U.S. patent application number 16/376768 was filed with the patent office on 2019-08-01 for coated substrate having corrosion resistant coating.
The applicant listed for this patent is Hamilton Sundstrand Corporation. Invention is credited to Theresa A. Hugener, Mark R. Jaworowski, Michael A. Kryzman, Blair A. Smith, Georgios S. Zafiris, Weilong Zhang.
Application Number | 20190233943 16/376768 |
Document ID | / |
Family ID | 58428069 |
Filed Date | 2019-08-01 |
United States Patent
Application |
20190233943 |
Kind Code |
A1 |
Jaworowski; Mark R. ; et
al. |
August 1, 2019 |
COATED SUBSTRATE HAVING CORROSION RESISTANT COATING
Abstract
A coated substrate made by a method that includes pre-treating a
metal substrate, immersing the metal substrate in a trivalent
chromium bath which does not contain hexavalent chromium,
post-treating the coated metal substrate with an oxidizer, and
curing the coated metal substrate in a controlled environment.
Inventors: |
Jaworowski; Mark R.;
(Sarasota, FL) ; Smith; Blair A.; (South Windsor,
CT) ; Zafiris; Georgios S.; (Glastonbury, CT)
; Zhang; Weilong; (Glastonbury, CT) ; Kryzman;
Michael A.; (West Hartford, CT) ; Hugener; Theresa
A.; (Coventry, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hamilton Sundstrand Corporation |
Charlotte |
NC |
US |
|
|
Family ID: |
58428069 |
Appl. No.: |
16/376768 |
Filed: |
April 5, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15095926 |
Apr 11, 2016 |
10294570 |
|
|
16376768 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 22/78 20130101;
C23C 22/34 20130101; C23C 22/82 20130101; C23C 22/83 20130101; C23C
2222/10 20130101 |
International
Class: |
C23C 22/34 20060101
C23C022/34; C23C 22/83 20060101 C23C022/83; C23C 22/78 20060101
C23C022/78; C23C 22/82 20060101 C23C022/82 |
Claims
1. A coated substrate made by a method comprising: pre-treating a
metal substrate such that a surface of the metal substrate is
de-oxidized, wherein the metal substrate is selected from the group
consisting of aluminum, zinc, cadmium, copper, silver, magnesium,
tin, iron, aluminum based alloys, zinc based alloys, cadmium based
alloys, copper based alloys, silver based alloys, magnesium based
alloys, iron based alloys, and tin based alloys; immersing the
metal substrate in a coating solution to produce a chromium-based
coating on the metal substrate, wherein the coating solution
comprises a trivalent chromium compound and a fluoride compound,
but does not contain hexavalent chromium; removing the coated metal
substrate from the coating solution; curing the chromium-based
coating in a controlled environment containing a gaseous atmosphere
to produce a hexavalent chromium enriched corrosion resistant
coating on the metal substrate; post-treating the coated metal
substrate with an oxidizer after removing the coated metal
substrate from the coating solution, but before curing the coating;
and exposing the coated metal substrate to ozone in the controlled
environment.
2. The coated substrate of claim 1, wherein exposing the coated
metal substrate to ozone in the controlled environment comprises
exposing the coated metal substrate to 1 ppm ozone for one
hour.
3. The coated substrate of claim 1, wherein exposing the coated
metal substrate to ozone in the controlled environment comprises
exposing the coated metal substrate to 0.1 ppm ozone for between
four and twenty-four hours.
4. The coated substrate of claim 1, wherein the gaseous atmosphere
has a relative humidity of over fifty percent.
5. The coated substrate of claim 1, wherein the metal substrate is
post-treated with 0.3 wt % to 3.5 wt % hydrogen peroxide
solution.
6. The coated substrate of claim 5, wherein the metal substrate is
post-treated with 1.0 wt % to 2.5 wt % hydrogen peroxide
solution.
7. A coated substrate made by the method comprising: pre-treating a
metal substrate such that a surface of the metal substrate is
de-oxidized; immersing the metal substrate in a coating solution to
produce a chromium based coating on the metal substrate, wherein
the coating solution comprises a trivalent chromium compound and a
fluoride compound, but does not contain hexavalent chromium;
removing the coated metal substrate from the coating solution;
curing the chromium-based coating in a controlled environment
containing a gaseous atmosphere to produce a hexavalent chromium
enriched corrosion resistant coating on the metal substrate,
wherein the gaseous atmosphere has a relative humidity of over
fifty percent; post-treating the coated metal substrate with an
oxidizer after removing the coated metal substrate from the coating
solution, but before curing the coating, wherein the metal
substrate is post-treated with 0.3 wt % to 3.5 wt % hydrogen
peroxide solution; and exposing the coated metal substrate to ozone
in the controlled environment.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. application Ser.
