CA1200740A - Method of improving the corrosion resistance of chemical conversion coated aluminum - Google Patents

Abstract

METHOD OF IMPROVING THE CORROSION RESISTANCE
OF CHEMICAL CONVERSION COATED ALUMINUM

ABSTRACT OF THE DISCLOSURE

A method for treating aluminum prior to applying a chemical conversion coating for improving its corrosion resistance especially when rigid requirements need to be met. The aluminum is alkaline cleaned and deoxidized, preferably with a non-chromated deoxidizer.
A supplementary treatment consists of immersing the deoxidized aluminum in a potassium or sodium nitrite solution. The aluminum is then coated with a standard chemical conversion coating, and dried at a temperature between 110°F and 130°F. The resulting product is able to pass salt spray test requirements as outlined in the military specification.

Description

BACKGROUND OF THE INVENTION
1. Field of the Inventiorl This invention relates in general to improving the corrosion resistance of chemical conversion coated aluminum, and more spe-cifically to a ~ethod for treating deoxidi2ed aluminum prior to conversion coating which improves the ability of the aluminum to pass rigid corrosion resistant requirements.

2. Discussion of the Prior Art Conversion coated aluminum has been used by airframe and other manufacturers, including the military, to improve alumi-num's corrosion resistance and paint adhesion properties. In the prior art, the aluminum is deoxidized using a chromated de-oxidizer, and then coated in the conversion process with a coat-ing resistant to corrosion~ When coated aluminum is manufactured for use in military applications, a stringent corrosion resistant test must be met as outlined in Military Specifications MIL-C-5541 and MIL-C-81706. These tests require subjecting the finished aluminum product to a salt spray for a lengthy period of time, such as 16~ to 328 hours, respectively.
Prior to the present invention, it was extremely difficult to pass the salt spray requirement imposed by the military for aluminum hardware. Special conversion coatings were developed, and various deoxidizers used. In general, the aluminum was treated using a chromated deoxidizer to etch away the natural ~5 and heat-treat oxide so that the finished product has a uniform ,,~

, color and texture. A chemical conversion coating i~ applied to th~ alum mum. One typical coatin~ is Plodine 1200S (trade mar.k) manufac-tured by the Amchem Products, Inc., Ambler, PA. This conversion coating conslsts of a corrosion resistant brown iridescent layer.
This film has potentially good resistance to normal corrosion;
however, in highly corrosive environments s~ch as that required by the salt spray test, the film may succ~mb to corrosion and not meet military specifications. In general, the prior art relied upon chromated deoxidizers to meet the stringent corrosion requirement tests. Non-~hromated deoxidizers were tested, but failed to meet the military 6pecification requirements after being chemically conversion coatedO Deoxidizers containing the chromate ion~ although producing better results in salt spray test requirements, have a pollution problem due to the effluent stream. Heavy metals, such as chrome, are highly toxic to the environment, and are therefore undesirable ~or release into closed or open bodies of water. Previo~s to the present in-vention, a workable non-chromated treatment process prior to chemical conversion coating aluminum was lacking if salt spray requirements were to be consistently met.
A search of the prior art disclosed the following patents to be of potential interest:
U = Inv_ntor Issue Dats

