US3531381A - Method of improving the corrosion resistance of oxidized metal surfaces - Google Patents

Description

United States Patent 3,531,381 METHOD OF IMPROVING THE CORROSION RE- SISTANCE OF OXIDIZED METAL SURFACES Charles J. Amore and James F. Murphy, Hamden, Conn., assignors to Olin Corporation, a corporation of Virginia No Drawing. Continuation-impart of application Ser. No. 354,162, Mar. 23, 1964. This application July 7, 1965, Ser. No. 470,248

Int. Cl. C23b /50 U.S. Cl. 204-35 12 Claims ABSTRACT OF THE DISCLOSURE Discloses a method of improving the corrosion resistance of oxidized metal articles and the metal article obtained thereby characterized by immersing the oxide coated metal in an aqueous solution maintained at an elevated temperature, immersing the oxide coated metal in a bath of an aqueous solution containing from 0.001% to saturation of an organic phosphate and passing an electric current through said bath between an inert cathode and the oxide coated metal as anode, with the bath being maintained at an elevated temperature.

The present application is a continuation-in-part of US. patent application Ser. No. 354,162, by Charles J. Amore and James P. Murphy, filed Mar. 23, 1964, now US. Pat. 3,351,540.

The present invention relates to an improved process for enhancing the corrosion resistance of oxidized metal surfaces, particularly of anodic coatings on aluminum and aluminum base alloy surfaces.

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 to 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 5 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 3,531,381 Patented Sept. 29, 1970 ice 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 at or near its boiling point, e.g., within about 10 F. of the boiling point.

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 oifer 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 A inch grid placed over the surface of the article; the number of squares containing one or more pits is determined. The ratio of squares containing 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 a current flow through the oxide. The resultant voltage drop across the cell is measured and the integral of the voltagetime 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.

In the aforesaid co-pending application, of which the present application is a continuation-in-part, a significant improvement in the art is obtained by immersing the oxide coated metal in a bath of a soluble amphipathic material, for example, an organic phosphate or ester thereof, and passing an electric current of at least 2 volts through said bath between an inert cathode and said oxide coated metal as anode, said bath being maintained at a temperature of from about 50 to C.

The foregoing process represents a significant improvement in the corrosion resistance of the oxidized metal surface. However, When an organic phosphate is employed as the amphipathic material, it has been found that a bluish haze or tint is frequently imparted to the oxide coating. This haze or tint may be objectionable for some uses especially since the bluish color is not readily removed after the process treatment.

It is, therefore, a principal object of the present inven tion to provide a process for improving the corrosion re- 3 sistance 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 unobtainable while avoiding the disadvantage of a bluish haze or tint to the treated metal.

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

In accordance with the present invention, it has now been found that the foregoing objects and advantages may be readily accomplished and a method provided for improving the corrosion resistance of oxidized metal surfaces without obtaining an objectionable blue color, 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 an aqueous solution maintained at an elevated temperature for at least 10 seconds; (2) immersing said metal in a bath of an aqueous solution containing from 0.001% to saturation of an organic phosphate; (3) passing an electric current under at least 2 volts anodic potential through said bath between an inert cathode and said oxide coated metal as an anode, said bath being maintained at a temperature of from 50 to 150 C.

Various alternative processes or modifications of the foregoing process are contemplated by the present invention. For example, the oxide coated metal may be immersed for at least 10 seconds in an aqueous solution maintained at a temperature of from 20 to 40 C. containing an inorganic material selected from the group consisting of inorganic carbonates, inorganic hydroxides, inorganic chromates and inorganic bicarbonates or mixtures thereof, followed by subsequently immersing said metal in the phosphate solution followed by said anodizing step.

Alternatively, the oxide coated metal may simply be immersed in water, for example distilled water or tap water, at a temperature of from 50 to 100 C., preferably 100 C., for at least 10 seconds, followed by immersing the metal in the phosphate solution, followed by the anodizing step.

