
16. An aluminum plating bath formulation comprising: an aluminum
halide; a nitrogen-containing compound selected from
N-alkyl-N'-alkyl' imidazolium halide or N-alkyl-N-alkyl'
pyrrolidinium halide; a brightening agent; and an alkali metal
halide.
17. The aluminum plating bath formulation of claim 16, comprising:
from 95.30 to 99.95 wt % of a mixture of aluminum trichloride and
1-ethyl-3-methylimidazolium chloride, wherein both components are
present in the mixture in a molar ratio ranging from 80:40 to
60:40, from 0.01 to 1.0 wt % of 1,10-phenanthroline, and from 0.04
to 3.7 wt % of KCl.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of European Application
Serial Number EP 15382212.7, filed Apr. 28, 2015, which is herein
incorporated by reference in its entirety.
FIELD OF DISCLOSURE
[0002] This specification refers to an environmentally friendly
electroplating process for coating a ferrous alloy steel substrate,
preferably a high strength steel alloy, using a novel aluminum bath
formulation comprising more safe-handling, less-hazardous and
environmentally friendly components than the formulation used in
the AlumiPlate.TM. process, which was the most promising aluminum
coating alternative for Cd replacement known until now.
Additionally, the present patent application refers to both the
aluminum coating and the coated ferrous alloy steel substrate
obtained by such process, as well as the use of both in
applications such as aeronautical, automotive, marine,
construction, industrial and household applications.
[0003] In particular, this specification provides an electroplating
process and aluminum bath formulation suitable for providing an
aluminum coating useful as a safe nickel-free alternative to the
cadmium coatings used in high strength steel components in
aerospace.
BACKGROUND OF THE DISCLOSURE
[0004] Numerous alternative technologies are being developed and
tested to replace cadmium sacrificial coatings for high strength
alloy steels and some of them are promising for some applications,
but none of them is yet authorized for all applications. Amongst
the most promising ones for some applications, low hydrogen
embrittlement Zn--Ni (LHE Zn--Ni) and AlumiPlate.TM. coatings offer
similar performance in most tests [Final Report WP-200022-Cadmium
Alternatives for High-Strength Steel, WP-200022, Steven A. Brown,
Naval Air Warfare Center Aircraft Division, Patuxent River, Md.,
Sep. 22, 2011; Approved for public release; distribution
unlimited], but it is still desirable to provide a safer handling,
less-hazardous and more environmentally friendly coating and/or
electroplating bath composition. AlumiPlate.TM. is a plating
technology that uses organic solvents as electrolytes, a
toluene-based very flammable and toxic solution which also contains
pyrophoric alkylaluminum constituent components. Therefore, it
involves handling hazardous and non-environmentally friendly
plating solutions.
[0005] A summary of promising technologies that have been assessed
or are under current assessment is described below.
[0006] Zinc-nickel electroplating is a promising candidate.
Zinc-nickel plating is possible in a wide range of pHs, using both
alkaline or acidic electrolyte baths, so several chemistries have
been developed in a wide range of pH leading to a range of Zn--Ni
alloy compositions. Amongst all of them, only 2 coating
specifications are compatible with high strength steel substrates:
the AMS 2417G and the ASTM B 841. These specifications allow both
alkaline and acid plating baths.
[0007] Aluminum is an excellent environmentally friendly
replacement for cadmium on aeronautical components of high strength
steel. Ion Vapour Deposition (IVD) aluminum technology has been
developed as a replacement of cadmium electroplating in some
aeronautic applications. Some of the disadvantages of this
technology include the limited ability to coat internal and deeply
recessed surfaces: depending on the orientation of the part's
surfaces in the chamber, the coating thickness may not be
equivalent in all areas (especially for internal diameters), the
coating does not pass the re-embrittlement test as per HSSJTP
[High-Strength Steel Joint Test Protocol, for Validation of
Alternatives to Low Hydrogen Embrittlement Cadmium For
High-Strength Steel Landing Gear and Component Applications, Jul.
31, 2003; AFRL/MLSC/WPAFB, OH 45433-7718; Approved for public
release; distribution unlimited (26 Mar. 2003)] and large
components may be physically restricted from IVD-Al coating by the
dimensions of the vacuum vessel.
[0008] The magnetron sputtered aluminum process was specifically
designed to coat internal diameters or recessed areas to overcome
the limitations of Ion Vapor Deposited Aluminum (IVD-Al). However,
some of the drawbacks of this technology are the high cost and the
fact that so far only limited applications are authorized.
[0009] Amongst the technologies available for low temperature
aluminum coating (such as IVD aluminum and sputtered aluminum),
electroplating is a versatile and economic alternatives. However,
because of the rather negative standard potential of aluminum, it
is not possible to electroplate aluminum from aqueous solutions
because hydrogen evolution occurs at the potentials at which
aluminum is plated, making the process not efficient and causing
hydrogen embrittlement to the substrate, which is not acceptable
for the high strength alloys used in aeronautical applications.
Therefore, only aprotic electrolytes such as nonaqueous inorganic
or organic electrolyte systems can be used to electroplate this
metal [Electrochimica Acta, Vol. 42, No 1, pages 3-13 (1997),
Yuguang Zhao and T. J. VanderNoot, Review: Electrodeposition of
aluminum from nonaqueous organic electrolytic systems and room
temperature molten salts].
[0010] AlumiPlate.TM. is an aluminum electroplating technology from
organic electrolyte systems. This technology is commercially
produced by means of the Siemens Sigal.RTM. process, which was
developed by Siemens AG (Germany), and most recently has also been
processed in Europe at Aluminal Corporation. The process was
licensed in the United States to AlumiPlate, Inc. in 1995. The
plating formulation of AlumiPlate.TM. comprises a toluene-based
solution containing a pyrophoric alkylaluminum constituent and
other compounds, such as ethers, aluminoxanes or ammonium salts
[US2007261966A1, 2007 Nov. 15, Alumiplate Inc. (US), Aluminum
Electroplating Formulations].
[0011] The aluminum coatings obtained with the AlumiPlate.TM.
plating process have demonstrated to have better performance than
cadmium in a number of tests, such as hydrogen embrittlement,
stress corrosion cracking, acidified (SO.sub.2) salt fog, fluid
corrosion resistance tests, etc. If compared to IVD-Al, it provides
coatings with better corrosion resistance and density. This process
can also lead to a similar throwing power or coverage than cadmium
plating by using auxiliary anodes to coat the internal recessed
surfaces.
[0012] A key disadvantage of this process is that it is not
environmentally friendly, since it employs a toluene-based toxic
and very flammable solution which also contains pyrophoric
alkylaluminum constituent components. Therefore, it is operated in
a humidity and oxygen controlled atmosphere line. Thus, the
elimination of the cadmium by this method addresses only one aspect
of cadmium substitution on high strength steel components, the
elimination of a toxic coating. However, the process still involves
handling toxic and non-environmentally friendly plating solutions
[US2007261966A1, 2007 Nov. 15, Alumiplate Inc. (US), Aluminum
Electroplating Formulations].
[0013] In view of the issues of the commercial aluminum
electroplating processes, new formulations involving more
environmentally preferred solvents are being developed. For
example, Global Ionix has developed a plating chemistry composed by
more environmentally preferred organic solvents for aluminum
electrodeposition. The plating formulation comprises non aromatic
organic solvents, such as ethanol, isopropanol or butanol, a
conductive additive and aluminum salts, such as aluminum alcoxides
and aluminum chloride. Global Ionix has reported that this
formulation provides coatings with throwing power comparable to
cadmium electroplating [WO2004079054A1, 2004 Sep. 16, Global Ionix
(CA), Electrodeposition of aluminum and refractory metals from
non-aromatic organic solvents].
[0014] Also, Hitachi Metals LTD has developed an aluminum
electroplating bath comprising dimethyl sulfone solvent and
ammonium chloride or a tetraalkylammonium chloride which is applied
by means of a barrel plating method. According to the inventors of
this formulation, this plating solution has improved the coatings
electrical conductivity, which in turn provides uniform aluminum
coatings. They also state that this bath possesses an extended bath
life [US2011253543A1, 2011 Oct. 20, Hitachi Metals Ltd. Aluminum
Electroplating Solution and Method for forming Aluminum Plating
Film].
[0015] It is noted that aluminum electroplating from ionic liquids
is an incipient technology compared to the rest of technologies
described before. Ionic liquids are novel fluids entirely
consisting of ionic species which usually have a melting point of
100.degree. C. or below. If the adequate chemical structure is
selected, they can have a wide electrochemical window,
negligible-volatility (which provides them with a non-flammable
nature), high solubility of metal salts, aprotic nature, or a high
conductivity in comparison to organic solvents [Phys. Chem. Chem.
Phys., 2006, 8, 4265-4279, Andrew P. Abbott and Katy J. McKenzie,
Application of ionic liquids to the electrodeposition of
metals].
[0016] The first studies of aluminum electroplating from ionic
liquids were reported in 1980s by Osteryoung et. al, although they
did not start being more actively studied until 2000s. Since then,
different ionic liquid categories have been explored, such as ionic
liquids based on dialkylimidazolium or dialkylammonium cations
combined with halide anions or more complex anions, such as
bis(trifluoromethyl sulfonyl)imide, etc. [Electrochimica Acta, Vol.
42, No 1, pages 3-13 (1997), Yuguang Zhao and T. J. VanderNoot,
Review: Electrodeposition of aluminium from nonaqueous organic
electrolytic systems and room temperature molten salts]. Most of
these electrolytes contained AlCl.sub.3 as the aluminium ion source
whose educts, once dissolved in the ionic liquid, made the
resulting electrolyte hygroscopic. This hygroscopic nature requires
this process to be handled under an inert gas atmosphere to keep
the electrolyte's stability. However, the electrolytes are not
flammable and do not have explosion's risk.
[0017] Most of the published studies have aimed at demonstrating
the feasibility of aluminum electroplating in different substrates
and the characterization and optimization of the coatings'
appearance or morphology. One of the most advanced ionic liquid
processes found, considering cadmium electroplating substitution,
is the development carried out by Dipsol. This company has patented
a formulation containing the ionic liquid
1-methyl-3-propylimidazolium bromide mixed with 10 to 50% by volume
of toluene, AlCl.sub.3, ZrCl.sub.2 polystyrene,
1,10-phenanthroline, isoniccotinic acid hydrazide and/or thiouracil
to electroplate Al and Al--Zr alloys. According to the patent
specification, 8 microns Al--Zr coatings electroplated with this
formulation have good adhesion (in the tape test, which is less
severe than the bend test), smooth cross section and can stand from
700 to 1500 hours in the SST (Salt Spray Test--JISZ2371) without
developing red rust [US2010285322A1, 2010 Nov. 11, Dipsol Chem
(Japan), Honda Motor Co. Ltd. (Japan), Electric Al--Zr Alloy
Plating Bath Using Room Temperature Molten Salt Bath and Plating
Method Using the Same; US2012205249A1, 2012 Aug. 16, Honda Motor
CO. LTD. (Japan) Dipsol Chem. (Japan), Aluminum or Aluminum Alloy
Barrel Electroplating Method]. However, no data was found regarding
the throwing power and the hydrogen embrittlement of this
formulation, key requirements in order to use the coating as an
alternative to Cd sacrificial coatings for high strength steel
alloys. In addition, this process contains aromatic organic
solvents so it still implies environmental, handling, and health
issues.
