Low temperature corrosion resistance improvement in anodized aluminum

Abstract: Anodized aluminum surfaces are improved in corrosion resistance by a low temperature sealing with long chain carboxylic acids and conditioning of the surface with a low molecular weight alcohol solvent for the acid.

Description: Field of Invention This invention has to do with improvements in the corrosion resistance of anodized aluminium. More particularly, the invention is concerned with treated anodized aluminium surfaces which have been coated to improve their corrosion resistance and specifically with novel methods for so coating such surfaces at ambient temperatures. In another aspect, the invention has to do with conditioning anodized aluminium surfaces to enhance subsequent sealing against corrosion deterioration. Background In United States Patent No. 3,510,411 to Kramer the use of straight chain aliphatic carboxylic acids on anodized aluminum surfaces is taught to be restricted to the use of high temperature conditions which effect a melt of the acid which is thus applied to the surface. This technique while effective to improve the corrosion resistance of anodized aluminium surfaces and to increase resistance to fatigue is inefficient, costly and cumbersome because of the tendency of the acid to oxidize at the elevated temperatures required by Kramer, and limited to less complex shapes of parts. Summary of the Invention It is accordingly an object of the present invention to provide methods of improving the corrosion and fatigue resistance of anodized aluminium which is unlimited by oxidation considerations, is usable with complex shapes of parts and more economical than past methods. This and other objects of the invention to become apparent hereinafter are realized in the method of increasing the resistance to corrosion of an anodized aluminium surface by the steps of: conditioning said surface with an alcohol having from 2 to 7 carbon atoms, and immediately applying an effective amount of a long chain carboxylic acid containing from 10 to 24 carbon atoms to the conditioned surface at ambient temperatures, said steps being carried out within thirty days after anodizing said surface. In this and like embodiments of the invention, the method further includes: dissolving the acid in the alcohol for simultaneous application of the alcohol and acid to the surface; selecting as the carboxylic acid an acid containing from 10 to 24 carbon atoms; selecting as the alcohol and alcohol having from 2 to 7 carbon atoms; applying the acid at ambient temperatures below about 35 degrees Centigrade; and, selecting isopropanol as the alcohol, stearic acid, isostearic acid or lauric acid as the carboxylic acid, and predissolving the acid in the alcohol at concentrations between 1% and 12% by weight before application to the surface. The coating with a long chain carboxylic acid is applied in corrosion resistance-increasing effective amount, which is preferably from 2.54 µm to 127 µm (0.1 mil to 5 mils) or more. Preferably the acid is dissolved in the alcohol for simultaneous application of a solution of the alcohol and the acid to the anodized aluminium surface. In a preferred arrangement the invention contemplates: selecting an alcohol having from 2 to 7 carbon atoms as the alcohol; selecting isopropanol as the alcohol; selecting a long chain carboxylic acid having from 10 to 24 carbon atoms as the acid; selecting the acid from stearic acid, isostearic acid and lauric acid; applying the solution at a temperature of about 25 degrees Centigrade; applying the solution at a concentration of acid in alcohol of between 1% by weight and saturation; applying the solution to the surface promptly after anodizing the surface; selecting the alcohol from alcohols having from 2 to 5 carbon atoms; selecting isopropanol as the alcohol; using the isopropanol at concentrations of 3% to 15% by weight and preferably about 5%; and selecting a sulphuric acid/oxalic acid anodized aluminium surface as the surface to be treated. The impregnation is preferably carried out within eight hours after anodizing said surface and most preferably immediately after anodizing said surface. Preferred Modes It will be evident from the foregoing that a novel means of applying the long chain carboxylic acids is provided, involving a pretreatment of the anodized aluminium surface, with an alcohol, to form, it