Method of rendering aluminum base metal resistant to water staining

A method of rendering an aluminum base metal surface resistant to water staining is provided which comprises providing an aluminum base metal surface and applying a stain resistant coating to the aluminum base metal surface, with the stain resistant coating containing a water soluble molybdate salt and water soluble nitrite compound in an amount sufficient to render the aluminum base metal surface resistant to water staining. An aluminum base metal article of manufacture which is resistant to water staining is also provided.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of treating an aluminum base 
metal to render it resistant to the corrosion or self anodization 
mechanism known in the art as water staining. More particularly, it 
concerns a method of rendering an aluminum base metal resistant to water 
staining by applying to the surface of the metal a coating which contains 
an effective amount of a water soluble molybdate salt, preferably sodium 
molybdate and a water soluble nitrite compound, preferably sodium nitrite. 
As used herein the term "aluminum base metal" includes pure aluminum, as 
well as alloys of aluminum which contain at least fifty weight percent of 
aluminum. Typical of such alloys are manganese and magnesium alloys of 
aluminum. 
Aluminum and its alloys, i.e., aluminum base metals, are frequently 
considered to be extremely corrosion resistant. This is attributed to the 
compact, adherent oxide film which forms on the metal surface. However, 
aluminum metal can easily experience the water staining form of corrosion 
or self anodization during shipment and storage. This is due to the 
ingress of water between coils or sheets of the metal from rain, humidity 
or temperature fluctuations past the ambient dew point. This water in 
crevices between aluminum surfaces can provide the conditions for a 
differential aeration cell. This differential aeration cell consists of 
regions into which oxygen can readily diffuse, like coil or sheet edges, 
and regions into which diffusion of oxygen is very limited, like the areas 
in from the edges of the metal. The oxygen poor region functions as the 
anode and the oxygen rich region becomes the cathode. Corrosion (i.e., 
self anodization) can occur in the anodic area, and under these 
conditions, the water staining form of aluminum corrosion results. Water 
staining requires nothing more than the presence of water and crevices to 
form the differential aeration condition. This staining can occur on 
commercially pure aluminum and all its alloys. 
In some case, staining is detrimental by virtue of its unsightliness alone. 
It can be brownish black, gray or whitish, and can roughen the surface 
significantly. Staining can also effect mechanical properties of the 
metal, ease of forming and weldability. Because of these problems, there 
is great interest in preventing water staining on aluminum. 
To date, many types of surface treatments have been tested in attempts to 
overcome water staining of aluminum. One compound which has been tried for 
aluminum and other metals is sodium molybdate. However, the use of sodium 
molybdate by itself on aluminum does not result in satisfactory water 
staining inhibition. 
From the foregoing, it is clear that water staining of aluminum is highly 
undesirable, and that while numerous techniques have been tried to obviate 
this problem, no solution has heretofore been advanced which is simple, 
effective and economical. 
Accordingly, the principal object of the present invention is to provide a 
simple, effective and economical means of rendering aluminum base metal 
resistant to water staining. 
The foregoing and other objects of the instant invention will become 
apparent to those skilled in the art from a reading of the following 
specification and claims. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention concerns a method of rendering an 
aluminum base metal surface resistant to water staining wherein the method 
comprises providing an aluminum base metal surface and applying a stain 
resistant coating to the aluminum base metal surface, the stain resistant 
coating containing a water soluble molybdate salt, preferably sodium 
molybdate, and a water soluble nitrite compound, preferably sodium 
nitrite, in an amount sufficient to render the aluminum base metal surface 
resistant to water staining. 
In another aspect, the instant invention relates to an article of 
manufacture comprising an aluminum base metal surface which is resistant 
to water staining, the surface having a coating thereon which includes a 
water soluble molybdate salt, preferably sodium molybdate, and a water 
soluble nitrite compound, preferably sodium nitrite, in an amount 
sufficient to render the aluminum base metal surface resistant to water 
staining.

