Non-chromated oxide coating for aluminum substrates

Abstract: (A.) A process for forming a cobalt conversion coating on a metal substrate, thereby imparting corrosion resistance and paint adhesion properties. The invention was developed as a replacement for the prior art chromic acid process. The process includes the steps of: (a) providing a cobalt conversion solution comprising an aqueous, alkaline solution containing a soluble cobalt-III hexavalent complex, the concentration of the cobalt-III hexavalent complex being from about 0.1 mole per gallon of solution to the saturation limit of the cobalt-III hexavalent complex; and (b) contacting the substrate with the solution for a sufficient amount of time, whereby the cobalt conversion coating is formed. The substrate may be aluminum or aluminum alloy, as well as Cd plated, Zn plated, Zn-Ni plated, and steel. The cobalt-III hexavalent complex may be present in the form of [Co(NH₃)₆]X₃ wherein X is Cl, Br, NO₃, CN, SCN, PO₄, SO₄, C₂H₃O₂, or CO₃. (B.) A chemical conversion coating solution for producing the cobalt conversion coating on a metal substrate, the solution including an aqueous, alkaline solution containing a soluble cobalt-III hexavalent complex, the concentration of the cobalt-III hexavalent complex being from about 0.1 mole per gallon of solution to the saturation limit of the cobalt-III hexavalent complex. The cobalt conversion solution may be prepared by a bath makeup sequence including the steps of: (a) dissolving an ammonium salt; (b) then dissolving a cobalt-II salt; and (c) then adding ammonium hydroxide (ammonia). (C.) A coated article exhibiting corrosion resistance and paint adhesion properties, the article including: (a) a metal substrate; and (b) a cobalt conversion coating formed on the substrate, the cobalt conversion coating including aluminum oxide Al₂O₃ as the largest volume percent, and cobalt oxides CoO, Co₃O₄, and Co₂O₃.

Description: This environmental-quality invention is in the field of chemical conversion coatings formed on metal substrates, for example, on aluminum substrates. More particularly, one aspect of the invention is a new type of oxide coating (which I refer to as a "cobalt conversion coating") which is chemically formed on metal substrates. The invention enhances the quality of the environment of mankind by contributing to the maintenance of air and water quality. In general, chemical conversion coatings are formed chemically by causing the surface of the metal to be "converted" into a tightly adherent coating, all or part of which consists of an oxidized form of the substrate metal. Chemical conversion coatings can provide high corrosion resistance as well as strong bonding affinity for paint. The industrial application of paint (organic finishes) to metals generally requires the use of a chemical conversion coating, particularly when the performance demands are high. Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly the high-copper 2000-series aluminum alloys, such as alloy 2024-T3, corrode much more rapidly than pure aluminum. In general, there are two types of processes for treating aluminum to form a beneficial conversion coating. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the aluminum component and the chemical bath. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes. The second type of process is by chemically producing a conversion coating, which is commonly referred to as a chemical conversion coating, by subjecting the aluminum component to a chemical solution, such as a chromic acid solution, but without using an electric current in the process. The chemical solution may be applied by immersion application, by manual application, or by spray application. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes. The present invention relates to this second type of process for producing chemical conversion coatings. The chemical solution may be applied by immersion application, by various types of manual application, or by spray application. US-A-3,905,838 discloses a bath for forming a film of aluminum oxide, comprising aqueous ammonia, aqueous hydrogen peroxide, an oxidizing agent, an inhibitor retarding the bath temperature increase and preventing the formed hydroxide from dissolution, and a bath stabilizer being triethanol amine functioning as a valency stabilizer for heavy metals present. This bath stabilizer inhibits the change of the valency by oxidation of for instance Co ++ into Co +++ . EP-A-405,340 (considered state of the art for novelty pursuant to Article 54(3) and (4) EPC) discloses a surface blackening treatment for zinciferous surfaces, using an aqueous solution comprising Ni ++ and/or Co ++ ions, a complexing agent for these ions, such as ammonia, and optionally an additional component. This additional component accelerates the tendency of zinc from the surface being treated to dissolve, and thereby accelerating precipitation on the metal surface of nickel and/or cobalt. DE-C-745,704 discloses a process for forming a glass-clear and colorless oxide layer on aluminum and on aluminum alloys. After a pretreatment of the substrate consisting of an acid treatment with cold, concentrated nitric acid or rubbing using a leather shammy, an aqueous solution is used comprising