is believed, an alkoxide, with the result that the application of the acid is effective at lowered temperatures, below about 35 degrees Centigrade, to seal the pores in the anodize and in that manner reduce the incursion of corrosive agents and increase the period during which the surface is corrosion resistant. While not wishing to be bound to any particular theory, it is believed that the prior art teaching of the necessary use of hot molten acid to get sufficient penetration of the anodize is the result of the art failing to appreciate the effect of pre-treating or simultaneously treating the anodize with a low molecular weight alcohol in aid of sealing the anodize surface with the high molecular weight acid. It has been found that the lower molecular weight alcohols used herein while not themselves useful as corrosion enhancing agents are especially effective in aid of sealing with the carboxylic acids disclosed herein, including on hard and other anodic coatings which are not amenable to prior art processes. Thus, the patentees in United States Patent No. 3,510,411 state that "hard anodic coatings formed by anodizing in an oxalate bath or by low temperature, ie. less than about 10 degrees Celsius (50 degrees F), anodizing are unsatisfactory for purposes of the present invention" and specify anodized coating thicknesses of 7.62µm to 17.78µm (0.3 to 0.7 mil) of medium hardness. Further, they state "Impregnating is advantageously accomplished by the use of an essentially undiluted, terminally substituted aliphatic carboxylic acid having a straight or branched chain and at least about sixteen carbon atoms per molecule and the impregnation is carried out at a temperature of at least 93.3 degrees Centigrade (200 degrees F)." As will be obvious from the invention examples hereinafter, excellent corrosion resistance can be obtained using hard (low temperature) anodized coatings formed in low temperature sulphuric acid-oxalic acid baths, even with coatings in the 17.78µm to 7.62µm (0.7 to 0.3 mil) thickness range. Dilute solutions in alcohol solvents (eg. ethanol, 2-propanol) of octadecanoic acid provide nearly as much resistance to corrosion as the molten acid. Excellent corrosion resistance was obtained during the 12 carbon lauric (dodecanoic) acid. Impregnation was effected at ambient temperature (25 degrees C) using alcohol solutions. Fairly good corrosion life was obtained from thick, medium hard coatings [76.2µm to 127µm (3 to 5 mil)], room temperature anodized, then impregnated with alcohol solutions of octadecanoic (stearic) acid as much as thirty days after anodizing. In tests 1008 hours to failure was obtained in salt spray for 2024 alloy as compared with 2850 hours with no failure for specimens treated immediately (within four hours). The preparation of medium-hard anodized coatings suitable for impregnation is described in United States Patent No. 3,510,411. Corresponding hard-anodized coatings can be prepared by anodizing at -2.2 degrees C to 4.4 degrees C (28 to 40 degrees F) in 15% to 25% sulphuric acid or in similar baths with 1% to 2% oxalic acid added. The impregnation is best carried out immediately after anodizing, rinsing, and drying, but delays of up to eight hours are acceptable. Appreciable protection is afforded with thick [101.6µm (4 mil)] anodized coatings treated as much as thirty days after anodizing. The concentration of long chain carboxylic acid in solvent can vary widely, from 1% to a saturated solution. Too low a concentration results in insufficient coverage of the surface; too high a concentration leaves excessive residue when solvent is evaporated. Best results are obtained with concentrations of 1% to 12% and preferably 2% to 5%. Lower alcohol solvents work best, perhaps because the aluminum alkoxide formed initially on the surface is readily replaced by the stronger carboxylic acid to form aluminium carboxylate. Satisfactory results can not be obtained using non-hydroxylic solvents such as ketones, esters, and fluorocarbons. Without being bound by theory, we postulate that the oxalic acid is prevented from attaching to the coating by the high concentration of the strong acid (sulphuric) present. A surface aluminum sulphate, being soluble, is then removed during rinsing, leaving a reactive aluminum oxide surface. United States Patent No. 3,510,441 teaches that temperatures of at least 121.1 degrees C (250 degrees