Still another alternative is to simply immerse the oxide coated metal in the phosphate solution, hold the metal in the said phosphate solution at a temperature of from 50 to 150 C. for at least 5 minutes and subsequently anodize the metal in said solution. Preferably, the voltage is raised slowly to the desired level at a rate of less than 2 volts per 20 seconds.

The optimum treatment is a combination of the foregoing steps, namely a process comprising either immersing the oxide coated metal in the inorganic solution in the above described manner or immersing the oxide coated metal in water in the above described manner, followed by immersion in the phosphate solution, followed by holding in the phosphate solution at an elevated temperature prior to the impression of voltage, followed by passing an electric current through the solution in the above described manner.

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.

A particular advantage of the present processes is that it produces a highly corrosion resistant article free from the disadvantageous blue haze or tint previously encotmtered.

The process of the present invention is readily applicable to any aluminum or aluminum base alloy in accordance with prior practices which contain sufiicient 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, 5052, 6061 and 6063. 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.

The oxide coated metal may first be immersed in an aqueous solution containing an inorganic material selected from the group consisting of an inorganic chromate, an inorganic carbonate, an inorganic hydroxide, an inorganic bicarbonate or mixtures thereof. Representative materials include ammonium, potassium or sodium carbonate, potassium, sodium, calcium, barium or ammonium hydroxide, potassium, sodium or ammonium chromate, ammonium, potassium or sodium bicarbonate, and so forth.

The anodized but unsealed metal is immersed in said aqueous solution for at least 10 seconds and preferably less than 5 minutes. The aqueous solution should be maintained at a temperature of from 20 to 40 C.

Naturally, other materials may be added to the inorganic aqueous solution provided that they do not alter the basic characteristics of the solution.

The concentration of said inorganic material in the aqueous solution should be maintained from 0.01% to 5% by weight, and preferably from 0.1% to 3% by weight.

Subsequent to the inorganic solution treatment step the oxide coated metal is preferably rinsed in water to remove remnants of the treatment solution. This is not essential however.

As an alternative to the foregoing treatment step, or in combination therewith the anoizde but unsealed oxide coated metal may be simply immersed in water maintained at a temperature of from 50 to C., preferably at around 100 C. for at least 10 seconds and preferably for from 10 seconds to 2 minutes. It is preferred to use distilled or tap water. However, the water may contain small amounts of other materials in order to achieve a particular result. For example, 3 to 10 parts of an inorganic phosphate may be added to the water to prevent powderable coatings.

As an alternative to the foregoing treatment steps or in combination with either or both of the foregoing treatment steps, the anodized but unsealed oxide coated metal is immersed in a bath of an aqueous solution containing an organic phosphate maintained at a concentration of from 0.001% to saturation and held in said solution prior to the impression of the desired voltage for a period of time of at least 5 minutes while said bath is maintained at a temperature of from 50 to C.

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, glycerine, dimethyl sulfoxide, etc.

The concentration of the phosphate in solution is generally in the range of from 0.0001 to 20 percent by weight, preferably from 0.001 to 10 percent by weight and optimally in the range of 0.01 to 1.0 percent.

In accordance with the present invention any organic phosphate, including esters thereof, may be conveniently utilized. For example, the following are representative phosphates contemplated in accordance with the present invention. Representative preferred phosphates are the following: alkyl phosphates or esters containing 8 to 20 carbon atoms; aryl phosphates or esters having various substituents on the aromatic ring; and alkyl aryl phosphates or esters where one or more of the substituted positions has 2 to 12 carbon atoms. The above may contain, for example, mono-, di-, or tri-esters.

