Method for producing resin-coated rust-proof steel sheets with properties suitable for electrodeposition coating

Abstract: Described herein is a method for producing a resin-coated rust-proof steel sheet with properties suitable for electrodeposition coating, the method including the step of, prior to chromate treatment and resin coating, immersing a zinc- or zinc alloy-electroplated steel sheet in an acid bath containing at least one member selected from the group consisting of sulfuric acid, chloric acid, phosphoric acid, boric acid and nitric acid and salts thereof and having a pH value smaller than 6, inclusive, to remove a surface layer of the electroplating on said steel sheet, thereby activating the plated surface in such a manner as to improve the quality of the following chromate treatment. Advantageously, the method further includes the step of grinding the electroplated surface with a brush or roll containing fine abrasive grains of #200 or a higher number to remove said surface layer simultaneously with or separately from the immersion in the acid bath.

Description: This invention relates to a method of producing an improved electroplated steel sheet, more particularly a steel sheet for use in automobile bodies. In order to meet the present demand for a high level of corrosion resistance, there has been a trend towards the use of surface treated corrosion resistant steel sheets in automobile bodies instead of the conventional cold rolled steel sheets. To this end, a variety of surface-treated steel sheets, including galvanized steel sheets and sheets plated in single or multiple layers with a zinc alloy containing one or more alloying elements such as, for example, nickel, iron, manganese, molybdenum, cobalt and aluminium, and having chromate and resin layers thereon have been widely employed. FR-A-2 550 227 teaches the treatment of a galvanized steel sheet by subjecting at least one face of the sheet to a chemical or electrochemical treatment before phosphatizing. Such treatments may include a soda treatment and an electrochemical passivating treatment with chromium in order to form a deposit of chromium and chromium oxide. The resulting steel sheet may also be subjected to the application of a cataphoretic coating and, furthermore, at least one of its faces may be brushed to remove the zinc coating, leaving thereon only a thin covering of an iron-zinc alloy. US-A-2 911 332 describes the coating of metals to improve corrosion resistance and the adhesion of organic top coatings such as paints, lacquers or resins. The method is particularly concerned with the treatment of non-ferrous metal surfaces and comprises coating the surface with an aqueous solution consisting essentially of water, chromic acid and a compatible reducing agent for the chromic acid, the coated surface being then heated to a temperature of 250 to 500°F, to cause a reaction between the reducing agent and the chromic acid, and to leave a water-insoluble layer on the metal surface. In order to avoid the detrimental effects of the formation of oxide on the electroplated steel surface, a pre-etching treatment using an aqueous solution of nitric acid is suggested. Furthermore, the disclosure suggests that the adhesion of a paint or lacquer is further improved by mechanical roughening of the surface before the mixed chromium oxide layer is applied. In many, the electrodeposition coatings formed on conventional steel sheets have been found to have an inferior quality from the viewpoint of outer appearance. We have accordingly sought to provide a method of producing an improved electroplated steel sheet provided with the above chromate and resin coatings. In accordance with the present invention, there is provided a method of producing an improved electroplated steel sheet, in which: (i) a zinc- or zinc alloy-electroplated steel sheet is immersed in an acid bath containing at least one member selected from the group consisting of sulfuric acid, chloric acid, phosphoric acid, boric acid and salts thereof, and having a pH value below 6, to remove a surface layer of the electroplating on said steel sheet; (ii) the acid treated zinc- or zinc alloy-electroplated steel sheet is subjected to a chromate treatment; and (iii) a resin is applied to the product of the said chromate treatment. According to a preferred aspect of the invention, the surface layer of the electroplated steel sheet is subjected to a grinding step with a brush or roll containing fine abrasive grains having a size number of 200 or above, to remove the surface layer simultaneously with, or separately from, the immersion in the acid bath, prior to the chromate treatment and resin coating steps. According to the method of the present invention, a steel sheet which has been electroplated with zinc or a zinc alloy is immersed in an acid bath of predetermined constitution to remove a surface layer of the plating, more advantageously, in combination with a grinding step using a brush or roll containing abrasive grains with a size number of 200 or above, in order to add a mechanical action to the chemical action employed to remove the surface layer, thereby activating the plated surface in such manner as to improve the