COMPOSITION AND PROCESS FOR CHROMATING METAL SURFACES

Abstract: A chromating treatment composition particularly suitable for preparing aluminum and stainless steel for clear coating comprises water, anions containing hexavalent chromium, trivalent chromium cations, phosphate ions, dry process finely divided silica, and acrylic and/or methacrylic acid polymer or copolymer in such amounts that: (A) the concentration of the total of hexavalent and trivalent chromium atoms is in the range from 1 to 60 g/L; (B) the (trivalent chromium atom)/(hexavalent chromium atom) weight ratio is in the range from 0.6 to 2.5; (C) the (phosphate ion)/(total chromium atoms in hexavalent chromium ions and trivalent chromium ions) weight ratio is in the range from 0.5 to 4.5; (D) the (dry process silica)/(total chromium atoms in hexavalent chromium ions and trivalent chromium ions) weight ratio is in the range from 0.1 to 5.0; and (E) the ratio of the weight of the specified water soluble polymer in the composition to the weight of the chromium atoms in the total of the hexavalent chromium ions and trivalent chromium ions in the composition is in the range from 0.01 to 1.0.

Description: COMPOSITION AND PROCESS FOR CHROMATING METAL SURFACES TBcmncAi, rra The present invention relates to a chromate treatment composition and process of using it. The composition is particularly well suited to forming a base or undercoating for a clear (substantially transparent) subsequent organic based overcoating on any metal surface, most particularly aluminum and stainless steels. BACKGROUND ART Numerous examples have appeared in recent years of the execution of clear coatings on metals (typically aluminum and stainless steel) for the purpose of producing a film which resists fingerprints, corrosion, and weathering and which simultaneously exploits the glossy appearance of the metal substrate. Clear coating in such cases has normally required a different type of undercoating or priming treat¬ ment from that optimized for underlying typical colored paints. Colored paints generally contain sufficient pigment to hide the appearance of any priming or undercoating treat- ment used underneath them, so that the aesthetic impact of the substrate color generated by the undercoating treatment is a matter of little concern. However, in the case of clear coatings, the color of the undercoating treatment di¬ rectly affects the post-painting appearance. In addition, it is highly desirable in the case of clear coatings to exploit the metallic texture of the coated metal. As a consequence, the undercoating treatment normally should be colorless or only very weakly colored. Moreover, clear coatings and color clear coatings usually impose limita- tions on the additives (pigments and the like) and employ special resins (e.g., fluorine containing resins) in con¬ tradistinction to colored paints, and as a result the known undercoating treatments in some cases may not provide a satisfactory adherence, corrosion resistance, and weather resistance. Phosphating treatments and chromate treatments have been heretofore employed as undercoating treatments for metals which are to be painted. Phosphating treatments are associated with the fol¬ lowing two problems: limitations on the treatable metals, and reduction of the metal gloss due to the formation of a conversion film on the metal surface. Chromate treatments are typically divided into the following 3 categories: reaction-type chromate treatments, electrolytic chromate treatments, and application- or coating-type chromate treatments. Reaction-type chromate treatments suffer from limitations on the treatable metals and from the general inability to avoid the coloration problem. Thus, when the associated coloration is reduced by limiting the film weight, the corrosion resistance and paint adherence become unsatisfactory because the film weight is then no longer adequate for these purposes. Limitations on the applicable metals are not encount¬ ered in the case of electrolytic chromate treatments, but this type of treatment has not generally provided a satis¬ factory corrosion resistance. Application-type chromate treatments are not limited with regard to applicable metals and provide a relatively good post-painting performance with typical colored paints. However, conventional appli¬ cation-type chromate treatments cannot avoid the coloration problem, and conventional application-type chromate treat- ments often give an unsatisfactory paint adherence with the fluorine-type paints used for contemporary clear coat¬ ings. This led to an examination of the application-type chromate treatments which have been disclosed in the pat¬ ent literature. Included among these are, for example, the treatments proposed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number 62-270781 [270,781/87] and Japanese Pat¬ ent Application Laid Open Number 63-270480 [270,480/88). Japanese Patent Application Laid Open Number 