U.S. patent application number 15/690329 was filed with the patent office on 2019-02-28 for corrosion mitigation of magnesium and magnesium alloys.
The applicant listed for this patent is GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Mahmoud Abd Elhamid, Yang Guo, Ming Liu, Anil K. Sachdev.
Application Number | 20190062926 15/690329 |
Document ID | / |
Family ID | 65321525 |
Filed Date | 2019-02-28 |
![](/patent/app/20190062926/US20190062926A1-20190228-D00000.png)
![](/patent/app/20190062926/US20190062926A1-20190228-D00001.png)
![](/patent/app/20190062926/US20190062926A1-20190228-D00002.png)
United States Patent
Application |
20190062926 |
Kind Code |
A1 |
Abd Elhamid; Mahmoud ; et
al. |
February 28, 2019 |
CORROSION MITIGATION OF MAGNESIUM AND MAGNESIUM ALLOYS
Abstract
A method is provided for reducing the corrosion rate of surfaces
of formed magnesium or magnesium alloy articles in which the formed
surface contains small embedded particles of iron. By exposing the
iron particle-containing formed surface to an acidic, aqueous
solution comprising alkali metal fluoride ions at a temperature of
between 20.degree. C. and 30.degree. C., an adherent passivating
layer of MgF.sub.2 is formed. Further, such exposure to the
acidified, aqueous, fluoride ion-containing solution reduces or
eliminates the concentration of cathodic, corrosion-promoting,
iron-containing particles on the article surface as the magnesium
fluoride layer is being formed. The development of the passivating
layer reduces corrosion in a water-containing environment, and even
if the passivating MgF.sub.2 layer is breached, the reduction in
surface iron-containing particles reduces the inherent corrosion
rate of the article.
Inventors: |
Abd Elhamid; Mahmoud; (Troy,
MI) ; Guo; Yang; (Shanghai, CN) ; Liu;
Ming; (Shanghai, CN) ; Sachdev; Anil K.;
(Rochester Hills, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM GLOBAL TECHNOLOGY OPERATIONS LLC |
Detroit |
MI |
US |
|
|
Family ID: |
65321525 |
Appl. No.: |
15/690329 |
Filed: |
August 30, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23F 13/02 20130101;
C23C 22/34 20130101 |
International
Class: |
C23F 13/02 20060101
C23F013/02 |
Claims
1. A method of reducing the corrosion rate of a formed magnesium or
magnesium-based alloy article, the formed article having a formed
surface which may be exposed to an aqueous environment in use, the
formed surface having embedded iron-containing particles; the
method comprising reacting the formed surface of the article with
an acidified aqueous solution of one or more of an alkali metal
fluoride compound and ammonium fluoride so as to form a coextensive
layer of MgF.sub.2 on the formed surface of the article, the
embedded iron-containing particles being removed from the formed
surface during the reaction, the acidified fluoride ion-containing
solution having a pH of less than 2.
2. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which an
acidified solution of one or more alkali metal fluoride compounds
is reacted with the surface of the formed article.
3. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the
reaction is conducted with the formed part and the aqueous solution
being at a temperature in the range of 20.degree. C. to 30.degree.
C.
4. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the
thickness of the formed MgF.sub.2 layer is in the range of 0.1
micrometer to 1000 micrometers
5. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the
fluoride ion-containing solution comprises hydrogen fluoride
(HF).
6. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the pH
value of the aqueous solution is obtained with the addition of one
of the group consisting of H.sub.2SO.sub.4 and HNO.sub.3.
7. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the alkali
metal earth compound is at least one of sodium fluoride or
potassium fluoride.
8. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the
fluoride ion-containing solution comprises between 0.1 mole and
28.9 moles of fluoride ion per liter.
9. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the formed
surface of the magnesium article is exposed to the fluoride
ion-containing solution for between 30 and 300 seconds.
10. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the formed
magnesium article is exposed to the fluoride ion-containing
solution by dipping the magnesium article into a bath of the
fluoride-containing solution.
11. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 8 in which the bath
of the fluoride-containing solution is agitated.
12. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 8 further comprising
agitating the article during exposure of the article to the
fluoride ion-containing solution.
13. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 in which the
magnesium article is exposed to the fluoride ion-containing
solution by spraying the fluoride-containing solution on the
magnesium article.