quality of the subsequent application type chromate treatment. The correlation between the activation of the surface of the zinc- or zinc alloy-plated steel sheet according to the invention and the effectiveness of the application type chromate treatment is not entirely clear at this stage, but appears to involve the following mechanism, it being understood that the invention is not restricted by any theory now set out. In general, a zinc- or zinc alloy-electroplated steel sheet is washed with water and dried in the stages subsequent to electroplating. During the transfer from the plating to the washing stage, the plated surface still bears some electrolyte thereon, which is normally adjusted to a pH in the range of from 1 to 4. In that pH range, the plated surface tends to redissolve and to form an hydrate of zinc or of zinc and alloying elements. Most of the hydrate is removed from the plated surface at the washing stage, but part remains on the surface. It is believed that the residual hydrate transformed into oxide in the drying stage of the steel sheet and the coating formed on the plated surface by the mixture of hydrate and oxide of zinc or zinc alloy have reduced reactivity thereby impeding the subsequent reaction with the chromate solution, and, as a result, causing a deterioration in the adhesion of the chromate film to the plated surface. Accordingly, when a resin is coated onto the steel sheet after such a chromate treatment, the resulting chromate layer has a tendency to be detached from the plated surface by the hydrogen gas which is generated in the electrodeposition coating stage. As a result, the appearance of the electrodeposited coating is very much impaired by peeling or loose film portions. In the method of the present invention, there is no particular restriction on the zinc- or zinc alloy-electroplated steel sheet itself and on the method for manufacturing such steel sheet. According to the method of the invention, a zinc-or zinc alloy-electroplated steel sheet is first immersed in an acid bath holding an aqueous solution containing at least one member selected from the group consisting of sulfuric acid, chloric acid, phosphoric acid and boric acid and salts thereof, and having a pH value below 6. Examples of useful salts include sodium sulfate, sodium borate and sodium phosphate. In the present invention, the acid bath containing the above inorganic acid or its salt needs to have a pH value below 6. If the pH value exceeds 6, it becomes difficult to remove the film layer of the above-mentioned inactive mixture from the plated surface, thereby failing to activate the surface to a sufficient degree and causing an inferior effect in the improvement of the electrodeposition coating. On the other hand, the lower the pH of the acid bath, the more the dissolution of the plated layer is accelerated to shorten the processing time. In such a case, however, there will arise problems such as overetching and irregularities in the degree of processing. Therefore, in view of the stability in quality and economy of the final products, the pH value of the acid bath is preferred to be larger then 3 and more specifically in the range of 3-5. According to the invention, resin-coated steel plates or sheets with properties suitable for electrodeposition coating can be obtained more advantageously with higher productivity and in a shorter processing time by grinding the plated surface of a steel sheet with a brush or roll containing #200 or finer abrasive grains, simultaneously with or before or after the step of immersing the electroplated steel sheet in the above-described acid bath, for removal of the surface layer of the plating. The abrasive grains on the brush or roll to be used for grinding the plated surface layer should be of #200 or finer grain size because the use of coarse abrasive grains larger than #200 is likely to result in overgrinding and a plated surface layer which is too coarse to form an electrodeposition coating with a surface of satisfactory appearance. On the contrary, if the abrasive grains are too fine, the grinding operation takes a longer time, inviting a drop in productivity. Accordingly, the size of the abrasive grains is preferred to be smaller than #500. The operation of grinding the plated surface layer of the steel sheet with a brush or roll containing abrasive grains may be effected simultaneously with or subsequent to the above-described immersion in the acid bath. If desired, the surface layer of the plating on the steel sheet may be ground prior to the immersion in the acid bath. As will be clear from the foregoing description, the method of the present invention makes it possible to obtain a resin coated rust-proof steel sheet having thereon an electrodeposition coating of satisfactory quality, by the immersion of a zinc- or zinc alloy-electroplated steel sheet in an acid bath prior to a chromate treatment and resin coating step, advantageously in combination with the step of grinding the plated surface with a brush or roll containing abrasive grains to remove a surface layer of the plating by chemical and/or mechanical actions whereby the plated surface is in an activated state which improves the succeeding