TREATMENT FOR THE FORMATION OF A CORROSION RESISTANT FILM ON METAL SURFACES

Abstract: A phosphate surface-treatment bath which contains a cationic organic polymeric compound (or salt thereof) that contains at least one cationic nitrogen atom and has a molecular weight of 1,000 to 1,000,000 forms a highly corrosion resistant film on metal surfaces (e.g., iron, zinc, aluminum, etc.) that is also an excellent paint undercoat. The film preferably is one in which the resin composed of the cationic organic compound (or salt thereof) has penetrated into the grain boundaries between phosphate crystals with an accompanying formation of a phosphate crystal/resin composite. A film formed in accordance with the present invention affords a remarkable improvement in corrosion resistance and paint adherence, compared with the phosphate conversion achieved with the same treatment, except for the omission of the organic polymer containing cationic nitrogen. In addition, even when the primer coating step is omitted, the present invention exhibits a performance approximately equivalent to the application of a primer coating on a conventional phosphate film.

Description: Description TREATMENT FOR THE FORMATION OF A CORROSION RESISTANT FILM ON METAL SURFACES Technical Field The present invention relates to a novel treatment ag¬ ent that forms a strongly corrosion-resistant film on the surface of metals such as iron, zinc, aluminum, and the like. This film is also an excellent paint undercoat. Background Art At present, increasingly high levels of corrosion re¬ sistance are being reguired of painted objects such as au¬ tomobile bodies and construction materials. Metal surfaces have heretofore been treated by phosphate treatments, chro- ate treatments, etc.; however, the prior metal surface treatment agents do not always exhibit a satisfactory cor¬ rosion resistance and paint adherence. The art is already familiar with the formation of a phosphate film on a metal surface as a generally applicable pretreatment method when organic films, e.g., paints, ad- hesives, and the like, are applied on the surfaces of met¬ als such as iron, zinc, aluminum, and the like. The cor¬ rosion resistance and paint adherence are improved by this addition of a phosphate film on the metal surface prior to the application of the organic coating (typically paint) . When higher levels of corrosion resistance are required, both a phosphate treatment and a post-treatment (e.g. , sealing with chromic acid and the like) or primer appli- cation are carried out prior to painting. However, these post-treatments (e.g., sealing with chromic acid and the like) contribute only minor benefits. Moreover, while the application of a primer does improve the paintability, this also expands the painting process and thus substantially impairs the workability. Nevertheless, primer application is currently required for painted objects such as automobile bodies and construe- SUBSTITUTESHEET tion materials. In the case of automobile bodies, a zinc phosphate film is formed on the metal surface, which is then immersed in aqueous paint for electrodeposition coat¬ ing. This coating operation is a type of primer coating, and its purpose is to increase the corrosion resistance of the painted surface and to secure corrosion resistance for the interior surfaces of the automobile body, which can be difficult to finish coat. A primer coating is similarly applied on construction materials prior to finish coating in order to increase the corrosion resistance. While improvements in operating efficiency and cost reduction in painting operations require a contraction of the process, a satisfactory response to this goal has yet to be developed. Thus, phosphate films alone have a poor corrosion resistance, while painting alone does not satisfy the requirements for corrosion resistance and paint adher¬ ence. When higher levels of corrosion resistance are re¬ quired, both a phosphate film and a primer coating are re¬ quired. This results in high painting costs and requires large work areas. The development of a metal surface treatment agent that exhibits a high corrosion resistance and paint adherence is therefore desired. The development of a metal-surface-treatment agent that permits the omis¬ sion of primer coating is also desired. Disclosure of the Invention Problems to Be Solved by the Invention As a means of responding to the above-listed problems, the present invention takes as its object the introduction of a metal-surface-treatment agent that can impart a high corrosion resistance and paint adherence to metals that are suitable for phosphate film treatment, such as iron, zinc, aluminum, and the like (hereinafter briefly denoted simply as "metal") . A further object of the present invention is the introduction of a metal surface treatment agent that permits the omission of primer coating while at the same time retaining the level of corrosion resistance currently available. Summary of the Invention As the result of extensive research directed at solv¬ ing the aforementioned problems, it has now been discovered that a high corrosion resistance and paint adherence are obtained by film formation from a