14. The method of reducing the corrosion rate of a formed magnesium
or magnesium-based article as stated in claim 1 further comprising
rinsing, drying and optionally baking the article, after
terminating exposure of the magnesium article to the fluoride
ion-containing solution.
15. A formed magnesium article, the formed article having a formed
surface which may be exposed to an aqueous environment in use, the
formed magnesium article having on its surface, a co-extensive
layer of MgF.sub.2 with a thickness ranging from 0.1 micrometer to
1000 micrometers, the article having a surface concentration of
iron-containing particles which is less than 50% of the volume
concentration of iron-containing particles in the body of the
article.
16. A formed magnesium article as stated in claim 15 in which the
layer of magnesium fluoride was formed by reacting the formed
surface of the article with an acidified aqueous solution of one or
more of an alkali metal fluoride compound and ammonium
fluoride.
17. A formed magnesium article as stated in claim 15 in which the
layer of magnesium fluoride was formed by reacting the formed
surface of the article with an acidified aqueous solution of an
alkali metal fluoride compound.
18. A formed magnesium article as stated in claim 15 in which the
layer of magnesium fluoride was formed by reacting the formed
surface of the article with an acidified aqueous solution of at
least one of sodium fluoride and potassium fluoride.
Description
TECHNICAL FIELD
[0001] This disclosure describes methods of reducing the rate of
corrosion at formed surfaces of manufactured articles composed of
magnesium and its alloys. Magnesium alloy articles which are
exposed to water, particularly salt-containing water, tend to be
vulnerable to reactions on their surfaces which lead to
unattractive and debilitating corrosion of the magnesium.
BACKGROUND OF THE INVENTION
[0002] Magnesium and magnesium-based alloys offer attractive
strength-to-weight ratios when compared to other structural alloys
such as steels and aluminum alloys. For this reason, magnesium is
favorably considered when substitution of higher strength-to-weight
materials is considered, for example, in aerospace applications and
for weight reduction in automotive vehicles and other consumer
goods.
[0003] Cars, vans, trucks, and motorcycles may have components
formed of magnesium-based alloys (typically containing 85-90% by
weight or more magnesium) such as door inner panels, wheels,
control arms, oil pans, and engine blocks. The surfaces of such
manufactured components on vehicles may be exposed to environmental
conditions, especially water-containing conditions, which may
promote corrosion. Magnesium is highly reactive, and, unlike
aluminum and its alloys, does not form a protective oxide coating.
In addition, magnesium and its commercial alloys commonly contain
metal impurities which may set up galvanic corrosion cells within a
magnesium article and accelerate its corrosion. Corrosion of
magnesium may be exacerbated when any aqueous environment to which
the magnesium is exposed incorporates high concentrations of ions,
such when the road de-icing salts, typically employed in snow-prone
and ice-prone regions, are used. These road de-icing salts,
intended to promote melting of ice and snow at temperatures of less
than 0.degree. C., dissolve into the water formed from the melted
snow and ice.
[0004] Thus, there a continuing need to fortify formed articles of
magnesium and its alloys against corrosion, especially in
aggressively-corrosive environments, so that its full potential for
reduction of weight in vehicle components may be realized.
SUMMARY OF THE INVENTION
[0005] This disclosure describes a method of enhancing the
corrosion resistance of surfaces of formed articles of magnesium or
magnesium alloys, hereinafter typically referred to as a formed
magnesium alloy article. The enhanced corrosion resistance is
particularly helpful when the magnesium alloy article may be
exposed to an aqueous environment, possibly with a high ion
concentration from the dissolution of road de-icing salts. An
underlying aspect of the subject method is our recognition that
iron, when present as elemental iron or as an iron-containing phase
or compound on the surface of a magnesium alloy article (or
embedded in the surface of the article), presents a local
electrochemical environment that renders the surrounding magnesium
susceptible to galvanic corrosion. The iron or iron-containing
material, even as a micro-scale particle on the surface of a
magnesium alloy article, may function as a cathode in an
electrochemical corrosion reaction that leads to oxidation of
nearby anodic magnesium. The iron may be present in the original
alloy composition, although the specifications of most magnesium
alloys limit iron content to less than fifty parts per million of
total magnesium alloy content. More commonly, the iron may have
been deposited into or onto the article as the magnesium alloy was
being cast into the shape of the article or when the article was
being machined or otherwise formed with iron-containing tools.
