It is known that a plated zinc or zinc-based alloy layer on a steel substrate exhibits an excellent galvanic protecting activity for the steel substrate. Also, it is known that the plated zinc or zinc-based alloy layer is effective for forming a passive state film on a surface of the plated zinc layer in a corrosive environment so as to protect the steel material from corrosion. Therefore, the zinc- or zinc-based alloy-plated steel materials are widely useful as corrosion-resistant materials in the field of motor vehicles, home electrical appliances and building and construction materials.
In recent years, especially, in the field of the motor vehicle industry, a cationic electrodeposition method became widely utilized for the primer coating process of steel materials. However, in this process, it was found that the cationic electrodeposition method applied to the conventional zinc- or zinc-based alloy-plated steel material caused the following disadvantages. That is, the cationic electrodeposition procedure results in formation of undesirable protuberances having a size of about 0.3 to about 2 mm or pin holes in the resultant coating layer. The protuberances and pin holes serve as starting points of locally rusting the steel substrate and result in defects in appearance which cannot be removed by means of upper-coating. This phenomenon will be explained in detail hereinafter by referring to FIG. 1 of the accompanying drawing.
According to the results of research conducted by the inventors of the present invention, it was found that the protuberances each contain therein pores. It was assumed that the pores were formed by hydrogen gas which was generated in the form of bubbles during the electrodeposition procedure. That is, in the electrodeposition procedure, cationic lacquer particles deposit on the surface of the steel material and also, water which is used as a medium, is electrolyzed to generate hydrogen gas bubbles. Sometimes, the hydrogen gas bubbles are generated below the lacquer coating layer so as to form the protuberances and/or pin holes on and/or in the lacquer coating layer. The inventors of the present invention studied the adaptability of various types of metals and alloys to the cationic electrodeposition method and found that the above-mentioned defects on and/or in the lacquer coating were created significantly when the cationic electrodeposition procedure was applied to zinc- or zinc-based alloy-plated steel materials.
Accordingly, it is strongly desired to provide a new type of zinc- or zinc-based alloy-plate steel material which does not cause the undesirable protuberances and/or pin holes to be formed on or in the lacquer coating even when the cationic electrodeposition procedure is applied thereto.
Further, it is known that in order to increase the rust-preventing effect on the lacquer coating, it is necessary to enhance the bonding property of the lacquer coating to the surface of the steel material in a corrosive environment. This necessity is attained by applying a phosphate treatment to the surface of the steel material.
For example, in the lacquer coating of a car body, the cationic electrodeposition coating method is widely distributed as stated above. This is due to the fact that when the lacquer coating formed by the cationic electrodeposition method is placed in a corrosive environment, and a local cell is formed on the coating film, the coating film in the cathode portion of the resultant local cell exhibits an excellent resistance to creep. However, the conventional zinc phosphate treatment is not adequate for forming a base coating film for the lacquer coating layer formed by the cationic electrodeposition method. When the conventional zinc phosphate treatment is applied onto the steel material, the resultant phosphate coating film mainly comprises a hopeite type zinc phosphate (Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O) in the form of needle-like crystals. This type of coating film is easily soluble in an alkaline environment. Therefore, when placed in an alkaline environment, the hopeite coating film in the cathode portion under the lacquer coating layer is dissolved so that the bond of the lacquer coating layer to the surface of the steel substrate is deteriorated. That is, even when the cationic electrodeposition lacquer coating film, which is highly resistant to corrosion, is formed on the surface of the steel material through the zinc phosphate coating film, which is effective for protecting the steel material surface from rust, the resultant lacquer coated steel material exhibits a poor resistance to rusting under the alkaline environment, because of a poor bonding of the lacquer coating to the steel material through the zinc phosphate coating.
In recent years, a zinc-iron phosphate treatment has been developed as a base coating method for steel material. In the zinc-iron phosphate treatment, the resultant coating film mainly comprises a phosphophyllite type zinc iron phosphate (Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O) in the form of granular crystals. This type of coating film is highly resistive to an alkaline environment. Accordingly, the zinc-iron phosphate treatment is adequate and indispensable as a pre-treatment for forming a base coating layer on which the cationic electrodeposition lacquer coating layer is formed.
However, it should be noted that when the zinc-iron phosphate treatment is applied onto a zinc-plated surface of the steel material, no phosphophyllite is formed and only hopeite is formed on the zinc plated surface.
Therefore, even when the cationic electrodeposition lacquer coating film which is highly resistant to corrosion is formed on the zinc-plated steel material surface which is also resistive to corrosion, after the zinc-iron phosphate treatment is applied to the zinc-plate steel material, the resultant lacquer coated steel material exhibits an unsatisfactory resistance to rusting, due to the poor bonding of the lacquer coating film to the zinc-plated surface of the steel material under the alkaline environment. This phenomenon will be explained in detail by referring to FIG. 2 of the accompanying drawing hereinafter.