DETAILED DESCRIPTION OF THE PREFERRED PRACTICE OF THE INVENTION 
The corrosion process by which water staining (i.e., self anodization) of 
aluminum occurs is electrochemical in nature. It requires an electrolyte, 
a cathodic region (regions into which oxygen can readily diffuse), and an 
anodic area (regions which experience little oxygen diffusion). To inhibit 
or prevent corrosion from occurring, it is required that this 
electrochemical cell be interrupted in some manner. This has been 
accomplished via the instant invention by use of a coating on the aluminum 
base metal surface which contains both a water soluble molybdate salt, 
preferably sodium molybdate, and a water soluble nitrite compound, 
preferably sodium nitrite. 
In order to reasonably duplicate the problems frequently experienced in the 
aluminum industry with respect to water staining, two common alloys were 
chosen for evaluation: 3003, an aluminum-manganese alloy, and 5052, an 
aluminum-magnesium alloy. Throughout the course of this investigation, 
identical tests were performed on both alloys in the event that 
significantly different corrosion behavioral characteristics were 
exhibited as a function of major alloying elements. The experimental 
procedure was broken down into three divisions, the first being the 
analysis and reaction of the water staining problem, second the 
characterization of sodium molybdate behavior as an inhibitor, and 
finally, the determination of the combined effect of sodium molybdate with 
sodium nitrite. 
Duplicating Water Staining on 3003 and 5052 Aluminum 
Staining on aluminum sheets and coils is brought about by the presence of 
water between an aluminum surface and any other surface. To recreate these 
conditions required two things: aluminum surfaces in close contact with 
other surfaces and the presence of water between these surfaces. To 
provide the `crevices` on the metal surfaces, a test piece configuration 
was developed which consists of a stack of three sheets of aluminum alloy 
sandwiched between two sheets of cardboard which, in turn, are sandwiched 
between two Plexiglass holders. This design allows a sufficient contact 
area that assures both stagnant areas into which oxygen diffusion is 
difficult, and other areas into which oxygen is able to readily diffuse. 
It also allows the investigation to include an analysis of staining when 
aluminum surfaces contact other aluminum surfaces, as in the case of 
coiled aluminum with no liners, and staining when aluminum surfaces 
contact packing material, as when the metal is coiled or stacked with 
protective cardboard or paper liners. The test pieces are held together to 
provide maximum and continual contact for the duration of the test. 
To provide the ingress of water to the aluminum surfaces, the test pieces 
were placed, electrically isolated from one another and their 
surroundings, in glass dishes in a Model 434304 Hotpack 
Temperature-Humidity Chamber under constant temperature (32 degrees 
Centigrade), and at 100 percent humidity. Initially, tests were run to 
determine the time before staining became observable and the time until 
the staining did not visibly appear to worsen. These time values were used 
to establish test durations for subsequent tests. 
In this initial testing, the aluminum surfaces were not treated in any way 
other than by means of a surface polishing with Scotch-Brite to remove 
cosmetic scratches and burrs. Sufficient time was allowed after this 
polishing to re-establish the equilibrium aluminum oxide film on the metal 
surface. Afterwards, the test pieces were washed with soap to remove 
surface dirt and polish residue, degreased in an ultrasonic acetone bath, 
rinsed with distilled water, and fully dried. Throughout this 
investigation, all test pieces were polished and cleaned in this manner. 
Separate tests were performed, each for a duration of 5, 10, 15 and 20 
days and the results given in Table I. 
By way of background, test results present herein in tabular form are based 
on a minimum of three series of tests in each case to assure the 
reproducibility and validity of the reported observations. 
TABLE I 
______________________________________ 
WATER STAINING ON ALUMINUM SURFACES 
Exposure Time (Days) 
Alloy 2 5 10 15 20 
______________________________________ 
3003 1 1 1-2 2 2 
5052 1 1 1-2 2 2 
______________________________________ 
No Visible Staining 0 
Mild Staining 1 
Severe Staining 2 
Production of Water Staining on 3003 and 5052 Aluminum Surfaces 
To determine the conditions under which water staining of aluminum would 
take place on untreated metal surfaces, separate tests were run to assure 
that the conditions bringing about the staining were predictable and 
repeatable. Table I lists the test results as a function of alloy and 
exposure time. 
