CA2138790C - Non-chromated oxide coating for aluminum substrates - Google Patents

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 solution containing a soluble cobalt-III hexavalent complex, the concentration of the cobalt-III hexavalent complex being from about 0.01 mole per liter of solution to the solubility 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 Me m[Co(R)6]n, wherein Me is Na, Li, K, Ca, Zn, Mg, or Mn, and wherein m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C
atoms. (B) A chemical conversion coating solution for producing the cobalt conversion coating on a metal substrate, the solution including an aqueous solution containing a soluble cobalt-III hexavalent complex, the concentration of the cobalt-III
hexavalent complex being from about 0.01 mole per liter of solution to the solubility limit of the cobalt-III hexavalent complex. (C) A coated article exhibiting acceptable 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, Co2O3, and Co3O4.

Description

~~.3~790 WO 94/00619 ' PCT/EP93/01630 NON-CHROMATED OXIDE COATING FOR ALUMINUM SUBSTRATES
Field of the Invention 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.
Back~tround of the Invention 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 hieh.
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 of 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 cfuomic 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.
One widely-used chromic acid process for forming chemical conversion coatines on aluminum substrates is described in various embodiments in Ostrander et al. U.S. Patent 2,796,370 and Ostrander et al. U.S. Patent 2,796,371, in military process specification MIL-C-5541, and in Boeing Process Specification BAC 5719. These chromic acid chemical conversion baths contain hexavalent chromium, fluorides, and cyanides, all of which present significant environmental as well as health and safety problems. The constituents of a typical chromic acid conversion bath, such as ALODINEM1200, are as follows: Cr03 - "chromic acid"
(hexavalE:rt chromium); NaF - sodium fluoride; KBF4 - potassium tetrafluoroborate;
K2ZrF6 - potassium hexafluorozirconate; K3 Fe(CN)6 potassium ferricyanide;
and, HN03 - nitric acid (for pH control).
Many aluminum structural parts, as well as Cd plated, Zn pla~.ed, Zn-Ni plated, and steel parts. throughout the aircraft and aerospace industry are currently being treated using this chromic acid process technology. Chromic acid conversion alms, as formed on aluminum substrates, meet a 168 hours corrosion resistance criterion, but they primarily serve as a surface substrate for paint adhesion. Because of their relative thinness and low coating weights (40-150 milligrams/ft2), chromic acid conversion coatings do not cause a fatigue life reduction in the aluminum structure.
However, environmental regulations in the United States, particularly in California, and in other countries are drastically reducing the allowed levels of hexavalent chromium compounds ,in effluents and emissions from metal finishing processes. Accordingly, chemical conversion processes employing hexavalent chromium compounds must be replaced. The present invention, which does not employ hexavalent chromium compounds, is intended to replace the .previously used chromic acid process for forming conversion coatings on aluminum substrates.
Summary of the Invention A. In one respect, the invention is a process for forming a cobalt conversion coating on a metal substrate, thereby imparting corrosion resistance and paint adhesion propenies. 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 reaction solution containing a soluble cobalt-III hexacoordinated complex, the concentration of the cobalt-III
hexacoordinated complex being from about 0.01 mole per liter of solution to the saturation limit of the cobalt-III hexacoordinated complex; and (b) contacting the substrate with the aqueous reaction solution for a sufficient amount of time, whereby the cobalt conversion coating is formed. The substrate may be aluminum, aluminum alloy, as well as Cd plated, Zn plated, Zn-Ni plated, and steel. The cobalt-III hexacoordinated complex is preferably present in the form of Mem[Co(R)6)", wherein Me is Na, Li, K, Ca, Zn, Mg, or Mn, and wherein m is 2 or 3, n is 1 or 2, and R is a carboxylate having 1 to 6 C
atoms.
B. In another aspect, the invention is a chemical conversion coating solution for producing a cobalt conversion coating on a metal substrate, the solution including an aqueous reaction solution containing a soluble cobalt-III hexacarboxylate complex, the concentration of the cobalt-III hexacarboxylate complex being from about O.Ol mole per liter of solution to the saturation limit of the cobalt-III hexacarboxylate complex. The cobalt conversion solution may be prepared by a bath makeup sequence including the steps of (a) dissolving a metal carboxylate salt, such as sodium, magnesium, or calcium acetate; and (b) dissolving a soluble cobalt-II salt, preferably cobalt acetate, o form a cobalt conversion coating solution.