F) are required to reduce viscosity, so that acid can permeate the anodized coating. Contrary to this teaching, it has been found that the use of ethyl and isopropyl alcohol used as solvents enable ambient, eg. 25 degrees C to 35-50 degrees C coatings which give excellent results in extending corrosion resistance. EXAMPLE I Panels of several aluminum alloys measuring 7.6cm x 25.4cm x 0.32cm (3in. x 10in. x 1/8in.) were anodized in a 1.67 degrees C (35 degrees F) bath of 20% sulphuric acid, 2% oxalic acid, balance water to a 17.78µm (0.7 mil) thickness of anodized coating, rinsed and dried. Panels were immersed in 5% solutions of various carboxylic acids in various invention (low molecular weight alcohol) and control (non-alcohol) solvents (1% concentration in the fluorocarbon) for 3 to 5 minutes at 27 degrees C, allowed to air dry, then scraped free of excess fatty acid with a plastic scraper and wiped with a paper towel soaked in the solvent. Results are given in Table 1. Salt spray testing was conducted as in ASTM Method B117, using 5% salt solution. Failure was defined as 5 or more pits per panel. IPA=2-PROPANOL; FC=FREON; AA=AMYL ACETATE; MIBK=METHYLISOBUTYL KETONE EXAMPLE II Samples were anodized as in Example I, but to a thickness of 76.2µm (3 mil). Impregnation, within 6 hours, was by immersion in 5% stearic acid/2-propanol solution. No corrosion was noted at 1350 hours with any of the 6061 alloy samples, including an anodized, unsealed control. There was no failure at 2350 hours with treated 7075 or treated 2024, but an anodized, unsealed control of these metals failed in less than 336 hours. EXAMPLE III Panels were anodized to a thickness of 101.6µm (4 mils) in a 23.9 degrees C (75 degrees F) bath of 20% sulphuric acid, rinsed, and dried. After a period of 30 days, they were impregnated with 7.5% solutions of various acids dissolved in isopropyl alcohol. After 336 hours, lauric acid treated specimens of 2024 had failed, but stearic acid and 50% palmitic-50% stearic acid treated specimens were not corroded. Testing of these was continued to failure (5 or more pits) at 1008 and 648 hours, respectively. Neither a 6061 control or treated specimen failed in 2080 hours of testing. Although the invention has been particularly described with reference to the above examples, the invention may include: (i) applying the solution of a long chain carboxylic acid in an alcohol at a temperature of about 25 degrees C; (ii) selecting the alcohol from alcohols having from 2 to 5 carbon atoms; (iii) selecting isopropanol as the alcohol at concentrations of 3% to 15% by weight.

Claims: Method of increasing the resistance to corrosion of an anodized aluminum surface by the steps of: conditioning said surface with an alcohol having from 2 to 7 carbon atoms, and immediately applying an effective amount of a long chain carboxylic acid containing from 10 to 24 carbon atoms to the conditioned surface at ambient temperatures, said steps being carried out within thirty days after anodizing said surface. The method according to claim 1, including also dissolving the acid in the alcohol for simultaneous application of said alcohol and said acid to said surface. The method according to claim 1 or 2, including also applying said acid at ambient temperatures below about 35 degrees C. The method according to claim 1, including also selecting isopropanol as said alcohol, stearic acid, isostearic acid or lauric acid as said carboxylic acid, and predissolving said acid in the alcohol at concentrations between 1% and 12% by weight before application to the surface. The method according to claim 1, including selecting isopropanol as said alcohol. The method according to claim 1, including selecting said acid from stearic acid, isostearic acid, palmitic and lauric acid. The method according to claim 1, including selecting a sulphuric acid/oxalic acid anodized aluminum surface as the surface to be treated. The method according to any preceding claim, wherein said steps are carried out within eight hours of anodizing said surface. The method according to claim 8, in which said steps are carried out immediately after anodizing said surface.

Inventors: BAUMAN ALBERT J, SHULMAN GARSON P, BAUMAN, ALBERT J., SHULMAN, GARSON P.
Applicants: BAUMAN ALBERT J, SHULMAN GARSON P, BAUMAN, ALBERT J., SHULMAN, GARSON P.
Application year: 1990
Main classification: C25D11
IPC codes: C25D11