Tests indicated that at 32 degrees centigrade and at 100 percent humidity, 
with the water at pH values ranging between 5.6 and 6, noticeable staining 
of both the 3003 and 5052 aluminum surfaces was observed within a time 
period of 2 days. In virtually all cases, it was also observed that the 
staining was more immediate and more severe on the aluminum surfaces in 
contact with cardboard than on the metal-metal contact surface. This can 
be explained because the contact of the damp cardboard to the aluminum 
surface is more intimate and more complete and because salts contained in 
the cardboard can leach out, enhancing the conductivity of the electrolyte 
on the metal surface and increasing the corrosion or staining rate. 
As the results in Table I show, fairly severe staining appeared on all test 
pieces between 5 and 10 days. After 10 days, staining was quite severe, 
and could upon occasion, join two pieces of aluminum such that they had to 
be mechanically separated. Again, staining on the cardboard-metal 
interface more completely covered the aluminum surface. 
It was concluded upon the completion of this series of tests, that test 
exposure times of 5,10, 15 and 20 days under conditions of 100 percent 
humidity and 32 degrees centigrade, would allow water staining to develop 
and grow significantly, possibly exceeding worst-case conditions that 
could be experienced in the commercial storage and shipment of aluminum. 
The Characterization of Sodium Molybdate Methods to Maintain Sodium 
Molybdate on the Aluminum Surface 
For sodium molybdate to function as an inhibitor it must be in contact with 
corrosion or self anodization processes at the metal surface. It is 
commercially available in dry powder form and is readily dissolved in 
water. 
It is obvious that in its dry form, sodium molybdate could not break down 
to release its functional ion which works as an inhibitor. To provide the 
necessary aqueous environment and to provide adhesion of the inhibitor to 
the surface being treated, it was decided to use a nontoxic, commercially 
available binding agent. Two different types were utilized in the initial 
tests: polyvinyl alcohol and hydroxypropyl methylcellulose. Both of these 
binders dissolve in distilled water with agitation to prevent 
agglomeration of the dry powders. They are nontoxic and widely used in a 
variety of applications. A series of brief tests were run on these two 
binders to determine the concentrations necessary to achieve adequate 
viscosity of each. The viscosity of the binder solutions had to be such 
that the molybdate in the solutions would remain in contact with the 
aluminum surface in an environment of 100 percent humidity with gravity 
effects on standing test pieces. The viscosity at the same time could not 
be excessive so that the test pieces were gummy or unable to make intimate 
contact with other surfaces. Once the binding solution concentrations were 
established (values shown in Table II below), a series of tests to 
determine the effects of the binding solutions on water staining of 
aluminum had to be run to isolate the effects of sodium molybdate from 
those of the binder. 
TABLE II 
______________________________________ 
Concentrations of Organic Binders 
Used on Corrosion Tests 
Binder Optimal Concentration 
______________________________________ 
Polyvinyl Alcohol 4 grams per liter 
Hydroxypropyl Methylcellulose 
1.5 grams per liter 
______________________________________ 
Characterization of Sodium Molydbate as a Water Stain Inhibitor 
Using the test configuration described before, a series of tests were 
conducted to determine the degree to which water staining would occur 
under identical environmental conditions and time periods: 
a. on plain aluminum surfaces in contact with each other; 
b. on plain aluminum surfaces in contact with packing cardboard; 
c. on aluminum surfaces coated with a thin layer of binder (both binder 
solutions) in contact with other similarly coated aluminum surfaces; 
d. on aluminum surfaces coated with a thin layer of both binders in contact 
with packing cardboard; 
e. on aluminum surfaces coated with varying concentrations of sodium 
molybdate in each of the two binding solutions in contact with similarly 
coated aluminum surfaces; and 
f. on aluminum surfaces coated with varying concentrations of sodium 
molybdate in each of the two binding solutions in contact with packing 
cardboard. 
It must be noted at this time that distilled water was used to make up all 
binder solutions. The pH of the distilled water used was 5.6 before and 
after the addition of both types of binder. Therefore, all tests run in 
this series of evaluations were run at a naturally occurring pH of 5.6. It 
must also be noted that the pH of the condensate inside the humidity 
chamber had a value of 5.6. This acidic pH can be attributed to the 
presence of dissolved CO.sub.2 from the air, and the resulting equilibrium 
which is established. Results from this test sequence must consequently be 
assumed to be valid, in part, as a function of the pH of 5.6 in the 
electrolyte. 