No. 15/095,926 filed Apr. 11, 2016 for "METHOD FOR MAKING CORROSION
RESISTANT COATING" by M. R. Jaworowski, B. A. Smith, G. S. Zafiris,
W. Zhang, M. A. Kryzman, T. A. Hugener and B. A. Van Hassel.
BACKGROUND
[0002] Chromium based conversion coatings are used to passivate
metals such as aluminum, copper, cadmium, zinc, magnesium, tin,
silver, iron, and their alloys to reduce and slow corrosion of the
metal, or as a finishing coating. Chromium conversion coatings can
be applied to everyday items such as tools or hardware to prevent
corrosion, and to aerospace and commercial equipment with high
requirements for corrosion durability.
[0003] Traditionally, chromic acid was used to create conversion
coating. However, chromic acid contains high levels of hexavalent
chromium. Hexavalent chromium was used to create conversion
coatings due to its high oxidation state, resulting in highly
effective anti-corrosion coatings. Specifically, hexavalent
chromium based conversion coatings prevent oxide formation on the
surface of the metal, are conductive, thin and flexible, provide
adhesion for other coatings such as adhesives, paints, sealants,
and substantially slow corrosion.
[0004] However, hexavalent chromium is now known to be a dangerous
toxin and a known carcinogen. Chronic inhalation of hexavalent
chromium increases risk of lung cancer among other health
complications. The presence of hexavalent chromium in drinking
water has created substantial health risk as well. For this reason,
hexavalent chromium is heavily regulated in both the U.S. and
abroad. In 2017, the EU will ban hexavalent chromium for many
applications unless an authorization for a specific application or
use has been granted.
[0005] Corrosion resistance of a conversion coating is a function
of the amount of hexavalent chromium on the surface of the coating.
Thus, industry has been actively trying to find a substitute for
hexavalent chromium based conversion coatings. No alternatives to
hexavalent chromium coatings have exhibited as high a corrosion
resistance. Specifically, many inventors have tried to use
non-toxic trivalent chromium solutions to passivate metals and
create corrosion resistant coatings. In these methods, trivalent
chromium solutions are used during processing rather than
hexavalent chromium solutions. The suggested methods, such as that
disclosed in U.S. Pat. No. 5,304,257 to Pearlstein et al.
("Pearlstein"), do not create corrosion coatings that are as
effective as the previous hexavalent chromium based coatings.
[0006] Pearlstein discloses a method of making a coating which uses
an immersion bath of aqueous trivalent chromium to coat an aluminum
substrate. After the substrate is removed from the bath, the
coating is exposed to an oxidizing solution such that a small
portion of the trivalent chromium is converted to hexavalent
chromium. However, the method disclosed in Pearlstein contaminates
the oxidizing solution and any subsequent rinse waters with
hexavalent chromium, creating a chemical waste stream and a
chemical exposure hazard.
SUMMARY
[0007] A coated substrate made by a method that includes
pre-treating a metal substrate such that a surface of the metal
substrate is de-oxidized; immersing the metal substrate in a
coating solution to produce a chromium based coating on the metal
substrate, wherein the coating solution consists of a trivalent
chromium compound, and a fluoride compound, but not containing
hexavalent chromium; removing the coated metal substrate from the
coating solution; and curing the chromium based coating in a
controlled environment containing a gaseous atmosphere to produce a
hexavalent chromium enriched corrosion resistant coating on the
metal substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a flow chart depicting a method of producing a
corrosion resistant coating.
[0009] FIG. 2 is a graph depicting the corrosion resistance rating
of coatings as a function of varying processing and curing
conditions.
[0010] FIG. 3 is a series of photographs showing the morphology of
chromium based corrosion resistant coatings processed and cured
under different conditions.
[0011] FIG. 4A is a second series of photographs showing the
morphology of chromium based corrosion resistant coatings processed
and cured under different conditions as described in Table 5.