62-270,781 does not give a satisfactory basis for the clear coating art; coloration is still a problem because it employs a (trivalent chromium)/(hexavalent chromium) weight ratio in the range of 0.2 to 1.0. Furthermore, its paint adherence remains unsatisfactory. On the other hand, while Japanese Patent Application Laid Open Number 63-270480 is silent with regard to clear coatings, it nevertheless provides im- provement with regard to post-treatment appearance and post-painting performance. However, this method places em¬ phasis on obtaining a transparent whiteness for the post- treatment appearance in the case of no subsequent paint¬ ing, and it requires the addition of an inorganic colloidal compound (silica sol or alumina sol) . As a result, prob¬ lems still remain with the paint adherence and the long term durability after painting. In addition to these processes, other tactics include the application of paint after only a degreasing step and the use of silane coupling agent in the undercoating treat¬ ment (an example of the latter is Japanese Patent Publica¬ tion Number 63-35712 [35,712/88]). No coloration problem is encountered in either approach, but the former approach suffers from an unsatisfactory paint adherence, corrosion resistance, and weather resistance while the latter ap¬ proach suffers from an unsatisfactory corrosion resistance and weather resistance, although it does have an effect on the paint adherence. DESCRIPTION OF THE INVENTION Problem to Be Solved by the Invention The present invention takes as its major object the provision of a chromate treatment composition (also called "bath" for brevity) which produces a conversion coating that is not only almost colorless, but also exhibits an ex- cellent paint adherence, corrosion resistance, and weath¬ er resistance. Summary of the Invention It has been found that an excellent paint adherence could be obtained through the addition of dry process sil- ica and water soluble carboxyl containing polymer to a com¬ position containing both hexavalent and trivalent chromium and by limiting the weight ratios of silica and polymer relative to total chromixim (sum of trivalent chromium ions and chromium content of hexavalent chromium ions) to within specific, suitable ranges. The addition of only silica or only water soluble carboxyl containing polymer does affect the paint adherence to some degree, but the addition of both provides a remarkable improvement in the paint adher¬ ence. The chromate treatment bath obtained based on the pre¬ ceding comprises, more preferably consists essentially of, or most preferably consists of water and from 1 to 60 g/L total chromium (total as chromium atoms for hexavalent chromium ions plus trivalent chromium ions) , phosphate ions, dry process silica, and water soluble carboxyl containing polymer, with the following limits on ratios among the various constituents: a (trivalent chromium atom)/(hexavalent chromium atom) weight ratio in the range from 0.6 to 2.5, a (phosphate ion)/(total chromium atoms in hexavalent chromium ions and trivalent chromium ions) weight ratio in the range from 0.5 to 4.5, a (dry process silica)/(total chromium atoms in hexa¬ valent chromium ions and trivalent chromium ions) weight ratio in the range from 0.1 to 5.0, and a (water soluble carboxyl containing polymer)/(total chromium atoms in hexavalent chromium ions and trivalent chromium ions) weight ratio in the range from 0.01 to 1.0. Details of Preferred Embodiments of the Invention The treatment bath under consideration can be prepared as follows: Chromic anhydride and phosphoric acid are dis¬ solved in water, part of the hexavalent chromium ion is then reduced to trivalent chromium ion using a reductant, and the dry process silica is subsequently dispersed in the bath and the water soluble carboxyl containing polymer is dissolved in the bath. The specific technique for prepar¬ ing the treatment bath should be selected as appropriate. The chromic anhydride can be replaced by dichromate, chrom- ate, or any other water soluble hexavalent chromium con¬ taining substance. The phosphoric acid can be replaced by other phosphate ion containing compounds, such as the poly- phosphoric acids, ammonium phosphate, etc. (The stoichio- metric equivalent as phosphate ions of any type of phos¬ phorus containing anions or acids present is considered as the total phosphate for the purpose of calculating the ra¬ tios specified above.) The reductant may be selected as appropriate from compounds which exhibit a reducing aσtiv- ity, such as hydrogen peroxide, alcohols such as methanol and the like, polyvinyl alcohol, starch, tannic acid, hydrazine, etc. Suitable silicas comprise dry process silicas with an average primary particle diameter of 7 to 100 nm. The use of wet process silica (silica sol) as the silica tends to result in blistering in post painting water resistance testing, although the post-treatment appearance will norm¬ ally be