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.

Background 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 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 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 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.

One widely-used chromic acid process for forming chemical conversion coatings on aluminum substrates is described in various embodiments in Ostrader et al. U.S. Patent 2,796,370 and Ostrader 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 ALODINE 1200, are as follows: Crθ3 - "chromic acid" (hexavalent chromium); NaF - sodium fluoride; KBF4 - potassium tetrafluoroborate;

K^ZrFg - potassium hexafluorozirconate; K ₃ Fe(CN) ₆ potassium ferricyanide; and,

HNO3 - nitric acid (for pH control).

Many aluminum structural parts, as well as Cd plated, Zn plated, Zn-Ni plated, and steel pans, throughout the aircraft and aerospace industry are currently being

treated using this chromic acid process technology. Chromic acid conversion films, 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/ft^), 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 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 to the cobalt-III hexavalent complex being from about 0.01 mole per liter 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, aluminum alloy, as well as Cd plated, Zn plated, Zn-Ni plated, and steel.

The cobalt-III hexavalent complex is preferably present in the form of Me _m [Co(R)6] _n , wherein Me is Na, Li, K, Ca, Zn, Mg, or Mn, and wherein πi 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 solution containing a soluble cobalt-III hexacarboxylate complex, the concentration of the cobalt-III hexacarboxylate complex being from about 0.01 mole per liter 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 a soluble cobalt-II salt, preferably cobalt acetate, and (b) dissolving a metal acetate salt such as sodium, magnesium or calcium acetate to form a 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. Brief Description of the Drawings

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 a cobalt conversion coatings 410 formed by a 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 XI 0,000 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 photomicron at XI 0,000 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 \ 5 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.

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 sealing step is no longer required. This result is achieved by adding metal fluorides and wetting agents such as alkyl fluorides and fluorocarbons to the 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 A 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(NO ₃ 6H ₂ O, CoCl ₂ « 6H ₂ O, NH4NO3, NH4CI and NH4OH. 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 X ₂ + 4 NH ₄ X + 20 NH3 + O ₂ > 4[Co(NH ₃ ) ₆ ] X3 + water, ( 1 )

wherein X = Cl, Br, NO ₃ , CN, SCN, PO ₄ , SO ₄ , C ₂ H ₃ O ₂ , and CO ₃ . In the absence of activated charcoal, replacement will occur to give:

[Co(NH ₃ ) ₅ X]2+ (2)

These generic reactions are based on the fact that cobalt-II salts have a very strong tendency to oxidize to cobalt-III complexes, i.e.,

[Co(NH ₃ ) ₆ X]2+ — [Co(NH ₃ ) ₆ X]3 ⁺ + e- (3)

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(NO ₃ ) *6H ₂ O + 4 NH4NO3 + 20 NH4OH + oxygen >

4 [Co(NH ₃ ) ₆ ] (NO ₃ ) ₃ + H ₂ O (4)

The ammonium nitrate is used to prevent initial precipitation of the reaction products. This chemistry is extensively reviewed in prior copending application Serial No. 07/525,800, filed May 17, 1990. An examination of reaction (4) revealed however, that while well defined iridescent coatings could be formed on aluminum substrates, an excess of ammonia, i.e., NH4OH was required to drive this reaction. As a consequence, it is difficult 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 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)) 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 amine complexes, i.e.,

[Co(NH ₃ ) ₆ ]X ₃ (5) wherein X = CI, Br, NO3, CN, SCN, PO , SO ₄ , C ₂ H ₃ O _2> CO3, and wherein the ^" portion of the complex that includes the ligand is positively charged, i.e.,

[Co(NH ₃ ) ₆ ]3+ (6)

With the substitution of nitrite compounds for the ammonium compounds the formation of 3 -valent cobalt complexes such as

Me ₃ [Co(NO )6], wherein Me = Na, Li, K (7) was achieved where the portion of the complex that includes the ligand is negatively charged, i.e.,

[Co(NO ₂ ) ₆ ]3- (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 my prior copending application Serial No. 07/621, 132. 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 rήg/ft2 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/ft^.

Successful formulations were finally made with the introduction of acetates to form complexes such as

Me ₃ [Co(C ₂ H ₃ O ₂ ) ₆ ] (9) where the portion of the complex that includes the ligand is also negatively charged, i.e.,

[Co(C ₂ H ₃ O ₂ ) ₆ ]3- (10) Coatings formed by this method are strongly colored, similar to prior chromated coatings in appearance. Coating weights in excess of 350 mg/ft^ can be achieved. Nitrates such as NaNO3, Mg(NO3) ₂ ^» 6H ₂ O or Ca(NO3) ₂ * 6H O 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(NO ₃ ) ₂ • 6H ₂ O + 6NaNO + l/2H ₂ O ₂ →

Na ₃ [Co(NO ₂ )6] + 2NaNO ₃ + 2NaOH (11)

Colored Coat:

Co(NO ₃ ) ₂ • 6H ₂ O + 6CH3COONH4 l/2H ₂ O ₂ →

Na ₃ [Co(C ₂ H ₃ O ₂ ) ₆ ] + 2NH4NO3 + NH4OH (12)

When coatings are produced by equations (11) and (12) and are sealed subsequently in special formulated solutions, up to 140 hours of salt spray corrosion resistance are obtained when tested in accordance with ASTM Bl 17. This chemistry is extensively discussed in my prior copending application Serial No. 07/732,568.