[0012] FIG. 4B is a third series of photographs showing the
morphology of chromium based corrosion resistant coatings processed
and cured under different conditions as described in Table 5.
DETAILED DESCRIPTION
[0013] The present invention discloses a method for making a
corrosion resistant coating using non-toxic trivalent chromium
during processing. Trivalent chromium conversion coatings, once
applied to a metal substrate, include varying amounts of naturally
occurring hexavalent chromium compounds in the coating. The
relative amount of hexavalent chromium to trivalent chromium in
conversion coatings varies systemically with types of
pre-treatment, processing conditions, and post-treatment. The
relative amount of hexavalent chromium to trivalent chromium
correlates strongly with corrosion resistance, as hexavalent
chromium compounds are corrosion inhibitors.
[0014] Thus, when the trivalent chromium based coating is cured
under a certain set of conditions, the oxidation state of trivalent
chromium will change to hexavalent chromium, increasing the
corrosion resistant properties of the coating without utilizing
hexavalent chromium during processing. Specifically, the coating is
cured in an environment where relative humidity, temperature,
atmosphere and other variables can be controlled. The controlled
curing allows for optimization of hexavalent chromium on the
surface of the metal substrate, and higher corrosion resistance,
without the toxin impact of using hexavalent chromium
solutions.
[0015] FIG. 1 depicts method 10 of making a trivalent chromium
based corrosion resistant coating. Method 10 includes pre-treating
a metal substrate (step 12), immersing the metal substrate in a
solution including trivalent chromium but not including hexavalent
chromium (step 14), removing the coated metal substrate from the
solution (step 16), post-treating the coated metal substrate with
an oxidizer (step 18), and curing the metal substrate in a
controlled environment to produce a hexavalent chromium enriched,
corrosion resistant coating on the metal substrate (step 20).
[0016] First, the metal substrate is pre-treated (step 12). The
metal substrate may be aluminum, zinc, cadmium, copper, silver,
magnesium, tin, iron, or alloys of those metals, such as aluminum
based alloys, zinc based alloys, cadmium based alloys, copper based
alloys, silver based alloys, magnesium based alloys, tin based
alloys, or iron based alloys. The pre-treatment may be chemical or
mechanical. A chemical pre-treatment can include degreasing with an
alkaline degreaser or with a different solvent, such as acetone or
isopropanol, rinsing with water, and using an acid cleaner, such as
nitric acid, to de-oxidize the surface of the metal substrate. In
contrast, a mechanical pre-treatment can include grit-blasting,
sanding, pumice scrubbing, or abrasive pad processing of the metal
substrate, and subsequently degreasing the metal substrate in
acetone, a different solvent, or a different degreaser.
[0017] After the metal substrate is pre-treated, it is immersed in
a trivalent chromium rich bath (step 14). The bath is a solution
containing a trivalent chromium salt, such as chromium sulfate, a
fluoride compound, such as an alkali metal flurosilicate, and a
sufficient amount of alkali to maintain the solution pH. Various
trivalent chromium process (TCP) solutions are readily
available.
[0018] During the bath, a trivalent chromium coating adheres to the
metal substrate. When the coating is formed on the metal substrate,
it is initially in the form of a hydrated gel on the surface of the
metal substrate. While still in the bath, the hydrated gel is
surrounded by the solution. The hydrated gel is permeable to oxygen
and fluoride ions from the solution. These molecules remove a
native oxide layer from the metal substrate to allow formation of
the chromium based coating on the surface of the metal substrate,
and allow for oxidation of trivalent chromium on the surface to
corrosion resistant hexavalent chromium.
[0019] When the chromium coating is formed, a small amount
trivalent chromium is naturally converted to hexavalent chromium.