phosphate treatment bath that contains an organic polymeric compound (or salt there¬ of) having certain properties. The present invention was developed as a result of this discovery. That is, a film that exhibits the highly desirable properties specified above can be formed by treating the metal surface with a treatment agent for the formation of a film on metal surfaces, wherein said treatment agent characteristically comprises a phosphate surface treatment bath j hich contains a cationic organic polymeric compound (or salt thereof) that contains at least 1 cationic nitro¬ gen atom and has a molecular weight of 1,000 to 1,000,000. The film according to the present invention preferably is a composite film in which the resin composed of the cation¬ ic organic compound (or salt thereof) has penetrated into the grain boundaries between phosphate crystals with an accompanying formation of a phosphate crystal/resin com¬ posite. The surface treatment agent according to the present invention comprises the solution or stable dispersion of a cationic organic polymeric compound (or salt thereof) in a phosphate treatment bath. Said phosphate treatment bath comprises any surface-treatment bath that is capable of forming a phosphate film on the surface of a metal such as iron, zinc, aluminum, etc. , either for a single species of metal alone or simultaneously on the surfaces of two or more species of the preceding metals. In general, its es¬ sential components are zinc ions and phosphate ions, but it may optionally contain nitrate ions, other metal ions (nickel, manganese, calcium, and the like), fluoride, and various types of oxidants. However, the phosphate treat¬ ment bath is not specifically restricted within the context of the present invention, and the present invention encom- passes all known phosphate treatment baths. The cationic organic polymeric compound should contain at least 1 cationic nitrogen atom and should have a molecu¬ lar weight of 1,000 to 1,000,000. Although its structure is not restricted in the broadest embodiments of the inven¬ tion, organic polymers are particularly preferred that have a resin skeleton comprising at least one selection from epoxy resins, urethane resins, polybutadiene resins, acryl¬ ic resins, and maleic anhydride resins, wherein these resins contain a cationic nitrogen-containing group. The salts of the cationic organic polymeric compound encompass its inorganic salts and organic salts. The inor¬ ganic salts are exemplified by phosphate, nitrate, sulfate, and the like, and the organic salts are exemplified by ace- tate, propionate, glyconate, and the like. " A single spe¬ cies or two or more species of this cationic organic poly¬ meric compound (or salt thereof) can be employed. Only a weak improvement in corrosion resistance is obtained at molecular weights below 1,000, while it is very difficult to obtain dissolution or stable dispersion in zinc phos¬ phate baths at molecular weights in excess of 1,000,000. Moreover, paint additives (such as pigment and the like) , other types of resins, activators, and the like may be added on an optional basis. With respect to the technique for treating the metal surface, the present invention can employ conversion treat¬ ment by spray or immersion as well as electrolytic metho¬ dologies, and the treatment agent according to the present invention is not limited to a particular treatment method. Examples The effects of the present invention will be explained in detail in the following using illustrative examples and comparison examples; however, the present invention is not limited to the examples, which describe merely a typical pre-paint phosphate.treatment and a typical painting sys¬ tem. In comparison examples, evaluation and treatment were conducted as in the examples, but with the use of organic 4 SUBSTITUTESHEET polymeric compound outside the scope of the present inven¬ tion and with omission of the cationic organic polymer altogether. The general conditions for the examples and comparison examples were as follows: Workpieces; Cold-rolled steel sheet; electroplated steel sheet (zinc coating = 20 g/m 2 ) ; aluminum sheet (JIS 5052). Surface treatment agents: The cationic organic polymeric compounds used in the examples and the polymeric compounds used in the comparison examples are reported in Table I. PARBOND™ L3020 (surface-treatment agent for automotive applications from Nihon Parkerizing Company, Limited) was used for the organic polymer-free phosphate surface treat¬ ment bath. This was a typical phosphate surface-treatment bath. Treatment sequence: 1) Degreasing: 2 % solution of FINECLEANER™ L4460 (fro-a Nihon Parkerizing Company, Ltd.), 120 second spray at 42° C 2) Water wash: 30 second spray at room temperature 3) Surface conditioning: 0.1 % solution of PARCOLENE™ ZN (from Nihon Parkerizing Company, Ltd.) , 20 second spray at room temperature 4) The surface-treatment agents according to the present invention and the surface-treatment agents in the com¬ parison examples were both applied under the following conditions: immersion at 42° C for 120 seconds. 