[0018] With respect to the last point, a more recent patent of
Dipsol discloses new formulations including the same ionic liquid,
a brightening agent, an organic polymer but without any organic
solvent. These formulations also contain dimethylamine borane and
hydrides, such as aluminum lithium hydride. They have demonstrated
that this process has a good throwing power [US2013292255 A1, 2013
Nov. 7, Dipsol Chem. (Japan), Electrical Aluminium or Aluminium
alloy fused salt plating bath having good throwing power, and
electroplating method and pretreatment using the same]. However, in
spite of the elimination of organic solvents, this formulation
still has some handling and health risks since the hydrides in this
bath liberate flammable gases in contact with water, which may
cause burns. Also, in this particular case, no hydrogen
embrittlement performance has yet been reported.
[0019] In conclusion, despite the potential and promising results
achieved in aluminum electroplating with these novel electrolytes,
it seems that all the ionic liquid based formulations developed so
far that can provide a balanced compromise between the basic
properties required for Cd replacement still require
non-environmentally friendly, toxic and/or hazardous additives and
solvents.
SUMMARY OF THE DISCLOSURE
[0020] This specification provides a safer handling, less-hazardous
and more environmentally friendly process, compared with other
known processes such as AlumiPlate.TM., for coating ferrous alloy
steel such as high strength steel alloy with an aluminum coating.
In particular, this patent application provides a process suitable
for complying with environmental and occupational health and safety
regulations. This aluminum coating can be useful in applications
such as aeronautical, automotive, marine, construction, industrial
and household applications, in particular as a Ni-free Cd
replacement for high strength steel alloys.
[0021] In particularly preferred embodiments, this specification
provides a process for obtaining aluminum metallic coatings as a
Ni-free Cd replacement for ferrous alloy steel such as high
strength steel alloys, with the main objective of achieving similar
or better performance than the LHE Cd or the AlumiPlate.TM.
methods, but plating using safer handling, less-hazardous and more
environmentally friendly electrolytes, i.e., the developed ionic
liquid electrolytes for aluminum plating described in this patent
application.
[0022] Thus, the Al coating obtained by the electroplating process
described herein is suitable for complying with environmental and
occupational health and safety regulations, while passing the
structural and functional requirements established for Cd
replacement process qualification in high strength ferrous alloy
steels. The coatings proposed in this patent application show
comparable behavior to Low Hydrogen Embrittlement Cd (LHE Cd) and
AlumiPlate.TM. reference coatings with respect to compliance with
most or, more preferably, all the preliminary acceptance criteria
established for Cd replacement in ferrous alloy steel such as high
strength steel alloys, i.e. coating appearance, morphology,
throwing power, adhesion, corrosion resistance and hydrogen
embrittlement performance. But, advantageously, they are produced
with an environmentally friendly and safe handling plating bath and
plating process.
[0023] Only high strength steel alloys, for example, steel alloys
with tensile strength higher than 1000 MPa or hardness higher than
30HRc, are sensitive to hydrogen embrittlement. Therefore, the
compliance with this test is not required when the aluminum coating
is applied to other ferrous alloy steel not susceptible to hydrogen
embrittlement. On the other side, an aluminum coating not complying
with the bend adhesion requirement can be useful as Cd replacement
sacrificial coating in less exigent applications, or,
alternatively, this coating may be used in combination with other
means to improve the bend adhesion such as the application of an
electrocleaning step during surface preparation or the application
of a nickel strike bond layer between the ferrous alloy steel
substrate and the Al coating.
[0024] Therefore, both the aluminum coating and the ferrous alloy
steel substrate, preferably high strength steel alloy, coated with
an aluminum coating by the process described in this patent
application are particularly useful in aerospace applications.
Specifically, the aluminum coating obtained by the process
described herein may be particularly useful as sacrificial coatings
in such applications wherein Cd sacrificial coatings were used, for
example, high strength steel landing gear, high strength steel
actuators, steel fasteners (bolts, rivets) or electrical connector
shells.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0025] This specification provides an electroplating process for
coating a ferrous alloy steel cathode substrate with an aluminum
coating, characterised in that the process comprises:
[0026] a) immersing an aluminum anode substrate in an aluminum
plating bath formulation comprising: [0027] a source of aluminum,
[0028] an ionic liquid, [0029] a brightening agent, and [0030] a
metal salt;
[0031] b) etching a ferrous steel alloy cathode substrate by
immersing it into the aluminum plating bath formulation of step a)
and performing an anodic polarization step;
[0032] c) electroplating the etched ferrous alloy steel cathode
substrate of step b) with the aluminum plating bath formulation of
step a), wherein this step is carried out with a current density
ranging from 1 to 100 mA/cm.sup.2, at a temperature ranging from 20
to 100.degree. C., preferably stirring, and under a dry inert gas,
for example, nitrogen, helium or argon; and
[0033] d) rinsing the aluminum coated ferrous steel alloy obtained
in step c). Preferably, the aluminum coated ferrous steel alloy is
rinsed with alcohol and water, followed by drying.
[0034] The aluminum plating bath formulation is preferably
anhydrous and the electroplating is conducted under a dry inert gas
stream in order to prevent contact of the electrolyte with the
ambient's moisture. However, an accurate control of oxygen and
moisture in the electrochemical cell is not needed.
[0035] The process described herein can be applied to different
types of ferrous alloy steel substrates [ASM Handbook Volume 1,
Properties and Selection: Irons, Steels, and High Performance
Alloys]. Specifically, the process can be applied to plain carbon
steels with low-carbon (lower than 0.2% C), medium-carbon (between
0.2-0.5% C) or high-carbon (more than 0.5% C); to low alloy steels
(alloys with not more than 8% of alloying elements) and to
high-alloy steels (alloys with more than 8% alloying elements).
[0036] In preferred embodiments, the ferrous alloy steel substrate
is a medium-carbon ultra-high strength structural low-alloy steel,
e.g., a steel alloy containing between 0.2 and 0.5% of C, not more
than 8% of alloying elements and with an ultra-high structural
strength. This substrate is also referred to as "high strength
steel alloy" in this patent application.
[0037] Low-alloy steels constitute a category of ferrous materials
that exhibit mechanical properties superior to plain carbon steels
as the result of additions of such alloying elements as nickel,
chromium, and molybdenum. Total alloy content in low-alloy steels
can range from 2% up to levels just below that of stainless steels,
which contain a minimum of 10% Cr. For many low-alloy steels, the
primary function of the alloying elements is to increase
hardenability in order to optimize mechanical properties and
toughness after heat treatment. Among low-alloy steels,
medium-carbon ultra-high strength steels are structural steels with
yield strengths that can exceed 1380 MPa (200 ksi). Many of these
steels are covered by SAE-AISI designations or are proprietary
compositions and include AISI/SAE 4130, the higher-strength
AISI/SAE 4140, and the deeper hardening, higher-strength AISI/SAE
4340. [ASM Handbook Volume 1, Properties and Selection: Irons,
Steels, and High Performance Alloys].
[0038] The high strength steel alloys may be those which currently
are being electroplated with cadmium as sacrificial coating in
aerospace applications.
[0039] In particular, medium carbon ultra-high strength low-alloy
steels may include, for example: [0040] Alloy Steel 4130,
conforming to AMS 6350 steel, sheet, and plate; and [0041] Alloy
Steel 4340, conforming to AMS 6414P steel, bars, forgings and
tubing, 260-280 KSI.
[0042] The source of aluminum may be an aluminum compound such as,
for example, aluminum halide, aluminum sulfate, aluminum
methanesulfonate, aluminum trifluoromethanesulfonate, an aluminum
salt formed with other anions or oxoanions such as isopropoxide or
ethoxide, or any combination of the mentioned aluminum compounds.
In preferred embodiments, the source of aluminum is an aluminum
halide such as fluoride, chloride, bromide or iodide. More
preferably, the aluminum halide is aluminum trichloride as it
provides good performance for Al electroplating and is cost
effective.
[0043] Thus, the aluminum bath formulation described herein
comprises a source of aluminum and a further compound, referred to
as "ionic liquid" in this specification, which is an ionic compound
or salt in the liquid state. The admixture of the source of
aluminium and the ionic liquid described herein is liquid in the
electroplating working condition, giving rise to an electrolyte
solution capable to electroplate aluminum. More specifically, the
term "ionic liquid" may be understood as ionic compounds or salts
whose melting point is below some established temperature, such as
100.degree. C. While ordinary liquids are predominantly made of
electrically neutral molecules, ionic liquids are largely made of
ions and short-lived ion pairs. The term ionic liquid was coined to
distinguish these lower temperature ionic liquids from the high
temperature analogues (i.e. high temperature molten salts) which
are composed predominantly of inorganic ions.
[0044] In preferred embodiments, the ionic liquid comprised in the
aluminum plating bath formulation is a nitrogen-containing compound
selected from N-alkyl-pyridinium salts, N-alkyl-N'-alkyl'
imidazolium salts, N-alkyl-N-alkyl' pyrrolidinium salts,
N-alkyl-N-alkyl' piperidinium salts, quaternary ammonium salts and
combinations thereof. Additionally, phosphor-containing compounds
such as, for example, quaternary phosphonium salts or
sulfur-containing compounds such as, for example, tertiary
sulfonium salts may be also used as ionic liquid in the aluminum
plating bath formulation described herein. The counter-anion of any
of these salts may be, for example, a complex anion such as
bis(trifluoromethylsulfonyl)imide, a cyano-containing anion such as
dicyanamide, a sulphur-containing anion such as sulfate (for
example, methylsulfate) or sulfonate (for example,
methanesulfonate, tosylate or trifluoromethanesulfonate), a
phosphate such as hexafluorophosphate, a borate such as
tetrafluoroborate, or a halide such as fluoride, chloride, bromide
or iodide.
[0045] In some embodiments, the counter-anion of the source of
aluminum and the counter-anion of the ionic liquid may be the same.
As a result, the solubility of both components may be improved.
[0046] Moreover, alternatively to the ionic liquids defined above,
the bath formulation described herein may comprise compounds which
may form an ionic liquid-type electrolyte solution by reaction with
a source of aluminum such as aluminum halide. These "ionic
liquid-type" compounds may be acetamide, urea, or a derivative
thereof, for example, the acetamide or urea derivatives described
in patent application US2013/0001092 A1.
[0047] In more preferred embodiments, the nitrogen-containing
compound comprises a counter-anion defined as mentioned above.