The oxidation of trivalent chromium to hexavalent chromium occurs
as hydrogen peroxide is generated from the interaction of the metal
substrate with the TCP solution. Specifically, the metal substrate
undergoes oxidation and produces electrons. If the substrate is
aluminum, the aluminum is oxidized, forming an inner alumina film
that stretches across the aluminum surface, as shown in Equation
1:
2Al+3H.sub.2O.fwdarw.Al.sub.2O.sub.3+6H.sup.++6e.sup.- Equation
1
[0020] The alumina film dissolves through a reaction with
hydrofluoric acid. At the same time, in a water-rich environment,
such as the aqueous bath in this coating method, the electrons
produced through metal dissolution generate hydrogen peroxide, as
shown in Equation 2:
O.sub.2+2H.sub.2O+2e.sup.-.fwdarw.H.sub.2O.sub.2+2OH.sup.- Equation
2
[0021] The production of hydrogen peroxide on the surface of the
metal substrate allows for an oxidizing environment which oxidizes
some of the trivalent chromium on the surface to hexavalent
chromium, as shown in Equation 3:
2Cr(OH).sub.3+3H.sub.2O.sub.2.fwdarw.2Cr(OH).sub.6 Equation 3
[0022] Two other chemical reactions compete with the hydrogen
peroxide-producing reaction: the reduction of oxygen to hydroxyl
ions (pictured in Equation 2), and the evolution of hydrogen gas
from hydrogen ions, as shown in Equation 4 below.
2H.sup.++2e.sup.-.fwdarw.H.sub.2 Equation 4
[0023] Even though some hexavalent chromium is formed on the
surface of the metal substrate as it is exposed to the trivalent
chromium bath, there is still a high ratio of trivalent chromium on
the surface of the metal substrate when it is removed from the bath
(step 16).
[0024] After the coated metal substrate is removed from the bath,
the coating is post-treated with an oxidizer, such as hydrogen
peroxide (step 18). This post-treatment increases the oxidation
environment on the coating, and induces further oxidation of
trivalent chromium to hexavalent chromium (as shown in Equation 3),
and increases corrosion resistance of the coating. The hydrogen
peroxide solution can be 0.3 wt % to 3.5 wt %. The post-treatment
further oxidizes trivalent chromium on the surface of the coating
to increase the amount of hexavalent chromium on the surface of the
coating before it is placed in the controlled environment for
curing. Raman spectral data shows samples treated with hydrogen
peroxide at this stage have a higher ratio of hexavalent chromium
to total chromium on the surface of the coating, as shown in Table
1. The ratio of hexavalent chromium to total chromium content given
here is not an actual concentration, but is a relative proportion
estimated from detected Raman spectra peak heights, where
hexavalent chromium is obtained from a peak height of 880 cm.sup.-1
and a mixture of hexavalent chromium and trivalent chromium is
obtained from 860 cm.sup.-1. The peak assigned for trivalent
chromium is measured at 535 cm.sup.-1.
TABLE-US-00001 TABLE 1 Post-Treatment None 0.3 wt % H.sub.2O.sub.2
Average Ratio of 0.38 0.51 Cr.sup.6+/[Cr.sup.6+ + Cr.sup.3+]
[0025] Finally, the coated metal substrate is cured in a controlled
environment (step 20). In the controlled environment, atmosphere,
temperature, relative humidity, curing gas, and exposure to
oxidizer is controlled. Each of these variables can enrich the
hexavalent chromium on the surface of the chromium conversion
coating by creating an environment favorable to oxidation of the
trivalent chromium, and consequently increase the corrosion
resistance of the coating. The coatings resulting from these
conditions were analyzed using Raman spectroscopy to show the
presence of hexavalent chromium, trivalent chromium, and total
chromium was on the surface of the conversion coatings (Table 2).
In these tests, the temperature of the controlled environment was
kept consistent at 22.degree. C. to 24.degree. C.