All tests were run to determine staining after 5, 10, 15 and 20 days 
respectively. For the control tests run with and without binder, staining 
appeared almost always within the first 5 days and was quite severe after 
10 and 15 days. There was little difference in every case between the 15 
and 20 day test results, and so it was determined that 20 days was a 
sufficient test duration in which to characterize the effects of test 
variables. 
Once the effects of no binder/no inhibitor, binder/no-inhibitor and 
binder/inhibitor between adjacent aluminum surfaces and aluminum-cardboard 
surfaces were determined, the same series of tests were run to define pH 
effects on those same systems. In summary at this point, Table III lists 
the constant and the variables being considered in the tests which have 
been run thus far. 
TABLE III 
______________________________________ 
Test Parameters 
Constants Variables 
______________________________________ 
Temperature - 32.degree. Celsius 
Surface Treatment: 
Relative Humidity - 100% 
No Binder-No Inhibitor 
Test Piece Size - 3 .times. 3 .times. 0.25 inches 
Binder-No Inhibitor 
Test Durations - 5, 10, 15, 20 days 
Binder-Inhibitor 
Alloy Types - 3003, 5052 Al 
Inhibitor Concentration 
pH 
Nature of Surface Contact: 
Al--Al 
Al-Cardboard 
______________________________________ 
Further, series of tests were conducted to determine the effect of sodium 
molybdate on water staining. These results are set forth in Table IV. 
TABLE IV 
______________________________________ 
Corrosion Test Results 
Inhibitor Exposure Time, Days 
Alloy Binder mg/cm.sup.2 
pH 5 10 15 20 
______________________________________ 
3003 P -- 5.6 2 2 2 2 
3003 M -- 5.6 1 2 2 2 
5052 P -- 5.6 1 2 2 2 
5052 M -- 5.6 1 2 2 2 
3003 P -- 7.0 0 0 1 1 
3003 M -- 7.0 0 0 1 1 
5052 P -- 7.0 0 0 1 1 
5052 M -- 7.0 0 0 1 1 
3003 P -- 9.0 1 1 1 1 
3003 M -- 9.0 1 1 1 1 
5052 P -- 9.0 1 1 1 2 
5052 M -- 9.0 1 1 1 1 
3003 P .005 5.6 2 2 2 2 
3003 M .005 5.6 2 2 2 2 
5052 P .005 5.6 2 2 2 2 
5052 M .005 5.6 2 2 2 2 
3003 P .01 5.6 1 2 2 2 
3003 M .01 5.6 1 1 2 2 
5052 P .01 5.6 1 1 2 2 
5052 M .01 5.6 1 2 2 2 
3003 P .01 7.0 1 1 1 1 
3003 M .01 7.0 1 1 1 1 
5052 P .01 7.0 1 1 1 1 
5052 M .01 7.0 1 1 1 1 
3003 P .05 5.6 1 1-2 1-2 1-2 
3003 M .05 5.6 1 1-2 1-2 1-2 
5052 P .05 5.6 0-1 2 2 2 
5052 M .05 5.6 0-1 2 2 2 
3003 P .05 7.0 0-1 1 1 1 
3003 M .05 7.0 1 1 1 1 
5052 P .05 7.0 1 1 1 1 
5052 M .05 7.0 1 1 1 1 
3003 P .05 9.0 1-2 1-2 1-2 1-2 
3003 M .05 9.0 1-2 1-2 1-2 1-2 
5052 P .05 9.0 1-2 1-2 1-2 1-2 
5052 M .05 9.0 1-2 1-2 1-2 1-2 
3003 M 0.1 5.6 1-2 1-2 1-2 1-2 
5052 M 0.1 5.6 1-2 1-2 1-2 1-2 
3003 M 0.1 7.0 1 1 1 1 
5052 M 0.1 7.0 1 1 1 1 
3003 M 0.5 5.6 1 1 1 1-2 
5052 M 0.5 5.6 1 1 1 1 
3003 M 0.5 7.0 1 1 1 1 
5052 M 0.5 7.0 1 1 1 1 
______________________________________ 
No Visible Staining 0 
Mild Staining 1 
Severe Staining 2 
P -- Polyvinyl Alcohol 
M -- Methylcellulose 
From the foregoing tests, it was observed that even though sodium molybdate 
was shown to delay the onset of water staining, it did so only by a matter 
of a few days. It also appeared to reduce the severity of water staining 
after the 10 to 20 day exposure times, but only to a minor degree. The use 
of sodium molybdate alone did not function as a satisfactory water 
staining inhibitor for aluminum base metals. 