Preferably this counter-anion is a halide, and a cation is selected
from N-alkyl-N'-alkyl' imidazolium (I) and N-alkyl-N-alkyl'
pyrrolidinium (II), wherein the substituents R and R' independently
represent an alkyl group. More specifically, any of these radicals
may represent a C.sub.1-C.sub.8 alkyl group such as, among others,
methyl, ethyl, propyl, butyl or octyl.
##STR00001##
[0048] The aluminum plating bath formulation may comprise a mixture
of an aluminum trichloride with a nitrogen-containing compound
selected from N-alkyl-N'-alkyl' imidazolium chloride and
N-alkyl-N-alkyl' pyrrolidinium chloride. More specifically, the
molar ratio between aluminum trichloride and the
nitrogen-containing compound may range from 80:40 to 60:40. For a
fixed anion and alkyl substituents, imidazolium based ionic liquids
generally offer lower viscosity and higher conductivity than the
pyrrolidinium based ones. A high conductivity and low viscosity are
beneficial to increase the throwing power and decrease the ohmic
losses of the electrodeposition process.
[0049] If the molar ratio of the aluminum trichloride and the
nitrogen-containing compound, for example,
1-ethyl-3-methylimidazolium chloride, is too low, there will be not
enough concentration of active aluminum species to electrodeposit
aluminum coatings.
[0050] The aluminum bath formulation described herein comprises a
brightening agent, which is an organic compound that may be
selected, for example, from a large organic cyclic compound, a
bicyclic compound, a monocyclic compound or an acyclic
compound.
[0051] Examples of large organic cyclic compounds are, azine or
oxazine dyes (e.g azine dye--methylene blue dye), bipyridine
compounds (e.g. 1,10 phenanthroline), amino polyaryl methanes (e.g.
triphenyl methane dye--magenta dye) or proteins (e.g. casein).
Examples of monocyclic and bicyclic compounds are azines (e.g.
phthalazine), hydrazides (isoniazid), thiazolines (e.g.
mercaptothiazoline), thiazole derivatives (e.g. 2-aminothiazole),
aromatic sulfonic acids (e.g. benzene sulfonic acid, 1,3,6
naphtalene sulfonic acid), aromatic sulfonamides (e.g. p-toluene
sulfonamide), aromatic sulfonimides (e.g. saccharin), heterocyclic
sulfonic acids (e.g. thiophen-2-sulfonic acid), aromatic sulfinic
acids (e.g. benzene sulfinic acid), sulfonated aryl aldehydes (e.g.
.delta.-sulpho benzaldehyde), saturated and unsaturated carboxylic
acids and their esters (e.g. nicotinic acid, isonicotinic acid,
.delta.-hydroxy-cinnamic acid), 1,2 benzo pyrones (e.g. coumarin),
benzodioxole (e.g. 3,4-methylenedioxy toluene), aromatic alcohols
(e.g. .beta.-naphtol, catechol, phenol or resorcinol), quinolinium,
quinaldinium, pyridinium compounds (e.g. N-methyl quinolinium
iodide), imidazole compounds (e.g. methylimidazole), quinidines,
pyrazoles, indazoles and pyrimidines (e.g. cytosine), azo dyes
(e.g. p-aminoazobenzene) and thiourea derivatives (e.g. o-phenylene
thiourea-2-mercaptobenzimidazole). Examples of acyclic compounds
are ethylenic aliphatic sulfonic acids (e.g. allyl sulphonic acid),
aldehydes (e.g. formaldehyde), chloro and bromo substituted
aldehydes (e.g. chloral hydrate), allyl and vinyl compounds (e.g.
allyl sulfonic acid), saturated carboxylic acids and their esters
(e.g. oxalic acid, sodium oxalate), unsaturated carboxylic acids
and their esters (e.g. diethyl maleate), acetylenic compounds such
as alcohols (e.g. 2-butyne 1,4-diol), carboxylic acids (e.g. phenyl
propiolic acid), sulfonic acids (e.g. 2-butyne 1,4-sulfonic acid),
amines (e.g. 3-dimethylamino 1-propyne) and aldehydes (e.g.
propargyl aldehyde), nitriles (e.g. ethyl cyanohydrin),
thionitriles (e.g. .beta.-cyanoethyl thioether), amines and
polyamines (e.g. tetraethylene pentamine, sulfobetaines), thiourea
and derivatives (e.g. allyl thiourea), alcohols (e.g. glycerol),
polyethylene glycols and sulfur compounds (e.g. carbon
disulfide).
[0052] As the other components included in the bath formulation of
this patent application, the brightening agent is preferably
less-hazardous and more environmentally friendly than the
constituents of other aluminum plating baths such as the
AlumiPlate.TM. plating baths. Therefore, preferred brightening
agents may be, for example, 1,10-phenanthroline, phthalazine,
saccharin, isoniazid, coumarin, isonicotinic acid, nicotinic acid,
3,4-(methylenedioxy)toluene, 1,4-butynediol, 2-aminothiazole,
2-mercaptothiazoline, 1-methylimidazole or combinations
thereof.
[0053] More preferably, the brightening agent is
1,10-phenanthroline since its use allows the electrodeposition of
uniform, highly leveled aluminum coatings.
[0054] In other preferred embodiments, the brightening agent,
specifically wherein this agent is 1,10-phenanthroline, is present
in the aluminum bath formulation in an amount ranging from 0.01 to
1.0 by weight with respect to the total weight of the aluminum
plating bath formulation.
[0055] The cation of the metal salt comprised in the aluminum
plating bath formulation described herein may be selected, for
example, from an alkali-metal, alkali-earth metal, transition
metal, post-transition metal, rare-earth metal and combinations
thereof. On the other side, the counter-anion may be selected, for
example, from halide, sulfate, sulfonate, an oxoanion and
combinations thereof. Specifically, the halide may be fluoride,
chloride, bromide or iodide. Examples of oxoanions are isopropoxide
or ethoxide.
[0056] In some embodiments, the counter-anion of the metal salt and
the counter-anion of the source of aluminum and/or the
counter-anion of the ionic liquid may be the same. As a result, the
solubility of the components may be improved.
[0057] In other preferred embodiments, the metal salt is an alkali
metal halide such as, for example, potassium chloride, potassium
bromide, sodium chloride or lithium chloride. When the alkali metal
halide is potassium chloride, this compound is preferably present
in an amount ranging from 0.04 to 3.70% by weight with respect to
the total weight of the aluminum plating bath formulation, which
corresponds to a range from 5 g/L to 50 g/L.
[0058] In other preferred embodiments, the aluminum plating bath
formulation used in the electroplating process as described
therein, consist of: [0059] a range from 95.30 to 99.95 wt % of a
mixture of aluminum trichloride and 1-ethyl-3-methylimidazolium
chloride, wherein both components are present in the mixture in a
molar ratio ranging from 80:40 to 60:40, [0060] a range from 0.01
to 1.0 wt % of 1,10-phenanthroline, and [0061] a range from 0.04 to
3.7 wt % of KCl.
[0062] In more preferred embodiments, the aluminum plating bath
formulation consists of: a range from 95.3 to 99.5 wt % of a
mixture of aluminum trichloride and 1-ethyl-3-methylimidazolium
chloride in a molar ratio of 60:40; a range from 0.1 to 1.0 wt % of
1,10-phenanthroline and a range from 0.4 to 3.7 wt % of KCl.
[0063] The preferred bath formulations described in the above
paragraphs comprise the required amounts of all the components in
order to get aluminum coatings with improved performance and
particularly suitable to be used as Ni-free Cd replacement coating.
Thus, a molar ratio of the aluminum trichloride and
1-ethyl-3-methylimidazolium chloride of 60:40 provides enough
concentration of active aluminum species and, therefore, to get a
suitable Al coating. Additionally, the reported amounts of
1,10-phenanthroline and KCl give rise to an improvement in the
balance between the parameters for use of aluminum coatings as
Ni-free Cd replacement coatings.
[0064] Prior to the anodic polarization step, the plating bath
formulation may be conditioned by purging the electrolyte with a
dry inert gas stream inside the plating bath formulation during at
least 30 minutes. Once the electrolyte has been appropriately
conditioned, the anodic polarization and electroplating steps may
be performed with dry inert gas outside the electrolyte.
[0065] In other preferred embodiments, the electroplating step c)
is carried out with a current density ranging from 5 to 25
mA/cm.sup.2, a temperature ranging from 40 to 75.degree. C. and
stirring the electrolyte in a range from 500 to 1000 rpm. The
throwing power of the aluminum coatings usually decreases when
increasing the temperature with respect to the range stated above.
Aluminum coatings composed by multiple consecutive layers, poorly
adhered to each other, are usually obtained when plating without
stirring the electrolyte. Aluminum coatings with a more brittle
appearance may be produced when higher current densities than 25
mA/cm.sup.2 are applied.
[0066] In the electroplating process described herein, the aluminum
anode substrate is immersed in the etching/plating bath, and then
the bath formulation may be conditioned, for example, as previously
described.
[0067] On the other side, the ferrous alloy steel cathode
substrate, preferably high strength steel alloy, is immersed in the
conditioned bath formulation described in this patent application,
which will be used afterwards for aluminum plating, and an anodic
polarization step ranging from 0.6 to 1.2 V may be applied during a
period ranging from 10 to 30 seconds. This etching step b) may be
done, for example, at the same temperature as the plating step
c).
[0068] In other preferred embodiments, the aluminum anode substrate
used in the electroplating process described in this patent
application is polished, cleaned, deoxidized and dried aluminum.
Thus, the electroplating process is more easily performed if the
aluminum substrate is polished, e.g., is free from oxides and
compounds formed upon the exposure of the anode to the air or
during previous electrodeposition processes. Moreover, the aluminum
substrate should also be cleaned and dried to avoid contamination
of the plating formulation bath.
[0069] When the aluminum anode substrate is not correctly polished,
cleaned, deoxidized and/or dried, stains and an accused dendritic
growth of the aluminum coating may arise in the borders of the
aluminum electrodeposited high strength steel cathodes.
[0070] The aluminum anode substrate used in the electroplating
process described herein could have been subjected to a
pre-treatment in order to get a polished, clean, deoxidixed and dry
aluminum anode substrate. This pre-treatment may comprise one or
more of the steps described in the following paragraphs.
[0071] In preferred embodiments, the aluminum anode substrate used
in the electroplating described herein is subject to a
pre-treatment comprising:
[0072] i) mechanical polishing an aluminum anode substrate,
[0073] ii) alkaline cleaning the polished aluminum anode substrate
followed by water rinsing,
[0074] iii) deoxidizing the cleaned aluminum substrate followed by
water rinsing, and
[0075] iv) drying the deoxidized aluminum anode substrate to obtain
a polished, clean, deoxidized and dry aluminum anode substrate.
[0076] In some embodiments, the step iv) may comprise the drying of
the aluminum substrate with hot air at a temperature of at least
60.degree. C. during at least 1 minute, until constant weight is
achieved.