Molten magnesium alloy may also dissolve iron from casting
dies.
[0006] In accordance with the methods of this invention, surfaces
of a manufactured (formed) magnesium alloy article are suitably
cleaned of residual material from the tools and processes by which
it was formed. The cleaning methods are conducted to suitably
expose the surface of the magnesium alloy article, including
regions in which small (sometimes micrometer-size) particles of
iron (or containing iron) are firmly attached or embedded. When
iron in such forms may be present in the surface of a magnesium
alloy article, the whole surface or selected surface regions of the
article are subjected to the following processing.
[0007] A reactive aqueous solution is formed of acidified fluoride
ions that is composed to remove iron-based cathode sites from the
surface(s) of the magnesium alloy article while concurrently
forming a thin protective, water-resistant layer of magnesium
fluoride (MgF.sub.2) on the article surface. For example, the
aqueous solution may contain up to about ten percent by weight of
sodium fluoride and/or potassium fluoride at a pH<2. Ammonium
fluoride may also be used in combination with or in place of an
alkali metal fluoride compound. The solution of the alkali metal
fluoride and/or ammonium fluoride salts may be acidified by the
addition of a suitable amount of hydrogen fluoride or of nitric
acid or sulfuric acid. The solution may be applied to the surface
of the magnesium alloy article, for example, at a temperature in
the range of 20.degree. C. to 30.degree. C., such as a typical
ambient or room temperature. The aqueous solution reacts with the
article surface to progressively form a thin protective layer of
magnesium fluoride and to dissolve iron material as the protective
layer is formed around the iron-containing site on the surface of
the article. The fluoride salt-containing aqueous solution is
maintained in contact with the surface for a period of minutes
(e.g., thirty seconds to five minutes) to form the conforming
magnesium fluoride surface layer having a desired thickness,
typically in the range of 0.1 micrometer to 1000 micrometers.
[0008] Following the formation of the MgF.sub.2 protective layer
and the removal of the iron sites, the treated surface may be
rinsed with water, dried, and, optionally, baked to consolidate the
protective layer.
[0009] The above-described process promotes enhanced corrosion
resistance of the formed magnesium article for two reasons. First,
the passivating MgF.sub.2 layer will inhibit magnesium corrosion.
Second, even if the passivating MgF.sub.2 layer is breached, the
thus-exposed treated magnesium surface will be depleted in
iron-containing particles resulting in a treated magnesium surface
which is inherently less corrodible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows, in schematic enlarged cross-section, a portion
of a magnesium alloy article, such as a portion of a die cast
transfer case housing or an inner door panel for an automobile. The
formed magnesium alloy article or part has iron-containing
particles. Some of the iron-containing particles are located within
the microstructural cross-section of the formed article, while
other particles are embedded into a surface of the article and are
at least partially exposed to an environment. The iron particles in
the surface of the formed article present cathode sites,
potentially leading to electrochemical conversion (corrosion) of
adjacent magnesium atoms.
[0011] FIG. 2 illustrates a flow diagram for a process which
improves the corrosion resistance of a magnesium alloy article such
as that shown at FIG. 1. The process comprises (i) cleaning and
removing surface contaminants from the formed magnesium alloy
panel, (ii) exposing the surface of the panel to an acidic,
aqueous, fluoride ion-containing solution followed by (iii)
rinsing, drying and, optionally, baking the solution-treated
article.
