US3351540A

US3351540A - Method of improving the corrosion resistance of oxidized metal surfaces

Info

Publication number: US3351540A
Application number: US354162A
Authority: US
Inventors: Charles J Amore; James F Murphy
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1964-03-23
Filing date: 1964-03-23
Publication date: 1967-11-07
Anticipated expiration: 1984-11-07
Also published as: GB1036400A; DE1496947A1; US3531381A

Description

United States Patent Ofiice 3,351,540 Patented Nov. 7, 1967 3,351,540 METHGD F IMPROVING THE CQRRO- SHQN REESTANCE 0F ()XIDIZED METAL SURFAEES Charles J. Amore, New Haven, and James F. Murphy, Hamden, (Ionm, assigners to ()iin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Mar. 23, 1964, Ser. No. 354,162 9 Claims. (1. ant-35 ABSTRACT OF THE DISCLQSURE The present invention relates to a method of improving the corrosion resistance of oxidized metal surfaces, particularly of anodic coatings on aluminum or aluminum base alloy surfaces, by electrolytic treatment in a bath of a soluble amphipathic material.

For many purposes an oxide layer is formed as a protective coating on aluminum surfaces. This is accomplished by making the aluminum the anode in an electrolytic cell having an electrolyte formed of about 2 to 70 percent sulfuric acid or other acids or acid salts of the type, chromic acid, oxalic acid, sulfamic acid and the like. Any suitable metal, such as lead, for example, may form the cathode. A voltage of about 10 to 20 volts is impressed upon the cell while the electrolyte is held at a suitable temperature, such as from 10 to 50 C., while an oxide coating of the desired thickness is formed. Generally, the time of treatment varies from to 60 minutes or more depending upon the thickness of the coating desired.

In addition to the foregoing, there are a number of less commercial processes for producing oxide layers on aluminum or aluminum base alloys.

The oxide layer formed under these conditions consists generally of relatively anhydrous alumina, A1 0 containing sulfate ion. Particularly when formed by anodic oxidation, the layer of aluminum oxide is hard, porous, highly absorbent, and of substantial thickness, depending upon the particular aluminum sample treated and the specific process used for forming the oxide coating.

The formation of an oxide coating on the aluminum surface is intended, inter alia, to improve corrosion resistance, to improve resistance to abrasion, and to improve absorption of coloring in order to provide permanent coloration on the aluminum surface.

It has been found heretofore that the characteristics of the oxide film can be markedly improved by a process generally known as sealing. The sealing process renders the oxide film relatively impervious and less porous, The sealing process is generally carried out commercially by immersing the anodized aluminum article in water maintained near its boiling point, i.e., within about F. of the boiling point. The sealing process is believed to result primarily in the conversion of a substantial part of the porous and pervious oxidefilm of anhydrous alumina to a hydrated product, such as alumi num monohydrate, Al O H O, with resultant swelling or volume increase of the oxide particles to partially close or seal the pores.

This sealing process improves the resistance to corrosion significantly and markedly. It is highly desirable, however, to obtain a still further improvement in the corrosion resistance of the anodized aluminum article. Pursuant to this goal, numerous methods have been proposed aiming towards improving the corrosion resistance still further than that which is obtained by conventional sealing processes. For example, the sealing bath has been modified by various additions, such as chromic acid or boric acid or metal salts, such as nickel acetate and others. These modified processes offer some improvement but still leave considerable room for still greater improvement in the corrosion resistance.

conventionally, various accelerated tests have been devised to determine the corrosion resistance of an aluminum specimen, for example, the CASS test and a cathodic electrochemical breakdown test. These tests enable us to determine quickly and expeditiously the corrosion resistance of an aluminum surface. In the CASS test, a fine spray of a solution containing 58 gm. per liter sodium chloride, 0.264 gm. per liter cupric chloride, pH 3.0 adjusted with glacial acetic acid and at F., is allowed to impinge upon the aluminum article .for several hours. The sample is then removed, cleaned, and a 4 inch grid placed over the surface of the article; the number of squares containing one or more pits is. determined. The ratio of squares containin one or more pits to the total number of squares, multiplied by 100, gives the percent area affected. In the cathodic breakdown test the anodized aluminum article is made cathode in a cell containing an electrolyte and platinum anode. A voltage is impressed across the cell and associated circuitry, which results in current flow through the oxide. The resultant voltage drop across the cell is measured and the integral of the voltage-time curve over a 3-minute interval determined. The integrated value in volt-seconds is the test number.