a heavy metal-amine salt, an excess amount of ammonia, and an alkali persulfate. The heavy metal amine salt is a cobalt-II hexammine complex. (A.) In one aspect the invention relates to a process for forming an oxide film cobalt conversion coating on a metal substrate, said process comprising the steps of: (a) providing a cobalt conversion solution having a pH of 7.5 to 9.5, and comprising an aqueous, alkaline reaction solution comprising a soluble cobalt-III hexammine complex, where said cobalt-III hexammine complex is present in the form of [Co(NH 3 ) 6 ]X 3 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and where said cobalt-III hexammine complex is made by reacting (1) a cobalt-II salt CoX 2 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (2) an ammonium salt NH 4 X, where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (3) ammonium hydroxide (ammonia), such that the cobalt-II complex is oxidized to the cobalt-III complex, with the proviso that bromate and hydrogen peroxide are excluded as oxidizing agent, and the concentration of said cobalt-III hexammine complex is from 0.026 mole/l (0.1 mole per gallon) of solution up to the saturation limit of said cobalt-III hexammine complex; and (b) contacting said substrate with said reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt version coating is formed. (B.) In another aspect the invention relates to a cobalt conversion solution having a pH of 7.5 to 9.5, and comprising an aqueous, alkaline reaction solution, for forming an oxide film cobalt conversion coating on a metal substrate, comprising a soluble cobalt-III hexammine complex present in the form of [Co(NH 3 ) 6 ]X 3 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and where said cobalt-III hexammine complex is made by reacting (1) a cobalt-II salt CoX 2 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (2) an ammonium salt NH 4 X, where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (3) ammonium hydroxide (ammonia), wherein the molar ratio of the ammonium salt to the cobalt-II salt is 8 to 1 to 14.7 to 1, such that the cobalt-II complex is oxidized to the cobalt-III complex, with the proviso that bromate and hydrogen peroxide are excluded as oxidizing agent, and the concentration of said cobalt-III hexammine complex is from 0.026 mole/l (0.1 mole per gallon) of solution up to the saturation limit of said cobalt-III hexammine complex. (C.) In yet another aspect the invention relates to the use of a cobalt conversion solution according to the invention for producing an oxide film cobalt conversion coating on an aluminum or aluminum alloy substrate. Finally the invention relates to a coated article exhibiting corrosion resistance and paint adhesion properties, said articles comprising: (a) an aluminum or aluminum alloy substrate; and (b) an oxide film cobalt conversion coating formed on said substrate, said cobalt conversion coating comprising aluminum oxide Al 2 O 3 as the largest volume percent, and one or more cobalt oxides from the group consisting of CoO, Co 3 O 4 , and Co 2 O 3 , wherein said cobalt conversion coating has a thickness of 0.4 to 0.6 µm. The figures herein are photomicrographs of images produced by a scanning electron microscope of coatings on aluminum alloy test panels. FIGS. 1-8 are photomicrographs (scanning electron microscope operated at 30 KV) of alloy 2024-T3 test panels with cobalt conversion coatings made by the invention. FIGS. 1-4 show a cobalt conversion coating 10 formed by a 10 minute immersion in a typical cobalt coating solution. FIGS. 5-8 show a cobalt conversion coating 50 formed by a 15 minute immersion in a typical cobalt coating solution. There does not appear to be any significant difference between coating 10 and coating 50. FIGS. 9-12 are photomicrographs (scanning electron microscope operated at 20 KV) of alloy 2024-T3 test panels with chromic acid conversion coatings made by the prior art ALODINE 1200 process, which are shown for comparison. FIG. 1 is a photomicrograph at X10,000 magnification of a test panel showing a cobalt conversion coating 10 of the invention. The photomicrograph is a top view, from an elevated angle, of the upper surface of oxide coating 10. The top of oxide coating 10 is porous and looks like a layer of chow mein noodles. This test panel was immersed in the cobalt conversion coating solution for 10 minutes. The white bar is a length of 1 micron. The roundish objects indicated by reference numeral 12 are unidentified impurities on the surface of the oxide coating. FIG. 2 is a photomicrograph at X50,000 magnification of the test panel of FIG. 1. The photomicrograph is a top view, from an elevated angle, of the upper surface of oxide coating 10. FIG. 2 is a close-up, at higher magnification, of a small area of FIG. 1. The white bar is a length of 1 micron. FIG. 3 is a photomicrograph at X10,000 magnification of another test panel showing a side view of a fractured cross section of a cobalt conversion coating 10 of the invention. The fractured cross section of the aluminum substrate of the test panel is indicated by reference numeral 32. This test panel was immersed in the coating bath for 10 minutes. To make the photomicrograph, the test panel was bent and broken off to expose a cross section of