5 A critical step in the process of the present invention is the passage of an electric current under at least 2 volts anodic potential through the bath between an inert cathode and the oxide coated aluminum as an anode. During the electric treatment the bath is maintained at a tem- (5) Filter all of the organic phosphate solution with a 0.5 micron filter after every 1, 2, 3, 4 or 5 days of use. Add only enough phosphate to replenish loss by drag-out, for example, 0.15 gm. per square foot of anodized aluminum processed. This method can also be used on the basis perature of between about 50 and 150 C. The preferred 5 of square feet processed, i.e., filter after 10 or more square temperature is 70 to 100 C. since normally water will feet per gallon of solution has been processed. be a major portion of the solvent. Naturally, inert mate- (6) The solution may be brought into contact with an rials may be added to the bath to either raise or depress absorbent such as aluminum oxide powder for 1 hour to the boiling point of the bath depending on treatment re- 10 3 y after 3, 4 r y or after r m r quirements. square feet per gallon of solutlon has been processed. The treatment time is not especially critical and gen- Only th amount of Phosphate removed y drag-Out heed erally will depend on other variables such as current, he added to the bathparticular alloy, temperature, etc. Generally, however, In the Preferred embodlmeht the hp Sealed Subsethe treatment should be maintained for at least thirty qlleht t0 the treatmeht 0f the Present lhveh'tloh; I Seconds and f bl not over 60 i ple may be sealed prior to the treatment but this mcreases An electric current is passed through the bath as aforee efalablhty the eeatlhg- T seehhg 15 aeeem' said. A minimum of at least 2 volts anodic potential is P e y lthmerslhg h e p 1h wfltef h q ee required; but the upper voltage limit is not especially $01'11t10h malhtalhed h about 10 Of Its belhhg critical. It is preferred, however, for convenience of han- 2 P t t Preferably bolhhg Watef- Preferably, y Short dling that a voltage of less than 150 volts be employed and Seahhg tlmes, e as mlhutes 9 less are P Y optimally a voltage in the range of from 2 to 70 volts. The P e lhvehtloh e Improvements reshltlhg The electric current is passed through the bath between therefrom W111 be e lf apparent from a Consldem' an inert cathode and the oxide coated metal as anode. tloh of the tollowlhg llhlstratlve eXaIhples- The particular cathode employed is not especially critical other than the requirement that the cathode be substan- EXAMPLE I tially inert. For example, the alloy being treated may be used as cathode or stainless steel, lead, mild steel, carbon, A plurality of 4 X 6 h panels of alummum alloy 5457 were anodized for 20 minutes at 12 amps/sq. ft. and at aluminum and Its alloys m general any men mate- 22 C This treatment formed an anodized oxide coatin rial which does not add deleterious cations to the solution. on the'qluminum Sam ]es of about 0 3 mil in thickness g In the preferred embodiment when the anodized but P unsealed oxide coated aluminum is held in the phosphate EXAMPLE II solution prior to the impression of voltage, 1t 1s preferred to Slowly l t velt'age t0 the desired level- The P Samples treated in accordance with Example I Were mum eohdltloh 13 to false the voltageless h 2 Volts P immersed in an inorganic solution as indicated below for 20 seconds uhtll fleshed Voltage 15 a chlevedvarying periods of time with the solution being main- In use the Orgahle Phosphate Solution becomes e tained at a temperature between 20 and 30 C. as indipleted of phosphate due to attachment to the anodic Gated b 1 eoatlhg and f Conventional means of p 40 Following the immersion in the inorganic solution the mg the Sohltloh are eohtemplated- SeVefa1TeP1eh1shmeht samples were immersed in a dilute aqueous solution conmethods have been found to be particularly advantageous. t i i (15% of a m terial containing about 6 5% by These are hsted helOWI weight of an ethoxylated octyl phenol phosphate ester, (1) Remove l-l5% of the total volume of the organic about 12% water and about 23% ethoxylated octyl phos hate solut on every 24 hours and replace it with phenol. The dilute aqueous solution was maintained at fresh 0.5% solution. 100 C. and an electric current was passed through said (2) Remove 575% of the total volume of the organic bath between an inert cathode of unanodized aluminum phosphate solution after square feet of anodized alumialloy 5457 and the oxide coated aluminum as anode under num per gallon of solution has been processed through the an anodic potential of 30 volts for about 5 minutes. solution and replace it with fresh solution. 50 An additional sample was treated in the phosphate so- (3) Remove 5-75 of the total volume of the organic lution as above but was not treated in the inorganic so phosphate solution every 5 days and replace it with fresh lution. solution. All samples were CASS tested for 16 hour periods and (fl) Remove 0.1-1.5% of the total volume of the orthe percentage area affected by pitting corrosion on each game phosphate solution when 1 square foot of anodized was determined in the manner previously described. The aluminum per gallon of solution has been processed. results of this evaluation are shown in the following table.