[0077] The mechanical polishing may comprise a first manual polish
with P-120 emery paper and then removing the powder remaining on
the surface, for example, with a white cloth.
[0078] The alkaline cleaning may be done by immersing the aluminum
anode substrate in an aqueous alkaline cleaning agent such as, for
example, a range from 45 to 60 g/L of Turco-4215 NC LT and a range
from 1 to 3 g/L of T-4215 NC LT ADD (additive) during a period
ranging from 5 to 30 min. The alkaline cleaning may be carried out,
for example, stirring in a range from 200 to 500 rpm, at a
temperature ranging from 45 to 55.degree. C. After the cleaning
step, the aluminum anode substrate may be rinsed, for example,
first with tap water followed by deionized water.
[0079] The deoxidizing step may be carried out by immersing the
aluminum anode substrate in the deoxidizing bath containing a
deoxidizing agent such as, for example, a range from 60 to 120 g/L
of Turco Smut Go NC and a range from 15 to 30 g/L of HNO.sub.3
(42.degree.Be) during a period ranging from 1 to 10 min. The
deoxidizing step may be carried out, for example, at a temperature
in the range from 20 to 50.degree. C. After that, the aluminum
anode substrate may be rinsed, for example, first with tap water
followed by deionized water.
[0080] Finally, the pretreated aluminum anode substrate is dried,
for example, using hot air. Prior to the drying step, it may be
rinsed with a more volatile solvent such as acetone in order to
remove part of water with this solvent.
[0081] In other preferred embodiments, the ferrous alloy steel
cathode substrate is a degreased and blasted ferrous alloy steel,
preferably a degreased and blasted high strength steel alloy. Thus,
the electroplating process is more easily performed if the steel
substrate is degreased, i.e. it is free from any grease or oil on
its surface that could hinder a uniform aluminum electrodeposition.
It is also preferred that the steel substrate would be blasted in
order to get a mechanical etching of the surface and subsequent
good adhesion of the electrodeposited aluminum layer.
Advantageously, this mechanical etching helps coating adhesion but
does not provoke any risk of hydrogen embrittlement for the
substrate, on the contrary to conventional chemical acid or
alkaline pre-treatments.
[0082] The ferrous alloy steel used in the electroplating process
may be subjected to a pre-treatment in order to get a degreased and
blasted ferrous alloy steel. This pre-treatment may comprise one or
more of the steps as described in the following paragraphs.
[0083] In preferred embodiments, the ferrous alloy steel cathode is
subjected to a pre-treatment comprising:
[0084] v) degreasing a steel alloy substrate, and
[0085] vi) dry-blasting the degreased steel alloy, followed by
removing any powder remaining in the surface of the stripped steel
alloy to obtain a degreased and dry-blasted ferrous steel alloy
substrate. Preferably, the ferrous alloy steel is a high strength
steel as described above.
[0086] The ferrous alloy steel cathode substrate may be degreased
using any degreasing agent such as, for example, acetone or an
aqueous alkaline degreasing agent. This step may comprise the
immersion of the steel in a degreasing agent, manual cleaning with
the help of a white cloth and the application of ultrasonic
agitation for at least 10 minutes, until neither oil nor grease
remains on their surface. Additionally, after the degreasing step,
the steel may be dried, for example, using hot air.
[0087] The degreased steel surface may be blasted, for example,
with alumina grit, silicon carbide grit, glass beads or steel grit
to remove any possible oxide and impurities off the steel
substrate. The powder remaining on the surface after blasting may
be removed with compressed air. Preferably, the blasting agent has
a particle size from F-36 to F-80 macrogrits (i.e. a mean diameter
ranging from 185 to 525 microns), since the use of this blasting
agent in the electroplating process described herein results in an
improvement in the bend adhesion of the aluminum coating obtained.
Examples of those preferred blasting agents are F-80 and F-36
alumina grit.
[0088] The electroplating process described in this patent
application preferably comprises rinsing the aluminum coated
ferrous alloy steel with alcohol and water. In particular, it may
comprise rinsing first with ethyl alcohol followed by water rinsing
such as deionized water rinsing, until a clean surface free of any
rest of ionic liquid is obtained.
[0089] The aluminum plated specimens may be stored in a humidity
controlled atmosphere.
[0090] In other preferred embodiments, the electroplating coating
process described herein further comprises step e), wherein a heat
treatment is applied to the aluminum coated ferrous steel alloy
obtained in step d).
[0091] In more preferred embodiments, the aluminum coated specimens
are baked at 190.+-.14.degree. C. for at least 23 hours. The
addition of step e) is preferably included in order to the aluminum
coated specimens obtained comply with the hydrogen embrittlement
requirements. Therefore, this step is preferably included to the
electroplating coating process when the ferrous steel alloy is a
high strength steel alloy, substrates which are susceptible to
hydrogen embrittlement.
[0092] In other preferred embodiments, the electroplating coating
process described herein further comprises applying a conversion
coating to the aluminum coated ferrous steel alloy obtained in step
d), or preferably the ones obtained in step e), wherein this
conversion coating is selected from hexavalent chromium conversion
coating and a Cr-free conversion coating, in particular Cr-free
conversion coating with a similar performance to the hexavalent
chromium conversion coating.
[0093] The aluminum plated specimens may be optionally conversion
coated using conventional Cr VI based conversion treatments, such
as Alodine 1200S or similar. Optionally, Cr-free conversion
treatments such as those described in U.S. Pat. No. 8,298,350 B2
and US 2013/0052352 A1 patent disclosures or similar products and
developments may be used.
[0094] Thus, this specification provides a safer handling and more
environmentally friendly electroplating process and bath
formulation for coating ferrous alloy steel, preferably a high
strength steel alloy, with an aluminum coating. Additionally, the
Al coatings obtained by the process described herein are also more
environmentally friendly than Cd coatings and other known Cd
alternative coatings (e.g. Zn--Ni). Thus, this specification
provides a process to obtain an aluminum coating useful in the
applications such as aeronautical, automotive, marine,
construction, industrial and household applications. Particularly,
the coating obtained by the process described in this patent
application can be used as Cd replacement in sacrificial coatings
for high strength steel alloys.
[0095] In particularly preferred embodiments, the electroplating
process described herein comprises: the pre-treatment of the high
strength steel alloy cathode substrate and the aluminum anode
substrate as described in this patent application; the
electroplating treatment using an aluminum plating bath formulation
which comprises: a range from 95.30 to 99.95 wt % of a mixture of
aluminum trichloride and 1-ethyl-3-methylimidazolium chloride in a
molar ratio ranging from 80:40 to 60:40, a range from 0.01 to 1.0
wt % of 1,10-phenanthroline, and a range from 0.04 to 3.7 wt % of
KCl; and the post-treatment of the obtained coating as described in
this patent application. This specific combination of process steps
and bath composition provides an aluminum coating with particularly
improved properties that makes the product obtained a particularly
preferred candidate for Cd replacement as sacrificial coatings for
high strength steel alloys.
[0096] Thus, according to particularly preferred embodiments, the
electroplating process for coating a high strength steel alloy
substrate with an aluminum coating comprises:
[0097] 1) pre-treatment of an aluminum anode substrate, wherein
this pre-treatment further comprises:
[0098] i) mechanical polishing an aluminum anode substrate,
[0099] ii) alkaline cleaning the polished aluminum substrate
followed by water rinsing,
[0100] iii) deoxidizing the cleaned aluminum substrate followed by
water rinsing, and
[0101] iv) drying the deoxidized aluminum substrate to obtain a
polished, clean, deoxidized and dry aluminum anode substrate;
[0102] 2) pre-treatment of a high strength steel alloy cathode
substrate, wherein this pre-treatment comprises:
[0103] v) degreasing the steel alloy cathode substrate, and
[0104] vi) dry-blasting the degreased steel alloy, preferably with
a blasting agent with a particle size ranging from F-36 to F-80
macrogrits such as F-80 or F-36 alumina grit, followed by removing
any powder remaining in the surface of the stripped steel alloy to
obtain a degreased and blasted high strength steel alloy
substrate;
[0105] 3) electroplating treatment comprising:
[0106] a) immersing the aluminum anode substrate obtained in the
pre-treatment of 1) in an aluminum plating bath formulation
comprising: [0107] a range from 95.30 to 99.95 wt % of a mixture of
aluminum trichloride and 1-ethyl-3-methylimidazolium chloride,
wherein both components are present in the mixture in a molar ratio
ranging from 80:40 to 60:40, [0108] a range from 0.01 to 1.0 wt %
of 1,10-phenanthroline, and [0109] a range from 0.04 to 3.7 wt % of
KCl;
[0110] b) etching the high strength steel alloy cathode substrate
obtained in the pre-treatment of 2) by immersing it into the
aluminum plating bath formulation of step 3a) and performing an
anodic polarization step;
[0111] c) electroplating the etched high strength steel alloy
cathode substrate of step 3b) with the aluminum plating bath
formulation of step 3a), wherein this step is carried out with a
current density ranging from 1 to 100 mA/cm.sup.2, at a temperature
ranging from 20 to 100.degree. C., under a dry inert gas and
stirring;
[0112] d) rinsing the aluminum coated ferrous steel alloy obtained
in step 3c), preferably with alcohol and water followed by drying
until constant weight; and
[0113] e) heat treating the aluminum coated specimens at
190.+-.14.degree. C. for at least 23 hours.
[0114] This specification further refers to the aluminum coated
ferrous steel alloy obtained by the electroplating process
described herein. Preferably, the ferrous steel alloy is a high
strength steel alloy as described therein.
[0115] Additionally, this specification refers to the aluminum
coating obtained by the process described herein. The aluminum
coating obtained by the electroplating process described can be
used in the aeronautical industry, preferably as Ni-free Cd
replacement in sacrificial coatings for high strength steel
alloys.
[0116] Thus, the aluminum coating described herein can achieve a
similar or better performance than the one obtained by LHE Cd or
the AlumiPlate.TM. methods, but plating using safer handling,
less-hazardous and more environmentally friendly electrolytes.
[0117] A further object of this specification refers to an aluminum
plating bath formulation comprising: a source of aluminum, an ionic
liquid, a brightening agent and an alkali metal halide, wherein all
these components have the same meaning as previously described in
this specification.
[0118] More specifically, the aluminum plating bath formulation may
comprise, or consist of: [0119] an aluminum halide, [0120] a
nitrogen-containing compound selected from N-alkyl-N'-alkyl'
imidazolium halide and N-alkyl-N'-alkyl' pyrrolidinium halide,
[0121] a brightening agent such as, for example,
1,10-phenanthroline, phthalazine, saccharin, isoniazid, coumarin,
isonicotinic acid, nicotinic acid, 3,4-(methylenedioxy)toluene,
1,4-butynediol, 2-aminothiazole, 2-mercaptothiazoline,
1-methylimidazole, or combinations thereof, and [0122] an alkali
metal halide.