5) Water wash: 30 second spray at room temperature 6) Wash with deionized water (conductivity, 0.2 i- crosiemens/cm) : 20 second spray at room temperature After completion of the six process steps noted above and drying, painting was conducted by the following pro¬ cesses (the primer step was sometimes omitted, as noted in the tables below) : Table 1 Designa- Chemical Nature of the Organic Molecular tion of Polymeric Compound Used Weight Treatment A adduct of HN(CH 3 ) 2 with bisphenol A 8,800 epoxy resin B copolymer of methyl methacrylate 20,000 and dimethylaminoethyl methacrylate C adduct of H 2 NCH 2 N(CH 3 ) 2 with maleic 2,000 anhydride resin a adduct of H 2 NCH 2 N(CH 3 ) 2 with maleic 800 maleic anhydride resin b polyvinyl alcohol 3,000 1) Primer coating: Electrodeposition painting with ELE¬ CTION™ 9410 from Kansai Paint Kabushiki Kaisha to pro¬ duce a film thickness = 20 micrometers; baking at 175° C for 30 minutes 2) Intermediate coating: KPX36 from Kansai Paint Kabu¬ shiki Kaisha; film thickness = 30 micrometers; baking at 140° C for 30 minutes 3) Finish coating: RUGABAKE™ B 531 from Kansai Paint Ka- bushiki Kaisha; film thickness 40 micrometers; baking at 140° C for 30 minutes. All the process steps noted above were carried out in the same manner for both the examples and the comparison examples, except for the chemical nature of the surface- treatment agents used (in step 4) . Performance evaluation: 1) Water-resistant secondary adhesion: The painted sheet was immersed in deionized water at 40° C for 240 hours, and a checkerboard of 100 squares (2 mm x 2 mm) was scribed into the base material using a sharp cut¬ ter. After peeling with cellophane tape, the number of peeled squares was counted. A smaller number of peeled squares indicates a better score. 2) Composite cycle test: A cross was scribed in the painted sheet through to the base material, using a sharp cutter, and the painted sheet was then subjected to 14 repetitions of the T λ - T 2 cycle (see below) . After the test, evaluation was carried out by measur¬ ing the maximum one side film blister width from the inscribed cross. T x : salt-spray test (JIS Z 2371) : 24 hours T 2 : wetting test (50° C, 70 % RH) : 216 hours Benefits of the Invention Tables 2, 3, and 4 report the paint adherence and cor- rosion resistance of the films obtained by surface treat¬ ment, respectively, of cold-rolled steel sheet, electrogal- vanized steel sheet, and aluminum sheet. These tables also include the results for the comparison examples. In the case of treatment by a metal-surface-treatment agent for composite film formation in accordance with the present invention, the results confirm a remarkable im¬ provement in corrosion resistance and paint adherence over phosphate treatment. In addition, even with omission of primer coating, the present invention exhibits a perform- ance approximately equivalent to the application of a pri¬ mer coat on phosphate film. When an organic polymeric compound was used that was not within the scope of the present invention, either sub¬ stantially no effect was obtained or the performance was in fact degraded. As discussed hereinbefore, the metal surface treatment agent for composite film formation in accordance with the invention increases the corrosion resistance and paint ad¬ herence and makes possible the omission of primer coating. SUBSTITUTE SHEET

Claims: Claims 1. A phosphate conversion coating treatment composition for metal surfaces, characterized in that the treatment composition comprises a cationic organic polymeric compound that contains at least 1 cationic nitrogen atom and has a molecular weight of 1,000 to 1,000,000, or a salt of such a cationic organic polymeric compound. 2. A phosphate conversion coating treatment according to claim 1, wherein the cationic organic polymeric compound is a compound having a resin skeleton comprising at least one selection from epoxy resins, urethane resins, polybutadiene resins, acrylic resins, and maleic anhydride resins. 3. A process for forming a protective coating on a metal by contacting the metal with a phosphate conversion coating solution, characterized in that the phosphate conversion coating solution has a composition according to claim 1 or 2. 4. A process according to claim 3, wherein the metal is iron, zinc, or aluminum. 5. A process according to claim 4, wherein the protective coating formed has a composite structure, in which the res¬ in containing the cationic organic compound or salt thereof penetrates into grain boundaries between phosphate crys¬ tals. 6. An article of manufacture comprising an outer surface of an organic protective coating, an intermediate film in¬ cluding phosphate ions underlying the organic protective coating, and a metal underlying the intermediate coating, characterized in that the intermediate coating has a com- posite structure, in which a resin containing a cationic nitrogen containing organic polymer or a salt thereof pene¬ trates into grain boundaries between phosphate crystals.

Inventors: FURUYAMA OSAMU, ISHII HITOSHI, MORITA RYOJI, FURUYAMA, OSAMU, ISHII, HITOSHI, MORITA, RYOJI
Applicants: HENKEL CORP, HENKEL CORPORATION
Application year: 1992
Main classification: C23C22
IPC codes: B05D7, C09D5, C09D133, C09D163, C23C22, C23C28