Metal surface treatment liquid for cation electrodeposition coating

Abstract

A surface treatment with a zirconium ion that enables sufficient throwing power, and superior anti-corrosion properties to be exhibited when thus surface treated metal base material is subjected to cation electrodeposition coating is provided. A metal surface treatment liquid for cation electrodeposition coating includes zirconium ions and tin ions, and has a pH of 1.5 to 6.5, in which: the concentration of zirconium ions is in the range of 10 to 10,000 ppm; and the content of the tin ions to the zirconium ions is 0.005 to 1 on a mass basis. Furthermore, a polyamine compound, copper ions, fluorine ions, and a chelate compound may also be included.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)[0001]The present application is a continuation of PCT application no. PCT / JP2007 / 074536 filed Dec. 20, 2007.TECHNICAL FIELD[0002]The present invention relates to a metal surface treatment liquid, particularly to a metal surface treatment liquid suited for cation electrodeposition coating, and a method of metal surface treatment.BACKGROUND ART[0003]In order to impart anti-corrosion properties to various metal base materials, surface treatments have thus far been performed. Particularly, a zinc phosphate treatment has been generally employed on metal base materials which constitute automobiles. However, this zinc phosphate treatment has a problem of sludge generation as a by-product. Accordingly, a surface treatment without use of zinc phosphate for a next generation has been demanded, and a surface treatment with zirconium ions is one of such treatments (see, for example, Patent Document 1).[0004]Meanwhile, metal base materials which constitu...

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Benefits of technology

[0026]It is believed that the throwing power attained by the metal surface treatment liquid for cation electrodeposition coating of the present invention can be improved by including tin ions in addition to zirconium ions when the cation electrodeposition coating is carried out after forming a conversion coating film with this treatment liquid. Although not clarified, the grounds are conceived as follows.

[0027]When zirconium ions are used alone, formation of their oxide coating film is believed to be executed simultaneously with etching of the metal base material in an acidic medium. However, since segregation materials and the like of compounds containing silicon or carbon in addition to silica may be present on cold-rolled steel plates, such parts are not susceptible to etching. Therefore, the coating film cannot be uniformly formed with zirconium oxide, whereby portions without coating film formation can be present. Since a difference in electric current flow is believed to be generated between the parts with and without formation of the coating film, the electrodeposition is not uniformly executed, and consequently, the throwing power cannot be sufficiently attained.

[0028]When tin ions are additionally present, it is further considered as in the following. Since the tin ions are less likely to be affected on the steel plate as compared with the zirconium ions, their oxide coating film can be more easily formed on the base material. Although formation of the coating film of the tin ions is not specific to the parts where the zirconium ions are not significantly deposited, formation of the oxide coating film of the tin ions is not restricted to a specific part while having another part remain without formation of the film. As a result, the tin ions would form the coating film such that it covers the part where the zirconium ion could not form the coating film.

[0029]The metal surface treatment liquid for cation electrodeposition coating of the present invention can improve adhesiveness to the coated film by cation electrodeposition through including the polyamine compound, and consequently, it can pass SDT test under more stringent conditions. In addition, the metal surface treatment liquid for cation electrodeposition coating of the present invention can improve anti-corrosion properties by including the copper ion. Although the grounds are not clarified, it is believed that some interaction may be caused between copper and zirconium in forming the coating film. Furthermore, the metal surface treatment liquid for cation electrodeposition coating of the present invention can form a zirconium oxide coating film in a stable manner by including a chelate compound when a metal other than zirconium is included in large quantity. This occurrence is believed to result from capture by the chelate compound of metal ions that are more likely to be deposited than zirconium.

Problems solved by technology

However, this zinc phosphate treatment has a problem of sludge generation as a by-product.

However, it has been recently proven that when a metal base material which had been surface treated with the zirconium ions is subjected to the cation electrodeposition coating, there may be a case in which no significant effect in terms of the throwing power is achieved, for example, the throwing power may not be sufficient for cold-rolled steel plates in some cases.

Accordingly, when the cation electrodeposition coating is carried out, sufficient anti-corrosion properties cannot be achieved if the throwing power is insufficient.

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