Sodium Molybdate in Conjunction with Sodium Nitrite 
Under the identical conditions the previous tests experienced, a new test 
series was created to determine the effects of sodium nitrite (oxidizing 
agent) in combination with sodium molybdate (inhibitor). In order to 
determine whether observed effects of the combined chemicals were real or 
could be attributed merely to the presence of the additional oxidizing 
agent, a series of controls were used in all tests. The control in each 
case was a set of binder coated surfaces with the oxidizing agent present 
in the binder alone (in the same concentrations as that used in 
conjunction with the inhibitor). 
Synergistic Effects of Sodium Molybdate and Sodium Nitrite 
The effects of sodium molybdate in conjunction with sodium nitrite 
(NaNO.sub.2) at the pH of distilled water (5.6) and neutral distilled 
water buffered with a phosphate were evaluated. Sodium nitrite 
concentrations of 0.0125, 0.025 and 0.5 mg/cm.sup.2 were evaluated. 
Within the scope of these tests, the 0.025 mg/cm.sup.2 showed the most 
favorable results. As in previous evaluations, only at a neutral pH was 
the staining of the aluminum surface preventable. At neutral pH and at a 
level of 0.025 mg/cm.sup.2 nitrite and 0.05 mg/cm.sup.2 sodium molybdate, 
all visible staining on the Al-Al interface and the Al-cardboard interface 
was eliminated for all exposure times. 
As shown in Table V at a pH of 5.6, with 0.06 mg/cm.sup.2 sodium nitrite 
and 0.05 mg/cm.sup.2 sodium molybdate, the severity was reduced and water 
staining was delayed up to 10 days on the 3003 alloy. A sample was taken 
at 7.5 days for this test and no water staining was found, confirming that 
the first evidence of water staining was at 10 days. This can be 
attributed in part to the oxidizing effect of the sodium nitrite, but it 
is apparent that effect cannot overcome the pH effect on the aluminum 
oxide film stability. It cannot be, however, just the oxidizing impact of 
the sodium nitrite that is reducing the staining, for if that were the 
case, the more nitrite present, the more diminished would be the staining, 
and that is not the case. 
For the range of pH's, and at the same level of sodium molybdate, staining 
on the aluminum surfaces was always less at a sodium nitrite level of 
0.025 to 0.05 mg/cm.sup.2. Therefore, the desired synergistic effect 
between sodium molybdate and sodium nitrite occurs at a neutral pH at a 
level of 0.025 mg/cm.sup.2 sodium nitrite and 0.05 mg/cm.sup.2 sodium 
molybdate and at levels of 0.05 mg/cm.sup.2 sodium nitrite and 0.05 
mg/cm.sup.2 sodium molybdate. 
The test results are illustrated in Table V. 