oxide coating 10. The white bar is a length of 1 micron. FIG. 4 is a photomicrograph at X50,000 magnification of the test panel of FIG. 3 showing a side view of a fractured cross section of cobalt conversion coating 10 of the invention. FIG. 4 is a close-up, at higher magnification, of a small area of FIG. 3. The cross section of oxide coating 10 looks like the side wall of a glacier. The aluminum substrate of the test panel is indicated by reference numeral 32. The white bar is a length of 1 micron. Oxide coating 10 has a vertical thickness of about 0.4-0.6 micron. FIG. 5 is a photomicrograph at X10,000 magnification of another test panel showing another cobalt conversion coating 50 of the invention. The photomicrograph is a top view, from an elevated angle, of the upper surface of oxide coating 50. The top of oxide coating 50 is porous and looks like a layer of chow mein noodles. This test panel was immersed in the cobalt conversion coating solution for 15 minutes. The white bar is a length of 1 micron. The roundish objects 52 are unidentified impurities on the surface of oxide coating 50. FIG. 6 is a photomicrograph at X50,000 magnification of the test panel of FIG. 5. The photomicrograph is a top view, from an elevated angle, of the upper surface of oxide coating 50. FIG. 6 is a close-up, at higher magnification, of a small area of FIG. 5. The white bar is a length of 1 micron. FIG. 7 is a photomicrograph at X10,000 magnification of another test panel showing a side view of a fractured cross section of cobalt conversion coating 50 of the invention. The aluminum substrate of the test panel is indicated by reference numeral 72. This test panel was immersed in the coating bath for 15 minutes. To make the photomicrograph, the test panel was bent and broken off to expose a cross section of oxide coating 50. The white bar is a length of 1 micron. FIG. 8 is a photomicrograph at X50,000 magnification of the test panel of FIG. 7 showing a side view of a fractured cross section of cobalt conversion coating 50 of the invention. FIG. 8 is a close-up, at higher magnification, of a small area of FIG. 7. The exposed side of oxide coating 50 looks like the wall of a glacier. The aluminum substrate of the test panel is indicated by reference numeral 72. The white bar is a length of 1 micron. Oxide coating 50 has a vertical thickness of about 0.4-0.6 micron. FIG. 9 is a photomicrograph at X10000 magnification of another aluminum test panel showing a chromic acid conversion coating 90 made by the prior art ALODINE 1200 process. The photomicrograph is a top view, from an elevated angle, of the upper surface of chromic acid conversion coating 90. Coating 90 is characterized by the numerous fissures or channels that are visible in FIG. 9. The white bar is a length of 1 micron. FIG. 10 is a photomicrograph at X50,000 magnification of the test panel of FIG. 9. It is a top view, from an elevated angle, of the upper surface of conversion coating 90. FIG. 10 is a close-up view, at higher magnification, of a small area of FIG. 9. The white bar is a length of 1 micron. FIG. 11 is a photomicrograph at X10,000 magnification of another test panel showing chromic acid conversion coating 90 made by the prior art ALODINE 1200 process. FIG. 11 is a side view, from a slightly elevated angle, of a fractured cross section of the chromic acid conversion coating 90. The aluminum substrate of the test panel is indicated by reference numeral 112. To make this photomicrograph, the test panel was bent and broken off to expose a cross section of conversion coating 90. The white bar is a length of 1 micron. FIG. 12 is a photomicrograph at X50,000 magnification of the test panel of FIG. 11 showing a side view, taken from a slightly elevated angle, of the fractured cross section of the conversion coating 90. FIG. 12 is a close-up, at higher magnification, of a small area of FIG. 11. The aluminum substrate is 112. The exposed side of conversion coating 90 looks like a smooth wall. The white bar is a length of 1 micron. Chromic acid conversion coating 90 has a vertical thickness of about 0.2 micron. A considerable amount of empirical research was conducted in order to arrive at the present invention. A variety of multivalent compounds was investigated, used either by themselves or in combination with alkalies, acids, or fluorides. Among these compounds were vanadates, molybdates, cerates, ferrates and a variety of borates. While film deposition of compounds containing these elements on aluminum alloy substrates has been achieved, none afforded any appreciable corrosion protection nor paint adhesion. A significant increase in corrosion protection was observed, however, when aluminum substrates were immersed in aqueous solutions of simple cobalt-II (Co 2+ ) salts heated to 82°C (180°F). This led to an investigation of a number of cobalt-II and cobalt-III (Co 3+ ) reactions, in particular as described below. When a stream of air is drawn for several hours through an aqueous solution containing a cobalt-II salt, such as CoX 2 (where X = Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 ,½CO 3 ), and the corresponding ammonium salt NH 4 X, in the presence of ammonium hydroxide (ammonia) and activated charcoal (as catalyst), then a cobalt-III