C. In yet another aspect to the invention, wetting agents such as alkyl fluorides, fluorocarbons, and metal fluorides can be added to the conversion coating solutions.
Addition of these wetting agents eliminate the need for a costly sealing step following formation of the conversion coating.
In specific embodiments, the invention provides:
A process for forming an oxide film cobalt conversion coating on a metal substrate, said process comprising the steps of: (a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution comprising a soluble cobalt-III hexacoordinated complex, where said cobalt-III hexacoordinated complex is present in the form of Mem[Co(R)6]n, wherein Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, Mn and a mixture thereof, m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C atoms, the concentration of said cobalt-III hexacoordinated complex being from about 0.01 mole per liter of said aqueous reaction solution up to the saturation limit of said cobalt-III hexacoordinated complex; and (b) contacting said substrate with said aqueous reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed.
A process for forming an oxide film cobalt conversion coating on a metal substrate, said process comprising the steps of: (a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution comprising a soluble cobalt-III hexacarboxylate complex, the concentration of said cobalt-III
hexacarboxylate complex being from about 0.01 mole per liter -4a-of said aqueous reaction solution up to the saturation limit of said cobalt-III hexacarboxylate complex; and (b) contacting said substrate with said aqueous reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed.
A process for forming an oxide film cobalt conversion coating on a metal substrate, said process comprising the steps of: a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting a cobalt-II salt with a metal carboxylate having from 1 to 5 carbon atoms, wherein the concentration of said cobalt-II salt is from about 0.04 mole per liter of final solution up to about 0.15 mole per liter of final solution, and the concentration of metal carboxylate is from about 0.03 to 2.5 mole per liter of final solution; and b) contacting said substrate with said aqueous reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed.
A process for forming an oxide film cobalt conversion coating on a substrate, wherein said substrate is aluminum or an aluminum alloy, said process comprising the steps of: a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting a cobalt-II salt with a metal carboxylate having from 1 to 5 carbon atoms, wherein the concentration of said cobalt-II salt is from about 0.04 mole per liter of final solution up to about 0.15 mole per liter of final solution, and the concentration of metal carboxylate is from about 0.03 to 2.5 mole per liter of final solution; and b) contacting said substrate with said aqueous reaction solution for a sufficient amount of time to -4b-oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed.
A chemical conversion coating solution for producing an oxide film cobalt conversion coating on a metal substrate, said solution comprising an aqueous reaction solution comprising a soluble cobalt-III hexacoordinated complex, wherein said cobalt-III hexacoordinated complex is present in the form of Mem[Co(R)6]n, wherein Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, Mn and a mixture thereof, m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C atoms, the concentration of said cobalt-III hexacoordinated complex being from about 0.01 mole per liter of said aqueous reaction solution up to the saturation limit of said cobalt-III hexacoordinated complex.
A chemical conversion coating solution for producing an oxide film cobalt conversion coating on a metal substrate, said solution comprising: an aqueous reaction solution comprising a soluble cobalt-III carboxylate complex, the concentration of said cobalt-III carboxylate complex being from about 0.01 mole per liter of said aqueous reaction solution up to the saturation limit of said cobalt-III carboxylate complex, wherein said aqueous reaction solution is prepared by reacting a cobalt-II salt with a metal carboxylate salt having from 1 to 5 C atoms, wherein the concentration of said cobalt-II salt is from about 0.01 mole per liter of final solution up to the saturation limit of the cobalt-II salt employed, and the concentration of said metal carboxylate salt is from about 0.03 to 2.5 mole per liter of final solution.
A process for forming an oxide film cobalt conversion coating exhibiting corrosion resistance and paint -4c-adhesion properties on a substrate, wherein said substrate is aluminum or an aluminum alloy, said process comprising the steps of: (a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting cobalt acetate with a metal acetate selected from the group consisting of Mg, Ca, and Na acetate, wherein the concentration of said cobalt acetate is about 30 to 35 grams per liter of final solution and the concentration of said metal acetate is about 65 to 130 grams per liter of final solution; and (b) contacting said substrate with said aqueous reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed, thereby imparting corrosion resistance and paint adhesion properties to said substrate.
A process for forming an oxide film cobalt conversion coating exhibiting corrosion resistance and paint adhesion properties on a substrate, wherein said substrate is aluminum or an aluminum alloy, said process comprising the steps of: (a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution of a soluble cobalt-III hexacarboxylate complex, wherein said cobalt-III hexacarboxylate complex is present in the form of Mem [Co (R) 6) n, wherein Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, Mn and a mixture thereof, m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C atoms, and wherein said cobalt-III
hexacarboxylate complex was made by reacting a cobalt-II
carboxylate salt with a metal carboxylate such that the concentration of said cobalt-III hexacarboxylate complex is from about 0.01 mole per liter of said aqueous reaction solution to the solubility limit of said cobalt-III
hexacarboxylate complex; and (b) contacting said metal 21766-&90 -4d-substrate with said aqueous reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed, thereby imparting corrosion resistance and paint adhesion properties to said substrate.
A process for forming an oxide film cobalt conversion coating exhibiting corrosion resistance and paint adhesion properties on a substrate, wherein said substrate is aluminum, an aluminum alloy, magnesium, a magnesium alloy, a Cd plated substrate, or a Zn-Ni plated substrate, said process comprising the steps of: (a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting a cobalt-IT
salt with a metal carboxylate salt, wherein the concentration of said cobalt-II salt is from about 0.1 moles per liter of final solution to the solubility limit of the cobalt-II salt employed and the concentration of said metal carboxylate salt is from about 0.03 to 2.5 moles per liter of final solution; and (b) contacting said substrate with said aqueous reaction solution for a sufficient amount of time to oxidize the surface of said substrate, whereby said oxide film cobalt conversion coating is formed, thereby imparting corrosion resistance and paint adhesion properties to said substrate.
Brief Descri tion of the Drawin s The figures herein contained are photomicrographs of images produced by a scanning electron microscope of coatings on aluminum alloy test panels. FIGURES 1 through 4 are photomicrographs (scanning electron microscope operated at 20 KV) of alloy 2024-T3 test panels with cobalt conversion coatings made by the invention. FIGURES 1 through 4 show cobalt conversion coatings formed by a -4e-15 minute immersion in a typical cobalt conversion coating solution at 140°F.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
FIGURE 1 is a photomicrograph at X10,000 magnification of a test panel showing a cobalt conversation coating 410 of the invention. The photomicrograph is a top view of the upper surface of oxide coating 410. The top of oxide coating 410 is porous and looks like a sponge. This test panel was immersed in the cobalt conversion coating solution for 15 minutes. The white bar is a length of 1 micron.
FIGURE 2 is a photomicrograph at X70,000 magnification of the test panel of FIGURE 1. The photomicrograph is a top view of the upper surface of oxide coating 410. FIGURE 2 is a close-up, at higher magnification, of a small area of the test panel. The white bar is a length of 1 micron.
FIGURE 3 is a photomicrograph at X10,000 magnification of another test panel showing a side view, from an elevated angle, of a fractured cross section of a cobalt conversion coating 420 of the invention. The fractured cross section of the aluminum substrate of the test panel is indicated by reference numeral 422. 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 420. The white bar is a length of 1 micron.

-4f-FIGURE 4 is a photomicrograph at X70,000 magnification of the test panel of FIGURE 3 showing a side view, from an elevated angle, of a fractured cross section of cobalt conversion coating 420 of the invention. FIGURE 4 is a close-up, at higher magnification, of a small area of the test panel. The aluminum substrate of the test panel is indicated by reference numeral 422. The white bar is a length of 1 micron.