The research advances described up to this point involve conversion coatings formed from the reaction of a 2-valent cobalt salt such as Co(NO3) ₂ ^« 6H ₂ O with ammonium acetate (CH3COONH4) to form a 3 -valent 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 weights 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(C ₂ H ₃ O ₂ ) ₆ ]3- _{( 13)} would gradually be replaced by an ammonia to form the complex

[Co(NH ₃ ) ₆ ] ³⁺ (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(C ₂ H ₃ O ₂ ) ^• 3H ₂ O, Mg(C ₂ H ₃ O ₂ ) ₂ ^• 4H ₂ O, or Ca(C ₂ H ₃ O ₂ ) ₂ ^• H ₂ O for the

NH ₄ (C ₂ H3O ₂ ) in Equation (12) would eliminate the above competing reactions described in conjunction with complexes (13) and (14), and result in the same strong

colored coatings as the original ammonium acetate solutions. Sodium acetate is the most preferred metal carboxylate. Other metal carboxylates such as zinc, lithium, potassium, and manganese acetate will work but are not preferred. The typical reactions are

2Co(C ₂ H ₃ O ₂ ) ₂ • 4H ₂ O + 3 Mg(C H ₃ O ₂ ) ₂ • 4H ₂ O

+2HC ₂ H ₃ O ₂ ► Mg ₃ [Co(C ₂ H ₃ O2)6]2 + 21H ₂ O (15)

Co(C ₂ H ₃ O ₂ ) ₂ • 4H ₂ O + 3Ca(C ₂ H ₃ O ₂ ) ₂ ^• H ₂ O +1/2 O ₂ + 2HC ₂ H ₃ O ₂ > Ca ₃ [Co(C ₂ H ₃ O ₂ ) ₆ ] ₂ + 21H ₂ O (16)

Co(C ₂ H ₃ O ₂ ) ₂ • 4H ₂ O + 3Na(C ₂ H ₃ O ₂ ) • 3H ₂ O + l/4O ₂ + HC ₂ H ₃ O ₂ >Na ₃ [Co(C ₂ H ₃ O ₂ ) ₆ ] + 13 l/2H ₂ O (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 Bl 17. 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 ►Me _m [ ^Co ( ^R ) ₆ ] _n , (18) 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 MgF ₂ and CaF ₂ 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 IV) to improve corrosion protection and 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 invention.

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 aforementioned prior copending 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 Using Metal Acetates for Complexing Component Corrosion Resistance Paint Adhesion

Cobalt Acetate + Me Acetate 1 2 Cobalt Nitrate f Me Acetate 3 1

Cobalt Sulfate + Me Acetate 2 4

Cobalt Chloride -t- Me Acetate 4 3

Rating: 1 = best, 4 = worst

It can be seen from Table I that the cobalt acetate formulations present the best 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 complexed salts

Cobalt acetate is the most preferred soluble cobalt-II salt. Other water soluble cobalt salts such as Co NO3) ₂ , C0SO ₄ , CoCl ₂ , C0PO ₄ , C0CO3, 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 5 carbon atoms, although metal salts of acetic acid are most preferred. The carboxylate salts of Ca, Mg, and Na are preferred, with the Na carboxylate being most preferred. 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. Although not required, fluorinated wetting agents may be added to the bath as discussed above. When these wetting agents are employed, a conversion coating is created that does not need to be subjected to a conventional sealing step in order to exhibit satisfactory corrosion resistance.

Chemical Concentration. pH Control. Temperature, and Immersion Time 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 The concentration of the cobalt-III hexcarboxylate 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 carboxylate 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 dynes per centimeter at 20°C. The metal fluorides, MgF and CaF ₂ , 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 5 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/ft ² . 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.

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 which maintains a continuous water film for a period of at least 30 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 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 parts, 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 necessary for adequate mechanical support.

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 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

Components- Makeup Per Liter Control Range

Cobalt Acetate 33.0 gm 30 - 35 gm Co(C ₂ H ₃ O ₂ ) ₂ *4H ₂ 0

Magnesium Acetate 85.0 gm 80 - 90 gm Mg(C ₂ H ₃ O ₂ ) ₂ -4H ₂ 0

or

Calcium Acetate 70.0 gm 65 - 75 gm Ca(C ₂ H ₃ O ₂ ) ₂ -H ₂ 0

or

Operating Temperature

Immersion Time

\'Coatings formed with this technology do not require sealing for corrosion resistance. ^Maintain solution surface tension between 30 to 40 dynes per centimeter.

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(C ₂ H ₃ O ₂ ) 5 - 8 min @ 140°F

For Mg(C ₂ H ₃ O ₂ ) ₂ 15 . 0 min @ 140°F

For Ca(C ₂ H ₃ O ₂ ) 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 9.0. 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(C ₂ H3θ ) • 3 H ₂ O or magnesium acetate Mg(C ₂ H3θ ₂ ) ₂ • 4H ₂ O. 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 Bl 17 and all specimens were

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(C ₂ H3θ ₂ ) • 3H ₂ O was chosen as the most preferred complexer over magnesium acetate Mg(C ₂ H_3θ ₂ ) ₂ • 4H O. 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.

Table HI

Table IV

TYPE I - Chromated epoxy primer (aircraft high-performance coating)

TYPE II - Non-chromated epoxy enamel topcoat (aircraft high performance coating)

- Boeing Material Specification BMS 10-11 is a highly cross-linked epoxy primer system and details the performance requirements of these coaungs.

Qxide 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" 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. 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 ₂ O ₃ , Co ₃ O4, and Al O ₃ . And the data shows that in the top portion of the oxide coating, the largest volume percent is a mixture of Co ₂ O ₃ and Co ₃ O ₄ . Additional characterization of the cobalt conversion coatings of the invention may be found in FIGS. 1 through 4 and in t ie 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

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.