An additional disadvantage of conventional sealing practices, for example, sealing in boiling water or in nickel acetate solution, is that they generally lead to changes in the susceptibility of the anodic coating to crazing on either bending or thermal expansion.

It is, therefore, the principal object of the present invention to provide a method for improving corrosion resistance of oxidized metal surfaces, and particularly of an oxide coated aluminum surface.

It is a still further object of the present invention to provide a process as aforesaid which simply, effectively, and expeditiously attains an improved level of corrosion resistance heretofore unattainable.

It is a still further object of the present invention to obtain a highly corrosion resistant oxidized metal article and especially an oxide covered aluminum or aluminum alloy article.

It is an additional object of the present invention to provide a process for obtaining a highly corrosion resistant article as aforesaid which in addition will not craze easily as do conventionally sealed articles.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention, is has now been found that the foregoing objects and advantages may be readily accomplished and a method provided for improving the corrosion resistanct of oxidized metal surfaces and particularly of an anodized oxide coating formed on aluminum or aluminum base alloys.

The foregoing objects are obtained by (l) immersing the oxide coated metal in a bath of a soluble amphipathic material, preferably in a bath of a fatty acid salt; (2) passing an electric current of at least 2 volts through said bath between an inert cathode and said oxide coated said amphipathic material in an amount from 0.0001

percent to saturation.

The improved article of the present invention is a metal article containing an oxide coating thereon of at least 0.01 mil -in thickness, at least the major portion of said oxide coating, including the surfaces of the pores, containing adsorbed therein an amphipathic material.

The process of the present invention provides a simple method for improving the corrosion resistance of oxidized metal surfaces and particularly of an oxide coated aluminum surface and provides a highly corrosion resistant oxide covered article. In addition, the present process produces an article which will not craze easily, as do conventionally sealed coatings.

When an oxide coated metal is simply immersed in a solution of an amphipathic material, only the outer, easily accessible surfaces are treated. Thus, the barrier to corrosive reaction is considerably less than in accordance with the present invention wherein at least the major portion of the oxide coating, including the surfaces of the pores, contains the amphipathic material adsorbed therein. While it is believed that in accordance with the present invention the amphipathic material is adsorbed in the oxide coating, the present invention is not intended to preclude the possibility that some or even considerable reaction is occurring in the oxide.

An additional, surprising feature of the present invention is that amphipathic materials are wetting agents and in accordance with the present invention we are obtaining a hydrophobic or nonwetting surface by treatment with a wetting agent.

The process of the present invention is readily applicable to any aluminum or aluminum base alloy in accordance with prior practices which contains sufficient aluminum to anodize in the conventional fashion to produce a reasonably thick anodic coating. This would include all of the standard and non-standard aluminum base alloys. Exemplificative aluminum base alloys which may be readily employed include but are not limited to aluminum alloys 1100, 3003, 5453, 5053, 5252, 6061 and 60 63. More generally, the present invention is applicable to any oxidized metal surface, the metal of which forms an insoluble compound with the treating solution, for example, magnesium, iron, copper, zinc and alloys thereof.

In accordance with the present invention, the oxide coated aluminum, for example, is immersed in a bath of a soluble amphipathic material (fatty acid salt, for example) and preferably a solution containing said material in an amount of from 0.0001 to saturation. The bath may be, for example, the molten amphipathic material provided that the pure material is stable under the treatment conditions. In the preferred embodiment of the present invention, a solution of the material is used and the major portion of the solvent is water for reasons of economy; however, all or part of the solvent may include water miscible organic materials, e.g., isopropyl alcohol, ethylene glycol, glyce-rine, dirnethyl sulfoxide, etc. The concentration of the material in solution is generally in the range of from 0.0001 to 20 percent by weight, preferably from 0.001 to percent by weight and optimally in the range of 0.01 to 1.0 percent.

