JP3345010B2 - Chromium-free oxide coating for aluminum support - 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 Mem(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

Description: FIELD OF THE INVENTION The present invention, which is environmentally friendly, comprises a metal substratum.
ate), for example, a chemical conversion coating formed on an aluminum support
About. In particular, according to one aspect of the present invention, there is provided a new type of oxide coating that is chemically formed on a metal support. Hereinafter, this type of oxide coating is referred to as "cobalt conversion coating".
The present invention improves the quality of the environment for mankind by contributing to the maintenance of air and water quality.

BACKGROUND OF THE INVENTION Generally, chemical conversion coatings are formed chemically by converting the surface of the metal to be converted into a tight, adherent coating, all or part of which consists of an oxide of the support metal. Chemical conversion coatings exhibit high corrosion resistance and strong binding affinity for paint. Industrial applications of paints on metals, i.e. organic finishes, generally require a chemical conversion coating, especially where high performance is required.

Aluminum protects itself from corrosion by a naturally formed oxide coating, but cannot protect it completely. In the presence of moisture and electrolytes,
Aluminum alloys, especially 2000 series aluminum alloys with a high copper content, such as the 2024-T3 alloy, corrode much faster than pure aluminum.

In general, there are two types of treatment of aluminum to form a beneficial conversion coating. The first type of method is the anodizing method. In this case, the aluminum component is immersed in a chemical bath, for example, a chromic acid bath or a sulfuric acid bath, and then the aluminum component and the chemical bath are energized. The conversion coating formed on the aluminum component surface is corrosion resistant and provides a bonding surface for the organic finish.

The second type of method is a method of chemically forming a conversion coating, which is generally called a chemical conversion coating. In this case, the aluminum part is treated with a chemical, for example, a chromic acid solution. However, no current is used. The chemical may be applied by dip coating, manual coating or spray coating. The conversion coating formed on the aluminum component surface is corrosion resistant and provides a bonding surface for the organic finish. The present invention relates to a second type of chemical conversion coating forming method. The chemical solution may be applied by dip coating, manual coating in various modes, or spray coating.

Various embodiments of one widely used chromic acid method for forming chemical conversion coatings on aluminum supports are described in U.S. Pat. Nos. 2,796,370 and 2,796,371 to Ostrada et al. Each specification, military process (military process) specification MIL-C-5
541 and Boeing Process Specification BAC
5719. These chromic acid chemical conversion baths contain hexavalent chromium, fluoride and cyanide. All of these bath components pose significant problems from an environmental, health and safety perspective. Representative chromic acid conversion baths, such as AL
The components of ODINE 1200 are: CrO ₃ chromate containing hexavalent chromium, sodium fluoride NaF, potassium tetrafluoroborate K ₂ ZrF ₆ , potassium ferricyanate K ₃
Fe (CN) ₆ and HNO ₃ nitrate. In this case, nitric acid is a pH adjusting component.

Many aluminum structural parts and Cd-plated, Zn-plated or Zn-Ni-plated structural parts and steel parts used in the aviation and aerospace industries are:
At present, treatment is performed by the chromic acid method.
The chromic acid conversion film formed on the aluminum support meets the corrosion resistance standard of 180 hours,
First, it must serve as a surface support for the paint to adhere. The film is relatively thin,
Because the coating amount is also low as 40-150 mg / sq.
The chromate conversion coating does not reduce the fatigue life of the aluminum structure.

However, environmental regulations in the United States, particularly California and other countries, have greatly reduced the tolerance of hexavalent chromium compounds in wastewater and effluents from metal finishing processes. Therefore, the chemical conversion method using hexavalent chromium compounds must be replaced by another method. The present invention is a method that does not use a hexavalent chromium compound and has been made in order to provide a method that can replace the chromic acid method conventionally used for forming a conversion coating on an aluminum support. is there.

SUMMARY OF THE INVENTION A. According to one aspect of the present invention, there is provided a method of imparting corrosion resistance and paint adhesion to a metal support by forming a cobalt conversion coating on the metal support. The present invention has been developed as an alternative to the conventional chromic acid method. The method includes the following steps (a) and (b).

(A) Soluble cobalt-III hexavalent complex of about 0.01 mol /
An aqueous solution containing a concentration from liters to a saturated amount is provided, then (b) contacting the metal support with the aqueous solution for a time sufficient to form a cobalt conversion coating.

The metal support may be aluminum, aluminum alloy, Cd plated, Zn plated, Zn-Ni plated and steel. The cobalt-III hexavalent complex has the following formula: Me _m [Co (R) ₆ ] _n (where Me represents Na, Li, K, Ca, Zn, Mg or Mn;
m represents 2 or 3, n represents 1 or 2, and R represents a carboxylate having 1 to 6 carbon atoms).