[0026] The atmosphere in the controlled environment may be air,
oxygen, nitrogen, argon, other inert or oxidizing gases, or some
combination of those gases. When tested, the use of air containing
oxygen resulted in a higher ratio of hexavalent chromium to total
chromium content on the surface of the conversion coating. Test
data shows the effect of curing gas environment on the relative
amount of hexavalent chromium on the surface of the coating, as
shown in Table 2:
TABLE-US-00002 TABLE 2 Curing Atmosphere Ar Air Average Ratio of
0.42 0.45 Cr.sup.6+/[Cr.sup.6+ + Cr.sup.3+]
[0027] Relative humidity of the controlled curing environment can
also significantly alter the amount of hexavalent chromium on the
surface of the conversion coating. When relative humidity is
increased, the coating environment contains a higher concentration
of water, which naturally induces the production of hydrogen
peroxide, as discussed above, and allows for oxidation of the
trivalent chromium. When tested, relative humidity of <20% was
ineffective at curing a conversion coating such that a high ratio
of hexavalent chromium formed. Relative humidity 50% or higher
produced the best results. Relative humidity of <20%, which
induced rapid drying, induces cracks in the coating, which minimize
corrosion protection. This is summarized in Table 3, where the
relative amount of surface hexavalent chromium was determined from
both Cr(VI) and Cr(III) characteristic peak heights in area around
860 cm-1 and 535 cm-1, individually using Raman spectroscopy:
TABLE-US-00003 TABLE 3 Relative Humidity <20% ~50% >90%
Average Ratio of 0.41 0.56 0.44 Cr.sup.6+/[Cr.sup.6+ +
Cr.sup.3+]
[0028] The controlled environment can be altered to be an oxidizing
environment through the use of irradiation with ultra-violet light,
or through the injection of an oxidizing, non-corrosive gas, such
as ozone. If UV is used, the preferred UV integrated flux is 360
kJ/m.sup.2, but metal substrates with favorable geometries, such as
flat surfaces, many require less, while metal substrates with
complex geometries may require diffuse exposure with greater
nominal flux. UV wavelengths in the UV-A class, of 315-400 nm, is
preferred due to safety concerns; but UV-B and UV-C wavelengths can
be used to the same effect. The exposure of the coating to UV
radiation will induce further oxidation of trivalent chromium to
hexavalent chromium, creating a more corrosion resistant
coating.
[0029] The coating can alternatively be exposed to ozone to oxidize
the trivalent chromium in the coating to hexavalent chromium. In
this case, the preferred ozone exposure is 1 ppm O.sub.3 for one
hour. Alternatively, for safety reasons, the controlled environment
can be exposed to 0.1 ppm O.sub.3 for anywhere between four and
twenty-four hours. Preferably, ozone exposure is conducted when the
environment has over 50% relative humidity. Ozone, a strong
oxidizer, also induces the oxidation of trivalent chromium to
hexavalent chromium.
[0030] Finally, shorter curing times in the controlled environment
produced more hexavalent chromium-rich coatings (Table 4). Curing
the coatings for a time between one hour and one day produced the
most corrosion resistant coatings, while curing the coatings for
one week long (168 hrs) did not.
TABLE-US-00004 TABLE 4 Cure Time 1 Hour 24 Hours 168 Hours Average
Ratio of 0.49 0.43 0.43 Cr.sup.6+/[Cr.sup.6+ + Cr.sup.3+]
[0031] FIG. 2 is a series of graphs summarizing the ASTM B 117
salt-fog test corrosion performance results of varying curing
conditions, such as the type of trivalent chromium bath vendor and
chemistry (graph 22), curing time (graph 24), relative humidity
(graph 26), curing atmosphere (graph 28), hydrogen peroxide
post-treatment (graph 30), and surface pre-treatments (graph 32).
In each set of tests, the other variables were held constant. After
curing, each sample set was ASTM B 117 neutral salt-fog tested for
corrosion resistance. ASTM B 117 neutral salt-fog testing is an
industry-known method of testing the corrosion resistance of
surface coatings. With this method, ASTM B 117 neutral salt fog
testing was completed on test panels of coatings cured under
different conditions. The samples were then analyzed for corrosion
resistance and rated on a scale from 0.00 to 5.00, where 5.00 is an
ideal corrosion resistance coating, with no pits. Thus, a coating
with a 5.00 rating exhibited high corrosion resistance, a large
ratio of hexavalent chromium to total chromium, and good surface
morphology with no pits.
[0032] Graph 22 shows the use of six types of commercially
available trivalent chromium processes ("TCP"): "A", "B", "C", "D",
"E", and "F". The effects of different TCP was unpredictable, as
some trivalent bath solutions resulted in a better corrosion
resistance rating, and others did not. Specifically, TCP solutions
D and E performed the best, resulting in ASTM B 117 salt-fog test
ratings of 3.75 and 3.63 on average, while TCP solution F produced
the lowest rating during salt-spray testing, only 3.01 on
average.
[0033] Graph 24 shows the salt-spray corrosion rating as a function
of curing time. If the sample was cured between one hour and
twenty-four hours, and then removed from the controlled
environment, the rating averaged between about 3.31 and 3.36 out of
5.00. At curing time greater than twenty-four hours the rating
dropped, for example down to 2.44 or lower after 168 hours of
curing. Thus, shorter curing times resulted in better corrosion
resistance.