_;_ FIGURE 5 is a graph showing the tradeoff between paint adhesion and corrosion resistance as a function of immersion time.
Detailed Description of the Preferred Embodiment The present invention relates to a new cobalt conversion coating. The cobalt conversion coating can be made so resistant to corrosion that the conventional sealine step is no longer required. This result is achieved by adding metal fluorides and wetting agents such as alkyl fluorides and fluorocarbons to the cobalt conversion coating solution. It is believed that the combination of the wetting agents and the metal fluorides impart a small etch effect on the aluminum substrate 'surface which is believed to aid in the coating formation.
By way of background leading up to this invention, the subject matter set forth in my prior copending applications will be reviewed. A considerable amount of empirical research was conducted in order to arrive at the present invention.
:~
variety of multivalent compounds was investigated, used either by themselves or in combination with alkalis, acids, or fluorides. Among these compounds were vanadates, molybdates, cerates, ferrates and 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.
Cobalt ammine .complexes were thus produced with a number of reactants, i.e., Co(N03)~ ~H20, CoCl2~ 6H20, NH~N03, NH~CI and NH40H. The resultant coatings formed on aluminum substrates were found to have substantially improved corrosion resistant over the simple salt immersion described earlier. A review of cobalt complexing chemistry yielded the following information:
When a stream of air is drawn for several hours through a solution of cobalt-II
salt, containing ammonium hydroxide, the corresponding ammonium salt and activated charcoal, then hexammine salts are obtained:
4 Co X2 + 4 NH4X + 20 NH3 + 02 ---~ 4[Co(NH3 )~] X3 + water, ( 1 ) wherein X = Cl, Br, N03, CN, SCN, 1/3 POa, 1/2 504, CZH302, and 1/2 C03.
In the absence of activated charcoal, replacement will occur to give:
[Co(NH3)5 X]2+ (?) These generic reactions are based on the fact that cobalt-II salts have a strong tendency to oxidize to cobalt-III complexes, i.e., [Co(~3)6 ~2+ - = [Co(NH3)6 x]'~ '' a (') These reactions are not new and have been studied extensively. Cobalt-III
complexes are typically made in the photo development industry as oxidizers to enhance clarity of color photography. What is surprising, however, is that cobalt complexes are capable of forming oxide structures on aluminum substrates. The exact reaction mechanism of this oxide formation is not completely understood, but is believed to function on the chemical equilibrium in Equation (3).
This oxidizing tendency is considered to be responsible for the formation of oxide films on aluminum substrates. Bath formulations were made up successfully using the following chemical balances below:
4 Co(N03)2~6H20 + 4 NH4N03 + 20 NH40H + oxygen --~
4 [Co(NH3)6] (N03)3 + H20 (4) The ammonium nitrate is used to prevent initial precivitation of the reaction products.
This chemistry is extensively reviewed in EP-A-0 458 020.
An examination of reaction (4) revealed however, that while well defined iridescent coatinQS could be formed on aluminum substrates, an excess of ammonia, i.e., NH40H was required to drive this reaction. As a consequence, it is difFtcuh to control the pH of the bath due to the high evaporation rates of ammonia from the solution. Furthermore, the amount of excess ammonia in the bath had a pronounced effect on the paint adhesion and corrosion resistance characteristics of coatings formed by this method. Paint adhesion and corrosion resistance performance ranged from superb to complete failure, depending on the amount of ammonia in the bath.
A major advancement of this technology was achieved when a change in the portion of the complex that includes the ligand (the bracketed portion of formula (6)) '_5 within the cobalt complex was attempted. The development of the ammoniated cobalt complexes as described under reactions ( 1 ) through (4) established the bath chemistries as 3 valent cobalt ammine complexes, i.e., [Co~3)6~x3 . ( wherein X = C1, Br, N03, CN, SCN, 1/3 P04, 1/2 S04, CzH302, 1/2 C03, and wherein the portion of the complex that includes the ugand is positively charged, i.e., [Co~3)6)3+
With the substitution of nitrite compounds for the ammonium compounds the formation of 3-valent cobalt complexes such as _7_ Me3[Co(N02)6J, wherein Me = Na, Li, K (7) was achieved where the portion of the complex that includes the ligand is negatively charged, i.e., [Co(N02)6J3 (8) This nitrite complexed chemistry was found to be completely self controlling with a stable pH in the region 6.8 to 7.2. Paint adhesion performance was also found to be substantially superior,to prior chromated films. This chemistry is extensively discussed in EP-A-0 488 430. Two pronounced characteristics of conversion films formed with this nitrite technology was the almost clear (very light iridescence) ~ surface appearance and the coating film weights which were limited at 90 mglft2 maximum. These two limitations led to further research work to investigate the feasibility of obtaining coatings with strong, easily inspectable color indication along with higher coating weights, i.e., in excess of 90 mg/ft2.
Successful formulations were finally made with the introduction of acetates to form complexes such as Me3 [Co(C2H302)6J (9) where the portion of the complex that includes the ligand is also negatively charged, ~. e., [Co(C~H302)6)3 ( 10) Coatings formed by this method are strongly colored, similar to prior chromated coatings in appearance. Coating weights in excess of 350 mglft2 can be achieved.
Nitrates such as NaN03, Mg(N03)2~ 6H~0 or Ca(N03)2~ 6H~0 were added to this formulation to aid in coating firmness at higher coating weight thus avoiding powdery loose deposits.
As a result of this research, two bath formulations that resulted in a clear coating and a colored coating were selected for further testing. The following two equations below establish the chemistries for the clear and colored cobalt conversion coatings.
Clear Coat:
Co(N03)~ ~ 6H~0 + 6NaN0~ + 1/2H202 --~
~ Na3[Co(NO~)6] + 2NaN03 + 2NaOH ( 1 I ) -s-Colored Coat:
Co(N03)2 ~ 6H20 + 6CH3COONH4 + 1/2H202-Na3(Co(C2H302)6] + 2NH4N03 + NH40H (12) When coatings are produced by equations ( 11 ) and ( 12) and are sealed subsequently _5 in special formulated solutions, up to 140 hours of salt spray corrosion resistance are obtained when tested in accordance with ASTM B I 17. This chemistry is extensively discussed in EP-A-0 523 288.
The research advances described up to this point involve conversion coatings formed from the reaction of a 2-valent cobalt salt such as Co(N03)2~ 6H20 with ammonium acetate (CH3COON'H4) to form a 3-vaient cobalt complex. The resultant coatings are of superb quality with regard to defined performance criteria;
however, the bath life of solutions utilizing ammonium acetate are rather short, i.e., on the order of 30 to 40 days. The desire to extend this bath life was the basis for further research that was just recently completed. This work has now progressed to overall process and coating performance improvements in accordance with the present invention, as discussed in further detail below.
During testing of the ammonium acetate complexed cobalt solution (Equation (12)), it was noticed that after several weeks of normal tank operation the coating weiehts on aluminum substrates would gradually drop off and the color intensity would become lighter. In order to compensate for this, ever increasing immersion times were required. It was also noticed that a gradual solution appearance change would occur over time, from a dark brown to wine red color.
Analysis finally established that a competing reaction was taking place over time, where the acetate in the complex (Co(C2H302)6]3- ( 13 ) would gradually be replaced by an ammonia to form the complex (Co~3)6J3~ ( 14) Notice the valence change in these bracketed ionic species.
In an effort to solve this problem, it was discovered, in accordance with the present invention, that the substitution of a metal acetate such as Na(C2H302) ~ 3H~0, Mg(C~H302)2 ~ 4H20, or Ca(C?H30~)~ ~ H~0 for the NH4(C2H302) in Equation (12) would eliminate the above competing reactions described in conjunction with complexes ( 13 ) and ( 14), and result in the same strong _g_ colored coatings as the original ammonium acetate solutions. Sodium acetate is the most preferred metal carboxyiate. Other metal carboxylates such as zinc, lithium, potassium, and manganese acetate wil! work but are not preferred. The typical reactions are 2Co(C2H302)2 ~ 4H~0 + 3 Mg(C2H302)2 ~ 4H20 +2HC~H30~ --~ Mg3[Co(C2H302)6)2 + 21H20 (15) Co(C2H302)2 ~ 4H20 + 3Ca(C2H302)2 ~ H20 +1/2 02 + 2HC2H302---~ Ca3[Co(C2H302)6]2 + 21H20 (16) Co(C2H302)2 ~ 4H20 + 3Na(C2H302) ~ 3H20 + 1/402 + HC~H302--->Na3(Co(C2H302)6J + 13 1/2H20 (17) These reactions are carried out by bubbling air through the solution without the use of hydrogen peroxide. The acetic acid is not added as a reactant, but is formed in solution as part of the complex chemical reaction. The resultant conversion coatings are further improved in corrosion resistance over those conversion coating solutions produced by the reactions of Equations ( 11 ) and ( 12) when subjected to salt spray testing per ASTM B 117. In the broader context, it has been found that a cobalt conversion coating having superior performance characteristics can be produced by reacting a soluble cobalt salt with a metal carboxylate in accordance with the following general formula Soluble Cobalt Salt + Me(R)X---~Mem[Co(R)6Jn, (1 g) wherein x can be 1 or 2, m is 3, n is 1 or 2, Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, and Mn, and wherein R is a carboxylate having from 1 to 5 carbon (C) atoms, provided the carboxylates are soluble in the reaction solution.
Metal fluorides such as MgF2 and CaF2 and wetting agents such as water soluble alkyl,fluorides and fluorocarbons can also be added in very small quantities to these solutions (Tables III and I~ to improve corrosion protection and ~M
manufacturing ease. In particular,alkyl fluoride wetting agents such as MSP-ST
alkyl fluoride by M&T Harshaw, Cleveland, Ohio, and fluorocarbons FC99 or FC95 wetting agents by 3M Company, St. Paul, Minnesota, have been successfully used.
The presence of the fluorinated wetting agents, metal fluorides, or mixTUres thereof raise the corrosion performance level of resultant coatings to such a degree that sealing of these conversion coatings in a secondary seal step is no longer required. In principle, any water soluble fluorinated wetting agent capable of lowering liquid surface tension into the range of 30 to 40 dynes per centimeter at 20°C
is usable.
Solutions operated as detailed below yield coatings that pass a 168 hours salt spray corrosion resistance criteria in accordance with Boeing Process Specification BAC 5719, "Chromated Conversion Coatings." The single step conversion coating has yielded in excess of 240 hours of salt spray resistance before showing any sign of ' corrosion pitting.
It became apparent during experimentation with formulations (16), (17) and ( 18) that certain parameters are critical with respect to both make-up, chemistry and consistency of coating performance. These parameters were found to be bath makeup and process sequence, reactant selection and bath concentrations, temperature control, and immersion times. It must also be emphasized that the reaction system created is very complex and involves several simultaneous reversible reactions. It is believed that the entire reaction system must be present at or near an equilibrium condition in order to achieve optimum results in accordance with the present mvent~on.
Reactant Selection and Solution Concentrations The most critical parameters affecting performance of conversion coatings with respect to paint adhesion and corrosion resistance were found to be selection of the reactants and their concentrations in solution. It was found that coating performance was affected foremost by these factors rather than bath temperature or immersion time, although temperature and immersion time do impart their effects over larger variations of these parameters.
It is known that with respect to surface treatments of aluminum, paint adhesion and corrosion resistance are divergent properties. In other words, maximizing paint adhesion usually occurs at the expense of corrosion performance and vice-versa. This surface treatment behavior was also found to exist with cobalt conversion coatings. The research carried out and described in this and the aformentioned published EP applications has established a preferential list with regard to the various cobalt compounds and their corrosion resistance effectiveness versus paint adhesion.