TABLE V 
______________________________________ 
Test Results with Sodium Molybdate and Sodium Nitrite 
in a Binder of Methylcellulose 
Inhibitor Oxi- Oxidizer Exposure Time, Days 
Alloy mg/cm.sup.2 
dizer mg/cm.sup.2 
pH .5 10 15 20 
______________________________________ 
3003 0.05 S.N. .0125 5.6 1-2 2 2 2 
5052 0.05 " .0125 5.6 1-2 1-2 2 2 
3003 0.05 " .0125 7.0 0 0 0 1 
5052 0.05 " .0125 7.0 0 0 0 1 
3003 0.05 S.N. .025 5.6 1-2 1-2 2 2 
5052 0.05 " .025 5.6 1-2 2 2 2 
3003 0.05 " .025 7.0 0 0 0 0 
5052 0.05 " .025 7.0 0 0 0 0 
3003 0.05 S.N. .05 5.6 0 1 1 1 
5052 0.05 " .05 5.6 1-2 2 2 2 
3003 0.05 " .05 7.0 0 0 0 1 
5052 0.05 " .05 7.0 0 0 0 0-1 
3003 -- S.N. .025 5.6 1 1-2 1-2 2 
5052 -- " .025 5.6 1 1-2 2 2 
3003 -- " .025 7.0 1 1 1 1 
5052 -- " .025 7.0 1 1 1 1 
3003 -- S.N. .05 5.6 1 1 1-2 1-2 
5052 -- " .05 5.6 1 1 1-2 1-2 
3003 -- " .05 7.0 1 1 1 1 
5052 -- " .05 7.0 1 1 1 1 
3003 -- S.N. 0.5 5.6 1 1-2 1-2 1-2 
5052 -- " 0.5 5.6 1 1-2 1-2 1-2 
3003 -- " 0.5 7.0 1 1 1 1 
5052 -- " 0.5 7.0 1 1 1 1 
______________________________________ 
S.N. -- Sodium Nitrite 
No Visible Staining 0 
Mild Staining 1 
Severe Staining 2 
Based upon corrosion tests of 3003 and 5052 aluminum plate at 32 degrees 
Centigrade at 100 percent humidity for periods of 5, 10, 15 and 20 days, 
the following conclusions can be drawn: 
1. Sodium molybdate alone cannot prevent water staining of aluminum at pH 
values of 5.6, 7.0 or 9.0. 
2. Solutions of sodium molybdate and sodium nitrite can prevent or delay 
water staining of 3003 and 5052 aluminum depending upon the pH of the 
system. In systems which include a binder buffered to a pH of 7.0, sodium 
molybdate and sodium nitrite completely inhibit water staining for 20 
days. In a system which includes an unbuffered binder at a natural 
occurring of pH of 5.6, water staining is delayed from 1 day in a control 
group without sodium nitrite or inhibitor to 10 days with the use of the 
inhibitor and sodium nitrite for the 3003 aluminum alloy. 
3. It appears that the addition of sodium nitrite to the sodium molybdate 
systems serves to maintain the oxidation state of the molybdenum at a high 
enough value (+6) that it can co-precipitate with aluminum oxide on the 
protective oxide film and prevent further oxidation of the film. 
In order to practice the instant invention, sodium molybdate and sodium 
nitrite can be solubilized in any convenient solvent which when applied 
effectively deposits these materials on the surface being treated. As 
noted herein, an organic binder, such as polyvinyl alcohol or 
methylcellulose, is preferred. 
The resultant solution can then be applied to the surface being treated by 
any convenient means, such as brushing, spraying or dipping. All that is 
required is that an effective amount of sodium molybdate-sodium nitrite be 
deposited. This, obviously, can be determined empirically. 
Again, the benefits of the instant invention are realized when an effective 
amount of sodium molybdate-sodium nitrite is applied to the surface of the 
aluminum base metal being treated. The exact amount utilized is not 
critical. Based on tests to date, excellent results are obtained when the 
sodium molybdate is deposited in the range of about 0.005 to about 0.05 
mg/cm.sup.2 and sodium nitrite in the range of about 0.0125 to about 0.5 
mg/cm.sup.2, with optimal results being achieved when the concentrate of 
sodium molybdate is about 0.55 mg/cm.sup.2 and the concentration of sodium 
nitrite is 0.025 mg/cm.sup.2. 
While the preferred practice of the invention employs a sodium 
molybdate/sodium nitrite system, other water soluble alkali and alkaline 
earth molybdates, ammonium molybdate, and amine and alkanolamine 
molybdates can be utilized together with other alkali and alkaline earth 
nitrites, including ammonium nitrite, amine nitrite and alkanolamine 
nitrite. 
Likewise, while the use of polyvinyl alcohol and hydroxypropyl 
methylcellulose as binders has been discussed herein, other water soluble 
agents which form flexible films on drying may be used. These include the 
alginates, gums, glycerol, and various forms of methyl and ethyl 
cellulose. 
While there have been described what are at present considered to be the 
preferred embodiments of this invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the invention, and it is, therefore, 
intended in the appended claims to cover all such changes and 
modifications as fall within the true spirit and scope of the invention.