clear. The water soluble carboxyl containing polymer is se- lected from the polymers and copolymers of acrylic acid and methacrylic acid. Suitable polymers of this type are com¬ mercially available. The bases for the restrictions imposed on the compo¬ sition of the chromate treatment bath of the present in- vention are considered below. A total chromium concentration less than 1 g/L cannot usually produce an adequate film weight, and the corrosion resistance, adherence, and weather resistance will be in¬ ferior as a result. When 60 g/L is exceeded, the film weight becomes too large and the color becomes noticeable. When the (trivalent chromium atoms)/(hexavalent chromium atoms) weight ratio falls below 0.6, the resulting film takes on noticeable color because too much hexavalent chromium ion is present. When this ratio exceeds a value of 2.5, the corrosion resistance will be inferior because too little hexavalent chromium ion is present. When the (phosphate ion)/(total chromium atoms) weight ratio is less than 0.5, the film assumes a noticeable color. On the oth¬ er hand, the secondary adherence and weather resistance will be poor when this ratio takes on values in excess of 4.5. When the (silica)/(total chromium) ratio is less than 0.1, the film's primary and secondary adherence will be in¬ ferior because an adequate silica add-on will not be ob¬ tained. When this ratio assumes values in excess of 5.0, the primary and secondary adherence will be inferior due to the presence of too much silica. With respect to the (water soluble carboxyl containing polymer)/(total chromium) weight ratio, values less than 0.01 result in an unsatisfactory primary and secondary ad¬ herence while values in excess of 1.0 result in an iπfer- ior secondary adherence and weather resistance. Film formation using the treatment bath according to the present invention will now be considered. This rreat- ment bath is preferably applied or coated so as to produce on the clean metal surface a conversion coating containing from 5 to 60 milligrams of chromium metal per square meter of surface treated (hereinafter abbreviated as M mg/m 2'•) , and this is followed by drying without a water rinse and then preferably by application of the particular clear coating desired. The application method is suitably se- lected from such methods as roll coating, immersion coat¬ ing, and wringer roll coating. The treatment bath under consideration is superbly qualified for use within the realm of clear coating, but of course it can also be used as an undercoating for ordinary pigmented paints. The present invention will be explained below in greater detail through illustrative, non-limiting examples and comparison examples. Examples and Comparison Examples (I, Preparation of the chromate treatment baths Example 1 50 grams ("g") of chromic anhydride and 41 g of phos- SUB phoric acid (75% aqueous solution) were dissolved in 500 g of water. This aqueous solution was reduced with starch to a (trivalent chromium atoms)/(hexavalent chromium atoms) weight ratio of 1:1. 50 grams of dry process silica (AEROSIL™ 200 from Nippon Aerosil Kabushiki Kaisha) was then dispersed into this bath, followed by dissolution into the dispersion thus obtained of 25 g of polyacrylic acid (JULYMER-AC-10H™ (20 % solids) from Nippon Junyaku Kabu¬ shiki Kaisha) . The treatment bath was subsequently brought to a total of 1 liter ("L") by the addition of water. Example 2 A treatment bath was prepared as in Example 1, but in this case using polymethacrylic acid (JULYMER-AC-30H™ {20 % solids} from Nippon Junyaku Kabushiki Kaisha) instead of the polyacrylic acid. Examples 3 to 12 Treatment baths were prepared as in Example 1, but using the respective component quantities given in Table 1. Comparison Examples 1 to 10 Treatment baths were prepared as in Example 1, but using the component quantities reported in Table 1. Comparison Example 11 A treatment bath was prepared as in Example 1, but in this case replacing the dry process silica with a wet pro- cess silica (SNOWTEX™ O {20 % solids} from Nissan Chemical Industries, Ltd.) . -2) Processing of the test samples The chromate coating baths prepared as above were each applied by roll coating to the surface of aluminum (Type A3005) and stainless steel sheet (Type SUS304) using the process sequence outlined below: Alkaline degreasing → water rinse → roll squeegee → dry¬ ing → chromate application → roll squeegee → drying (with¬ out rinsing) → painting → baking. Table 1. Composition of the treatment baths fTable 1 is continued on the next page) Table 1. Composition of the treatment baths Continued from the revious a e PAA = polyacrylic add PMA = polvme thacryiic add fTable 1 is continued on the next page) Table 1. Composition of the treatment baths (Continued from the previous page) fTable 1 is continued on the next page) PAA = polyacrylic add For the stainless steel sheet, alkaline degreasing in the process sequence outlined