[0034] Graph 26 shows the effect of relative humidity in the
controlled curing environment on the corrosion resistance of the
coating. The samples with the best corrosion resistance were cured
in an environment containing at least 50% relative humidity.
Samples cured in atmospheres with 90% or greater relative humidity
also produced good corrosion resistance, with a rating of up to
3.41. The amount of water in the curing environment continuously
drives the metal oxidation and oxygen reduction reactions (shown in
equations 1 and 2 above), resulting in hydrogen peroxide on the
surface of the coating which oxidizes trivalent chromium to
hexavalent chromium, increasing the ratio of hexavalent chromium to
total chromium, thus increasing corrosion resistance.
[0035] Graph 28 depicts the effect of a curing atmosphere that is
inert as opposed to a curing atmosphere which consists of air. When
argon was used as the inert curing environment, the average
corrosion resistance rating was lower than when air was used. When
air was used, the oxygen present in air may drive the oxygen
reduction reaction shown in Equation 2 above, producing hydrogen
peroxide which may oxidize trivalent chromium to hexavalent
chromium, and increase the corrosion resistance of the coating.
Samples cured in air had a rating of 3.31 on average.
[0036] Graph 30 depicts the difference between samples post-treated
with hydrogen peroxide after the trivalent chromium bath and those
not post-treated with hydrogen peroxide. Samples not post-treated
with an oxidizer had an average corrosion resistance rating below
3.00, whereas those samples treated with hydrogen peroxide
post-bath contained a rating of 3.64 or higher on average, up to
3.75. The post-treatment with hydrogen peroxide drives the
oxidation of trivalent chromium as shown in Equation 3.
[0037] Finally, graph 32 shows the difference in corrosion
resistance rating of samples pre-treated (step 12 of FIG. 1) with
mechanical means as opposed to chemical means. The pre-treatment
process (step 12 of FIG. 1) cleans the surface of the metal
substrate to allow for formation of the corrosion resistant
coating. When a mechanical pre-treatment was used, the corrosion
resistance rating was higher, up to 3.57 on average, than when a
chemical pre-treatment was used.
[0038] FIG. 3 depicts photographs of several corrosion resistant
coating created under different conditions. Each photograph 34, 36,
38, shows two 3 inch by 5 inch metal test panels which were coated
according to the method in FIG. 1, but each sample 34, 36, 38 was
cured under different conditions. Each test panel was then tested
for corrosion resistance with salt fog testing, as discussed above.
The samples were analyzed for corrosion resistance and rated on a
scale from 0.0 to 5.0, where 5.0 is an ideal corrosion resistance
coating. A coating with a 5.0 rating exhibited high corrosion
resistance, a large ratio of hexavalent chromium to total chromium,
and good morphology.
[0039] Photograph 34 shows a corrosion resistant coating processed
under conditions that included a mechanical pre-treatment, a 0.3 wt
% hydrogen-peroxide post-treatment, and curing for a short time in
a controlled environment containing air at a relative humidity of
around 50%. The morphology of the samples in photograph 34 is
smooth and lighter in color with no discernable pits.
[0040] In contrast, photograph 36 shows a corrosion resistant
coating processed under the same conditions, but without the
hydrogen-peroxide post-treatment. The morphology of the samples in
photograph 36 is also smooth, but it is darker in color, indicating
the early stages of coating failure and aluminum substrate pitting
compared to the sample treated with hydrogen peroxide after the TCP
bath.
[0041] Finally, photograph 38 shows non-ideal processing where the
curing method of the coating is not controlled. Instead, the
samples were removed from the TCP and left to dry without any
post-treatment or controlled environment. The corrosion in the
coating and aluminum substrate corrosion is visible as compared to
the other samples.