B. According to a second aspect of the present invention, there is provided a chemical conversion coating solution for forming a cobalt conversion coating on a metal support, wherein the soluble cobalt-III hexacarboxylate complex is prepared from about 0.01 mol / liter. A chemical conversion coating solution is provided that includes an aqueous solution containing up to a saturated amount. The cobalt conversion solution forms a conversion coating solution by (i) dissolving a cobalt-II salt, preferably cobalt acetate, and then (ii) dissolving a metal salt of acetic acid, such as a sodium, magnesium or calcium salt. Alternatively, it may be prepared by a bath preparation method including a step of performing the above.

C. In yet another embodiment of the present invention, wetting agents such as alkyl fluorides, fluorocarbons and metal fluorides can be added to the conversion coating solution. The addition of a wetting agent eliminates the costly sealing step required after the formation of the conversion coating.

BRIEF DESCRIPTION OF THE DRAWINGS The attached figure is a photomicrograph of an image obtained by scanning electron microscopy of a coating on an aluminum alloy test panel. 1 to 4 are photomicrographs of a 2024-T3 alloy test panel having a cobalt conversion coating formed according to the present invention, taken with a 20 KV scanning electron microscope. FIGS. 1-4 show a cobalt conversion coating 410 formed by immersion in a representative cobalt conversion coating solution at 140 ° F. for 15 minutes.

The foregoing aspects and many of the attendant advantages thereof according to the present invention can be better and more readily understood based on the following detailed description, taken in conjunction with the accompanying drawings.

FIG. 1 is a 10,000 × magnification photomicrograph of a test panel showing a cobalt conversion coating 410 according to the present invention. This micrograph is a top view of the top surface of oxide coating 410. Oxide coating 410
Is porous and presents a sponge-like appearance.
Place this test panel in a cobalt conversion coating solution for 15 minutes.
Immerse for a minute. The length of the white bar is 1 micron.

FIG. 2 is a photomicrograph of the test panel shown in FIG. 1 at a magnification of 70,000. This micrograph shows the oxide coating 41
It is a photograph from the upper surface of 0 upper surface. FIG. 2 is a higher magnification enlarged photograph of a small area of the test panel. The length of the white bar is 1 micron.

FIG. 3 is a photomicrograph at 10,000 × magnification of another test panel, showing a side view from elevation of a fractured surface of a cobalt conversion coating 420 according to the present invention. The fracture surface of the aluminum support of the test panel is indicated by reference numeral 422. The test panel was immersed in the coating bath for 15 minutes. The cross section of the oxide coating 420 was exposed by bending and breaking the test panel for microscopy. The length of the white bar is 1 micron.

FIG. 4 is a photomicrograph of the test panel shown in FIG.
The side view from the elevation angle of the fracture surface of 420 is shown. FIG. 4 is a higher magnification enlarged photograph of a small area of the test panel. The aluminum support of the test panel is designated by reference numeral 422. The length of the white bar is 1 micron.

FIG. 5 is a graph showing an offset relationship between corrosion resistance and paint adhesion as a function of immersion time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to novel cobalt conversion coatings.
The cobalt conversion coating can be prepared to exhibit corrosion resistance to the extent that a conventional sealing step is not required. This effect is obtained by adding metal fluorides and wetting agents such as alkyl fluorides and fluorocarbons to the conversion coating solution. It is believed that such an effect results in a small corrosive effect on the aluminum support surface due to the combined use of the wetting agent and the metal fluoride, which aids in the formation of the coating.

In light of the background that led to the present invention, the subject matter of the inventor's earlier pending patent application was reviewed. To arrive at the present invention, a great deal of empirical research has been performed.
Various polyvalent compounds were studied and used alone or in combination with alkalis, acids or fluorides. These compounds include vanadate, molybdate, cerate, ferrate and various borates. Although films of compounds containing these elements could be deposited on the aluminum alloy support, none of them exhibited the corrosion resistance and paint adhesion that deserve evaluation.

Cobaltamine complexes are used in many reactive reagents, namely Co
(NO ₃ ) 6H ₂ O, CoCl ₂ 6H ₂ O, NH ₄ NO ₃ , NH ₄ Cl and NH ₄
Prepared using OH. It has been found that the coating formed on the aluminum support is substantially improved over the simple salt dip described above. A review of the complexation chemistry of cobalt provided the following findings.

Blowing air into a solution of ammonium hydroxide, the corresponding ammonium salt and a cobalt-II salt containing activated carbon gives the hexamine salt, as follows: 4CoX ₂ + 4NH ₄ X + 20NH ₃ + O ₂ → 4 [Co ( NH ₃ ) ₆ ] X ₃ + H ₂ O (1) wherein X is Cl, Br, NO ₃ , CN, SCN, PO ₄ , SO ₄ , C ₂ H ₃ O ₂
Or indicate CO ₃ .

In the absence of activated carbon, another reaction takes place, producing a compound of formula (2): [Co (NH ₃ ) ₅ X] ² (2) These general reactions involve cobalt-II Based on the fact that the salt has a very strong tendency to change to a cobalt-III complex by oxidation, see the following formula (3).