TABLE I - Performance Rating of Cobalt Salts Usine Metal Acetates for ComplexinQ
Com onent Corrosion Resistance Paint Adhesion Cobalt Acetate + Me Acetate 1 '-Cobalt Nitrate + Me Acetate ~ 1 Cobalt Sulfate + Me Acetate '-C'nhah Chloride + Me Acetate 4 3 Rating: 1 = best, 4 = worst It can be seen from Table I that the cobalt acetate formulations present the best ~ 0 possible combination with respect to corrosion resistance while maintaining good paint adhesion performance. It should be noted, however, that in cases where corrosion performance is not a factor, nitrates or nitrites yield the best paint adhesion performance that is achievable with these cobalt compiexed salts.
Cobalt acetate is the most preferred soluble cobalt-II salt. Other water soluble i 5 cobalt salts such as Co(N03)2, CoS04, CoCl2, CoP04, CoC03, may be substituted for cobalt acetate, but are not preferred for the reasons illustrated in Table I. These cobalt salts are preferably reacted with soluble metal carboxylates having from 1 to ~
carbon atoms, although metal salts of acetic acid are most preferred. The carboxyiate salts of Ca, Mg, and Na are preferred, with the Na carboxylate being most preferred, 20 while Zn, Li, K, and Mn may also be used. The limitations on using carboxylates other than the acetates is water solubility. Other carboxylates that will work are for example sodium propionate. The minimum solubility needed to produce an effective coating is about 0.01 moles of cobalt-II salt per liter of water at 20 ° C. (68 °F) The salts may be used up to their solubility limits.
25 Although not required, fluorinated wetting agents may be added to the bath as discussed above. When these wetting agents are employed, a conversion coanng ~s created that does not need to be subjected to a conventional sealing step in order to exhibit satisfactory corrosion resistance.
_Chemical Concentration ~H Control Temperature. and Immersion Time 30 With respect to chemical concentrations, the concentration of dissolved cobalt-II salt used may be from about 0.01 mole per liter of final solution up to the solubility limit of the cobalt-II salt employed at 20°C (68°F).
Preferably, the concentration of dissolved cobalt-II salt used may be from about 0.04 mole per liter of final solution up to 0.15 mole per liter of final solution.
35 The concentration of the cobalt-III hexcarboxyiate coordination complex may be from about 0.01 mole per liter of final solution up to the solubility limit of the cobalt-III hexcarboxylate coordination complex employed. Preferably, the concentration of the cobalt-III hexcarboxylate coordination complex may be from about 0.04 mole per liter of final solution up to 0.15 mole per liter of final solution.
The concentration of dissolved metal carboxylate, preferably a metal acetate, may be from about 0.03 to 2.5 moles per liter of final solution. Preferably, the concentration of dissolved metal carboxyiate used may be from about 0.05 mole per liter of final solution up to 0.2 mole per liter of final solution.
When employed, the concentration of the fluorinated wetting agents is preferably sufficient to hold solution surface tension between 30 'to 40 dunes per centimeter at 20°C. The metal fluorides, MgF2 and CaF2, may be present in a concentration from 0 to solubility limit. It is to be understood that the fluorinated wetting agents, metal fluorides, or mixtures thereof are not required, but are preferred. If the wetting agents and metal fluorides are not used, the conversion coating must be subjected to a sealing step to achieve high corrosion resistance. By using the wetting agents and fluorides, the sealing step can be eliminated, thus making the use of the present invention even more economical.
The pH of the bath may be from about 5.0 to 9.0 with 6.0 to 7.5 being preferred and 6.5 being most preferred. The temperature of the bath may be from about 68°F to 160°F. Above 160°F, gradual decomposition of the cobalt-III
hexcarboxylate complex may occur. The optimum temperature is 140 ~ 5 °F. The immersion time may be from about 3 minutes to 60 minutes, more preferably from to 30 minutes. When sodium acetate is employed, the immersion time can be reduced to 5 to 8 minutes. Use of these parameters will result in coating weights ranging for example from 20 to 240 mg/ft2.
Preferred Bath Preparation Sequence The following bath make up sequence is preferred for the acetate complexed cobalt solution:
1. A-stainless~ steel tank is equipped with air agitation and temperature control equipment capable of controlling temperature within =5 °F.
(The tank may be lined with an inert material capable of withstanding 150°F continuous operation.) 2. The tank is filled to 3/4-full with deionized water and heated to 120°F.
Air agitation is commenced to achieve a gentle boil.