hexammine coordination complex is obtained. For example, 4 CoX 2 +4 NH 4 X + 20 NH 3 0 2 / 4 [Co(NH 3 ) 6 ]X 3 + water If the above reaction is carried out in the absence of activated charcoal, the reaction takes a longer time to produce the cobalt-III complex in equation (1) and initially produces an intermediate cobalt-II complex ion: [Co(NH 3 ) 5 X] 2+ An important aspect of cobalt chemical reactions is the fact that cobalt-II complexes have a strong tendency to oxidize to cobalt-III complexes, i.e., It was discovered that when aluminum alloy substrates (such as alloy 2024-T3) are immersed in an aqueous solution containing the cobalt-III complex above, bright iridescent coatings are formed on the aluminum alloy, which give excellent corrosion resistance properties. The above cobalt complexes are not new. Typical use of cobalt-III complexes is made in the photo development industry as oxidizers to enhance the clarity of color photography. For example, Bissonette U.S. Patent 4,088,486 teaches this photographic use of cobalt-III ammine complexes. However, it is surprising that these cobalt-III hexammine complexes are capable of forming oxide structures on aluminum substrates. The exact reaction mechanism of the oxide formation is not completely understood at this time; while not desiring to be bound by a particular theory, the oxide formation is believed to function on the chemical equilibrium (3) shown above. The oxidizing ability of the cobalt-III hexammine complex is believed to be responsible for the formation of the observed oxide films (which I refer to as "cobalt conversion coatings") on aluminum substrates. The formation of oxide structures has been confirmed by instrumental analysis (Auger analysis and electron microscopy) of the coating. The photomicrographs in FIGS. 1-8 illustrate the appearance of the cobalt conversion coating of the invention. Initial bath formulations were made up using the following chemicals: CoCl 2 ·6H 2 O; NH 4 Cl; and NH 4 OH (ammonia). Reaction quantities were used in accordance with stoichiometric balances as shown: 4 CoCl 2 ·6H 2 O + 4 NH 4 Cl + 20 NH 3 O 2 / 4 [Co(NH 3 ) 6 ]Cl 3 + water Activated charcoal (catalyst) was not used as it presented significant bath purity problems. It became apparent during experimentation with this initial formulation that a number of parameters are important from the standpoint of bath chemistry and uniform formation of oxide coating films. Some of these parameters are: chemical reactant selection; chemical reactant concentrations; bath makeup sequence; pH control; temperature; and immersion time. Chemical Reactant Selection Regarding reactant selection, a wide variety of cobalt salts and ammonium salts are operable for cobalt complexing. Among the other cobalt-II salts which are operable in water solution are cobaltous cyanide dihydrate Co(CN) 2 ·2H 2 O, cobaltous thiocyanate Co(SCN) 2 ·3H 2 O, cobaltous orthophosphate Co 3 (PO 4 ) 2 , cobaltous sulfate CoSO 4 , cobaltous acetate Co(C 2 H 3 O 2 ) 2 ·4H 2 O, and cobaltous basic carbonate 2CoCO 3 ·Co(OH) 2 ·H 2 O. Each of the foregoing cobalt-II salts may be reacted with its corresponding ammonium salt and ammonium hydroxide (ammonia). From an environmental standpoint, the cyanide and thiocyanate salts are not preferred. Furthermore, other cobalt-II salts may be used if they possess a minimum solubility in water or in a water solution containing the corresponding ammonium salt and ammonium hydroxide (ammonia). Moreover, if the minimum solubility of the cobalt-II salt is sufficient, it is not necessary to use the ammonium salt that corresponds to the cobalt salt, e.g., other ammonium salts may be used such as NH 4 Cl, NH 4 NO 3 , (NH 4 ) 2 CO 3 , NH 4 SCN. Also, the water solution may employ a combination of ammonium salts (see Example 5 below). It may also be noted that the preferred ammonium salts are NH 4 NO 3 and (NH 4 ) 2 CO 3 , or a combination thereof, since cobalt salts tend to have a strong reaction affinity for nitrates and carbonates. Chemical Reactant Concentration, pH Control, Temperature, And Immersion Time With respect to chemical reactant concentrations, the concentration of dissolved cobalt-II salt used is from 0.026 mole/l (0.1 moles per gallon) of final solution up to the saturation limit of the cobalt-II salt employed. The concentration of dissolved ammonium salt may be from about 0.6 to 12 moles per 3.8 liters (gallon) of final solution. The concentration of ammonium hydroxide (ammonia) may be from about 0.26 to 2.11 mole of NH 3 per liter (1 to 8 moles of NH 3 per gallon) of final solution; a stoichiometric excess of ammonium hydroxide (ammonia) is preferred. The pH of the bath is from 7.5 to 9.5. The temperature of the bath may be from about 20°C to 65°C (68°F to 150°F); above 65°C (150°F) gradual decomposition of the cobalt-III hexammine complex occurs. The immersion time may be from about 3 minutes to 60 minutes. Preferred Bath Preparation Sequence 1. A stainless steel tank fitted with air agitation plumbing and heating coils is filled to 3/4 with deionized water at room temperature 20°C (68°F) . Air agitation is commenced to a gentle bubble. (The tank may be equipped with a filter unit to remove any solid impurities (dust, aluminum silt, etc.) during processing.) 