[Co (NH ₃ ) ₆ X] ² = [Co (NH ₃ ) ₆ X] ³ + e ⁻ (3) These reactions are not new and are widely studied. Cobalt-III complexes are commonly produced in the photographic development field as oxidizing agents to enhance the sharpness of color photographs. However, it has surprisingly been found that cobalt complexes can form oxide structures on aluminum supports. The exact mechanism of this oxide formation is not completely understood, but is thought to work based on the chemical equilibrium shown in equation (3).

It is considered that an oxide film is formed on the aluminum support due to the tendency to oxidize. The bath composition is prepared continuously by utilizing the following equation.

4Co (NO ₃ ) ₂ · 6H ₂ O + 4NH ₄ NO ₃ + 20NH ₄ OH + O ₂ → 4 [Co (NH ₃ ) ₆ ] (NO ₃ ) ₃ + H ₂ O (4) By using ammonium nitrate, the initial reaction product Precipitation can be prevented. About this chemistry, 1990
No. 07/52, filed May 17, 2006
This is discussed in detail in US Pat. No. 5,800.

However, experiments in reaction (4) revealed that a white, well-defined iridescent coating was formed on the aluminum support, and an excess of ammonia, ie NH ₄ , was required to trigger this reaction. OH was required. This makes it difficult to adjust the pH of the bath due to the high evaporation rate of ammonia from the solution. In addition, excess ammonia in the bath has a significant effect on the paint adhesion and corrosion resistance of the coating formed by this method. Paint adhesion and corrosion resistance vary from excellent properties to complete disappearance depending on the amount of ammonia in the bath.

A major improvement in this method was made by changing the part containing the ligand of the cobalt complex, ie the part in parentheses of formula (6). By improving the cobalt complex combined with ammonia as described in the formulas (1) to (4), a bath composition containing a trivalent cobalt amine complex represented by the following formula (5) was obtained.

[Co (NH ₃ ) ₆ ] X ₃ (5) (where X is Cl, Br, NO ₃ , CN, SCN, PO ₄ , SO ₄ , C ₂ H ₃ O ₂
Alternatively, the moiety containing CO ₃ and containing the ligand of the complex is positively charged.) In this regard, see the following formula (6).

[Co (NH ₃ ) ₆ ] ³ (6) By replacing the aluminum compound with a nitrite compound, a trivalent cobalt complex represented by the following formula (7) is obtained. In this case, the part of the complex containing the ligand is negatively charged. See Equation (8) in this regard.

Me ₃ [Co (NO ₂ ) ₆ ] (7) (where Me represents Na, Li or K) [Co (NO ₂ ) ₆ ] ^3- (8) The nitrite complex has a pH of 6.8 to 7.2. It was found that the self-adjustment was stable in the range. It has also been found that paint adhesion is substantially better than conventional chromium-containing films. This point is discussed in detail in the inventor's earlier pending patent application Ser. No. 07 / 672,132. Two notable features of the conversion film formed using this nitrite complex are its almost transparent and very thin iridescent surface appearance, and the weight of the coating film is limited to a maximum of 90 mg / sq ft. Is Rukoto. Based on these two characteristics, the availability of high strength coatings with colors that are easy to inspect and with coatings greater than 90 mg / sq.ft. Was considered.

A useful formulation was obtained by blending acetate to form a complex represented by the following formula (9).

Me ₃ [Co (C ₂ H ₃ O ₂ ) ₆ ] (9) In this case, the portion of the complex containing the ligand is negatively charged. See equation (10) in this regard.

_{[Co (C 2 H 3 O} 2) 6] 3- (10) Coatings formed by this method, like the conventional chromium-containing coating, exhibits a strongly colored appearance. Coatings weighing over 350 mg / sq. Ft. Were obtained. Nitrates, for example _{NaNO 3, Mg (NO 3)} 2 · 6H 2 O
Alternatively, the addition of Ca (NO ₃ ) ₂ .6H ₂ O to this formulation assists in the tightness of the coating at high coating weights and avoids the formation of powdery, low adhesion deposits.

As a result of the above discussion, two additional bath formulations that formed a clear coating and a colored coating were selected for further testing. The following two reaction formulas (11) and (12)
Supports the formation of transparent or colored cobalt conversion coatings.

Transparent coating _{Co (NO 3) 2 · 6H} 2 O + 6NaNO 2 + 1 / 2H 2 O 2 → Na 3 [Co (NO 2) 6] + 2NaNO 3 + 2NaOH (11) colored coating _{Co (No 3) 2 · 6H} 2 O + 6CH 3 COONH ₄ + 1 / 2H ₂ O ₂ → Na ₃ [Co (C ₂ H ₃ O ₂ ) ₆ ] + 2NH ₄ NO ₃ + NH ₄ OH (12) A coating is formed according to formulas (11) and (12), In a test according to ASTM B117, by subjecting it to a sealing treatment in
Salt spray corrosion resistance for up to hours was obtained. In this regard,
This is discussed in detail in a prior pending patent application 07 / 732,568 by the inventor.