[0123] In preferred embodiments, the aluminum plating bath
formulation may comprise, or consists of:
[0124] a range from 95.30 to 99.95 wt % of a mixture of aluminum
trichloride and 1-ethyl-3-methylimidazolium chloride, wherein both
components are present in the mixture in a molar ratio ranging from
80:40 to 60:40,
[0125] a range from 0.01 to 1.0 wt % of 1,10-phenanthroline,
and
[0126] a range from 0.04 to 3.7 wt % of KCl.
[0127] This environmentally friendly formulation is particularly
suitable for use in the aluminum electroplating process described
herein, providing an aluminum coating particularly useful as
Ni-free Cd replacement in sacrificial coatings for high strength
steel alloys. Thus, the aluminum coating obtained with this
environmentally friendly formulation shows similar or better
performance than the LHE Cd or the AlumiPlate.TM. methods but
plating using safer handling, less hazardous and more
environmentally friendly electrolytes.
[0128] The aluminum bath formulation described herein may be
synthesized as follows: The ionic liquid, for example the
nitrogen-containing compound, may be dried at 70.degree. C. under
vacuum. Then, the required amount of aluminum halide may be added
slowly under inert gas, such as argon, flow. Finally, the ionic
liquid may be cooled down and, optionally, stored in a
humidity-free atmosphere. Alternatively, commercial ionic liquid
comprising the required ratio of ionic liquid (for example,
nitrogen-containing compound) and aluminum halide may also be
used.
[0129] Then, the ionic liquid may be heated up to 80.degree. C. in
a closed vessel under a dry inert gas stream while stirring, and
the brightening agent may be added to the heated ionic liquid.
[0130] After that, this mixture may be stirred for 2 h at
80.degree. C. in the closed vessel under a dry inert gas stream.
Then, the alkali metal halide may be added to the mixture, and the
formulation may be stirred for 2 h at 80.degree. C. in the closed
vessel under a dry inert gas stream.
[0131] Finally, the bath formulation may be cooled down and stored
in a humidity-free atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
[0132] FIG. 1: Aluminum electroplating process flow-diagram
according to a particularly preferred embodiment.
[0133] FIGS. 2a and 2b: Cross section micrographs of each coating.
(1) With Cr-VI post-treatment; (2) Bare aluminum without conversion
coating post-treatment; (3) In-house formulated AlCl.sub.3-EMIC
60:40; (4) Basionics.TM. Al01 based; (5) Water rinsing during
post-treatment; (6) Ethyl-alcohol rinsing during post-treatment;
(8) F-220 alumina grit blasting during pre-treatment; (9) F-80
alumina grit blasting during pre-treatment; (10) F-36 alumina grit
blasting during pre-treatment.
[0134] FIG. 3: Representative photographs of aluminum electroplated
panels subjected to the scribe-grid+tape adhesion tests. (1) With
Cr-VI post-treatment; (2) Bare aluminum without conversion coating
post-treatment; (3) In-house formulated AlCl.sub.3-EMIC 60:40; (4)
Basionics.TM. Al01 based; (5) Water rinsing during post-treatment;
(6) Ethyl-alcohol rinsing during post-treatment; (8) F-220 alumina
grit blasting during pre-treatment.
[0135] FIG. 4: Representative photographs of aluminum electroplated
panels subjected to the bend adhesion tests. (1) With Cr-VI
post-treatment; (2) Bare aluminum without conversion coating
post-treatment; (3) In-house formulated AlCl.sub.3-EMIC 60:40; (4)
Basionics.TM. Al01 based; (5) Water rinsing during post-treatment;
(6) Ethyl-alcohol rinsing during post-treatment; (8) F-220 alumina
grit blasting during pre-treatment; (9) F-80 alumina grit blasting
during pre-treatment; (10) F-36 alumina grit blasting during
pre-treatment.
[0136] FIG. 5: Representative panels of each coating after the
corrosion tests, unscribed panels. (1) With Cr-VI post-treatment;
(2) Bare aluminum without conversion coating post-treatment; (3)
In-house formulated AlCl.sub.3-EMIC 60:40; (4) Basionics.TM. Al01
based; (5) Water rinsing during post-treatment; (6) Ethyl-alcohol
rinsing during post-treatment; (8) F-220 alumina grit blasting
during pre-treatment.
[0137] FIG. 6: Representative panels of each coating after the
corrosion tests, scribed panels. (1) With Cr-VI post-treatment; (2)
Bare aluminum without conversion coating post-treatment; (3)
In-house formulated AlCl.sub.3-EMIC 60:40; (4) Basionics.TM. Al01
based; (5) Water rinsing during post-treatment; (6) Ethyl-alcohol
rinsing during post-treatment; (8) F-220 alumina grit blasting
during pre-treatment.
[0138] FIGS. 7a and 7b: Results of the throwing power assessment.
(1) With Cr-VI post-treatment; (2) Bare aluminum without conversion
coating post-treatment; (3) In-house formulated AlCl.sub.3-EMIC
60:40; (4) Basionics.TM. Al01 based; (5) Water rinsing during
post-treatment; (6) Ethyl-alcohol rinsing during post-treatment;
(8) F-220 alumina grit blasting during pre-treatment.
EXAMPLES
1. Ionic Liquid Electrolytes
[0139] The ionic liquid electrolytes used in these examples were
synthesized as follows:
[0140] B01: Either the as-received Basionics.TM.Al01 ionic liquid
electrolyte from BASF or the house-made AlCl.sub.3-EMIC 60:40
electrolyte (see below) were independently used as baseline
electrolytes to be modified with the different additives.
[0141] AlCl.sub.3-EMIC 60:40 electrolyte was prepared by mixing the
corresponding amounts of aluminum trichloride and
1-ethyl-3-methyl-imidazolium chloride, as follows: The
1-ethyl-3-methylimidazolium chloride [EMIC](Fluka Ref. 30764,
purity min 93%), was dried at 70.degree. C. under vacuum for
several hours. Then, it was placed into a glass vessel. The
aluminum trichloride [AlCl.sub.3](Across Organics Ref. 19578,
anhydrous, 99%, granules) was weighted (as received) inside a
glovebox filled with argon inside a glass dispenser; then, it was
transferred to an addition funnel, taken out of the glovebox, and
placed on top of the glass vessel already containing the EMIC. The
ionic liquid electrolyte was synthesized by slowly adding the
AlCl.sub.3 to the EMIC under an argon flow. Finally, the ionic
liquid electrolyte was cooled down and stored in a humidity-free
atmosphere.
[0142] B01-phen: The baseline electrolyte was heated up to
80.degree. C. in a closed vessel under a dry inert gas stream while
stirring. Then, a range from 0.1 to 1.0% wt of 1,10-phenanthroline
was added. The Basionics.TM.Al01 ionic liquid modified with the
1,10-phenanthroline was stirred for 2 h at 80.degree. C. in the
closed vessel under a dry inert gas stream. Finally, the ionic
liquid formulation obtained was cooled down and stored in a
humidity-free atmosphere.
[0143] B01-phen-KCl: An ionic liquid formulation comprising
1,10-phenanthroline obtained as described above (B01-phen) was
heated at 80.degree. C. in a closed vessel under a dry inert
atmosphere gas stream while stirring. Then, a range from 5 to 50
g/L of KCl was added and the formulation obtained was stirred for 2
h at 80.degree. C. in the closed vessel under a dry inert gas
stream. Finally, the aluminum plating formulation bath was cooled
down and stored in a humidity-free atmosphere.
2. Electroplating Process
2.1 Pre-Treatment of the Aluminum Anode Substrates
[0144] To plate onto flat rectangular panels, a rectangular slot in
the center of the electrochemical cell's cover fixes the cathode.
The cathode was a high strength steel rectangular sheet panel. In
particular, the cathode was a rectangular flat panel machined from
4130 alloy steel conforming to AMS 6350 steel sheet. The anodes
were 2 rectangular 99.999% purity aluminum sheets and were
positioned at both sides of the cathode.
[0145] To plate onto specimens with cylindrical geometry, there was
a cylindrical hole in the center of the electrochemical cell's
cover to fix the cathode. The cathode was a high strength steel
cylindrical specimen. In particular, the cathode was a cylindrical
1.a.1 geometry type AISI 4340/SAE AMS-S-5000 steel specimen with
the size and geometry required by the ASTM F-519 standard. The
material was certified by the supplier according to the
requirements of the ASTM F-519 standard. The anode was an A11050
aluminum cylindrical sheet, which was positioned around the
cathode.
[0146] The aluminum anode substrates were all pre-treated following
a same procedure, independently of the plating bath's formulation
and the electroplating conditions. These pre-treatments involved:
[0147] Mechanical polishing: The aluminum anode substrates were
first manually polished with P-120 emery paper and the powder
remaining on the surface was then removed with a white cloth.
[0148] Alkaline cleaning: The polished aluminum anode substrates
were immersed in a cleaning bath which contained a range from 45 to
60 g/L of Turco-4215 NC LT and a range from 1 to 3 g/L of T-4215 NC
LT ADD (additive) during a period ranging from 5 to 30 min.
Alkaline cleaning was carried out stirring at a range from 200 to
500 rpm, at a temperature ranging from 45 to 55.degree. C. After
that, the aluminum anode substrates were manually rinsed with tap
water followed by deionized water rinsing. [0149] Deoxidizing: The
cleaned aluminum anode substrates were immersed in an deoxidizing
bath which contained a range from 60 to 120 g/L of Turco Smut Go NC
and a range from 15 to 30 g/L of HNO.sub.3 (42.degree.Be) during a
period ranging from 1 to 10 min. Deoxidizing was carried out at a
temperature in the range from 20 to 50.degree. C. After that, the
aluminum anode substrates are manually rinsed with tap water
followed by deionized water rinsing. [0150] Drying: The deoxidized
aluminum anode substrates were manually rinsed with acetone and
were dried using hot air. [0151] Racking: Finally, the aluminum
anode substrates were placed in the holding rack.
2.2 Pre-Treatment of the High Strength Steel Alloy Cathode
Substrates
[0152] The steel cathode substrates were all pre-treated following
the same procedure, independently of the plating bath formulation
and the electroplating conditions. These pre-treatments involved:
[0153] Degreasing: The steel cathode substrates were first manually
degreased with acetone and then they were immersed in acetone which
was placed in an ultrasonic bath for 10 minutes, until neither oil
nor grease remained on their surface. After that, the specimens
were dried using hot air. [0154] Stripping: The degreased steel
surface was then dry-blasted with F-220, F-80 or F-36 grit alumina
to remove any possible oxide and impurities off the steel
substrate. The powder remaining on the surface after blasting was
removed with compressed air. [0155] Masking: The areas of a part
which do not need to be plated were masked using conventional means
which do not contaminate the plating bath formulation, such as
masking tape. [0156] Racking: The pre-treated steel cathode
substrates were placed in the holding rack.