chromate treatment. The invention is more particularly illustrated by the following examples, which however should not be restrictive of the invention in any way whatsoever. In the following description, the pH of the acid bath was adjusted by the use of an acid having the same anion as sodium hydroxide or its salt. Example 1 According to ordinary procedures, after degreasing and acid washing, a Zn-Ni alloy was electroplated on 8mm thick cold rolled steel sheets by the use of an acid bath at a deposition rate of 20g/m². Thereafter, each steel sheet was immersed in one of the acid baths having the composition shown in Table 1 for 5 seconds at room temperature, and, after water washing and drying, subjected to a chromate treatment (i.e. the application of a 40wt% aqueous solution of reduced chromate and drying for 1 minute at 150°C to have a total chromate application rate of 40-50g/m²). Then, a water-soluble resin was applied with a bar coater in a thickness of 1µm, and baked at a temperature of 180°C for 1 minute to yield a resincoated rust-proof steel sheet. The resulting resin-coated rust-proof steel sheets were coated with a cationic electrodeposition paint under an electrodeposition voltage of 200V, building-up control of 30 seconds and coating time of 2.5 minutes, followed by baking at 170°C for 20 minutes to yield a steel sheet with an electrodeposition coating. The existence of pimple-like defects on the surface of the electrodeposition coating of each steel sheet was checked to assess the quality of the electrodeposition coating. The results are shown in Table 1. Example 2 Zn-Ni was electroplated on the same cold rolled steel sheets as in Example 1, which were then immersed in one of the acid baths of the compositions shown in Table 2 for 3 seconds at room temperature while simultaneously grinding the plated surface for 3 seconds with a brush or roll containing abrasive grains as indicated in Table 2. Thereafter, each steel sheet was subjected to a chromate treatment and resin coating in the same manner as in Example 1 to yield a resin-coated rust-proof steel sheet. After coating the steel sheet by electrodeposition under the same conditions as in Example 1, the quality of the electrodeposition coating was assessed. The results are shown in Table 2. Example 3 Zn-Ni alloy was electroplated on the same cold rolled steel sheets as in Example 1, and, after grinding the plated surface with a brush or roll containing the abrasive grains of Table 3 for 2 seconds, each plated steel sheet was immersed in one of the acid baths having the compositions indicated in Table 3 for 3 seconds at room temperature. Thereafter, chromate treatment and resin coating were carried out in the same manner as in Example 1 to obtain resin-coated rust-proof steel sheets. Then, electrodeposition coating was formed on each of these steel sheets under the same conditions as in Example 1, and the quality of the electrodeposition coating was assessed. The results are shown in Table 3. Example 4 Zn-Ni alloy was electroplated on the same cold rolled steel sheets as in Example 1, and the plated steel sheets were immersed in one of the acid baths of the composition shown in Table 4 for 2 seconds at room temperature, thereafter grinding the plated surface of each steel sheet with a brush or roll containing abrasive grains as shown in Table 4 for 2 seconds. Then, chromate treatment and resin coating were carried out in the same manner as in Example 1 to obtain resin-coated rust-proof steel sheets, followed by electrodeposition coating under the same conditions as in Example 1 and assessment of the resulting electrodeposition coating. The results are shown in Table 4. As will be clear from the foregoing description, the method of the present invention makes it possible to obtain resin-coated rust-proof steel sheets or plates with properties particularly suitable for electrodeposition coating.

Claims: A method of producing an improved electroplated steel sheet, in which: (i) a zinc- or zinc alloy-electroplated steel sheet is immersed in an acid bath containing at least one member selected from the group consisting of sulfuric acid, chloric acid, phosphoric acid, boric acid and salts thereof, and having a pH value below 6, to remove a surface layer of the electroplating on said steel sheet; (ii) the acid treated zinc- or zinc alloy-electroplated steel sheet is subjected to a chromate treatment; and (iii) a resin is applied to the product of the said chromate treatment. A method according to Claim 1, in which: (iv)the electroplated surface of said electroplated steel sheet is subjected to a grinding treatment with a brush or roll carrying fine abrasive grains with a size number of 200 or above, said grinding step being carried out concurrently with, or before, or after, the step of immersing said electroplated steel plate in said acid bath [step (i)], in a stage prior to said chromate treatment step (ii) and said resin coated treatment step (iii), respectively.

Inventors: MIKI KENJI, NAKAMURA KANJI, NOMURA SHINGO, MIKI, KENJI, NAKAMURA, KANJI, NOMURA, SHINGO
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1990
Main classification: C23C22
IPC codes: C25D5, C23C22, C25D11, C25D13