The above-mentioned improved technique, is formed by a reaction to form a Co (NO ₃₎ trivalent cobalt complex of divalent cobalt salt is reacted with ammonium acetate (CH ₃ COONH _4), such as 2 _· 6H ₂ O conversion Coating included. The resulting coatings have excellent properties with respect to defined performance criteria, but the bath life of solutions using ammonium acetate is rather short, on the order of 30 to 40 days. This requirement to extend bath life provided the basis for further consideration, but this requirement was met by more recent studies. This study
As further detailed below, the invention has been further developed by improving the overall process and coating properties.

During the course of testing the solution of cobalt complexed with ammonium acetate, it was found that after a few weeks of normal tank operation, the coating weight on the aluminum support gradually decreased and the color faded. For the test process, equation (12)
Please refer to. To compensate for this, a longer immersion time was required. It was also found that the appearance of the solution gradually changed from dark brown to wine red over time.
The final analysis revealed that the gradual replacement of the acetate in the complex represented by the formula (13) with ammonium causes a competitive reaction to form the complex represented by the formula (14) over time. .

_{[Co (C 2 H 3 O} 2) 6] 3- (13) [Co (NH 3) 6] 3 (14) In this case, a change in ionic species valency in parentheses.

In the course of solving this problem, according to the present invention,
NH ₄ (C ₂ H ₃ O ₂ ) in the formula (12) is converted to a metal acetate such as Na (C
_{2 H 3 O 2) · 3H} 2 O, Mg (C 2 H 3 O 3) 2 · 4H 2 O or Ca (C ₂ H ₃ O
₂ ) By substitution with ₂ · H ₂ O, the formulas (13) and (14)
The competitive reaction related to the complex represented by does not occur,
It was found that the same highly colored coating was formed as the original ammonium acetate solution. Sodium acetate is the most preferred metal carboxylate, and other metals such as carboxylate such as Zn, Li, K or Mn can be used, but are not preferred. Representative reactions are shown in the following equation (15) ~ (17): 2Co (C 2 H 3 O 2) 2 · 4H 2 O + 3Mg (C 2 H 3 O 2) 2 · 4H 2 O + 2HC 2 H 3 O 2 _{→ Mg 3 [Co (C 2} H 3 O 2) 6] 2 + 21H 2 O (15) Co (C 2 G 3 O 2) 2 · 4H 2 O + 3Ca (C 2 H 3 O 2) 2 · H 2 O +1 _{/ 2O 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 + 3Nz (C 2 H ₃ O ₂ · 3H ₂ O + 1 / 4O ₂ + HC ₂ H ₃ O ₂ → Na ₃ [Co (C ₂ H ₃ O ₂ ) ₆ ] +13 1 / 2H ₂ O (17) These reactions produce hydrogen peroxide. This is done by blowing air into the solution without using it.Acetic acid is not added as a reactant, but is formed in the solution as a result of the chemical reaction of the complex.The resulting conversion coating has the formula (11) And the conversion coating obtained from the conversion coating solution produced by the reaction represented by (12) Shows improved corrosion resistance in the fog test and furthermore, reacts the soluble cobalt salt with the metal carboxylate according to the general formula of formula (18) to obtain a cobalt conversion coating with excellent properties It was found that a carboxylate which was soluble in the reaction solution was used.

Soluble cobalt salt + Me (R) _x → Me _m [Co (R) ₆ ] _n (18) (where x is 1 or 2, m is 3 and n is 1
Or 2, and Me is Na, Li, K, Ca, Zn, Mg and Mn
A group selected from the group consisting of:
5 of showing the carboxylate) metal fluorides, for example, MgF ₂ and CaF ₂ and wetting agents,
For example, by adding small amounts of soluble alkyl fluorides and fluorocarbons to these solutions, corrosion resistance and manufacturability can be improved. See Tables III and IV for the solutions. In particular, alkyl fluoride wetting agents such as the commercial product "M" from Harshaw, located in Cleveland, Ohio "M
Good results are obtained by using "SP-ST alkyl fluoride" and a commercial product "Fluorocarbon FC99 and FC95 Wetting Agents" from 3M Company, located in St. Paul, Minn. The presence of the fluoride wetting agent, metal fluoride or mixtures thereof can increase the corrosion resistance of the resulting conversion coating to such an extent that subsequent secondary sealing steps of the coating can be omitted. In principle, any water-soluble fluoride wetting that reduces the surface tension of the liquid at 20 ° C. to 30-40 dyne / cm can be used. By performing the solution processing described in detail below, the Boeing Law Specification BA
A coating is obtained which exhibits properties of 168 hours on a salt spray corrosion resistance basis according to C5719 "Chromium-containing conversion coating". This single-step conversion coating exhibits salt spray resistance of 240 hours or more without showing corrosion pits.

In the course of the experiments according to equations (16), (17) and (18), certain parameters relating to bath composition and coating performance were found to be important. These parameters are bath composition, operating procedure, choice of reactants, bath temperature control and immersion time. It should also be noted that the above reaction system is very complicated and several reversible reactions occur simultaneously. For optimal results according to the invention,
It is believed that all reaction systems must be in or near equilibrium.