2. A quantity of the ammonium salt is added and completely dissolved. The amount used is determined by the prime objective. If the prime objective is for the oxide coating to exhibit high corrosion resistance, the preferred molar ratio of ammonium salt to cobalt salt is about 8 to 1 and the preferred concentration of ammonium salt is about 0.63 moles/l (2.4 moles per gallon) of final solution. Or if the prime objective is for the oxide coating to exhibit high paint adhesion, the preferred molar ratio of ammonium salt to cobalt salt is about 12 to 1 and the preferred concentration of ammonium salt is about 0.95 moles/l (3.6 moles per gallon) of final solution. 3. The cobalt-II salt is now added and dissolved. The preferred concentration is about 0.08 moles/l (0.3 moles per gallon) of final solution. Thus, when the prime objective is high corrosion resistance, this concentration of the cobalt salt, when added to a solution already containing 0.63 moles/l 2.4 (moles per gallon) of the ammonium salt, achieves the preferred molar ratio of ammonium salt to cobalt salt of about 8 to 1. Or when the prime objective is high paint adhesion, this concentration of the cobalt salt, when added to a solution already containing 0.95 moles/l (3.6 moles per gallon) of the ammonium salt, achieves the preferred molar ratio of ammonium salt to cobalt salt of 12 to 1. Air bubbling is maintained. 4. A stoichiometric excess of ammonium hydroxide NH 4 OH (ammonia) is now added with air agitation. Preferably, 250-300 ml of ammonium hydroxide (28-30% wt. NH 3 ) per gallon of final solution is added; this is a concentration of about 0.92-1.16 moles NH 3 /l (3.5-4.4 moles of NH 3 per gallon) of final solution. A precipitate may form depending on the concentration of ammonium salt being used, but the precipitate will gradually dissolve as the cobalt-II complex and the cobalt-III complex in equilibrium equation (3) above are formed. 5. The tank is filled to the final volume with deionized water. Air agitation of this solution is maintained for 2-3 hours at room temperature and then the solution is heated to preferably 62 ± 3°C (145 ± 5°F). 6. The solution is maintained at preferably 62 ± 3°C (145 ± 5°F) for at least an additional 2 hrs. to allow the cobalt complexing reaction (e.g., equations (1) and (4) above) to make a minimum operable amount of the cobalt-III complex, which is indicated by a burgundy or purple/red color in the solution. Preferably, the solution is maintained at 62 ± 3°C (145 ± 5°F) for an additional 8 hrs. to make a larger amount of the cobalt-III complex to facilitate efficient practice of the cobalt conversion coating process. 7. Optionally, a second stainless steel tank (to be used for a silicate immersion step) is prepared with air agitation plumbing and heating coils and is filled 3/4 with deionized water. This post-cobalt conversion coating step serves as a paint adhesion promoter. The tank is heated to 48-60°C (120-140°F) with air agitating. 8. The preferred amount of sodium meta-silicate Na 2 SiO 3 ·9H 2 O (about 0.07-0.09 mole/l (0.28-0.35 moles per gallon) of final solution) is added and dissolved. The tank is then filled to final volume with deionized water. No further air agitation is needed. A substantial amount of sludge may be created in the tank if the above makeup sequence is not employed. Preferred Overall Processing Sequence The preferred overall processing sequence for formation of cobalt conversion coatings, meeting the performance criteria of MIL-C-5541 and BAC 5719, is as described below. The overall processing sequence is: (1) vapor degrease (for example, according to Boeing Process Specification BAC 5408); (2) alkaline clean (for example, according to Boeing Process Specification BAC 5749); (3) rinse at room temperature; (4) deoxidize (for example, according to Boeing Process Specification BAC 5765); (5) rinse at room temperature; (6) form oxide coating according to the present invention (preferably for 5-10 minutes at 48-65°C (120-150°F)); (7) (optional) silicate immersion treat (preferably for 3 minutes at 48-60°C (120-140°F)); (8) rinse at room temperature; and (9) dry (do not exceed 60°C (140°F)). The preferred overall processing sequence may be summarized as follows: PROCESS SEQUENCE FLOWCHART (1) VAPOR DEGREASE - BAC 5408 (2) ALKALINE CLEAN - BAC 5749 (3) RINSE - ROOM TEMP. (4) DEOXIDIZE - BAC 5765 (5) RINSE - ROOM TEMP. (6) FORM OXIDE COATING - 5-10 MIN. AT 57 ± 9°C (135 ± 15°F) (7) RINSE - ROOM TEMP. (8) SILICATE IMMERSION 54 ± 6°C (130 ± 10°F)- 3 MIN. MINIMUM (9) RINSE - ROOM TEMP. (10) DRY - DO NOT EXCEED 60°C (140°F) Examples of specific solution formulations within the scope of the invention are as follows: Example 1. Component Make-Up Per liter (Gallon) Of Final Solution Control Limits Cobalt(ous) nitrate Co(NO 3 ) 2 ·6H 2 O (hexahydrate)(about 0.29 mole) 22,3 g (85 g) Ammonium nitrate NH 4 NO 3 (about 2.32 moles) 48,9 g (186 g) Ammonium hydroxide NH 4 OH 65,8-79 ml (250-300 ml) (28-30% wt. NH 3 ) (about 3.5-4.4 moles of NH 3 ) Water balance Temperature 48-65°C (120-150°F) pH 8.0 - 9.0 The formulation of Example 1, with a molar ratio of ammonium salt to cobalt salt of about 8 to 1, is useful for producing oxide coatings exhibiting high corrosion resistance. Example 