2.3 Bath Conditioning and Etching
[0157] Prior to the anodic polarization step, the aluminum anode
substrates were immersed in the plating bath and the electrolyte
was conditioned by purging the plating bath formulation with a dry
inert gas stream placed inside the plating bath during 30 minutes.
Once the electrolyte had been conditioned, the dry inert gas purger
was placed outside the electrolyte.
[0158] After that, the steel cathode substrates were immersed in
the conditioned ionic liquid bath, which was to be used afterwards
for aluminum plating, and an anodic polarization step of 0.6 V was
applied for 30 seconds. Etching was performed at the same
temperature as plating.
2.4 Electroplating
[0159] The experimental set-up for aluminum plating was the same
independently of the plating bath's composition and the plating
conditions. This set-up slightly changed depending on the geometry
of the specimens (cathode substrates) to be electroplated.
[0160] The electrochemical cell consisted of a closed vessel
containing a predetermined amount of the ionic liquid electrolyte.
The electroplating was conducted under a dry inert gas stream in
order to prevent contact of the electrolyte with the ambient's
moisture. However, an accurate control of oxygen and moisture in
the electrochemical cell was not needed. The cover of the vessel
had different slots and holes where the cathode, the anodes, the
temperature controller and the inert gas inlet and exhaust were
assembled.
[0161] The electroplating process comprised the immersion of the
pre-treated steel specimens in the bath formulation, closing the
electric circuit with the adequate fixtures and applying a
pre-determined cathodic direct current density to the cathode for a
pre-determined amount of time and temperature while the electrolyte
is kept at a pre-determined temperature.
[0162] The electroplating experiments were performed using a
current rectifier to provide the power supply under dry inert gas
stream. A hot plate with magnetic stirrer coupled to a temperature
controller provided heat and stirred the electrolyte at different
rpms.
[0163] The process conditions for aluminum coatings subjected to
the preliminary qualification tests are summarized in the following
table (Table I).
TABLE-US-00001 TABLE I Aluminum electroplating conditions Current
density Temperature Plating time Ref. (mA/cm.sup.2) (.degree. C.)
Bath agitation (minutes) B01-1 2.5-7.5 50 No 60-180 B01-2 5 40 No
270 B01-phen 5-25 40-75 Yes 30-120 B01-phen-KCl 5-25 40-75 Yes
30-120
2.5 Post-Treatment
[0164] After the cathode substrates were electroplated, the
aluminum coatings were all post-treated following the procedure
described below, independently of the plating bath's composition
and the plating conditions used.
[0165] The aluminum plated cathode substrates were manually rinsed
with deionized water or, alternatively, with ethyl alcohol followed
by deionized water until a clean surface free of any rest of ionic
liquid were obtained.
[0166] If rinsed only with water, a corrosion of the aluminum
coating was observed in the recessed areas of the cylindrical
specimens because of the resulting hydrolysis products, mainly
hydrochloric acid. Thus, rinsing with ethyl alcohol followed by
water rinsing was the preferred option.
[0167] The aluminum plated cathode substrates were dried using hot
air. Finally, some of the aluminum plated cathode substrates were
baked at 190.+-.14.degree. C. for 23 hours.
[0168] The aluminum plated cathode substrates were stored in a
controlled atmosphere without humidity.
2.6 Conversion Coating
[0169] Some of the aluminum plated cathode substrates were
conversion coated using the conventional Cr VI based conversion
treatment Alodine 1200S.
3. Performance-Qualification Tests
[0170] The high strength steel specimens Al plated using the
electrolyte compositions and process conditions described above
were tested in terms of coating's appearance, thickness,
composition, cross section morphology, adhesion, corrosion
resistance, throwing power and hydrogen embrittlement
susceptibility according to the test procedures described in Table
II (see below).
TABLE-US-00002 TABLE II Performance requirement and acceptance
criteria Reference Acceptance criteria Test method specifications
Appearance Coating must be continuous, smooth, Visual inspection
WP-200022 adherent, uniform in appearance, free HSSJTP from
blisters, pits, nodule, burning MIL-STD-870 contaminants, excessive
powder, and AMS-QQ-P-416 other apparent defects. Thickness Coating
thickness within 12-20 .mu.m Cross section MIL-STD-870 inspection
AMS-QQ-P-416 ASTM B-487 MIL-DTL 83488D Composition The composition
of the aluminum SEM/EDS analysis MIL-DTL 83488D coating less than
99.0 percent aluminum. Morphology Coating composition must be Cross
section -- uniform, non-dendritic, homogeneous inspection &
must cover the entire substrate Scribe-grid If a portion of coating
between the ASTM B 571 -- adhesion test lines breaks away from the
substrate, (section 13 but the adhesion is inadequate; applying a
pressure sensitive tape) Bend Little to no separation (peeling,
ASTM B 571 WP-200022 adhesion test flaking, fracturing or
blistering) of the (section 3) HSSJTP coating from the basis metal
or from MIL-STD-870 any under-plating at the rupture edge.
AMS-QQ-P-416 Cracking is acceptable in the bend MIL-DTL-83488D area
if the coating cannot be peeled back with a sharp instrument;
Unscribed Minimum of 3,000 h before Neutral Salt Spray WP-200022
salt spray appearance of red rust or comparable per ASTM B-117
HSSJTP corrosion to LHE Cd controls for CrVI (unscribed)
MIL-STD-870 resistance passivated alternative coatings QQ-P-416P
(acc. HSSJTP) Unscribed Test period (hours) No Neutral Salt Spray
MIL-DTL-83488D salt spray evidence of the base per ASTM B-117
corrosion metal corrosion (unscribed) resistance Type I Type 2
(acc. MIL- (as (Supplementary DTL-83488) Class coated) CrVI
treatment) 1 (min. 504 672 26 .mu.m) 2 (min. 336 504 13 .mu.m) 3
(min. 168 336 8 .mu.m) Scribed salt Minimum of 1,000 h before
Neutral Salt Spray WP-200022 spray appearance of red rust or
comparable per ASTM B-117 HSSJTP corrosion to LHE Cd controls for
CrVI (scribed) MIL-STD-870 resistance passivated alternative
coatings QQ-P-416P Coating The electrolyte able to properly throw
Visual/cross section MIL-STD-870 coverage/ and cover the entire
notch of 1.a.1 inspection QQ-P-416P Throwing geometry type (acc. to
ASTM F-519) MIL-DTL-83488D power specimens Hydrogen 4 type 1.a.1
specimens ASTM F-519 ASTM F-519 Embrittlement No fracture within
200 h, or if 1 WP-200022 sample fractures step load the rest by
HSSJTP 5% NFS every 2 h (fracture at >90% MIL-STD-870 NFS)
[0171] LHE Cd plated specimens conforming to MIL-STD-870B
specification Class 2 Type II were also tested for comparison. The
different aluminum plated coatings as well as the LHE Cd controls
were rated, at a minimum, providing pass/fail results according to
the success criteria agreed in Table II for each test. A "pass"
rating typically indicates a performance equivalent or better than
that of Cd. The results were also compared to those found for
AlumiPlate.TM. in the literature ([Final report WP-200022] and
[Report number JF130828, Juergen Fischer et. al, Electrodeposition
of aluminum with different ionic liquid based electrolytes and
their comparison with the AlumiPlate.RTM. layer, University of
North Dakota, January 2014]). The corrosion results were also
evaluated according to the MIL-DTL-83488D specification (Detail
Specification, Coating, Aluminum, High purity) standard considered
by the aerospace industry for the evaluation of Cd replacement
candidates whose composition is pure Al (e.g. IVD Al,
AlumiPlate.RTM., etc) (see Table II).
[0172] The types of substrates and test specimens that were used
for evaluating coating appearance, thickness, composition, cross
section morphology, adhesion and corrosion resistance were
rectangular flat panels machined from 4130 alloy steel conforming
to AMS 6350 steel sheet.
[0173] The test-specimens for thickness and composition
determination, cross section morphology examination and adhesion
tests were nominally 1 inch.times.4 inch.times.0.04 inches (25.4
mm.times.101.6 mm.times.0.10 mm). Unless otherwise specified, two
specimens were used for each test.
[0174] The test-specimens for corrosion resistance tests were
nominally 2 inch.times.4 inch.times.0.04 inches (50.8
mm.times.101.6 mm.times.0.10 mm). Unless otherwise specified, two
specimens were used for each test (2 scribed and 2 unscribed).
[0175] The types of substrates and test-specimens that were used
for hydrogen embrittlement were cylindrical 1.a.1 geometry type
AISI 4340/SAE AMS-S-5000 steel specimens with the size and geometry
required by the ASTM F-519 standard. The material and the
test-specimens were certified by the supplier according to the
requirements of the ASTM F-519 standard. Unless otherwise
specified, four specimens were used for hydrogen embrittlement
testing.
[0176] The test-specimens for the throwing power assessment were
cylindrical 1.a.1 geometry type AISI 4340/SAE AMS-S-5000 steel
specimens conforming to ASTM F-519 standard. Unless otherwise
specified, the coverage of the notch by the coating in all
specimens to be subjected to hydrogen embrittlement tests was
evaluated.
[0177] Tested samples are:
LHE Cd (1)
B01-1 (2)(3)(5)(8)
B01-2 (2)(3)(5)(8)
[0178] B01-phen (2)(4)(5)(8) B01-phen-KCl (2)(4)(6)(8) B01-phen-KCl
(1)(4)(6)(8) B01-phen-KCl (2)(4)(6)(7)(8) B01-phen-KCl (2)(4)(6)(9)
B01-phen-KCl (2)(4)(6)(10) wherein [0179] B01-phen: 0.1% wt
1,10-phenanthroline, [0180] B01-phen-KCl: 0.1% 1,10-phenanthroline
and 5 g/L KCl
[0181] (1) with Cr-VI post-treatment
[0182] (2) bare Al without conversion coating post-treatment
[0183] (3) in-house formulated AlCl.sub.3-EMIC 60:40
[0184] (4) Basionics.TM.Al01 based
[0185] (5) water rinsing during post-treatment
[0186] (6) ethyl-alcohol rinsing during post-treatment
[0187] (7) baking post-treatment
[0188] (8) F-220 alumina grit blasting during pre-treatment
[0189] (9) F-80 alumina grit blasting during pre-treatment
[0190] (10) F-36 alumina grit blasting during pre-treatment
3.1 Appearance
[0191] In general, the appearance of all tested coatings was
determined to be acceptable and all candidate coatings as well as
the baseline LHE Cd coating were given a "pass" rating for
appearance. The detailed results from the visual examination of the
coatings are shown in Table III.
TABLE-US-00003 TABLE III Appearance Coating Appearance results
Pass/Fail AlumiPlate .TM. Coating is continuous, uniform, smooth,
adherent, free from Pass blisters, pits, excessive powder and
contamination. LHE Cd Coating is continuous, uniform, smooth,
adherent, free from Pass (1)(8) blisters, pits, excessive powder
and contamination. B01-1 White-grey colored and dull appearance.