Any soluble amphipathic material may be used in the process of the present invention. The term amphipathic refers to ions which are highly polar and water soluble in one portion of the molecule and non-polar and water insoluble in another. The preferred materials are the water soluble fatty acid salts saturated or unsaturated, i.e., fatty carboxylates, especially the long chain fatty acids which contain one or more unsaturated bonds. Typical fatty acid salts include lithium, sodium, potassium, cesium salts of oleic, stearic, palmitic, erucic, ricinoleic, pelargonic, suberic, azeleic and lauric acids, and so forth. Other amphipathic materials include, but are not limited to anionic surfactants, such as alkyl or aryl or alkylaryl sulfonates, phosphonates, sulfates and phosphates. In addition, sulfonated fatty acid salts, cationic surfactants, such as primary, secondary or tertiary fatty amines, primary, secondary or tertiary fatty aminic salts and fatty quaternary aminic salts, and soforth.

As the anodized oxide coated article is immersed in the bath, an electric current of at least 2 volts anodic potential is passed through the bath between an inert cathode and the oxide coated aluminum as anode. During the electric treatment the bath is maintained at a temperature of between about 50 and 150 C. The preferred temperature is 70 to C. since normally water will be a major portion of the solvent. Naturally, inert materials may be added to the bath to either raise or depress the boiling point of the bath depending on treatment requirements.

The treatment time is not especially critical and generally will depend on other variables such as current, particular alloy, temperature, etc. Generally, however, the treatment should be maintained for at least thirty seconds and preferably not over 60 minutes.

An electric current of at least 2 volts anodic potential is passed through the bath as aforesaid. A minimum of at least 2 volts anodic potential is required; but the upper voltage limit is not especially critical. It is preferred, however, for convenience of handling that a voltage of less than 150 volts be employed and optimally a voltage in the range of from 70 to volts.

The electric current is passed through the bath between an inert cathode and the oxide coated metal as anode. The particular cathode employed is not especially critical other than the requirement that the cathode be substanial- 1y inert. For example, the alloy being treated may be used as cathode or stainless steel, lead, mild steel, carbon, aluminum and its alloys, or, in general, any inert material which does not add deleterious cations to the solution.

In the preferred embodiment the sample is sealed subsequent to the treatment of the present invention. The sample may be sealed prior to the treatment but this increases the crazability of the coating. The sealing is accomplished by immersing the sample in water or an aqueous solution maintained within about 10 F. of its boiling point and preferably boiling water. Preferably, very short sealing times, such as 2 minutes or less are employed.

The improved article of the present invention contains an oxide coating thereon of at least 0.01 mil in thickness, the major portion of said oxide coating, including the sur faces of the pores, containing adsorbed therein an amphipathic material. The improved article of the present invention has numerous advantages over conventional materials, for example, it is non-staining with organic dyes or other materials. The surface is hydrophobic in depth and does not craze under thermal cycling or thermal shock. The surface exhibits a lower friction than conventionally sealed surfaces and exhibits very low absorption of ionic materials. In addition, the article of the present invention shows a very high corrosion resistance and a very high resistance to cathodic attack.

A modification of the present invention includes pretreatment with a metal hydroxide solution, e.g., ammonium hydroxide, of a pH of at least 8 at a temperature of from 0 to 100 C., i.e., immersion in such a solution for at least thirty seconds.

The present invention and improvements resulting therefrom will be more readily apparent from a consideration of the following illustrative examples.

EXAMPLE I A popularity of 4 x 6 inch panels of aluminum alloy 5457 were anodized for 20 minutes at 12 amps./ sq. ft. and at 22 C. This treatment formed an anodized oxide coating on the aluminum samples of about 0.3 mil in thick ness.

Four of such samples were sealed in boiling distilled water for 15 minutes. Four additional samples were sealed in boiling water for 15 minutes and subsequently treated in accordance with the treatment of the present invention as follows: the oxide coated aluminum samples were immersed in a dilute aqueous solution containing 0.1 percent by weight of sodium stearate; the bath was maintained at 100 C. and an electric current was passed 5 through said bath between an inert cathode of unanodized aluminum alloy 5457 and the oxide coated aluminum as anode at an anodic potential of 30 volts for about 5 minutes.

A third set of four anodized oxide coated samples was treated in the same manner as the second set except that the third set of four samples was not sealed and the current was passed through the dilute solution for 15 minutes.

All samples were CASS tested for two 16-hour periods and the percentage area affected by pitting corrosion on each was determined in the manner previously described. The results of this evaluation are shown in the following table:

EXAMPLE II A water solution containing 1 percent sodium stearate was prepared and heated to 100 C. in a 4 liter beaker. A cylindrical sheet of aluminum alloy 5457 was placed in the beaker as cathode. The natural pH of the solution was 9.5.