2. Component Make-Up Per liter (Gallon) Of Final Solution Control Limits Cobalt(ous) chloride CoCl 2 ·6H 2 O (hexahydrate)(about 0.29 mole) 18,1 g (69 g) Ammonium nitrate NH 4 NO 3 (about 2.32 moles) 48,9 g (186 g) Ammonium hydroxide NH 4 OH (28-30% wt. NH 3 ) (about 3.5-4.4 moles of NH 3 ) 65,8-79 ml (250-300 ml) Water balance Temperature 48-65°C (120-150°F) pH 8.0 - 9.0 The formulation of Example 2, also having a molar ratio of ammonium salt to cobalt salt of about 8 to 1, is useful for producing oxide coatings possessing high corrosion resistance. When the molar ratio of ammonium salt to cobalt salt is increased (for example, to about 12 to 1 or 13 to 1), then oxide coatings exhibiting high paint adhesion are obtained. This, however, is accomplished with some reduction in corrosion resistance, i.e., 168 hrs. corrosion resistance testing produces marginal results. A specific formulation, having an ammonium salt to cobalt salt molar ratio of about 13.3 to 1, is as follows: Example 3. Component Make-Up Per liter (Gallon) Of Final Solution Control Limits Cobalt(ous) nitrate Co(NO 3 ) 2 ·6H 2 0 (about 0.29 mole) 22,3 g (85 g) Ammonium carbonate (NH 4 ) 2 CO 3 (about 3.87 moles) 97,9 g (372 g) Ammonium hydroxide NH 4 OH (28-30% wt. NH 3 ) (about 3.5-4.4 moles of NH 3 ) 65,8-79 ml (250-300 ml) Water balance Temperature 48-65°C (120-150°F) pH 8.0 - 9.0 The formulation of Example 3, with a molar ratio of ammonium salt to cobalt salt of about 13.3 to 1, is useful for producing oxide coatings exhibiting high paint adhesion. Another specific formulation, having an ammonium salt to cobalt salt molar ratio of about 13.3 to 1, is as follows: Example 4. Component Make-Up Per liter (Gallon) Of Final Solution Control Limits Cobalt(ous) chloride CoCl 2 ·6H 2 0 (about 0.29 mole) 18,1 g (69 g) Ammonium carbonate (NH 4 ) 2 CO 3 (about 3.87 moles) 97,9 g (372 g) Ammonium hydroxide NH 4 OH (28-30% wt. NH 3 ) (about 3.5-4.4 moles of NH 3 ) 65,8-79 ml (250-300 ml) Water balance Temperature 48-65°C (120-150°F) pH 8.0 - 9.0 The formulation of Example 4, with a molar ratio of ammonium salt to cobalt salt of about 13.3 to 1, is useful for producing oxide coatings exhibiting high paint adhesion. In summary, Examples 1 and 2 represent formulations for obtaining oxide coatings possessing high corrosion resistance, while Examples 3 and 4 represent formulations for obtaining oxide coating possessing high paint adhesion. Another formulation (Example 5 below), having a molar ratio of ammonium salt to cobalt salt of about 14.7 to 1, produces oxide conversion coatings meeting the 168 hours corrosion resistance requirement of MIL-C-5541 as well as showing superior paint adhesio properties over conventional conversion coatings made by the chromi acid process: Example 5. Component Make-Up Per liter (Gallon) Of Final Solution Control Limits Cobalt(ous) nitrate Co(NO 3 ) 2 ·6H 2 O (about 0.29 mole) or 22,3 g (85 g) Cobalt(ous) chloride CoCl 2 ·6H2O (about 0.29 mole) 18,1 g (69 g) Ammonium carbonate (NH 4 ) 2 CO 3 (about 0.97 mole) 24,5 g (93 g) Ammonium nitrate NH 4 NO 3 (about 1.16 moles) 24,5 g (93 g) Ammonium hydroxide NH 4 OH (28-30% wt. NH 3 ) (about 3.5-4.4 moles of NH 3 ) 65,8-79 ml (250 to 300 ml) Ammonium nitrate NH 4 NO 3 (about 0.97 mole) (after initial reaction) 24,5 g (93 g) Ammonium carbonate(NH 4 ) 2 CO 3 (about 1.16 moles) (after initial reaction) 24,5 g (93 g) Water balance Temperature 48-65°C (120-150°F) pH 8.0 - 9.0 The solution of Example 5, having a molar ratio of ammonium salt to cobalt salt of about 14.7 to 1, produces oxide coatings on alloy 2024-T3 panels having passing results on the corrosion resistance test of MIL-C-5541 and passing results on the paint adhesion test of BMS 10-11 and BMS 10-60. The paint adhesion test results were comparable to paint adhesion test results achieved by chromic acid conversion coatings, which pass the corrosion resistance test of MIL-C-5541. Example 5 also illustrates the use of a combination of ammonium salts and the concept of adding the ammonium salt in two steps, the second step being after the ammonium hydroxide, in the sequence shown. As mentioned above, an optional procedure may be used to enhance paint adhesion performance when formulations similar to Examples 1 and 2 are used. Water-soluble silicates, such as sodium meta-silicate and potassium meta-silicate, have been found to be particularly effective as paint adhesion promoters when used in a post-conversion coating step (step 8 in the Process Sequence Flowchart above). Tank makeup and controls are as follows: Example 6. Component Make-Up Per liter (Gallon) Of Final Solution Control Limits Sodium meta-silicate Na 2 Si0 3 ·9H 2 O 25 g (95 g) (about 0.33 mole) 21-26,3 g/l (80-100 g/gal) (about 0.28-0.35 mole Water balance Temperature 48-60°C (120-140°F) pH Control, Temperature, And Time The three parameters of pH control, temperature, and immersion time have been found to be important as relating to cobalt conversion coating performance. Preferred pH control is maintained between pH 8.0 to 9.0. This pH range is maintained with periodic small additions of NH 4 OH or (NH 4 ) 2 CO 3 . At a pH above 9.0, corrosion resistance performance wile decrease rapidly, while paint adhesion remains high. Coatings tend to lose their colored