TABLE I Pretreatment Area affected Time of 16 hour by pitting Cone. Temp. immersion Degree of CASS corrosion Treating solution (percent) pH C.) (see) bluish haze response (percent) No pretreatment Intense Very good 2 Ammonium earbonate 1.0 8.6 25 10 None do 2 Do 1.0 8.6 25 30 2 1.0 8.6 2:; 2 1.0 9.0 20 2 Sodium carbonate 0. 17 10. 8 25 2 Sodium bicarbonate 0. 17 22 2 Sodium chromate. 0.01 8.0 22 2 0.17 8.3 25 1.0 8.9 22 2 0.17 8.2 22

EXAMPLE 111 Samples treated in accordance with Example I were immersed in water maintained at 100 C. for varying periods of time as indicated below. The following samples were treated in the phosphate solution of Example II in the same manner as in Example II.

An additional sample was treated in the phosphate solution without the water pretreatment.

All samples were CASS tested as in Example II.

The results are shown in the following table.

8 (C) passing an electric current with at least two volts anodic potential through said bath between an inert cathode and said oxide coated metal as anode, said bath being maintained at a temperature of from 50 to 150 C.

3. A method according to claim 2 wherein said alumi mum is immersed in said inorganic solution for from seconds to 5 minutes.

4. A method according to claim 2 wherein said inorganic material is present in a concentration of from 0.01 to 5% by weight.

TABLE II Pretreatment Area. alfected Time of 16 hour by pitting Conc. Temp. immersion Degree of CASS corrosion Treating solution (percent) pH C.) (scc.) bluish haze response (percent) Distilled water 6. 5 100 30 None Do 6. 5 100 1 Do 6. 5 100 2 Do 5. 0 100 5. 5 100 Tap Water Intense 2 No pretreatment EXAMPLE IV Samples prepared in Example I were immersed in the organic phosphate solution of Example II and held in this solution at a temperature of 100 C. for 7 minutes before the voltage was applied to the specimen. The voltage was then raised to the level of Example II and the sample was treated as in Example II. The voltage was raised two volts every seconds to 30 volts.

The sample treated in this manner was found to have no bluish haze and gave a CASS response of 2.

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 anodized, oxide coated aluminous metal surfaces which comprises:

" (A) immersing the oxide coated metal in an aqueous solution maintained at an elevated temperature for at least 10 seconds;

(B) immersing the oxide coated metal in a bath of an aqueous solution containing from 0.001% to saturation of an organic phosphate selected from the group consisting of alkyl phosphates or esters thereof containing 8 to carbon atoms, aryl phosphates or es ters thereof, and alkyl aryl phosphates or esters thereof where one or more of the substituted positions has 2 to 12 carbon atoms;

(C) passing an electric current with at least two volts anodic potential through said bath between an inert cathode and said oxide coated metal as anode, said bath being maintained at a temperature of from 50 to 150 C.