Reactant Selection and Solution Concentration The most important parameters affecting the performance of the exchange coating with respect to paint adhesion and corrosion resistance are the selection of the reactants and the concentration of the reactants in the solution. The performance of the coating is more affected by these parameters than by bath temperature or immersion time. However, bath temperature and immersion time have their effects over a wide range of changes in these parameters.

Regarding the surface treatment of aluminum, paint adhesion and corrosion resistance are contradictory properties. That is, maximum paint adhesion is usually obtained at the expense of corrosion resistance, and vice versa. This surface treatment behavior is also seen in the case of the cobalt conversion coating. The experimental results described herein and in the above-pending prior patent application provide a favorable list of various cobalt compounds and their effect on corrosion resistance to paint adhesion.

As is evident from Table I, the cobalt acetate formulation exhibits the best corrosion resistance and retains good paint adhesion. However, if corrosion resistance is not a factor,
It should be noted that the use of nitrates or nitrites gives the best paint adhesion achievable when using these cobalt complexes.

Cobalt acetate is the most preferred soluble cobalt-II salt. Other water-soluble cobalt salts such as Co (NO ₃ ) ₂ , C
oSO ₄ , CoCl ₂ , CoPO ₄ or CoCO ₃ may be used instead of cobalt acetate, but these are not preferred for the reasons shown in Table I. Preferably, these cobalt salts are reacted with a soluble metal carboxylate having 1 to 5 carbon atoms, but most preferably a metal salt of acetic acid. C
Carboxylates of a, Mg and Na are preferred, especially N
While a carboxylate is most preferred, carboxylate of Zn, Li, K and Mn may be used. A limitation when using carboxylate besides acetate is solubility in water. Other carboxylate that can be used is exemplified by sodium propionate. The minimum solubility at 20 DEG C., or 68 DEG F., required to obtain an effective coating is about 0.01 mol of cobalt-II salt / liter of water. These salts may be used up to the limit solubility concentration.

Although not required, a fluoride wetting agent may be added to the bath as described above. The use of these wetting agents results in conversion coatings that do not need to be subjected to conventional sealing to obtain sufficient corrosion resistance.

Drug concentration, pH adjustment, temperature and immersion time For drug concentration, the cobalt-II salt is 20 ° C, ie
It may be used in a range from a concentration of about 0.01 molar per liter of the final solution at 68 ° F to the limit solubility. Preferably, the dissolved concentration of the cobalt-II salt used may vary from about 0.04 mol to 0.15 mol per liter final concentration.

The concentration of the cobalt-III hexacarboxylate coordination complex may vary from about 0.01 mole per liter of final solution to the limit solubility of the coordination complex. The preferred concentration of the coordination complex is about 0.04 per liter of final solution.
The molar concentration of the metal carboxylate, preferably metal acetate, is from about 0.03 to 2.5 moles per liter of final solution,
Preferably it is about 0.05-0.2 mol.

When using a fluoride wetting agent, the concentration is preferably sufficient to maintain the surface tension of the solution at 20 ° C. at 30 to 40 dyne / cm. Metal fluorides, for example Mg
The F ₂ and CaF ₂ may be present in an amount of from 0 to solubility limit. Fluoride wetting agents, metal fluorides or mixtures thereof are not required but are preferred mixture formulation components. If wetting agents and metal fluorides are not used,
To obtain high corrosion resistance, the conversion coating must be subjected to a sealing process. The use of wetting agents and metal fluorides makes the use of the present invention economical as it allows for the elimination of the sealing process.

The pH of the bath is about 5.0-9.0, preferably 6.0-7.5,
Most preferably, it is 6.5. Bath temperature is about 68-160 ° F. Above 160 ° F., the cobalt-III hexacarboxylate complex decomposes progressively. The optimal bath temperature is 140
± 5 ° F. The immersion time is about 3 to 60 minutes, preferably 5
~ 30 minutes. When using sodium acetate,
The immersion time can be reduced to 5 to 8 minutes. By employing these parameters, a coating of, for example, 20-240 mg / sq. Ft. Is obtained.

Preferred Bath Preparation Procedure When using an acetate complexed cobalt solution, the following bath preparation method is preferred.

1. Attach an air stirrer and a temperature controller capable of adjusting the temperature to ± 5 ° F in a stainless steel tank. The tank may be lined with an inert material capable of withstanding continuous operation at 150 ° F.

2. Fill the tank with deionized water to three quarters of the tank capacity and heat to 120 ° F. Start air stirring and gently boil.

3. Add and dissolve the applied amount of metal acetate. When a relatively large tank is used, a holding basket having a small mesh may be used as a holding device for promoting the dissolution of the component.

4. Add and dissolve the applied amount of cobalt salt. Gently boil the solution in the tank for another 4 hours. At this point, the reaction is almost complete. Also in this case, the dissolution of the additional component may be promoted by using a holding basket.

5. Heat the resulting solution to 140 ° F. and add a small amount of a fluoride wetting agent as desired. Stir the air for another 2 hours. This places the tank in an operable state.

Preferred Procedures for All Steps A summary of preferred procedures for all steps is as follows.