iridescence to approach nearly-colorless appearance. At a pH of 7.5, coatings tend towards maximum corrosion resistance performance with a decrease in paint adhesion performance. Below pH 7.5, coating iridescence also disappears and a brown "stain coat" forms instead. This brown deposit possesses neither corrosion resistance nor acceptable paint adhesion properties. Bath temperature was originally kept between 18 to 29°C (65 to 85°F). This was done in the early stages of development and resulted in acceptable oxide coats. However, a continuous operating range of 48-65°C (120 to 150°F) gives optimum results with respect to coating appearance and performance. Immersion times tend to be influenced by temperature control rather than solution concentration. At room temperature 20°C (68°F), immersion times of 15 to 20 minutes are required for satisfactory conversion coating formation. At elevated temperatures 48 to 65°C (120 to 150°F), consistent and well functioning conversion coatings are produced within 5 to 10 minutes. Corrosion Resistance Salt spray corrosion resistance of cobalt conversion coatings produced by the above processes varies over a wide range, depending on reactant selection, pH, and immersion times. Preferred results are obtained, however, when formulations similar to Examples 1 and 2 are utilized at immersion times of 10-15 minutes. In this way, oxide coatings have been produced with 200 hrs. of salt spray corrosion resistance when tested in accordance with the method of ASTM B117. Paint Adhesion Paint adhesion tests were conducted using aircraft paints qualified to Boeing Material Specification BMS 10-11 (a highly crosslinked epoxy primer) and BMS 10-60 (a highly crosslinked urethane topcoat). General trends observed with the present cobalt conversion coatings are consistent with conventional chromic acid conversion coatings, i.e., corrosion resistance and paint adhesion performance properties have an inverse relationship. In general, where corrosion resistance is at a maximum, paint adhesion is at a minimum, and vice versa. However, the post-conversion step consisting of immersion into a heated solution (at 48-60°C (120-140°F)) of sodium meta-silicate Na 2 SiO 3 ·9H 2 O (Example 6 above) solves this problem by obtaining strong paint adhesion values while maintaining high corrosion resistance properties. Table I below is presented to demonstrate the trend line behavior between corrosion resistance and paint adhesion properties as a function of solution formulation variation. FORMULATION (Co = cobalt salt) CORROSION RESISTANCE PAINT ADHESION / A) 12 NH 4 NO 3 /l Co Very Low Very High B) 12(NH 4 ) 2 CO 3 / l Co Low High C) 8(NH 4 ) 2 CO 3 /l Co Medium Medium D) 8 (NH 4 ) 2 CO 3 /l Co;Silicate Medium High E) 8 NH 4 NO 3 /l Co;Silicate High High F) 8 NH 4 NO 3 /l Co High Low G) 6 NH 4 NO 3 /l Co Low Very Low H) 6 (NH 4 ) 2 CO 3 /l Co Low High Oxide Coating Analyses ESCA surface analysis, using a Perkin-Elmer Model 550 surface analyzer, and Auger oxide profiles, using the same machine (in a different operating mode), have been performed in order to characterize the cobalt conversion coatings of the invention. (ESCA = electron spectroscopy for chemical analysis (also known as XPS or X-ray photoelectron spectroscopy).) These analyses show that the cobalt conversion coating consists of a mixture of oxides, namely, aluminum oxide Al 2 O 3 as the largest volume percent, and cobalt oxides CoO, Co 3 O 4 , and Co 2 O 3 . The term "largest volume percent" means that the volume of this oxide exceeds the volume of any other oxide which is present, but the term "largest volume percent" does not necessarily imply that the volume of this oxide is more than 50 volume percent. The data further shows that in the lower portion of the oxide coating (that is, next to the aluminum substrate), the largest volume percent is Al 2 O 3 . The middle portion of the oxide coating is a mixture of CoO, Co 3 O 4 , Co 2 O 3 , and Al 2 O 3 . And the data shows that in the top portion of the oxide coating, the largest volume percent is a mixture of Co 3 O 4 and Co 2 O 3 . Additional characterization of the cobalt conversion coatings of the invention may be found in FIGS. 1-8 and in the descriptions of FIGS. 1-8 above. FIGS. 1-4 show a cobalt conversion coating 10 formed by a 10 minute immersion in a typical cobalt conversion coating solution. FIGS. 5-8 show a cobalt conversion coating 50 formed by a 15 minute immersion in a typical cobalt conversion coating solution. Comparing FIGS. 1-4 and FIGS. 5-8, there does not appear to be any significant structural difference between coating 10 and coating 50. The top surface of the cobalt conversion coating, as shown in FIGS. 1, 2, 5, and 6, bears a resemblance to chow mein noodles, thus providing appreciable surface area and porosity for good paint adhesion. Below the top surface, the coating becomes more dense and solid (non-porous), as shown by FIG. 4. Other Methods Of Application The above examples illustrate producing cobalt conversion coatings by immersion application. The same principles apply to producing the conversion coating by manual application and by spray application. The patents, specifications, and other publications referenced above are incorporated herein by reference.