Pass (2)(3)(5)(8) Coating is continuous, uniform, smooth, adherent,
free from blisters, pits, excessive powder and contamination. B01-2
White-grey colored and dull appearance. Pass (2)(3)(5)(8) Coating
is continuous, uniform, smooth, adherent, free from blisters, pits,
excessive powder and contamination. B01-phen White-grey colored and
semi-bright reflective appearance. Pass (2)(4)(5)(8) Coating is
continuous, uniform, smooth, adherent, free from blisters, pits,
excessive powder and contamination. B01-phen- White-grey colored
and semi-bright reflective appearance. Pass KCl Coating is
continuous, uniform, smooth, adherent, free from (2)(4)(6)(8)
blisters, pits, excessive powder and contamination. B01-phen-
Yellowish colored and semi-bright appearance. Coating is Pass KCl
continuous, uniform, smooth, adherent, free from blisters,
(1)(4)(6)(8) pits, excessive powder and contamination. B01-phen-
White-grey colored and semi-bright reflective appearance. Pass KCl
Coating is continuous, uniform, smooth, adherent, free from
(2)(4)(6)(9) blisters, pits, excessive powder and contamination.
B01-phen- White-grey colored and semi-bright reflective appearance.
Pass KCl Coating is continuous, uniform, smooth, adherent, free
from (2)(4)(6)(10) blisters, pits, excessive powder and
contamination.
3.2 Thickness
[0192] The thickness of B01-phen and B01-phen-KCl coatings was
determined to be acceptable (between 12 and 20 .mu.m) as well as
that of the baseline LHE Cd coating. Thus, these coatings were
given a "pass" rating for thickness. The thickness of the B01-1 and
B01-2 coatings was not fine-tuned to be within 12-20 .mu.m and,
thus, they were given a "fail" rating. The detailed results of the
cross section's inspection of the coatings (according to ASTM
B-487) are shown in Table IV.
TABLE-US-00004 TABLE IV Thickness (cross-section examination ASTM
B-487) Reading average (.mu.m) Coating Specimen 1 Specimen 2
Pass/Fail AlumiPlate .TM. .gtoreq.13 (targeted 23) Pass LHE Cd
(1)(8) 12 12 Pass B01-1 (2)(3)(5)(8) 7.3 6.9 Fail B01-2
(2)(3)(5)(8) 23 25 Fail B01-phen (2)(4)(5)(8) 12 12 Pass
B01-phen-KCl (2)(4)(6)(8) 14 14 Pass B01-phen-KCl (1)(4)(6)(8) 17
16 Pass
3.3 Composition
[0193] The composition of the tested coatings was determined to be
acceptable (e.g., not less than 99% of Al). Thus, the coatings were
given a "pass" rating for composition. The composition of
B01-phen-KCl (1)(4)(6)(8) was less than 99% of Al due to the Cr-VI
post-treatment on top of the aluminum coating. The detailed results
of the surface SEM/EDS examination are shown in Table V.
TABLE-US-00005 TABLE V Composition (surface SEM/EDS examination)
Reading Average Wt Wt Wt Wt Wt Wt Wt % % % % % % % Pass/ Coating O
Al S Cl Cr Fe Cd Fail AlumiPlate .TM. 100 Pass LHE Cd 53.73 0 0.80
0 7.66 0 37.81 -- (1)(8) B01-1 0 100 0 0 0 0 0 Pass (2)(3)(5)(8)
B01-2 -- -- -- -- -- -- -- -- (2)(3)(5)(8) B01-phen 0 100 0 0 0 0 0
Pass (2)(4)(5)(8) B01-phen- 0 99.14 0 0.0 0 0.86 0 Pass KCl
(2)(4)(6)(8) B01-phen- 19.47 75.66 0.25 0 4.02 0.61 0 -- KCl
(1)(4)(6)(8)
3.4 Cross Section Morphology
[0194] The cross section morphology of the coatings electroplated
from the B01-phen and B01-phen-KCl electrolytes was determined to
be acceptable (uniform, adherent, dense and leveled coatings) as
well as that of the baseline LHE Cd coating. Thus, these coatings
were given a "pass" rating for cross section morphology. The
coatings electroplated from the B01-1 and B01-2 electrolytes failed
since non-uniform, non-dense coatings tending to dendritic
morphology were obtained.
[0195] The cross section morphology of the aluminum coatings was
radically improved when the AlCl.sub.3-EMIC 60:40
(Basionics.TM.Al01) baseline electrolyte was modified with the
1,10-phenanthroline additive. The addition of KCl did not
jeopardize the cross section morphology of the coatings while
improving other properties.
[0196] The cross section morphology for B01-phen-KCl coatings was
acceptable even if a bigger alumina particle size of F-80 grit was
used during blasting in the pre-treatment.
[0197] The detailed results from the cross section inspection of
the coatings are shown in Table VI. FIGS. 2a and 2b show
representative cross section's micrographs of each coating.
TABLE-US-00006 TABLE VI Cross section morphology Coating Cross
section examination results Pass/Fail AlumiPlate .TM. Coating
showing with complete coverage of the Pass substrate LHE Cd (1)(8)
Uniform adherent and levelled coatings that completely Pass covered
the substrate; Borders were uniformly and well covered. B01-1
Non-uniform coating, non-dense, non-compact, Fail (2)(3)(5)(8)
dendritic, especially in the edges B01-2 Non-uniform coating,
non-dense, non-compact, Fail (2)(3)(5)(8) dendritic, especially in
the edges B01-phen Uniform adherent and levelled coatings that
completely Pass (2)(4)(5)(8) covered the substrate; Borders were
uniformly and well covered. B01-phen-KCl Uniform adherent and
levelled coatings that completely Pass (2)(4)(6)(8) covered the
substrate; Borders were uniformly and well covered. B01-phen-KCl
Uniform adherent and levelled coatings that completely Pass
(1)(4)(6)(8) covered the substrate; Borders were uniformly and well
covered. B01-phen-KCl Uniform adherent and levelled coatings that
completely Pass (2)(4)(6)(9) covered the substrate; Borders were
uniformly and well covered. B01-phen-KCl Uniform adherent and
levelled coatings that completely Pass (2)(4)(6)(10) covered the
substrate; Borders were uniformly and well covered.
3.5 Scribe-Grid Tape Adhesion
[0198] In general, the scribe-grid tape adhesion of all tested
coatings was determined to be acceptable (no coating detachment
between the scribed lines) and all candidate coatings as well as
the baseline LHE Cd coating were given a "pass" rating for
scribe-grid tape adhesion. The detailed results of the visual
examination conducted after subjecting the specimens to the
adhesion test are shown in Table VII. FIG. 3 shows representative
panels of each candidate coating after the adhesion testing.
TABLE-US-00007 TABLE VII Scribe-grid and tape adhesion (ASTM
B571/s. 13 applying a pressure sensitive tape) Coating Scribe grid
+ tape adhesion results Pass/Fail LHE Cd (1)(8) No coating
detachment between the Pass scribed lines B01-1 (2)(3)(5)(8) No
coating detachment between the Pass scribed lines B01-2
(2)(3)(5)(8) No coating detachment between the Pass scribed lines
B01-phen (2)(4)(5)(8) No coating detachment between the Pass
scribed lines B01-phen-KCl No coating detachment between the Pass
(2)(4)(6)(8) scribed lines B01-phen-KCl No coating detachment
between the Pass (1)(4)(6)(8) scribed lines
3.6 Bend Adhesion
[0199] When the substrates were pre-treated using F-220 alumina
grit during pre-treatment, the bend adhesion of the coatings
electroplated from the B01-1 electrolyte was determined to be
acceptable, since little to no separation of the coating from the
basis metal at the rupture edge occurred, as well as that of the
baseline LHE Cd coating. These coatings were given a "pass" rating
for bend adhesion. The rest of the coatings failed, even if the
failure was only marginal for the coatings electroplated from the
B01-phen-KCl electrolyte.
[0200] The coatings electroplated from the B01-2 electrolyte were
considerably thicker than those plated from the B01-1 electrolyte,
which hindered the adhesion.
[0201] The bend adhesion of the aluminum coatings seemed to
decrease when using the 1,10-phenanthroline and KCl additives in
the AlCl.sub.3-EMIC 60:40 (Basionics.TM.Al01) baseline
electrolyte.
[0202] However, when increasing the particle size of the alumina
grit used during blasting (i.e. when F-80 or F-36 alumina grit was
used during pre-treatment instead of F-220 alumina grit), the
coatings electroplated from the B01-phen-KCl electrolyte passed the
adhesion test.
[0203] The detailed results of the visual examination conducted
after subjecting the specimens to the adhesion test are shown in
Table VIII. FIG. 4 shows representative panels of each candidate
coating after the adhesion tests.
TABLE-US-00008 TABLE VIII Bend adhesion (ASTM B 571/s. 3) Coating
Bend adhesion results Pass/Fail AlumiPlate .TM. Cracking of coating
up to 1/8 inch. Pass LHE Cd (1)(8) No separation of the coating
from the basis metal at Pass the rupture edge. B01-1 (2)(3)(5)(8)
No separation of the coating from the basis metal Pass at the
rupture edge. B01-2 (2)(3)(5)(8) Significant coating detachment in
the rupture edge Fail B01-phen Significant coating detachment in
the rupture edge Fail (2)(4)(5)(8) B01-phen-KCl Slight coating
detachment in the central area of Fail but (2)(4)(6)(8) the rupture
edge and especially at the side edges. marginal B01-phen-KCl
Coating detachment in the central area and at the Fail (1)(4)(6)(8)
borders of the rupture edge. B01-phen-KCl No cracks or coating
detachment in the central Pass (2)(4)(6)(9) area of the tested
face. Some coating detachment at the borders of the ruptured edge.
B01-phen-KCl No cracks or coating detachment in the central Pass
(2)(4)(6)(10) area of the tested face. Some coating detachment at
the borders of the ruptured edge.
3.7 Unscribed Salt Spray Corrosion Resistance
[0204] The corrosion resistance of unscribed panels of coatings
electroplated from the B01-phen-KCl electrolyte was determined to
be acceptable (more than 3,000 hours to red rust) as well as that
of the baseline LHE Cd coating, both with CrVI post-treatment on
top. These coatings were given a "pass" rating for unscribed salt
spray corrosion resistance according to HSSJTP criteria. The B01-2
coatings were also given a "pass" since they were able to withstand
more than 3,000 hours to red rust without any kind of conversion
coating post-treatment on top.
[0205] On the other hand, the corrosion resistance of the coatings
obtained with B01-1, B01-2 and B01-phen-KCl (without or with
conversion coating post-treatment on top) was determined to be
acceptable according to the criteria of the MIL-DTL-83488D standard
(Class 3 coatings--minimum of 8 micron thick: more than 168 hours
to red rust for unpassivated coatings; Class 2 coatings--minimum of
13 microns thick: more than 336 hours to red rust for unpassivated
coatings; Class 3 coatings--minimum of 8 micron thick: more than
336 hours to red rust for coatings with supplementary CrVI
treatment; Class 2 coatings--minimum of 13 microns thick: more than
504 hours to red rust for coatings with supplementary CrVI
treatment).