3,802,g73 Smith 4/9J74 3,510,430 Bland et al 5~5~70 3(i~

3,445,400 Everhart 5/20/69 3,433,577 Shick 3/18/69 3,340,001 Thornhill 9/5/67 3,335,096 Hatoh 8/8/67 3,275,562 Smith 9/27/66 3,140,203 Grunwald 6/7/64 2,883~311 Hulpert 4/~1/59 2,671,717 Ferguson 3/9/54 2,351~465 Wachter 6/13/44 None of these patents singularly or in combination anti~ipates the present invention~ Taking each of the above individuallyt U.S.
Patent No. 2,351,465 issued to Wachter deals with the use of sodium nitrite as an inhibitor to corrosion inside teel pipelines carry-ing petroleum distillates. This invention, specifically, needed to protect the pipelines from distillates containing hydrocarbon oils; and not salt spray as the present invention. Ferguson, in U.S. Patent No. 2,671,717 teaches the ~se of sodium nitrite in brightening aluminum, not for corrosion resistance. Hulpert, in U.S. Patent No. 2,883,371 teaches a chromated deoxidizer which the present invention would eliminate as apollution problem. A
non-chromated aluminum deoxidizer is disclosed in U.S. Patent No.
3,140,203 issued to Grunwald; however, this patent does not teach the applicability and compatability of this deoxidizer with a con-version coating. U.S. Patent No. 3,275,562, issued to Smith, shows one formulation of a non-chromated aluminum de-smutter, and does not teach the method of the present invention. Hatch, r~7~
in U.S. Patent No~ 3,335,096, deals with a corrosion protective chemical in a closed aystem which protects the metal surface only while in contact with the surface, and does not anticipate the present invention. As in the previously cited patent, Thornhill (U.S. Patent No. 3,340,001~ teaches a composition for protecting metal from corrosion which is constantly in contact with the metal.
This composition contains sodium nitrite; however/ it is not ob-vious from Thornhill that this compositon would haJe ~ood results when combined with chemical conversion coating. Shick, in U.S.
Patent No. 3,433,577, uses a composition containing sodium nitrite for protecting metal parts from corrosion. The purpose of this composition is for vapor phase corrosion inhibitors during storage, and does not anticipate or is applicable to the present invention.
U.S. Patent No~ 3,~4$~400 issued to Everhart contains a shromated formulation which the present invention seeks to avoid. Bland et al, in U.S. Patent No. 3,510,430 discloses a ferric sulfate non~
chromated deoxidizer which would not satisfactorily meet salt spray tests. The nitrites in this formulation are used to prevent pitting duriny deoxidization which is different from the function in the present invention~ U.S. Patent No. 3~802,973 teaches an oxidizin~ agent to uniformly etch aluminum in an alkaline so-lution ~or the purpose o hiding surface defec~s, and does not directly relate to the present invention.

SU~MARY OF THE IN~NTION
~. _~
The present invention discloses a method for treating aluminum metal to enhance corrosion resistance, and especially ~o meet salt spray test requirements imposed by the military. The method includes deoxidizing the aluminum with deoxidizing means, rinsing the aluminum and exposing the aluminum to a sodium nitrite solution with a pH lower than 5. The aluminum is then rinsed and coated with a shemical con-version coating such as "Alodine 1200S" and dried by suitable drying means such as warm air. The present invention teaches that the sodium nitrite treatment is especially effective if that solution is main-tained at a pH between 3 and 4. Further, it is preferable to use a deoxidizing solution of 25% to 50% co~rcial gYade nitric acid. Dryin~
temperat ~ s in the range of 110F and 130F ~ive ~e best res~Llts.
It is, therefore, the object of the present invention to provide a method for treating aluminum which uses a non-chromated deoxidizer and a chemical conversion coating with the final product able to with-stand the salt spray requirements of the military specification.
Another object of the present invention is to provide a method for treating aluminum which eliminates an effluent stream containing chLomate.
A further object of the present invention is to allow aluminum to be conversion coated without the use of a chromated deoxidizer.
These and other objects and advantages will become more apparent from the following detailed description of the invention, incudina tables and examples.

~,'f ~(~7~
Detailed DescriPtion of the Preferred Embodiment The pre~ent invention may be practiced on the following sequence of operation:
1) vapor degreasing the aluminum;
2) alkaline cleaning the al~minum;
3) rinsing the alurninum with water;

4) deoxidizing the aluminum, preferably with a non-chromated deoxidizer;

5) rinsing the deoxidized aluminum with water;

6) exposing the aluminum to a sodium nitrite sol~tion;

7) rinsing the aluminum with water;