3. The applicable amount of metal acetate salt is now added and dissolved. For larger tanks,a fine meshed holding basked will serve as a holding device aiding in the dissolution of the material.

~i3s79o 4. The applicable amount of cobalt salt is then added and dissolved. A
gentle air boil of the tank solution is maintained for another four hours at which time the reaction is mostly completed. A holding basket may also be used to aid in dissolution.

5. The solution is now heated to 140°F and the required small amounts of fluorinated wetting agent are added. Air agitation is maintained for an additional two hours. The tank is now ready for operation.
Preferred Overall Processing Sequence The preferred overall processing sequence may be summarized as follows:
1. Preclean 2. Mask and Rack 3. Alkaline Clean 4. Rinse at Room Temperature 5. Deoxidize 6. Rinse at Room Temperature 7. Form Oxide Conversion Coating 8. Rinse at Room Temperature 9. Dry General Notes With Respect To The Above Process Flow Charts The cobalt conversion coating should be applied after all trimming and fabrication have been completed. Parts, where solution entrapment is possible, should not be subjected to immersion alkaline cleaning or immersion deoxidizing;
manual cleaning and manual deoxidizing procedures should be used to obtain water break-free surfaces before applying cobalt conversion treatment. A water break-free surface is a surface ~.vhich maintains a continuous water film for a period of at least seconds after having been sprayed or immersion rinsed in clean water at a temperature below 100°F.
Thorough rinsing and draining throughout processing is necessary as each solution should be completely removed to avoid interference with the performance of 30 the next solution in the sequence. Parts should be processed from one step to the next without delay and without allowing the parts to dry. When it is necessary to handle wet pans, clean latex rubber gloves should be worn. After conversion coating, handle dry parts only with clean fabric gloves. For processing systems which require part clamping, the number and size of contact points should be kept to a minimum as 3 5 necessary for adequate mechanical support.

213 8'~ 9 0 1 Preclean Vapor degrease may be performed in accordance with Boeing Process Specification BAC 5408. Emulsion clean in accordance with Boeing Process Specification BAC 5763, or solvent clean in accordance with Boeing Process Specification BAC 5750 if parts are greasy or oily. Parts with open faying surfaces or spot-welded joints where solution entrapment is possible should be immersed in cold water (or in hot and cold water) for 2 minutes after precleaning.
Mask and Rack Areas which do not require cobalt conversion coatings should be masked with maskants. Dissimilar metal inserts (except chromium, nickel or cobalt alloy or plating, CRES, or titanium) and non-aluminum coated plasma flame sprayed area should be masked off. Parts are then racked in holding baskets or mounted on holding fixtures.
Alkaline Clean Alkaline clean and rinse may be performed in accordance with Boeing Process Specification BAC 5749, except for parts with open faying surfaces or spot welded joints, in which case, rinsing should be for at least 10 minutes using agitation with multiple immersions (a minimum of four times) followed by manual spray rinsing as required to prevent solution entrapment.
Deoxidize Deoxidize and rinse may be performed in accordance with Boeing Process Specification BAC 5765 except for parts where solution entrapment is possible, which parts may be rinsed using the method described above under "Alkaline Cleaning".
Castings may be deoxidized by either of the following methods:
a. Deoxidize in accordance with Boeing Process Specification ?5 BAC 5765, Solution 37, 38 or 39.
b. Dry abrasive blast castings in accordance with Boeing Process Specification BAC 5748, Type II, Class 1 and rinse.
A specific solution formulation within the scope of the invention is as set forth in TABLE II, below.

TABLE II Preferred Solution Formulation Component' Makeup Per Liter Control Ranee Cobalt Acetate 33.0 gm 30 - 35 gm J

Co(C2H302)24H20 Magnesium Acetate 85.0 gm 80 - 90 gm M8(C2H3 02)24H20 ~0 or Calcium Acetate ~ 70.0 gm 65 - 75 gm Ca(C2H302)2H20 or .5 Sodium Acetate 125.0 gm I20-I30 gm Na(C2H302)H20 Alkyl-Fluoride (MSP-ST) 4-5 ml =' Magnesium Fluoride 2 gm I-3 gm M~2 or Calcium Fluoride ~ gm I -3 gm 25 CaF2 Operating Temperature 100F 135-145F

(Makeup) ~ Operation) Immersion Time 5-20 min.

'~ (S to 8 Min, for Na Acetate) tCoatings formed with this technology do not require sealing for corrosion resistance.
'!viaintain solution surface tension between 30 to 40 d~~nes per centimeter.