[0012] FIGS. 3A, 3B, 3C illustrate a suggested sequence of changes
to the surface of the magnesium alloy panel of FIG. 1 resulting
from exposure of the surface shown in FIG. 1 to the aqueous,
acidic, fluoride ion-containing solution during the process
illustrated in FIG. 2. Initially, as shown at FIG. 3A, local
regions of the magnesium alloy surface react with the fluoride
ion-containing solution to form `patches` of a passivating layer of
MgF.sub.2 on the surface of the shaped and formed magnesium alloy
article. On continued exposure of the surface to the fluoride
ion-containing solution, the `patches` of MgF.sub.2 expand and
spread across the entire surface of the article, except for those
regions where the surface-located iron-containing particles occur
as shown at FIG. 3B. On yet further exposure of the surface of the
formed magnesium alloy article to the fluoride ion-containing
solution, the iron-containing particles dissolve into the acidic
solution and the MgF.sub.2 passivating layer becomes thicker, of
variable thickness, and extends across the entire treated article
surface as shown at FIG. 3C.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0013] This disclosure aims at enhancing the corrosion behavior of
magnesium in formed magnesium and magnesium alloy articles of
manufacture by the treatment of vulnerable surfaces of the sized
and shaped article with acidic aqueous solutions of alkali metal
fluorides. For example, vulnerable surfaces of the magnesium
article are those surfaces which are exposed to water, especially
salty water, in the use of the article. The treatment process is
practiced to eliminate the iron particulates in the surface of the
article, which particles otherwise present cathodic sites that
electrochemically accelerate the oxidation and dissolution
(corrosion) of adjacent magnesium atoms in the surface. The removal
of the iron particulates is achieved as a passivating magnesium
fluoride (MgF.sub.2) layer is deposited on the article surface
being treated with the alkali metal fluorides.
[0014] It is known that commercial magnesium alloys contain metal
impurities distributed as dispersed metal-containing phases such as
particles or precipitates in a magnesium matrix. These
metal-containing particles are cathodic to magnesium and so, in the
presence of a suitable electrolyte, a galvanic reaction is likely
to occur which leads to the anodic oxidation (corrosion) of nearby
magnesium atoms. Iron is a particularly problematic metal in
magnesium alloys, and it is generally accepted that, for acceptable
corrosion performance, the iron content of pure magnesium should be
maintained below 170 parts per million (ppm) by weight and, for
magnesium alloys, below 40 ppm by weight. Exceeding these limits
may lead to a dramatic increase in corrosion rate of magnesium.
Iron is substantially insoluble in magnesium, unlike the common
magnesium alloying agents, aluminum and zinc. Manganese, frequently
used in concentrations of up to 0.6 weight percent in zinc alloys,
is soluble in magnesium as are some other impurities commonly found
in magnesium alloys such as nickel, but generally, any iron present
exists as a dispersion of highly iron-rich, iron containing
particles in the magnesium-rich matrix.
[0015] These restrictions on iron concentrations result from the
requirement that, for galvanic corrosion to occur, any suitable
electrolyte must be able to simultaneously access the magnesium
matrix and any iron-containing phases. Thus, since a magnesium
alloy article may have an arbitrary surface, any section cut
through a magnesium body must expose sufficient iron-containing
phase to promote corrosion.
[0016] As an example, consider a die-cast transfer case housing,
which is a container for a driveline mechanism in an automotive
vehicle. Such a transfer case housing may be die-cast from a
magnesium alloy containing less than 0.004 percent by weight of
iron. The die-cast housing may then be further processed by
machining, for example by introduction of holes by drilling,
possibly followed by honing or threading, or the creation of
bearing support surfaces or sealing faces by milling. Thus, the
housing, when ready to receive the transfer gearing/mechanism, will
typically exhibit iron on its surfaces shaped by contact with the
die casting die and surfaces shaped by subtractive machining
processes. Any of these exterior surfaces may be expected to carry
exposed iron-containing particles, and any of these exterior
surfaces may be exposed to potentially corrosion-inducing aqueous
road splash in service.
[0017] A representative, schematic, cross-sectional view of a
portion of an article, such as transfer case housing, incorporating
iron-containing particles is shown at FIG. 1 which shows, in
fragmentary, cross-sectional view, a portion of a magnesium alloy
article 10 comprising a magnesium matrix or microstructure 12.
Iron-containing particles 14 are shown internal to the magnesium
alloy matrix 12 with additional particles 16 shown located
(typically at least partially embedded) at article surface 18 which
may be exposed to a corrosion-promoting environment. As the surface
of the magnesium alloy article corrodes, releasing surface-located,
iron-containing particles 16 and removing them from contact with
the magnesium matrix 12, previously internally-located,
iron-containing particles 14 will be exposed to maintain the
progress of the corrosion. It will be appreciated that other
particles, out of the plane of the illustrated section in FIG. 1,
that is, either above or below the plane of section, will also
contribute to the corrosion of the magnesium even though not
visible in the figure.