1. Pre-cleaning 2. Mask and rack 3. Alkaline cleaning 4. Rinsing at room temperature 5. Deoxygenation 6. Rinsing at room temperature 7. Oxide conversion coating of form 8. Rinsing at room temperature 9. Drying General notes on the above process flow chart.
It should be formed after the completion of imming and secondary processing. Parts that may incorporate the solution should not be subjected to immersion alkaline cleaning or immersion deoxygenation. In this case, prior to being subjected to the cobalt conversion treatment, water washing (water-breeding) is performed by performing manual washing and deoxygenation treatment.
ak) free surfaces should be obtained. A water-free surface is a surface that retains a continuous film of water for at least 30 seconds after spray or rinse immersion at 100 ° F or less using clean water.

Thorough rinsing and drainage is required throughout the process as each solution must be completely removed to avoid adversely affecting the performance of the next solution in the operating procedure. The parts to be processed should be transferred from one step to the next without drying without delay. If handling of wet parts is required, clean latex rubber gloves should be used. After performing the conversion coating process, the dried parts should be handled using clean textile gloves. In the case of processing systems that require component clamping, the number and size of the contact points should be minimized to provide adequate mechanical support.

Pre-cleaning Steam degreasing may be performed according to Boeing method specification BAC 5408. In the case of a component to be processed to which a large amount of fat or oil has adhered, emulsion cleaning according to Boeing method specification BAC5763 or solvent cleaning according to Boeing method specification BAC 5750 may be performed. Parts with open joints or spot welded joints where solution may be introduced should be immersed in cold or warm cold water for 2 minutes after pre-cleaning.

Masks and Racks Areas that do not require a cobalt conversion coating should be masked with a masking agent. Dissimilar metal inserts and plasma spray coated areas that do not contain aluminum should be unmasked. However, chromium, nickel or cobalt alloy or plating, CRES or titanium is excluded from the metal insert. The masked part is hooked into a holding basket or placed on a holding jig.

Alkaline cleaning Alkaline cleaning and rinsing are Boeing Act Specifications BAC 574
It may be performed according to 9. However, parts having open joint surfaces or spot welded joints are excluded. In this case, in order to prevent incorporation of the solution, rinsing is performed by subjecting to a plurality of immersion treatments for at least 10 minutes with shaking, followed by manual spray rinsing treatment. In this case, the immersion treatment is performed at least four times.

Deoxygenation Deoxygenation and rinsing may be performed in accordance with Boeing Law Specification BAC 5765. However, parts that may take solution are excluded. Parts of this kind may be rinsed by the method described in the section "Alkaline cleaning" above. Castings may be subjected to deoxygenation by any of the following methods.

(A) Deoxidation by Boeing method specification "BAC 5765, solution 37, 38 or 39" (b) Dry polishing injection casting by Boeing method specification "BAC 5748, Type II, Class 1 and Rinse" The composition of the particular solution within is shown in Table II below.

Fluctuations in bath temperature and immersion time also contribute to corrosiveness and paint adhesion, but to a lesser extent than the choice of solution reactants. It has been determined that variations in bath temperature and immersion time primarily affect variations in coating thickness, while reactants will primarily affect coating texture and density. The following general effects were observed: 1. The optimal bath temperature in terms of corrosion and adhesion was 140 ±
It was found to be 5 ° F.

2. Increasing the bath temperature above the optimum temperature results in a thicker and coarser coating, thus reducing paint adhesion and increasing corrosion resistance.

3. If the bath temperature is lower than the optimum temperature, the coating thickness becomes thinner, the paint adhesion increases, and the corrosion resistance decreases.

4. It was found that the optimal time depended on the choice of reactants.

5. For the cobalt salt complex of nitrous acid, the optimal immersion time was 20-25 minutes at the optimal bath temperature.

6. For the cobalt salt complex of acetic acid, the optimal immersion time at 140 ° F was found to be a function of the type of acetic acid used. That is, in the case of _{Na (C 2 H 3 O 2} ) was 5-8 minutes, Mg
(C ₂ H ₃ O ₂ ) ₂ takes 15 to 20 minutes, and Ca (C ₂ H ₃ O ₂ ) ₂
In the case of O _{2) 2} was 12 to 15 minutes.

FIG. 5 illustrates the general behavior of a cobalt conversion coating on the relationship between corrosion and paint adhesion. The intersection of the corrosion and adhesion curves represents the bath parameters, where the two opposing properties, corrosion and adhesion, are optimal for both.

Application has been performed between pH 5.0 and 9.0, but the pH is
It is preferably maintained between 6.0 and 7.5. Adjustment to that pH may be required after prolonged use of the solution.