~3g'~~3~
Bath temperature variations and immersion times also contribute to corrosion performance and paint adhesion, however, to a significantly lesser degree than the solution reactant selection. It was determined that the variations in both temperatures and immersion time will affect the coating thickness primarily while the reactant materials primarily influence coating structure and density. The following general performance effects were observed:
1. Optimum bath temperature from the standpoint of corrosion and adhesion was found to be 140°F ~ 5°F.
2. An increase in bath temperature from optimum does result in thicker and looser coatings and thus a decrease in paint adhesion with an increase in corrosion resistance.
3. A decrease in bath temperature from optimum results in thinner coatings with an increase in paint adhesion and a decrease in corrosion resistance.
4. The optimum time was found to be a function of reactant selection.
5. For nitrite complexed cobalt salts the optimum immersion time was 20-25 minutes at optimum bath temp.
6. For acetate complexed cobalt salts the optimum immersion was found to be a function of the type of acetate being used, i.e.
For Na(C2H302) 5 - 8 min @ 140°F
For Mg(C2H302)2 15 - 20 min @ 140°F
For Ca(C2H302)2 12 - 15 min @ 140°F
FIGURE 5 depicts the general behavior of cobalt conversion coatings with respect to corrosion performance vs. paint adhesion. The intercept point of the corrosion and adhesion curve represents the bath parameters where the two divergent properties (corrosion and adhesion) are at optimum with respect to each other.
It is preferred that the pH be maintained between pH 6.0 and 7.5, although coatings have been produced between pH 5.0 and 9Ø Adjustments to the pH may be required after the solutions have been used for extended periods.
Corrosion Resistance Utilizing the basic corrosion resistance behavior of complexed cobalt salts, as shown above, the cobalt acetate formulations were investigated extensively.
Cobalt acetate was complexed with sodium acetate Na(C2H302) ~ 3 H20 or magnesium acetate Mg(C2H302)2 ~ 4H20. The results show that these formulations have excellent corrosion resistance without a subsequent seal step. Salt spray corrosion testing was conducted in accordance with ASTM B 117 and all specimens were 2~3s7so subjected to 168 hr. exposure. See Tables III and IV for results. The test conditions were repeated two more times and near identical results to Tables III and IV
were obtained. Analysis of this data in conjunction with basic paint adhesion data, as well as other solution maintenance parameters has resulted in the optimum tank makeup and controls as listed under Table II, above. A final question on corrosion performance and color iridescence was answered with this work. This research established why sodium acetate Na(C2H302) ~ 3H20 was chosen as the most preferred complexer over magnesium acetate Mg(CZH302)2 ~ 4H20. The reason is that sodium acetate imparts a somewhat more aggressive etch effect on the aluminum substrate. This by itself was found to be very detrimental to corrosion performance.
However, when sodium acetate was chemically formulated with wetting agents and metal fluorides, it had the distinct advantage that bright color iridescence of coatings were maintained while corrosion resistance was not impaired. On the other hand, when magnesium acetate or calcium acetate were utilized in conjunction with the wetting agents and metal fluorides very little etch effect was imparted and resultant coatings were rather weak in color effect.

Tablg III
Salt Spray Corrosion Testing of Cobalt ComQlex Fortnufations To ASTM B 117 - Sin 1Q a Step Immersion - No Seal Immersion Salt Spray Corrosion Component Bath Temp Time Resistance (168 hrs. ASTM

Dee. F. (minutes) B117) Co(C2H302)6H20 and 120 5 fail Na(C2H302)3H20 120 10 fail complexed 120 I 5 fail 120 20 marginal plus 140 5 pass MgF2 and 140 10 pass Alkyl-fluoride (MSP-ST)140 15 pass 140 20 ass 150 5 pass I50 IO pass (powdery coat) 150 15 pass (powdery coat) 150 20 ass ( wderv coat) Co(C2H302)26H,0 and 120 5 fail 2H3~2)Z4H20 120 10 fail .
Mg( p 120 15 fail com lexed 120 20 fail plus 140 5 pass stained MgF2 and 140 10 pass stained Alkyl-fluoride (MSP-ST)140 IS pass stained 140 20 ass stained 150 5 pass stained 150 10 pass stained 150 15 pass stained 150 20 ass stained Control BAC 5719 pass 3 v ALpDINE 12005 (current ci~romated system) WO 94/00619 ,~ PCT/EP93/01630 Table IV
Paint Adhesion Test Results on BMS 10-1 I' Paint System ImmersionBMS 10-I1 Bath Tem Wet Drv Comvonent Time Coatine Co(C2H302)6H20 and 120 20 TYPE I 10 10 Mg(C2H302)24H20 120 20 TYPE I 10 10 & II

complexed 120 30 TYPE I 10 10 120 30 & II

& II

i 5 140 30 TYPE I 9 10 & II

Co(C2H302)26H20 120 10 TYPE I 10 10 Na(C2H302)3H20 120 10 TYPE I 10 10 & II

complexed 120 15 TYPE I 9 10 & I I

& II

& II

2~

Control BAC 57I9 TYPE I 9 10 OR

& II

Rating: 10 = Best s0 1 = Worst TYPE I - Chromated epoxy primer (aircraft high-performance coating) TYPE II - Non-chromated epoxy enamel topcoat (aircraft high performance coating) ~5 1 Hoeing Material Specification BMS 10-11 is a highly cross-linked epoxy primer system and details the performance requirements of these coatings.

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 A1203 as the largest volume percent, and cobalt oxides CoO, Co304, and Co203. 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 present" does not necessarily imply that the volume of this oxide is more than 50 volume percent.
I S 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 A 1203. 'The middle portion of the oxide coating is a mixture of CoO, Co20g, Co304, and A1203. And the data shows that in the top portion of the oxide coating, the largest volume percent is a mixture of Co203 and Co304.
Additional characterization of the cobalt conversion coatings of the invention may be found in FIGS. 1 through 4 and in the descriptions of FIGS. 1 through 4 above. FIGS. 1 through 4 show a cobalt conversion coating 410 and 420 formed by a 15 minute immersion in a typical cobalt conversion coating solution. The top surface of the cobalt conversion coating, as shown in FIGS. 1 through 4 bears a resemblance to a sponge, thus providing substantial surface area and porosity for good paint adhesion. Below the top surface, the coating becomes more dense and solid (non-porous).
Other Methods of Application The above formulation illustrates producing cobalt conversion coatings by immersion application. The same principles apply to producing the conversion coating by manual application and by spray application.
As will be apparent to those skilled in the art to which the invention is addressed, the present invention may be embodied in forms other than those specifically disclosed above, without departing from the spirit or essential characteristics of the invention. The particular embodiments of the invention described above and the particular details of the processes described are therefore to ~13g7gp be considered in all respects as illustrative and not restrictive. The scope of the present invention is as set forth in the appended claims rather than being limited to the examples set forth in the foregoing description. Any and all equivalents are intended to be embraced by the claims.