[0018] Although corrosion of magnesium promoted by iron-containing
phases or particles is largely dependent on the overall iron
content of the magnesium, any such corrosion may be exacerbated by
pick-up of iron during manufacture of a magnesium article. Examples
include the pick-up of iron particles from dies, such a stamping or
die-casting dies. Any iron particles picked up during article
processing tend to be particularly problematic since the iron is
typically located on the finished article surface. Thus, as
illustrated in FIG. 1, the concentration of iron-containing
particles 16 on surface 18 may be substantially greater than the
concentration of iron-containing particles 14 embedded in magnesium
matrix 12.
[0019] Most magnesium articles of commerce are formed of selected
magnesium based alloys. A few examples of such alloys include AZ91D
die cast or wrought (extruded or sheet) magnesium alloy, AZ31B die
cast or extruded (extruded or sheet) magnesium alloy, and AM60B die
cast magnesium alloy. As those of skill in the art appreciate,
these magnesium alloys vary in composition and microstructure, and
may incorporate, in their microstructure, a plurality of
metallurgical phases. The representation of the magnesium matrix
and its absence of any microstructural detail in FIG. 1 and also in
FIGS. 3A, 3B, and 3C is neither intended to, nor should be
interpreted as, representative of, or suggestive of, any specific
magnesium composition or alloy.
[0020] In general, corrosion of the surfaces of a magnesium alloy
article may be inhibited in two ways. The surface of the article
may be coated with a passivating, or non-corroding layer, thereby
denying any aqueous, corrosion-promoting composition access to the
magnesium surface. In a second approach, the sources of magnesium
corrosion such as iron-containing particles may be removed from the
surface of the magnesium alloy article. In accordance with
practices of this invention, an acidic aqueous solution of alkali
metal fluorides is used to react with the surface of the
magnesium-based alloy article to progressively form a thin layer of
magnesium fluoride (MgF.sub.2). Ammonium fluoride may also be used
to form the magnesium fluoride layer. And as magnesium fluoride is
formed around iron-containing particles, the particles are
dissolved in the acidic solution. Suitable alkali metal fluorides
include sodium fluoride and potassium fluoride. An inorganic acid
such as hydrogen fluoride or a mineral acid such as nitric acid
and/or sulfuric acid is added to the fluoride salt solution to
reduce the pH of the solution to 2 or lower. Preferably this
reactive, acidic, metal fluoride, and/or ammonium fluoride solution
is suitably applied to the surface(s) of a formed magnesium alloy
article after the surfaces have been cleaned of processing
lubricants or aids used during the forming of the article or
otherwise found on the surface of the article which could interfere
with the chemical action of the acidic metal fluoride solution.
[0021] A suitable procedure for effectuating both the development
of a passivating surface layer on a magnesium article and
substantially eliminating cathodic iron-containing particles from
the surface of a magnesium article is shown in FIG. 2. An
as-fabricated magnesium article to be rendered more corrosion
resistant is subjected to a process 100 which encompasses a
plurality of sequential operations 30, 32, 34, 36, 38, 40 and
(optionally 42). The process 100 may be conducted in a number of
different vessels, vats, pots, enclosures, containers or the like,
each having a unique environment with the article being transported
from one vat or like container to another. Alternatively, the
article may be placed in a single vat, pot, enclosure or container
and remain in that vat etc. as the environment in the vat is
successively modified to conduct the individual operations 30, 32
etc. Any such vat etc. may be sized and shaped to fully contain the
article to be processed.
[0022] Several of the steps of process 100 comprise exposing the
surface of the magnesium article to a liquid reactant or cleaning
solution. This exposure may result from immersing the article in
quiescent bath of liquid or may include agitating either or both of
the liquid and the article. Alternatively, liquid, dispensed at a
suitable pressure generally ranging from 50 to 2000 psi, may be
sprayed on the surface of the article. To assure substantially
uniform coverage of the surface if spray application is selected,
the liquid may be dispensed through either a plurality of
spray-heads arranged to ensure uniform or near-uniform coverage of
the surface or some means of moving the article relative to the
spray-head to achieve near-uniform coverage may be employed. Any
combination of these liquid application processes may be used to
accomplish the plurality of operations comprising process 100.