Claims: A process for forming an oxide film cobalt conversion coating on a metal substrate, said process comprising the steps of: (a) providing a cobalt conversion solution having a pH of 7.5 to 9.5, and comprising an aqueous, alkaline reaction solution comprising a soluble cobalt-III hexammine complex, where said cobalt-III hexammine complex is present in the form of [Co(NH 3 ) 6 ]X 3 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and where said cobalt-III hexammine complex is made by reacting (1) a cobalt-II salt CoX 2 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (2) an ammonium salt NH 4 X, where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (3) ammonium hydroxide (ammonia), such that the cobalt-II complex is oxidized to the cobalt-III complex, with the proviso that bromate and hydrogen peroxide are excluded as oxidizing agent, and the concentration of said cobalt-III hexammine complex is from 0.026 mole/l (0.1 mole per gallon) of solution up to the saturation limit of said cobalt-III hexammine complex; and (b) contacting said substrate with said reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt version coating is formed. The process of claim 1, wherein said cobalt conversion solution has a temperature of 20°C (68°F) to 66°C (150°F). The process of claim 1 or 2, wherein said substrate is contacted with said cobalt conversion solution for a time of 3 minutes to 60 minutes. The process of claim 1-3, wherein the concentration of said cobalt-II salt is from 0.026 mole/l (0.1 moles per gallon) of final solution to the saturation limit of the cobalt-II salt employed, the concentration of said ammonium salt is from 0.16 to 3.16 moles/l (0.6 to 12 moles per gallon) of final solution, and the concentration of ammonium hydroxide (ammonia) is from 0.26 to 2.11 mole of NH 3 per liter (1 to 8 moles of NH 3 per gallon) of final solution. The process of claim 1-4, wherein said cobalt conversion solution is prepared by a bath makeup sequence comprising: (a) dissolving said ammonium salt; (b) then dissolving said cobalt-II salt; and (c) then adding ammonium hydroxide (ammonia). The process of claim 1-5, wherein the molar ratio of ammonium salt to cobalt salt is 8 to 1 to 14.7 to 1. The process of claim 1-6, comprising the additional step of contacting said substrate with an aqueous solution comprising a silicate. The process of claim 7, wherein said silicate is selected from the group sodium meta-silicate and potassium meta-silicate. The process of claim 1-8, wherein said metal substrate is aluminum or aluminum alloy. A cobalt conversion solution having a pH of 7.5 to 9.5, and comprising an aqueous, alkaline reaction solution, for forming an oxide film cobalt conversion coating on a metal substrate, comprising, a soluble cobalt-III hexammine complex present in the form of [Co(NH 3 ) 6 ]X 3 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and where said cobalt-III hexammine complex is made by reacting (1) a cobalt-II salt CoX 2 , where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (2) an ammonium salt NH 4 X, where X is one or more selected from the group comprising Cl, Br, NO 3 , CN, SCN, 1 / 3 PO 4 , ½SO 4 , C 2 H 3 O 2 , or ½CO 3 , and (3) ammonium hydroxide (ammonia), wherein the molar ratio of the ammonium salt to the cobalt-II salt is 8 to 1 to 14.7 to 1, such that the cobalt-II complex is oxidized to the cobalt-III complex, with the proviso that bromate and hydrogen peroxide are excluded as oxidizing agent, and the concentration of said cobalt-III hexammine complex is from 0.026 mole/l (0.1 mole per gallon) of solution up to the saturation limit of said cobalt-III hexammine complex. The solution of claim 10, wherein the concentration of said cobalt-II salt is from 0.026 mole/l (0.1 moles per gallon) of final solution to the saturation limit of the cobalt-II salt employed, the concentration of said ammonium salt is from 0.16 to 3.16 moles/l (0.6 to 12 moles per gallon) of final solution, and the concentration of ammonium hydroxide (ammonia) is from 0.26 to 2.11 moles of NH 3 per liter (1 to 8 moles of NH 3 per gallon) of final solution. The solution of claim 10 or 11, wherein said cobalt conversion solution is prepared by a bath makeup sequence comprising the steps of: (a) dissolving said ammonium salt; (b) then dissolving said cobalt-II salt; and (c) then adding ammonium hydroxide (ammonia). The solution of claim 10-12, wherein said solution has a temperature of 20°C (68°F) to 66°C (150°F). The use of a cobalt conversion solution of claim 10-13 for producing an oxide film cobalt conversion coating on an aluminum or aluminum alloy substrate. An article comprising a metal substrate provided with an oxide film cobalt conversion coating formed in a process of claim 1-9, wherein said cobalt conversion coating has a thickness of 0.4 to 0.6 µm. A coated article exhibiting corrosion resistance and paint adhesion properties, said article comprising: (a) an aluminum or aluminum alloy substrate; and (b) an oxide film cobalt conversion coating formed on said substrate, said cobalt conversion coating comprising aluminum oxide Al 2 O 3 as the largest volume percent, and one or more cobalt oxides from the group consisting of CoO, Co 3 O 4 , and Co 2 O 3 , wherein said cobalt conversion coating has a thickness of 0.4 to 0.6 µm. The coated article of claim 16, wherein: (a) in the portion of said cobalt conversion coating adjacent said substrate, the largest volume percent of said coating consists essentially of a Al 2 O 3 ; (b) in the top portion of said cobalt conversion coating, the largest volume percent of said coating consists essentially of a mixture of Co 3 O 4 and Co 2 O 3 ; and (c) in the portion of said cobalt conversion coating therebetween, said coating consists essentially of a mixture of CoO, Co 3 O 4 and Al 2 O 3 , wherein said cobalt conversion coating has a thickness of 0.4 to 0.6 µm. The article of claim 15-17, wherein the top of said cobalt conversion coating (10, 50) is porous and has the appearance of chow mein noodles at X 10,000 and X 50,000. The article of claim 15-18, wherein said substrate being aluminum or aluminum alloy.

Inventors: SCHRIEVER MATHIAS P, SCHRIEVER, MATHIAS P.
Applicants: BOEING CO, THE BOEING COMPANY
Application year: 1991
Main classification: C23C22
IPC codes: C23C22, C01G51