Corrosion resistance Utilizing the basic corrosion resistance behavior of the above cobalt complex salt,
The cobalt acetate formulation was extensively studied. Cobalt acetate was complexed with sodium acetate _{Na (C 2 H 3 O 2} ) · 3H 2 O or magnesium acetate _{Mg (C 2 H 3 O 2} ) 2 · 4H 2 O. These formulations resulted in excellent corrosion resistance without a subsequent sealing step. ASTM B11 salt spray corrosion test
All samples were exposed for 168 hours according to 7 test methods. See Tables III and IV for results. The test was repeated two more times with almost the same results as in Tables III and IV. Analysis of this data in combination with basic paint adhesion data and other solution maintenance parameters gave the same results as the optimal tank configurations and comparisons listed above under Table II above. The last problem with corrosiveness and iridescence was solved by this study. The study of sodium acetate _{Na (C 2 H 3 O 2} ) · 3H 2 O is magnesium acetate _{Mg (C 2 H 3 O 2} ) why chosen clearly as best complexing agents than 2 _· 4H ₂ O did. The reason is,
That is, sodium acetate causes a corrosive etching effect on the aluminum support. This proved to be extremely inconvenient for corrosiveness. However, chemically formulating sodium acetate with wetting agents and metal fluorides has the distinct advantage of maintaining a bright iridescent color of the coating without compromising corrosion resistance. On the other hand, when magnesium acetate or calcium acetate was used with the wetting agent and the metal fluoride, very little etching effect was imparted, and the resulting coating had a rather weak color effect.

Oxide coating analysis E using a Perkin-Elmer model 550 surface analyzer to characterize the cobalt conversion coating of the present invention
Auger oxide profile analysis was performed using an SCA surface analyzer and the same instrument in different operating modes. Such analytical methods are known as ESCA, or electron spectroscopy for chemical analysis.
spectroscopy for chemical analysis) or XPS, that is, X-ray photoelectron spectroscopy
copy). From these analyses, the cobalt conversion coating was found to be oxide, ie, aluminum oxide Al ₂ O ₃ as the maximum volume percent, and cobalt oxide
It was found to consist of a mixture of O, Co ₃ O ₄ and Co ₂ O ₃ . The term "maximum volume%" means that the volume of this oxide is greater than the volume of any other oxide present,
The term "maximum volume present" does not necessarily mean that the volume of this oxide exceeds 50% by volume.

In addition, the data show that in the lower part of the oxide coating, ie, in contact with the aluminum support, the maximum volume percent is Al ₂ O ₃
Is shown. The middle part of the oxide coating is Co
It is a mixture of O, Co ₂ O ₃ , Co ₃ O ₄ and Al ₂ O ₃ . Furthermore, the data show that at the top of the oxide coating, the largest volume percent is a mixture of Co ₂ O ₃ and Co ₃ O ₄ .

Yet another feature of the cobalt conversion coating according to the present invention can be found in FIGS.
According to the description of the cobalt conversion coating 410 and 420
It can be seen that is formed by immersing in a typical cobalt conversion coating solution for 15 minutes. As shown in FIGS. 1-4, the top surface of the cobalt conversion coating is similar to a sponge and has sufficient surface area and porosity desired for paint adhesion. Below the top surface the coating becomes denser and harder and becomes non-porous.

Other Methods of Painting The above discussion describes the formation of a cobalt conversion coating by dip coating. The same principle applies to conversion coating formation by manual painting and spray coating.

As will be apparent to those skilled in the art who understand the present invention, the present invention may be in other forms than those disclosed above, without departing from the spirit and essential characteristics of the invention. Therefore, all details of the specific embodiments of the invention and the specific method described above are to be considered as illustrative and not restrictive. The scope of the invention is not limited to the examples described in the foregoing description, but is as set forth in the appended claims. Any and all equivalents should be encompassed by the following claims.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-5-44052 (JP, A) JP-A-5-9745 (JP, A) JP-A-4-231478 (JP, A) JP-A-50- 87937 (JP, A) JP-A-53-95139 (JP, A) JP-A-52-80240 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 22/56 C23C 22 / 66 C23C 22/68

Claims (33)