[0206] The coatings electroplated from the B01-2 electrolyte were
considerably thicker than those plated from the B01-1 electrolyte,
which provided the corrosion resistance.
[0207] The corrosion resistance of the aluminum coatings decreased
when using the 1,10-phenanthroline and KCl additives in the
AlCl.sub.3-EMIC 60:40 (Basionics.TM.Al01) baseline electrolyte.
[0208] The detailed results of the visual examination conducted
after subjecting the specimens to the corrosion test are shown in
Table IX. FIG. 5 shows representative panels of each coating after
the corrosion test.
TABLE-US-00009 TABLE IX NSSF Corrosion resistance - Unscribed
panels (ASTM B-117 angle 6.degree. off) Pass/Fail Reading Hours
HSSJTP average to red (with MIL- thickness rust (2 CrVI post- DTL-
Coating (.mu.m) speciments) treatment) 83488 AlumiPlate .TM.
.gtoreq.13 >3,000 Pass Pass (targeted 23) LHE Cd (1)(8) 12
>3,000 Pass -- B01-1 (2)(3)(5)(8) 10 216 -- Pass 216 -- Pass
B01-2 (2)(3)(5)(8) ~30 3,864 Pass Pass 5,208 Pass Pass B01-phen --
-- -- -- (2)(4)(5)(8) B01-phen-KCl ~17 504 -- Pass (2)(4)(6)(8) 504
B01-phen-KCl ~17 >3,500 Pass Pass (1)(4)(6)(8) >3,500
3.8 Scribed Salt Spray Corrosion Resistance
[0209] The corrosion resistance of scribed panels of the
B01-phen-KCl coatings and that of the baseline LHE Cd coating (both
with Cr-VI post-treatment on top) was determined to be acceptable
(requirement of more than 1,000 hours to red rust) and were given a
"pass" rating for scribed salt spray corrosion resistance according
to HSSJTP criteria.
[0210] The rest of the coatings tested, i.e. B01-1, B01-2 and
B01-phen-KCl without Cr-VI post-treatment on top, were not
evaluated with respect to the HSSJTP and the MIL-DTL-83488 criteria
since they do not set-up specifications respectively for coatings
without Cr-VI post-treatment and for scribed coatings.
[0211] The higher thickness of the coatings plated from the B01-2
electrolyte in comparison to those plated from the B01-1
electrolyte provided the coatings' corrosion resistance.
[0212] The corrosion resistance of the aluminum coatings seemed to
decrease when using the 1,10-phenanthroline and KCl additives in
the AlCl.sub.3-EMIC 60:40 (Basionics.TM.Al01) baseline
electrolyte.
[0213] The detailed results of the visual examination conducted
after subjecting the specimens to the corrosion test are shown in
Table X. FIG. 6 shows representative panels of each coating after
the corrosion tests.
TABLE-US-00010 TABLE X NSSF Corrosion resistance - Scribed panels
(ASTM B-117 angle 6.degree. off) Pass/Fail (HSSJTP) Reading average
Hours to red (with CrVI post- Coating thickness (.mu.m) (3) rust
treatment) AlumiPlate .TM. .gtoreq.13 (targeted 23) 1000 Pass LHE
Cd (1)(8) 12 >1,000 Pass B01-1 (2)(3)(5)(8) 9.5 192 -- 168 B01-2
(2)(3)(5)(8) ~30 1512 -- 336 B01-phen -- -- (2)(4)(5)(8)
B01-phen-KCl ~19 336 -- (2)(4)(6)(8) 336 B01-phen-KCl 21 >3,500
Pass (1)(4)(6)(8) 17 3,168
3.9 Throwing Power
[0214] The throwing power of the coatings electroplated from the
B01-2 and B01-phen-KCl electrolytes was determined to be
acceptable, since achieved full coating coverage in the notch, as
well as that of the baseline LHE Cd coating. Thus, these coatings
were given a "pass" rating for throwing power. The coatings
electroplated from the B01-1 and B01-phen electrolytes failed.
[0215] The throwing power of the aluminum coatings seemed to
decrease when using the 1,10-phenanthroline additive in the
AlCl.sub.3-EMIC 60:40 (Basionics.TM.Al01) baseline electrolyte.
However, the addition of KCl to the B01-phen electrolyte
considerably improved the throwing power without jeopardizing the
rest of the properties.
[0216] Rinsing with ethyl alcohol rather than water during
post-treatment helped to remove completely the remaining
electrolyte from the notch avoiding stains and preventing possible
corrosion due to the presence of electrolyte.
[0217] The detailed results of the visual examination of the
coatings are shown in Table XI. FIGS. 7a and 7b show representative
photographs of the notched areas of 1.a.1 geometry type
specimens.
TABLE-US-00011 TABLE XI Throwing power (surface/cross section
examination) Coverage of Pass/ Coating 1.a.1 geometry type notch
Fail AlumiPlate .TM. Full coating coverage in the notch Pass LHE Cd
(1)(8) Full coating coverage in the notch Pass B01-1 (2)(3)(5)(8)
Uncoated areas in the notch root Fail B01-2 (2)(3)(5)(8) Full
coating coverage in the notch Pass B01-phen (2)(4)(5)(8) Uncoated
areas in the notch root Fail B01-phen-KCl (2)(4)(6)(8) Full coating
coverage in the notch Pass B01-phen-KCl (1)(4)(6)(8) Full coating
coverage in the notch Pass B01-phen-KCl Full coating coverage in
the notch Pass (2)(4)(6)(7)(8)
3.10 Hydrogen Embrittlement
[0218] The aluminum coatings electroplated from the B01-1 and B01-2
electrolytes, not subjected to any baking post-treatment, passed
the hydrogen embrittlement test (e.g., minimum of 200 hours without
fracturing). Also, both the B01-phen-KCl and the LHE Cd coatings,
when subjected to a baking step after aluminum electroplating,
passed the hydrogen embrittlement test. All these coatings were
given a "pass" rating for hydrogen embrittlement.
[0219] The B01-phen-KCl specimens not subjected to a baking
post-treatment failed the test. The embrittling properties of the
aluminum electroplating process seemed to decrease when using the
1,10-phenanthroline and KCl additives in the AlCl.sub.3-EMIC 60:40
(Basionics.TM.Al01) baseline electrolyte. The detailed results of
the hydrogen embrittlement tests are shown in Table XII.
TABLE-US-00012 TABLE XII Hydrogen embrittlement (ASTM F-519)
Load/Loading time required by Hours without Pass/ Coating ASTM
F-519 fracturing Fail AlumiPlate .TM. 200 Pass 200 200 200 LHE Cd
(1)(8) 75% NFS/200 h 200 Pass 200 200 200 B01-1 (2)(3)(5)(8) 75%
NFS/200 h 200 Pass 200 200 200 B01-2 (2)(3)(5)(8) 75% NFS/200 h 200
Pass 200 200 200 B01-phen (2)(4)(5)(8) -- -- -- B01-phen-KCl
(2)(4)(6)(8) -- -- -- B01-phen-KCl (1)(4)(6)(8) 75% NFS/200 h 216
Fail 123.3 215.4 177.6 B01-phen-KCl 75% NFS/200 h 200 Pass
(2)(4)(6)(7)(8) 200 200 200
4. Summary of Results
[0220] The commercially available AlCl.sub.3-EMIC 60:40 ionic
liquid without any additives (B01) led to about 30 micron thick
coatings which complied with the requirements for appearance,
thickness, composition, throwing power, corrosion resistance,
hydrogen embrittlement and scribe-grid adhesion. However, they had
a dendritic morphology and insufficient bend adhesion for the
approximately 30 .mu.m thick coatings.
[0221] When plating from the AlCl.sub.3-EMIC
60:40+1,10-phenanthroline ionic liquid (B01-phen) the morphology
was improved, but jeopardizing the throwing power and the bend
adhesion for approximately 12 .mu.m thick coatings. Nonetheless,
achieving dense and leveled aluminum coatings over the grit blasted
high strength steel surfaces was an important breakthrough. This
electrolyte allowed an acceptable aluminum plating at higher
current density and higher temperature than the AlCl.sub.3-EMIC
60:40 baseline electrolyte (without any additives), which results
in higher electrodeposition rates.
[0222] Significant improved results were obtained with the
AlCl.sub.3-EMIC 60:40+1,10-phenanthroline+KCl electrolyte
(B01-phen-KCl) since the coatings plated were uniform, not powdery,
and had a semi-bright metallic appearance. In terms of coating's
appearance, the electroplating process was quite robust, since
coatings with very similar appearance were produced within all the
tested operating ranges, i.e., with a current density ranging from
5 to 25 mA/cm.sup.2, at a temperature ranging from 40 to 75.degree.
C. and under a dry inert gas. The coatings had continuous, uniform,
leveled and compact cross section morphology, comparable to that of
the coatings electroplated from the B01-phen. The adhesion was
similar to that of B01-phen coatings.
[0223] The throwing power of the electrolyte was considerably
improved with respect to that of the B01-phen. This electrolyte
also allowed an acceptable aluminum plating at higher current
density and higher temperature than the B01 (without any
additives), which results in higher electrodeposition rates.
[0224] The B01-phen-KCl electroplating bath achieves an improvement
of the electrical conductivity of the bath and facilitates the
deposition of aluminum because of the shift of the reduction
potential of Al, so that an improvement of the throwing power can
also be achieved.
[0225] The B01-phen-KCl electroplating process also achieved good
adhesion properties of the resulting aluminum coating when F-80 to
F-36 alumina grit blasting was used during pre-treatment.
[0226] The B01-phen-KCl electroplating process also achieved good
hydrogen embrittlement resistance when a baking step at
190.+-.14.degree. C. for at least 23 hours was used during
post-treatment.
[0227] These coatings showed comparable or superior behavior to LHE
Cd and Alumiplate.TM. reference coatings.
[0228] Moreover, the aluminum coating complying with all of the
tests reported may be considered a more environmentally friendly
coating than other sacrificial coatings for high strength ferrous
steel parts such as Cd and Zn--Ni. The process to achieve such
coating would be considered more environmentally friendly, more
safe and easier to handle than Cd plating, Zn/Ni plating, Al
plating from organic solvents or AlumiPlate.TM. plating
process.
[0229] Besides, the aluminum coating complying with most of the
tests reported (except bend adhesion and/or hydrogen embrittlement)
may still be considered a more environmentally friendly coating
than other sacrificial coatings for ferrous steel parts such as Cd
and Zn--Ni providing similar or superior corrosion resistance
performance than Cd or Zn/Ni. The process to achieve such coating
would be still considered more environmentally friendly, more safe
and easier to handle than Cd plating, Zn/Ni plating, Al plating
from organic solvents or AlumiPlate.TM. plating process.
* * * * *