8) chemical conversion coating the aluminum; and

9) drying the aluminum.
Vapor degreasing of the aluminum metal can be performed by standard techniques known to one skilled in the art. Similarly, alkaline clean-ing and rinsing of the aluminum are steps commonly known to those skilled in the art and insure a clean aluminum surface prior to de-oxidization.
The deoxidization step which is the next step is preferably pro-duced with a non-chromated deoxidizer. A chromated deoxidizer is undesirable from a pollution standpoint since chrome or chrome ions are toxic, and the effluent stream may be released to open bodies of water. The invention can be practiced, however, using a chromated deoxidizer if desired and the resulting product will readily meet the salt spray req~irements of the military. A non-chromated de-oxidizer that can be usecl for beneficial results, by way of example, is a nitric acid solution containing the fluoride ion. In the pre-erred embodiment of the invention, the fluoride ion would be presen~
in the deoxidizing solution in an adequate and normally practical amount to obtain an etch rate of the aluminum surface of up to ap-proximately 0.6 mils/side/hr~ The fluoride ion could be obtainedfrom hydrofluoric acid or any suitable salt containing the ion. Nitric acid can typically be used:Ln a cc2loe ntration Of 25% to 50% by vol~[~ of ccmnercial ~rad~ 1~03 to inhibit the redep~sit or copper, c~mmonly called "~ut".
The fluoride ion is preferred since it is one of the best etching agents of unfinished aluminum metal, and can etch uniformly without pitting. To supplement the etching action of the fluoride ion, nitric acid is used to remove undissolved silicon from the aluminum surface and prevent the formation of smut. Other non-chromated de oxidizers may be used in the practice of the invention without losing the beneficial efects of treatiny aluminum in the manner taught by the inventor. The aluminum may be immerse~ in the deoxidizer for approximately 5 minutes to 60 minutes or greater, depending on the dificulty of scale removal. The nitric acid deoxidizer performs best when a freshly prepared solution is buffered with some dissolved metallic aluminumO
After deoxidization, the aluminum is then rinsed with cold water, and a supplemental treatment, differirlg from the prior art, is per-formed to allow the final conversion coated product to pass salt spray requirements. In this step, the deoxidized aluminum is rinsed with a sodium nitrite solution. The aluminum may either be immersed in the solution or sprayed with the solution. The sodium nitrite solutiQn is maintained at a pH less than 5 and preferably at pH3 or below. Nitric acid can supply the necessary hydrogen ion concentra-tionr The concentration of sodium nitrite is not critical, assuming the pH is less than 5, and beneficial results can be obtained with a sodium nitrite solution of 1%. After the sodium nitrite treatment, a water rinse of the aluminum is performed prior to chemical conve~-sion coating. It sho~ld also be noted that potassium nitrite may be used in place of sodium nitrite with beneficial results.
The chemical conversion coating can be done with any of the coat-ing solutions known to the prior art. It has been fo~nd that Alodine1200S, manufact~red by Amchem Products, Inc., Ambler, PA, works sat-isfactorily.
To ~oat the aluminum, immersion in the coating solution is allowed to take place for a relatively short time such as up to five minutes.
After conversion coating, the aluminum is dried by warm air. In the preferred embodiment of the invention, the drying temperature will be in the range of 110 F to 130 F with an optimum at 120 F.
The present invention may be further understood by reference to the following examples and accompanying tables.

~ E~
Bare aluminum alloy specimens (10" by 3") were used in this and all other examples. Table 1 shows the results of this series of samples which ill~strates the effect of sodium nitrite treatment on the salt spray test res~lts. A partially depleted chromate-type 2~ deoxidizier was used called ~n Chem 6-17 (trade m~rk), ~ Product of ~n~h~m Products, I~c., Ambler, PA. As in all examples~ each specimen was , ~ ~ . . .~