above consisted of immersion for l minute in a 2 % aqueous solution of FINECLEANER™ 4360 (from Nihon Parkerizing Company, Limited) at 60° C. For the aluminum, degreasing was by immersion for 1 minute in a 2 % aqueous solution of FINECLEANER™ 315 (from Nihon Parkerizing Company, Limited) at 60° C. For the aluminum, painting in the process sequence outlined above consisted of applying a 10 micron thick polyester clear coating; for stainless steel, it was a 10 micron thick, fluorine-type coating. f3) Performance evaluation testing (a) appearance evaluation The color was visually evaluated after clear coating and is reported according to the following scale: + + no color + slight coloration x coloration x x substantial coloration (b) paint adherence Primary adherence The painted sheet was OT-folded, peeled with cellophane tape, and the residual film was then visually evaluated. Secondary adherence The painted sheet was immersed in boiling water for 2 hours and then evaluated as for the primary adherence. Evaluation of both primary and secondary adherence was reported according to the following scale: + + no film peeling + slight peeling over part of the surface x peeling over part of the surface x x substantial peeling (c) corrosion resistance A cut was scribed through the paint film to the base metal. This was followed by salt-spray testing for 2,000 hours (aluminum) or for 5,000 hours (stainless steel sheet) . The development of rust at both the cut and over the entire surface was visually evaluated and reported according to the following scale: ++ no rusting + modest rusting over part of the surface x rusting over part of the surface x x substantial rusting (d) weather resistance The paint film was scribed with a cut through to the base metal, followed by exposure in a Sunshine Weather-O-Meter for 500 hours (aluminum) or for 2,000 hours (stainless steel sheet) . Film exfoliation at both the cut and over the entire surface was then visually evaluated and reported according to the following scale. + + no film exfoliation + slight exfoliation over part of the surface x exfoliation over part of the surface x x substantial exfoliation The test results are reported in Table 2. They confirm that the chromate baths according to the present invention have excellent properties for application as a preparation for clear coating. Table 2. Test results fTable 2 is continued on the next page) fTable 2 is continued on the next page) - 16 - Table 2. Test results (Continued from the previous page) fTable 2 is continued on the next page) Table 2. Test results (Continued from the previous page)

Claims: CLAIMS 1. A liquid composition of matter comprising water, a source providing dissolved anions containing hexavalent chromium atoms, a source providing dissolved trivalent chromium cations, and a source of dissolved phosphate ions, characterized in that the composition also contains dry process silica and water soluble polymer selected from the group consisting of polymers and copolymers of acrylic acid and methacrylic acid and in that: (A) the concentration of the total of hexavalent and tri¬ valent chromium atoms is in the range from 1 to 60 g/L; (B) the (trivalent chromium atom) /(hexavalent chromium atom) weight ratio is in the range from 0.6 to 2.5; (C) the (phosphate ion) /(total chromium atoms in hexaval¬ ent chromium ions and trivalent chromium ions) weight ratio is in the range from 0.5 to 4.5; (D) the (dry process silica)/(total chromium atoms in hexavalent chromium ions and trivalent chromium ions) weight ratio is in the range from 0.1 to 5.0; and (E) the ratio of the weight of the specified water soluble polymer in the composition to the weight of the chrom¬ ium atoms in the total of the hexavalent chromium ions and trivalent chromium ions in the composition is in the range from 0.01 to 1.0. 2. A process for treating a metal surface, comprising contacting the metal surface with a liquid composition of matter according to claim 1 and subsequently drying the surface so contacted, without any intermediate rinsing between contacting and drying. 3. A process according to claim 2, wherein the metal treated is aluminum or stainless steel. 4. A process according to claim 3 in which the contacting is for a sufficient time to deposit a coating containing from 5 to 60 milligrams of chromium per square meter of metal surface treated. 5. A process according to claim 2 in which the contacting is for a sufficient time to deposit a coating containing from 5 to 60 milligrams of chromium per square meter of metal surface treated. 6. A process according to any one of claims 2 - 5, comprising an additional step of covering the contacted and dried surface with a clear organic based protective coating.

Inventors: HASEBE AKIHIKO, HATANO NORIFUMI, OYAMA KAZUYUKI, HASEBE, AKIHIKO, HATANO, NORIFUMI, OYAMA, KAZUYUKI
Applicants: HENKEL CORP, HENKEL CORPORATION
Application year: 1991
Main classification: C23C22
IPC codes: C23C22