2. A method of improving the corrosion resistance of anodized, oxide coated aluminous metal surfaces which comprises:

(A) immersing the oxide coated metal for at least 10 seconds in an aqueous solution maintained at a temperature from 20 to 40 C. containing an inorganic material selected from the group consisting of a carbonate, hydroxide, chromate and bicarbonate;

(B) subsequently immersing the oxide coated metal in a bath of an aqueous solution containing from 0.001% to saturation of an organic phosphate selected from the group consisting of alkyl phosphates or esters thereof containing 8 to 20 carbon atoms, aryl phosphates or esters thereof, and alkyl aryl phosphates or esters where one or more of the substituted positions has 2 to 12 carbon atoms;

5. A method according to claim 2 wherein said metal is rinsed in water subsequent to Step A.

6. A method of improving the corrosion resistance of anodized, oxide coated aluminous metal surfaces which comprises:

(A) immersing the oxide coated metal in water main tained at 50 to C. for at least 10 seconds;

(B) subsequently immersing the oxide coated metal in a bath of an aqueous solution containing from 0.001% to saturation of an organic phosphate selected from the group consisting of alkyl phosphates or esters thereof containing 8 to 20 carbon atoms, aryl phosphates or esters thereof, and alkyl aryl phosphates or esters where one or more of the substituted positions has 2 to 12 carbon atoms;

(C) passing an electric current with at least two volts anodic potential through said bath between an inert cathode and said oxide coated metal as anode, said bath :being maintained at a temperature of from 50 to C.

7. A process according to claim 6 wherein said water is maintained at a 100 C.

8. A process according to claim 6 wherein said oxide coated metal is immersed in water for from 10 seconds to 2 minutes.

9. A method of improving the corrosion resistance of anodized, oxide coated aluminous metal surfaces which comprises:

(A) immersing the oxide coated metal in a bath of an aqueous solution containing an organic phosphate at a concentration of from 0.001% to saturation wherein said organic phosphate is selected from the group consisting of alkyl phosphates or esters thereof containing 8 to 20 carbon atoms, aryl phosphates or esters thereof, and alkyl aryl phosphates or esters thereof where one or more of the substituted positions has 2 to 12 carbon atoms;

(B) holding said metal in said phosphate solution for at least 5 minutes;

(C) subsequently passing an electric current with at least two volts anodic potential through said both between an inert cathode and said oxide coated metal as anode, said bath being maintained at a temperature of from 50 to 150 C.

10. A process according to claim 9 wherein said current is raised at a rate of less than two volts per 20 seconds until the desired current level is reached.

11. A method of improving the corrosion resistance of anodized, oxide coated aluminous metal surfaces which comprises:

(A) immersing the oxide coated metal for at least 10 seconds in an aqueous solution maintained'at a temperature from 20 to 40 C. containing an inorganic material selected from the group consisting of a carbonate, hydroxide, chromate and bicarbonate; References Cited (B) rinsing said oxide coated metal in water;

(C) immersing the oxide coated metal in a bath of an UNITED STATES PATENTS aqueous solution containing an organic phosphate at 2,469,237 /1949 Mason et a1. 20458 a concentration of from 0.001% to saturation 2,515,934 7/1950 Verner et a1 148-6.15 wherein said organic phosphate is selected from the 5 3,012,917 12/1961 Riou et a1. 148-6.15 group consisting of alkyl phosphates or esters there- 3,345,544 10/1967 Metcalfe 317-230 of containing 8 to carbon atoms, aryl phosphates 2,080,299 5/1937 Benning et a1. 1486.15 or esters thereof, and alkyl aryl phosphates or esters 3,100,728 8/1963 Vullo et a1. 148-615 thereof where one or more of the substituted posi- 3,260,622 7/1966 Le Suer 148-615 tions has 2 to 12 carbon atoms; 10

(D) holding said metal in said phosphate solution for FOREIGN PATENTS at least 5 mmutes; 907,180 3/1946 France.

(E) subsequently passing an electric current with at least two volts anodic potential through said bath between an inert cathode and said oxide coated metal 15 JOHN MACK Pnmary Exammer as anode, said bath being maintained at a tempera- W. B. VAN SISE, Assistant Examiner ture of from to C. 12. An aluminous article having an oxide coating thereon produced in accordance with the process of 20 204-38, 42 claim 1.