,3t,ff~ 74~
fixst cleaned using an a~aline cleaner called Turco 2623 (trade mark) a Product o the Turco Product ~ivision of Purex Corporation Ltd., Wilmington, California. The alkaline cleaning w~s conducted for 30 minutes at 140 F. Deoxidation occurred in Amchem 6-17 with the etch rate ad-S justed to 0.25 mils/side/hr with hydrogen fluorid2. The next pro-cessing step for the'bottom four specimens on Table 1 was an immer-sion in the sodium nitrite solution for 30 minutesO The solution had a pH of 3 and a concentration of 3~. The top four specimens did not receive a sodium nitrite immersion. All specimens were then 1~ immersed in Alodine 1200S and dried for 30 minutes at a temperature of 110 - 130~F. It should be noted that all specimens in all ex-amples were rinsed with tap water after each processing step.
Table 1 shows that only the specimens which were immer~ed in the sodium nitrite olution passed the salt spxay test, i.e. the top ~our specimens of Table 1 had one or more corrosion pits visible to the unaided eye, indicating failure, while the bottom four spec-imens had no visible corrosion pits, indicating passing results.
In this example, as in all other examples, the tests were con-cluded after one week of salt spray immersion for vis~al examination and other evaluation tests.

Exam ~
Bare alloy specimens similar to those used in Exam~le 1 were used in Example 2. The main difference in the two examples was the use of a non-chromated ferrous ion-type deoxidizer instead of a chromated de-oxidizer. Ihe deoxidizer in this example was Isoprep 184 (-trade mark),a product of Allied-Kelite Products Division, Los Angeles, Califoxnia. It was ~ t~
used in a 25% concentration with an etch rate of 0,25 mils/side/hr.
maintained with hydrogen fluoride addition. The sodium nitrite so-lution was 2.3% sodium nitrite with a pH of 3Ø This example illus-trates that the only specimens which passed the salt spray test (no plts visible to the unaided eye~ were those immersed in the sodium nitrite solution.

Example 3 Example 3 is similar to Examples 1 and 2, differing only in the deoxidizer used. In ~xample 3, a nitric acid - fluoride ion type non-chromate deoxidizer was used~ The specimens were processed in the same manner as Examples 1 and 2. The deoxidizer consisted of 40~ by volume of concentrated nitric acid (technical grade~ with a sufficient fluoride ion concentration to produce an etch rate of 0.25 mils/side/hr. The sodium nitrite solution was of a 2.3~ con-centration with a pH of 3~3 The results outlined in the table showthat the specimens passing the salt spray test were those which were immersed in sodium nitrite. The specimens which were not immersed in the sodium nitrite solution failed the test.

In Example 4, a nitric acid - fluoride ion type non-chromated de-oxidizer was used as in Example 3. Instead of immersion in a sodium nitrite solution after this processing step, a potassium nitrite solution of 2.3% concentration with a 3~0 pH was used. The results show that the specimens which were not immersed in the potassiuM
nitrite solution failed to pass the salt spray test. The specimens 7~
that were immersed in potassi~m nitrite~ however, passed the salt spray test.

Exam~l _ Example 5 uses Isoprep 184, a ferrous ion-type non-chromated de-oxidizer as did Example 2. Instead of sodium nitrite, potassium nitrite was used in the next processing step. The solution was of 2.3% concentration with a pH of 3Ø As seen in Table 5, the only specimens passed were those treated with potassium nitrite.

Bare aluminum 20~4-T3 panels were processed in a conversion coat-ing line, using a chromated deoxidizer and a conversion coating solu-tion of Alodine 1200S. Immersion time in the Alodine 1200S varied slightly between 2~ and 3 minutes. All of the panels after drying were aged for 3 days prior to salt spray testing for 336 hours.
Drying temperatures were varied from ambient temperature to 160 F.
The results are summarized in Table 6. The panels dried at 120F
suffered no corrosion damage, and no detectable chanse was evident in color of the panels or surface finish. After the 336 hour salt spray test, the panels were resubjected to salt spray until failure.
Three of the four panels dried at 120F exceeded five weeks of salt spray when the tests were suspended. The panels dried at 140F
fai1ed in one week, which is typical of the results obtained prior to discovering the critical temperature factor. The panels dried at 100 F had marginal resistance to corrosion, while those dried at 70F-75F failed the test in a random pat~ern, perhaps due to insufficient time necessary for conditioning the Alodine 1200S coat-., ing at lower ternperatures~