(57) [Claims] 1. A method of forming an oxide film, a cobalt conversion coating on a metal support, comprising the steps of: (a) Soluble cobalt-III hexavalent complex Me _m [Co
(R) ₆ ] _n (where Me is one or more members selected from the group consisting of Na, Li, K, Ca, Zn, Mg, and Mn, m is 2 or 3, and n is 1 Or 2, wherein R is a carboxylate having 1 to 5 carbon atoms).
Providing an oxide film forming cobalt conversion solution wherein the concentration of the valent complex is from about 0.01 moles per liter of solution to the saturation limit; and (b) allowing the support to have a pH of about 5.0 to 9.0 for a sufficient time. Contacting the solution with the solution for a sufficient time to oxidize the surface of the support, thereby forming the cobalt conversion coating. 2. A liquid having a surface tension of 0.03 to 0.04 N / m, that is, 30 to 4 N / m.
The method of claim 1 wherein an alkyl fluoride or fluorocarbon wetting agent is added to said aqueous solution to maintain 0 dyn / cm. 3. The method according to claim 1, wherein R is acetate. 4. The solution according to claim 1, wherein the concentration of said cobalt-III hexavalent complex is in solution 1.
4. The method of any of claims 1-3, wherein the amount is from about 0.04 mole to about 0.15 mole per liter. 5. The process according to claim 1, wherein the cobalt conversion solution is prepared by reacting a cobalt-II salt with a metal carboxylate having 1 to 5 carbon atoms. 6. The method of claim 5 wherein the concentration of the cobalt-II salt is from about 0.04 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. Method. 7. The process of claim 5 wherein the cobalt-II salt is a cobalt-II salt having a minimum solubility of about 0.01 mole per liter of water at 20 ° C., 68 ° F. 8. The method according to claim 5, wherein the cobalt-II salt is cobalt acetate. 9. The method of claim 5 wherein said cobalt conversion solution is prepared by a bath composition sequence comprising: (a) dissolving a metal carboxylate; (b) then adding a cobalt-II salt. 10. The pH of the cobalt conversion solution is about 5.0 to 9.0.
The method of claim 1, comprising: 11. The temperature of the cobalt conversion solution is about 20-7.
The method of claim 1 wherein the temperature is 1.1 ° C, ie, 68-160 ° F. 12. The method of claim 1 wherein said support is contacted with said cobalt conversion solution for about 3 to 60 minutes. 13. The method of claim 1, wherein a fluorinated wetting agent is added to said cobalt conversion solution to assist in forming said cobalt conversion coating on said support. 14. The method according to claim 13, wherein said wetting agent is one or more wetting agents selected from the group consisting of alkyl fluorides, fluorocarbons, CaF ₂ , MgF ₂ and mixtures thereof. 15. The method according to claim 13, wherein the wetting agent is added in an amount necessary to produce a liquid surface tension of 0.03 to 0.04 N / m, that is, 30 to 40 dyn / cm. 16. The method according to claim 13, wherein MgF ₂ , CaF ₂ or a mixture thereof is added to the solution in an amount of 2 to 4 g per liter of the final solution. 17. The method according to claim 1, wherein the support is aluminum, aluminum alloy, magnesium, magnesium alloy, Cd-plated support or Zn-Ni-plated support. 18. An article obtained by the method according to claim 1. 19. A soluble cobalt-III hexavalent complex Me _m [Co
(R) ₆ ] _n (where Me is one or more members selected from the group consisting of Na, Li, K, Ca, Zn, Mg and Mn, m is 2 or 3, and n is 1 Or 2, wherein R is a carboxylate having 1 to 5 carbon atoms) containing a reactive aqueous solution having a pH of about 5.0 to 9.0, wherein the concentration of the cobalt-III hexavalent complex is 1 liter of the solution. A chemical conversion coating solution for forming an oxide film, a cobalt conversion coating, on a metal support, at a concentration from about 0.01 mole per mole to the saturation limit of the cobalt-III hexavalent complex. 20. The solution according to claim 19, wherein R is acetate. 21. A soluble cobalt-III hexavalent complex comprising a cobalt-II salt and a Me carboxylate having 1 to 5 carbon atoms (where Me is a group consisting of Na, Li, K, Ca, Zn, Mg and Mn). 20. The solution according to claim 19, which is a soluble cobalt-III carboxylate complex obtained by reaction with at least one selected from the group consisting of: 22. The concentration of the cobalt-II salt is from about 0.01 mole per liter of final solution to the saturation limit of the cobalt-II salt, and the concentration of metal carboxylate is from about 0.09 to 2.5 moles per liter of final solution. Claim 21 which is
The solution as described. 23. The solution of claim 21 wherein the cobalt-II salt is a cobalt-II salt having a minimum solubility of about 0.01 mole per liter of water at 20 ° C., 68 ° F. 24. The solution according to claim 21, wherein the cobalt-II salt is cobalt acetate. 25. The solution according to claim 21, wherein the metal carboxylate is a metal acetate. 26. The solution of claim 21 wherein the cobalt conversion solution is prepared by a bath composition sequence comprising: (a) dissolving a metal carboxylate; (b) then adding a cobalt-II salt. 27. The solution of claim 19, comprising a fluorinated wetting agent added to the cobalt conversion solution to assist in forming a cobalt conversion coating on the support. 28. The composition according to claim 1, which contains one or more wetting agents selected from the group consisting of alkyl fluorides, fluorocarbons, CaF ₂ , MgF ₂ and mixtures thereof.
The solution according to 27. 29. The solution according to claim 27, wherein the wetting agent is added in an amount necessary to produce a liquid surface tension of 0.03 to 0.04 N / m, ie 30 to 40 dyn / cm. 30. The solution according to claim 27, wherein 2-4 g of MgF ₂ , CaF ₂ or a mixture thereof are added per liter of the final solution. 31. A method for forming an oxide film, a cobalt conversion coating, on a metal support.
A method using a solution having a pH of about 5.0 to 9.0 described in 1. 32. A method for forming an oxide film, a cobalt conversion coating, on a metal support.
Temperature of about 20 to 71.7
C., ie, 68 to about 160 ° F. 33. A method for forming a cobalt conversion coating, which is an oxide film, on a support selected from the group consisting of aluminum, aluminum alloy, magnesium, magnesium alloy, Cd-plated support and Zn-Ni-plated support. 20. A method using one or more solutions according to 19.