[0042] FIGS. 4A and 4B show another set of 3'' by 5'' sample
coatings, similar to those exhibited in FIG. 3. The testing
completed on various samples showed that samples post-treated with
hydrogen peroxide, and cured in environments with a relative
humidity of >90% obtained the best corrosion-resistance
performance in salt fog testing. The results of select samples are
summarized in Table 5 below:
TABLE-US-00005 TABLE 5 Curing time Relative Curing Post- Pre-
Rating Rating Cr(6+)/ (hr) Humidity Gas treatment treatment at 168
hr at 336 hr [Cr(3+) + Cr(6+)] Morphology 1 24 <20% Ar None
Chemical 1.0 1.0 0.09 Corrosion + lots of pits 2 168 <20% Ar
None Chemical 2.5 1.0 0.15 Corrosion + lots of pits 3 24 >90%
Air None Chemical 3.5 2.5 0.33 About 20 pits 4 168 >90% Air None
Chemical 3.0 2.0 0.38 Greater than 20 pits 5 24 >90% Air None
Mechanical 4.3 3.8 0.70 1~2 pit 6 24 >90% Air 0.3 wt %
Mechanical 4.5 4.5 0.70 No pits H.sub.2O.sub.2 7 24 50% Air 0.3 wt
% Mechanical 4.3 4.3 0.73 No pits H.sub.2O.sub.2 8 1 50% Air 3.5 wt
% Chemical 5.0 4.5 0.92 No pits H.sub.2O.sub.2
[0043] Sample 1 was pre-treated with a chemical agent, was not
post-treated with an oxidizer, and was cured for 24 hours in a
controlled environment of argon with less than 20% relative
humidity. Sample 1 showed both a low ratio of hexavalent chromium
to total chromium of only 0.09, and a poor salt fog testing
performance rating of 1.0. The photograph of sample 1 in FIG. 4
shows cracking, apparent corrosion and pitting on the surface of
the coating.
[0044] Similarly, sample 2 was pre-treated with a chemical agent,
was not post-treated with an oxidizer, and was cured for 168 hours
in a controlled environment of argon with less than 20% relative
humidity. Sample 2 showed both a low ratio of hexavalent chromium
to total chromium of only 0.15, and a poor salt fog testing
performance rating of 2.5 initially, which dropped to 1.0 after 336
hours of exposure to the outside environment. The photograph of
sample 2 in FIG. 4 shows cracking, apparent corrosion and pitting
on the surface of the coating.
[0045] Samples 3 and 4 were pre-treated with a chemical agent, were
not post-treated with an oxidizer, but were cured in an environment
of air with greater than 90% relative humidity. Sample 3 was cured
for 24 hours in this environment, while sample 4 was cured for 168
hours. The ratio of hexavalent chromium to total chromium of the
sample has increased compared to samples 1 and 2 due to the
increased humidity and air in the controlled environment; sample 3
showed a ratio of 0.33 and sample 4 showed a ratio of 0.38.
Additionally, the salt fog performance testing ratings of samples 3
and 4 were 3.5 and 3.0, respectively, but dropped to 2.5 and 2.0
after 336 hours of exposure to the outside environment. Although
these samples fared better than samples 1 and 2, they still showed
pitting on the surface of the coating, with more than 20 pits in
each sample, as shown in FIG. 4A. Thus, increased relative humidity
and air in the controlled curing environment are beneficial, but do
not alone produce the optimal results.
[0046] Similarly, sample 5 was not post-treated with an oxidizer,
but was cured in an environment of air with greater than 90%
relative humidity for 24 hours. Sample 5 was mechanically
pretreated. The controlled relative humidity environment produced
samples which had some pitting on the surface, and salt fog testing
ratings of about 4.3 after removal from the environment.
[0047] In contrast, samples 6, 7 and 8, which were cured in
increased relative humidity and a curing environment consisting of
air, were also post-treated with an oxidizer conducted before
placing the coatings in the curing environment. Samples 6 and 7
were post-treated with 0.3 wt % H.sub.2O.sub.2, while sample 8 was
post-treated with 3.5 wt % H.sub.2O.sub.2. All three samples
produced coatings without any visible corrosion, cracking or pits
on the surface. Moreover, samples 6, 7, and 8, respectively,
produced salt fog test ratings of 4.5, 4.3 and 5.0. The amount of
hexavalent chromium on the surface of each sample, respectively,
was 0.70, 0.73 and 0.92. The samples treated with hydrogen peroxide
before being cured produced the best corrosion resistant
coatings.
[0048] Overall, curing in a controlled environment of air at a
relative humidity of above 20%, followed by hydrogen peroxide
post-treatment and exposure to ozone or ultra-violet radiation were
the most successful at producing hexavalent rich corrosion
resistant conversion coatings that withstood salt fog chamber
testing.
[0049] While the invention has been described with reference to an
exemplary embodiment(s), it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment(s) disclosed, but that the invention will
include all embodiments falling within the scope of the appended
claims.
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