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}7~D
rable 6 Drying Temperature Effect on Alodined 1200D
Aluminum Panels Alodine Immersion Drying Drying Panel No. T~ E~ ~ 3~ lt*
899 2~ Ambient 30 Failed, 2 weeks 900 2~ (70-75) 30 Passed, 5 weeks 901 2~ (70~75) 30 Failedt 2 weeks 902 2~ (70-75) 30 Passed, 6 weeks 914 3 100 30 Marginal**
915 3 lO0 30 "
916 3 100 30 "
gl7 3 lO0 30 "
910 3 120 30 Passed, 5 weeks 911 3 120 30 Passed, 5 weeks 912 3 120 30 Passed, 5 weeks 913 3 120 30 Passed, 2 weeks 906 3 140 30 Failed, 1 week 907 3 140 30 Eailed, 1 week 908 3 140 30 Failed, 1 week 9O9 3 140 30 Failed, 2 weeks 903 3 160 30 Failed, 3 days 904 3 160 30 Failed, 3 days 905 3 160 30 Failed, 3 days *Evaluated critically per MIL-C-5541A for a Class 1 coating (re-quired minimum salt-spray li~e of 14 days).
**A few pits, barely detectable with the unaided eye, were found after 2 weeks in salt spray. Upon re-exposure to salt-spray for another week, no further change in appearance was observed.

7~3 The invention may be embodied in other forms without depart-ing from the spiri-t or essential characteristics thereof.
The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes whlch come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for treating aluminum to enhance corrosion resistance of its surface, which includes:
a. deoxidizing the aluminum surface with deoxidizing means;
b. exposing the aluminum surface to a solution containing the nitrite ion and having a pH below 5;
c. coating the aluminum surface with a chemical conversion solution; and d. drying the aluminum with drying means.

2. The method of claim 1 wherein said deoxidizing means is a non-chromated deoxidizer.

3. The method of claim 2 wherein said non-chromated deoxidizer is a nitric acid solution with a concentration of nitric acid of at least 10% by volume of a 100% solution.

4. The method of claim 2 wherein said deoxidizing means con-tains a sufficient concentration of fluoride ion to etch the aluminum surface at a rate of at least .05 mil/hr.

The method of claim 1 wherein the pH of said nitrite solution is 3 or less.

6. The method of claim 1 wherein the drying is performed at a temperature between 110°F and 130°F.

7. The method of claim 1 wherein the nitrite ion in said solution is substantially contributed by sodium nitrite.

8. The method of claim 1 wherein the nitrite ion in said solution is substantially contributed by potassium nitrite.

9. The method of claim 1 wherein the nitrite ion in said solution is substantially contributed by a mixture of sodium and po-tassium nitrite.

10. In a process of treating the surface of aluminum metal to en-hance corrosion resistance which includes deoxidizing the surface with a deoxidizer and conversion coating the aluminum surface, the improvement of exposing the aluminum surface to a nitrite solution with a pH below 5 after deoxidizing the aluminum with said deoxidizer and before said conversion coating of the aluminum surface.

11. The improvement claimed in claim 10 which further includes drying the aluminum surface after said conversion coating at a temperature between 110°F and 130°F.

12. In a process of cleaning the surface of aluminum metal, which process includes deoxidizing with deoxidizing means, the im-provement of exposing the aluminum surface to a nitrite so-lution with a pH below 5 after deoxidizing the aluminum with the deoxidizing means.

13. The method of claim 2 wherein the non-chromated deoxidizer is nitric acid in a concentration of 15 to 25 percent by volume based on 100 percent nitric acid.

14. The method of claim 2 wherein the non-chromated deoxidizer is nitric acid in a concentration of 30 percent to 50 percent by volume based on commercial grade nitric acid.