Claims (59)

CLAIMS:

1. A process for forming an oxide film cobalt conversion coating on a metal substrate, the process comprising the steps of:
(a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution comprising a soluble cobalt-III hexacoordinated complex, where the cobalt-III hexacoordinated complex is present in the form of Me m[Co(R)6]n, wherein Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, Mn and a mixture thereof, m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C atoms, the cobalt-III hexacoordinated complex being present at a concentration of from about 0.01 mole per liter of the aqueous reaction solution up to the saturation limit of the cobalt-III hexacoordinated complex; and (b) contacting a surface of the substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed.

2. The process of claim 1, wherein an alkyl fluoride or fluorocarbon wetting agent is added to the aqueous reaction solution in an amount required to produce a liquid surface tension of 0.03 N/m to 0.04 N/m (30 to 40 dynes per centimeter).

3. The process of claim 1 or 2, wherein R is acetate.

4. The process of claim 1, 2 or 3, wherein the concentration of the cobalt-III hexacoordinated complex is from about 0.04 up to about 0.15 mole per liter of the aqueous reaction solution.

5. The process of any one of claims 1 to 4, wherein the aqueous reaction solution has a pH of about 5.0 to 9Ø

6. The process of any one of claims 1 to 5, wherein the aqueous reaction solution has a temperature of about 20°C to 71.1°C (about 68°F to 160°F).

7. The process of any one of claims 1 to 6, wherein the substrate is contacted with the aqueous reaction solution for a time of about 3 minutes to 60 minutes.

8. A process for forming an oxide film cobalt conversion coating on a metal substrate, the process comprising the steps of:
(a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution comprising a soluble cobalt-III hexacarboxylate complex, the cobalt-III hexacarboxylate complex being present at a concentration of from about 0.01 mole per liter of the aqueous reaction solution up to the saturation limit of the cobalt-III hexacarboxylate complex; and (b) contacting a surface of the substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed.

9. The process of claim 8, wherein the carboxylate is acetate.

10. The process of claim 8 or 9, wherein the concentration of the cobalt-III hexacarboxylate complex is from about 0.04 mole per liter of the aqueous reaction solution up to about 0.15 mole per liter of the aqueous reaction solution.

11. The process of claim 8, 9 or 10, wherein the aqueous reaction solution has a pH of about 5.0 to 9Ø

12. The process of any one of claims 8 to 11, wherein the aqueous reaction conversion solution has a temperature of about 20°C to 71.1°C (about 68°F to 160°F).

13. The process of any one of claims 8 to 12, wherein the substrate is contacted with the aqueous reaction solution for a time of about 3 minutes to 60 minutes.

14. A process for forming an oxide film cobalt conversion coating on a metal substrate, the process comprising the steps of:
a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting a cobalt-II salt with a metal carboxylate having from 1 to 5 carbon atoms, wherein the cobalt-II salt is employed at a concentration of from about 0.04 mole per liter of final solution up to about 0.15 mole per liter of final solution, and the metal carboxylate is employed at a concentration of from about 0.03 to 2.5 mole per liter of final solution; and b) contacting a surface of the substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed.

15. The process of claim 14, wherein the carboxylate is acetate.

16. The process of claim 14 or 15, wherein the cobalt-II salt is cobalt acetate.

17. The process of claim 14, 15 or 16, wherein the aqueous reaction solution has a pH of about 5.0 to 9Ø

18. The process of any one of claims 14 to 17, wherein the aqueous reaction solution has a temperature of about 20°C to 71.1°C (about 68°F to 160°F).

19. The process of any one of claims 14 to 18, wherein the substrate is contacted with the aqueous reaction solution for a time of about 3 minutes to 60 minutes.

20. The process of any one of claims 1 to 13, wherein the metal substrate is aluminum or an aluminum alloy.

21. A process for forming an oxide film cobalt conversion coating on a substrate, wherein the substrate is aluminum or an aluminum alloy, the process comprising the steps of:
a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting a cobalt-II salt with a metal carboxylate having from 1 to 5 carbon atoms, wherein the cobalt-II salt is employed at a concentration of from about 0.04 mole per liter of final solution up to about 0.15 mole per liter of final solution, and the metal carboxylate is employed at a concentration of from about 0.03 to 2.5 mole per liter of final solution; and b) contacting a surface of the substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed.

22. The process of claim 21, wherein the cobalt-II
salt is a cobalt-II salt which has a minimum solubility of about 0.01 moles of cobalt-II salt per liter of water at 20°C (68°F).

23. The process of claim 21, wherein the cobalt-II
salt is cobalt acetate.

24. The process of claim 21, 22 or 23, wherein the aqueous reaction solution is prepared by a bath makeup sequence comprising:

(a) dissolving the metal carboxylate salt; and (b) then adding the cobalt-II salt.

25. The process of any one of claims 21 to 24, wherein the aqueous reaction solution has a pH of about 5.0 to 9Ø

26. The process of any one of claims 21 to 25, wherein the aqueous reaction solution has a temperature of about 20°C to 71.1°C (about 68°F to 160°F).

27. The process of any one of claims 21 to 26, wherein the substrate is contacted with the aqueous reaction solution for a time of about 3 minutes to 60 minutes.

28. A chemical conversion coating solution for producing an oxide film cobalt conversion coating on a metal substrate, the solution comprising an aqueous reaction solution comprising a soluble cobalt-III hexacoordinated complex, wherein the cobalt-III hexacoordinated complex is present in the form of Me m[Co(R)6]n, wherein Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, Mn and a mixture thereof, m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C atoms, at a concentration of from about 0.01 mole per liter of the aqueous reaction solution up to the saturation limit of the cobalt-III
hexacoordinated complex.

29. The solution of claim 28, which further comprises an alkyl fluoride or fluorocarbon wetting agent added to the aqueous reaction solution in an amount required to produce a liquid surface tension of 0.03 N/m to 0.04 N/m (30 to 40 dynes per centimeter).

30. A chemical conversion coating solution for producing an oxide film cobalt conversion coating on a metal substrate, the solution comprising:
an aqueous reaction solution comprising a soluble cobalt-III hexacarboxylate complex, the cobalt-III
hexacarboxylate complex being present at a concentration of from about 0.01 mole per liter of the aqueous reaction solution up to the saturation limit of the cobalt-III
hexacarboxylate complex, wherein the aqueous reaction solution is prepared by reacting a cobalt-II salt with a metal carboxylate salt having from 1 to 5 C atoms, wherein the cobalt-II salt is employed at a concentration of from about 0.01 mole per liter of final solution up to the saturation limit of the cobalt-II salt employed, and the metal carboxylate salt is employed at a concentration of from about 0.03 to 2.5 mole per liter of final solution.