Several 4 X 4 inch panels of aluminum alloy 5252 were anodized in 18 percent sulfuric acid solution at 22 C. for 20 minutes. The current density was maintained at 12 amps per square foot; this treatment formed an oxide coating on the aluminum surface of about .0003 inch in thickness.

Two samples, identified as Samples A, were sealed in boiling water for 15 minutes at 100 C.

Two additional samples, identified as Samples B, were sealed in distilled water for 5 minutes and then immersed in the sodium stearate solution as anode and a current passed through the solution under a 100 volts D.C. ptential for 2 minutes at 100 C.

Two additional samples, identified as Samples C, were immersed in a water solution of ammonium hydroxide for 2 minutes and then anodically treated in the sodium stearate bath at 100 volts for 2 minutes at 100 C.

Two additional samples, identified as Samples D, were immersed directly in the stearate solution and anodically treated at 100 volts at 100 C.

Finally, two additional samples, identified as Samples E, were immersed in the ammonium hydroxide solution for 2 minutes and then anodically treated for 2 minutes at 60 volts at 100 C. in the sodium stearate bath.

All panels were CASS treated for 16 hours and the amount of pitting corrosion determined in the manner previously described. The results are shown in the following table:

TABLE II Treatment: Percent area affected Samples A-15 minute seal 41 Samples B- minute seal, 2 minutes, 100 v.

Samples C-2 minutes NH OH-pH 11.5, 2 minutes, 100 v.l00 C. Samples E--2 minutes NH.;OH--pH 11.5, 2 minutes, 60 v.100 C 9 Samples D--2 minutes, 100 v.--100 C.

All of the treated samples, except the conventional sealed Samples A, resisted crazing upon bending, or by thermal shock at 150 C. conventional sealed Samples A crazed badly under these conditions.

6 EXAMPLE 111' Two l-inch diameter disks from each treatment in Example II were immersed in a 0.1 percent sodium chloride solution for 64 hours. The chloride ion was tagged with the radioactive isotope of chlorine 36 so that the amount of the chloride adsorbed could be determined.

The chloride adsorbed by the anodic coating was greatly reduced by the method of the present invention as evidenced by the following data shown in the table below:

TABLE III Amount Treatment: of Cladsorbed Samples A-15 minute seal 34.0 ,ug./ sq.

inch. Samples B-15 minute seal, 2 minutes, v.-100 C 10.4 ,ug/sq.

inch. Samples C2 minutes NH OH pH 11.5, 2 minutes, 100 v.-100 C. 1.4 ,ug./sq.

inch. Samples E-2 minutes NH OH- pH 11.5, 2 minutes, 60 v.-l00 C. 0.9 ,ug/sq.

inch. Samples D-- 2 minutes, 100 v.100 C 0.6 ig/sq.

inch.

EXAMPLE IV In the cathodic breakdown test described above, a 0.3 mil anodic film sealed in the conventional manner by immersion in boiling water for 15 minutes gave an ampere second resistance of about 800.

Samples treated in accordance with the present invention gave considerably higher test numbers, thus indicating improved resistance to corrosive attack. For example, samples treated in a manner after Samples D in Example II in a solution containing 1 percent sodium stearate used at 100 C. with 100 volts applied gave a number of 5300. Similarly, a sample treated at only 20 volts gave a number of 1000. Voltages between these two limits gave intermediate values. Similar results were obtained over a range of concentrations of sodium stearate from about 0.01 to 3 percent and over.

EXAMPLE V In a manner after Example II, the treatment of Ex ample II, Samples D was repeated utilizing instead of sodium stearate, 1 percent aqueous solutions of sodium oleate, sodium laurate and potassium ricinoleate. Similar results were obtained, with all of the samples resisting crazing upon bending or by thermal shock.

EXAMPLE VI In a manner after Example II, the treatment of Example II, Samples D was repeated utilizing a 0.1 percent aqueous sodium lauroyl sarcosinate solution under a 60 volts DC. potential for 2 minutes at 100 C. After 16 hours of CASS testing, there were no signs of deterioration of the samples. In addition all samples resisted crazing upon bending or by thermal shock.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. A method of improving the corrosion resistance of oxidized aluminum surfaces which comprises:

(A) immersing the oxide coated aluminum in an aqueous solution containing from 0.0001% to saturation of a soluble organic amphipathic material selected from the group consisting of a fatty acid salt, an anionic surfactant and a cationic surfactant; and

(B) passing an electric current of from 2 to 150 volts through said bath between an inert cathode and said oxide coated metal as an anode for from 30 seconds to 60 minutes, said bath being maintained at a temperature of from 50 to 150 C., to provide an oxide coating of at least 0.01 mil in thickness, at least the major portion of said oxide coating containing adsorbed therein said amphipathic material.