[0023] Operations 30, 32, and 34 clean and prepare the article
surface and are intended to remove, among others, any loose debris,
water-soluble contaminants and grease from the article surface.
Operation 30 is a water rinse suitably conducted at a temperature
of from about 20.degree. C. and to about 50.degree. C. Suitably
operation 30 may be conducted for between 30 and 120 seconds, which
rinse period can be reduced using pressurized water. Operation 32
is a degreasing step conducted at between about 20.degree. C. and
60.degree. C. for a period of between 30 and 300 seconds using, for
example, trichloroethylene or tetrachloroethylene as the degreasing
agent. This can be followed by the use of an alkaline cleaner, such
as an aqueous solution of sodium carbonate and trisodium phosphate,
at 60-80.degree. C. for 60 to 180 seconds Operation 34 is a final
rinse, using de-ionized water at a temperature of between about
20.degree. C. and no greater than 50.degree. C., and carried out
for between 30 and 300 seconds.
[0024] Operation 36 is the operation which endows the magnesium
alloy article with its corrosion-resisting characteristics
resulting from forming a passivating MgF.sub.2 layer on the article
surface and eliminating any iron-containing particles on the
article surface. Operation 36 entails exposing the magnesium
article surface to an acidified, fluoride ion-containing, aqueous
solution with a pH of less than 2. Preferably, the fluoride ions
are provided by an alkali metal fluoride such as sodium fluoride
and/or potassium fluoride. Ammonium fluoride may be used alone or
in combination with one or more alkali metal fluorides. The
fluoride ion-containing solution is suitably maintained in an
ambient temperature range of, for example, between 20.degree. C.
and 30.degree. C. The magnesium alloy article is suitably exposed
to the solution for a period of between 30 and 300 seconds. Such
exposure is sufficient to generate, on the formed article surface,
an adherent, passivating, layer of MgF.sub.2 ranging in thickness
from 0.1 micrometer to about 1000 nanometers. A magnesium fluoride
layer thickness in the range of two to one hundred micrometers is
generally suitable. The acidification of the fluoride
ion-containing solution may be accomplished using HF, which serves
both as a source of fluoride ions as well as an acidifying agent,
or with a mineral acid such as H.sub.2SO.sub.4 and H.sub.2NO.sub.3,
added in sufficient concentration to generate the required acidity.
As stated, one or more alkali metal fluoride(s) is a suitable and
preferred source of the fluoride ions used to form the magnesium
fluoride passivation coating. Suitably the molar concentration of
fluoride ions should range from 0.1M to 28.9M.
[0025] Operation 38 is a rinse to remove remnant fluoride
ion-containing solution and employs water at a temperature of
between about 20.degree. C. and 30.degree. C. Suitably the article
should be exposed to the rinse water for a period 30-120
seconds.
[0026] Operation 40 is a drying operation which may be conducted by
exposing the magnesium article to a heated air flow at a
temperature of 100.degree. C. for a period of from 30 to 120
seconds. Alternatively, the rinsed parts may be simply dried in
ambient air.
[0027] Operation 42, which is optional, is a bake operation,
generally conducted at a higher temperature of up to 200.degree. C.
to more rapidly remove all retained or absorbed water from the
MgF.sub.2 layer. Operation 42 will commonly employ a heat source
such as an oven, furnace or the like maintained at a temperature of
300.degree. C. But forced hot air or heat lamps may also be used to
elevate the part temperature. Generally, exposure of the article to
the heat source for a period of 30-120 seconds will be sufficient
to raise the article temperature to between 40-200.degree. C. and
remove all water from the MgF.sub.2 layer. Alternatively, the
removal of residual water may be part of a subsequent heat
treatment of the magnesium alloy part that could also be used to
remove absorbed water.
[0028] Although not relied upon, it is believed that the
development of the passivating MgF.sub.2 layer and the reduction or
elimination of surface-located iron-containing particles occurs by
the mechanism shown in FIGS. 3A, 3B, and 3C. FIG. 3A illustrates a
representative portion of an outer surface 18 of a formed magnesium
alloy article 10' shortly after the magnesium particle 16
containing surface 18 was exposed to the acidic, fluoride
ion-containing solution employed in operation 36 of FIG. 2.