31. The solution of claim 30, wherein the cobalt-II
salt is a cobalt-II salt which has a minimum solubility of about 0.01 mole of cobalt-II salt per liter of water at 20°C (68°F).

32. The solution of claim 30, wherein the cobalt-II
salt is cobalt acetate.

33. The solution of claim 30, 31 or 32, wherein the metal is selected from the group consisting of Na, Li, K, Ca, Zn, Mg and Mn.

34. The solution of any one of claims 30 to 33, wherein the metal carboxylate is a metal acetate.

35. The solution of any one of claims 30 to 34, wherein the aqueous reaction solution is prepared by a bath makeup sequence comprising:

(a) dissolving the metal carboxylate; and (b) then adding the cobalt-II salt.

36. The solution of any one of claims 30 to 35, wherein the aqueous reaction solution has a pH of about 5.0 to 9Ø

37. The solution of any one of claims 30 to 36, wherein the aqueous reaction solution has a temperature of about 20°C to 71.1°C (about 68°F to 160°F).

38. The solution of any one of claims 30 to 37, wherein the substrate is aluminum or an aluminum alloy.

39. A process for forming an oxide film cobalt conversion coating exhibiting corrosion resistance and paint adhesion properties on a substrate, wherein the substrate is aluminum or an aluminum alloy, the process comprising the steps of:

(a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting cobalt acetate with a metal acetate selected from the group consisting of Mg, Ca, and Na acetates, wherein the cobalt acetate is employed at a concentration of about 30 to 35 grams per liter of final solution and the metal acetate is employed at a concentration of about 65 to 130 grams per liter of final solution; and (b) contacting a surface of the substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed, thereby imparting corrosion resistance and paint adhesion properties to the substrate.

40. The process of claim 39, wherein an alkyl fluoride or fluorocarbon wetting agent is added to the aqueous reaction solution in an amount required to produce a liquid surface tension of 0.03 N/m to 0.04 N/m (30 to 40 dynes per centimeter).

41. The process of claim 39, wherein MgF2, CaF2 or a mixture thereof is added to the aqueous reaction solution in an amount ranging from 2 to 4 grams per liter of final solution.

42. The process of claim 39, 40 or 41, wherein the aqueous reaction solution is prepared by a bath makeup sequence comprising:

(a) adding and dissolving the metal acetate;

(b) then adding and dissolving the cobalt acetate.

43. The process of any one of claims 39 to 42, wherein the aqueous reaction solution has a pH of about 6.0 to 7.5.

44. The process of claim 43, wherein the pH is about 6.0 to 7Ø

45. The process of any one of claims 39 to 44, wherein the aqueous reaction solution has a temperature of about 60 ~ 2.8°C (about 140 ~ 5°F).

46. The process of any one of claims 39 to 45, wherein the substrate is contacted with the aqueous reaction solution for a time of about 5 minutes to 30 minutes.

47. The process of claim 46, wherein the time is about 15 minutes to 25 minutes.

48. The process of any one of claims 39 to 47, wherein the concentration of the cobalt acetate is 33 grams per liter of final solution and of the metal acetate is 70 to 125 grams per liter of final solution.

49. A process for forming an oxide film cobalt conversion coating exhibiting corrosion resistance and paint adhesion properties on a substrate, wherein the substrate is aluminum or an aluminum alloy, the process comprising the steps of:

(a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution of a soluble cobalt-III hexacarboxylate complex, wherein the cobalt-III hexacarboxylate complex is present in the form of Mem[Co(R)6]n, wherein Me is selected from the group consisting of Na, Li, K, Ca, Zn, Mg, Mn and a mixture thereof, m is 2 or 3, n is 1 or 2, and R is a carboxylate having from 1 to 5 C atoms, and wherein the cobalt-III
hexacarboxylate complex is made by reacting a cobalt-II
carboxylate salt with a metal carboxylate such that the cobalt-III hexacarboxylate complex is employed at a concentration of from about 0.01 mole per liter of the aqueous reaction solution to the solubility limit of the cobalt-III hexacarboxylate complex; and (b) contacting a surface of the metal substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed, thereby imparting corrosion resistance and paint adhesion properties to the substrate.

50. The process of claim 49, wherein the aqueous reaction solution has a pH of about 6.0 to 7Ø

51. The process of claim 49 or 50, wherein the aqueous reaction solution has a temperature of about 57.2°C to 62.8°C (about 135°F to 145°F).

52. The process of claim 49, 50 or 51, wherein the substrate is contacted with the aqueous reaction solution for a time of about 3 minutes to 60 minutes.

53. A process for forming an oxide film cobalt conversion coating exhibiting corrosion resistance and paint adhesion properties on a substrate, wherein the substrate is aluminum, an aluminum alloy, magnesium, a magnesium alloy, a Cd plated substrate, or a Zn-Ni plated substrate, the process comprising the steps of:

(a) providing an oxide film forming cobalt conversion solution comprising an aqueous reaction solution prepared by reacting a cobalt-II salt with a metal carboxylate salt, wherein the cobalt-II salt is employed at a concentration of from about 0.1 moles per liter of final solution to the solubility limit of the cobalt-II salt employed and the metal carboxylate salt is employed at a concentration of from about 0.03 to 2.5 moles per liter of final solution; and (b) contacting a surface of the substrate with the aqueous reaction solution for a sufficient amount of time to oxidize the surface of the substrate, whereby the oxide film cobalt conversion coating is formed, thereby imparting corrosion resistance and paint adhesion properties to the substrate.

54. The process of claim 53, wherein the cobalt-II
salt is cobalt acetate.

55. The process of claim 53 or 54, wherein the metal of the metal carboxylate salt is selected from the group consisting of Na, K, Li, Ca, Zn, Mg and Mn.

56. The process of claim 53, 54 or 55, wherein a fluorinated wetting agent is added to the aqueous reaction solution to assist in the formation of the cobalt conversion coating on the substrate.

57. The process of claim 56, wherein the wetting agent is selected from the group consisting of an alkyl fluoride, a fluorocarbon, CaF2, MgF2 and mixtures thereof.

58. The process of any one of claims 53 to 57, wherein the aqueous reaction solution is prepared by a bath makeup sequence comprising:

(a) adding and dissolving the metal carboxylate;
(b) then adding and dissolving the cobalt-II salt.

59. The process of any one of claims 53 to 58, wherein the substrate is contacted with the aqueous reaction solution for a time of about 3 minutes to 60 minutes.