2. A method of improving the corrosion resistance of oxidized aluminum surfaces which comprises:

(A) immersing the oxide coated aluminum in an aqueous solution containing from 0.0001% to saturation of a fatty acid salt; and

(B) passing an electric current of from 2 to 150 volts through said bath for from 30 seconds to 60 minutes between an inert cathode and said oxide coated aluminum as an anode, said bath being maintained at a temperature of from 50 to 150 C., to provide an oxide coating of at least 0.01 mil in thickness, at least the major portion of said oxide coating containing adsorbed therein said fatty acid salt.

3. A method according to claim 2 wherein said fatty acid salt is present in an amount from 0.001 to 10 percent. percent.

4. A method according to claim 2 wherein said current is from 70 to 120 volts.

5. A method according to claim 2 wherein said temperature is from 70 to 100 C.

6. A method according to claim 2 wherein subsequent to said step (B) said oxide coated aluminum is sealed by immersion in an aqueous solution maintained within about 10 F. of its boiling point.

- 7. A method according to claim 2 wherein prior to said immersion the oxide coated aluminum is immersed for at least 30 seconds in a metal hydroxide solution maintained at a pH of at least 8 and a temperature of from to 100 C.

8. A method of improving the corrosion resistance of oxidized aluminum surfaces which comprises:

(A) immersing the oxide coated aluminum in a bath of an aqueous solution of a fatty acid salt, said fatty acid salt being present in an amount from 0.001 to 10 percent;

(B) passing an electric current of from 70 to 120 volts through said bath for from 30 seconds to minutes between an inert cathode and said oxide coated aluminum as an anode, said bath being maintained at a temperature of from to C.; and

(C) sealing said oxide coated aluminum by immersing in an aqueous solution maintained within about 10 F. of its boiling point, to provide an oxide coating of at least 0.01 mil in thickness, at least the major portion of said oxide coating containing adsorbed therein said fatty acid salt.

9. A method according to claim 1 wherein said soluble amphiphatic material is a phosphate.

References Cited UNITED STATES PATENTS 3,120,695 2/1964 Burnharn 2925.31 3,210,184 10/1965 Uhlig 961 2,262,967 11/1941 Schenk 20458 2,469,237 5/1949 Mason 20458 2,755,239 7/1956 Glauser 20435 2,776,918 1/1957 Bersworth 1486.14 3,016,339 1/1962 Riou et al 204--38 3,026,255 3/1962 Riou et a1. 204-33 3,230,162 1/1966 Gilchrist 204181' 3,265,597 8/1966 Neunzig et al. 204-58 FOREIGN PATENTS 548,033 9/1942 Great Britain. 656,566 2/ 1938 Germany.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

W. VAN SISE, Assistant Examiner.

Claims

1. A METHOD OF IMPROVING THE CORROSION RESISTANCE OF OXIDIZED ALUMINUM SURFACES WHICH COMPRISES: (A) IMMERSING THE OXIDE COATED ALUMINUM IN AN AQUEOUS SOLUTION CONTAINING FROM 0.0001% TO SATURATION OF A SOLUBLE ORGANIC AMPHIPATHIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF A FATTY ACID SALT, AN ANIONIC SURFACTANT AND A CATIONIC SURFACTANT; AND (B) PASSING AN ELECTRIC CURRENT OF FROM 2 TO 150 VOLTS THROUGH SAID BATH BETWEEN AN INERT CATHODE AND SAID OXIDE COATED METAL AS AN ANODE FOR FROM 30 SECONDS TO 60 MINUTES, SAID BATH BEING MAINTAINED AT A TEMPERATURE OF FROM 50 TO 150*C., TO PROVIDE AN OXIDE COATING OF AT LEAST 0.01 MIL IN THICKNESS, AT LEAST THE MAJOR PORTION OF SAID OXIDE COATING CONTAINING ABSORBED THEREIN SAID AMPHIPATHIC MATERIAL.