Magnesium particles 14 within the body of the article 10' are not
affected by the treatment of the surface 18. As shown at FIG. 3A,
localized regions or `islands` of MgF.sub.2 20 have formed on the
most reactive portions of the magnesium surface 18. With continued
exposure to the fluoride ion-containing solution, the extent of the
magnesium surface 18 which has reacted and is now covered by an
increased number of layers 20 of MgF.sub.2 continues to increase as
indicated in the changed article 10''. At the completion of the
layer forming process, as shown at FIG. 3C, layer 20' covers
substantially the entirety of the treated surface 18' of the
treated article 10''. The iron particles 14 within the
microstructure of the article 10''' are not affected by the
formation of the magnesium fluoride layer 18'.
[0029] When such nearly full coverage of the original article
surface 18 is achieved, the iron-containing particles which were
cathodic to magnesium, become anodic with respect to the MgF.sub.2
layer now covering the surface of the magnesium alloy article. The
iron-containing particles which did not initially react with the
fluoride ion-containing solution, because of the preference of the
solution to react with magnesium, now begin to react with, and be
dissolved by, the solution. Upon complete dissolution of the
iron-containing particles fresh magnesium surface is exposed below
the (now-removed) particles and additional MgF.sub.2 forms on the
newly-exposed magnesium surface. This, in combination with some
growth (thickening) of the MgF.sub.2 layer results in the formation
of the thickened MgF.sub.2 layer 20' overlying a slightly roughened
magnesium article surface 18'.
[0030] By the precepts of quantitative metallography, the area
fraction of iron-containing particles on any arbitrary plane is
equal to the volume fraction of iron-containing particles in the
bulk. Thus, dissolution of surface iron-containing particles will
impact the corrosion rate of the article analogously to reducing
the bulk iron concentration in the magnesium.
[0031] The above mechanism relies upon MgF.sub.2 initially
completely covering the magnesium article surface except for the
iron-containing particles, and then, after dissolution of the
iron-containing particles, the entirety of the magnesium surface.
With particular reference to magnesium alloys, a plurality of
phases, in addition to substantially pure magnesium, may be
present, including magnesium-rich phases such as Mg.sub.17Al.sub.12
as well as magnesium-free phases such as MnAl resulting from the
occurrence of manganese as an impurity in, for example, magnesium
AZ91 alloys.
[0032] In general, the magnesium-rich phases in a magnesium alloy
respond to the fluoride-ion containing solution analogously to
substantially pure magnesium and form MgF.sub.2. The behavior of
other, magnesium-free, phases on the surface is less clear, but it
will be appreciated that, provided coverage of any
magnesium-containing regions of the surface with MgF.sub.2 is
complete, any corrosion-inducing liquid will be denied access to
the underlying magnesium. Thus, even in the presence of
highly-corrosive liquids, corrosion of the magnesium will be
inhibited. In general, substantially complete coverage of the
surface is observed as a result of the formation of the
thermodynamically favored MgF.sub.2 layer thus formed on the
surface of the magnesium or magnesium alloy article.
[0033] Although such theory is not relied upon, it is supported by
the results of the following experiment. A 99.9% Mg electrode was
connected to a 99.95% Fe electrode, and the electrode pair was
immersed in a 2 wt. % HF aqueous solution (1M of fluoride ions)
prepared from de-ionized water and containing a trace quantity of
potassium ferricyanide as an indicator. Upon immersion, initially,
the Mg electrode turned black due to the formation of MgF.sub.2,
while H.sub.2 evolved on the Fe electrode. After the Mg electrode
was fully covered with MgF.sub.2, and thus passivated, the Fe
started to dissolve as indicated by the development of a blue color
in the solution--potassium ferricyanide reacts to form ferrous
ferricyanide (Prussian blue) in the presence of Fe.sup.++ ions.
Continued immersion of the electrodes resulted in the dissolution
of yet more iron as indicated by the deepening of the blue color of
the solution as a result of the formation of more ferrous
ferricyanide.
[0034] The above detailed description and the associated drawings
or figures are presented for illustration of suitable exemplary
embodiments and not for limitation of the following claims.
* * * * *