Rustproofing agent for zinc plated steel sheet

Abstract

A rustproofing agent for a zinc plated steel sheet is provided which contains (A) a chromium aqueous solution containing an organic substance and (B) an acid metal such as a metal salt of nitric acid or phosphoric acid. All of the chromium in the chromium aqueous solution of component. A is trivalent chromium. It is preferable if the organic substance in component A contains at least one type of oxy acid or oxide thereof. The metal in the acid metal salt of component B is an alkaline earth metal, cobalt, nickel, iron, zirconium, titanium, etc. It is also preferable if (C) a water-dispersible silica is contained as an additional component. The present invention also provides a zinc plated steel sheet, in which at least one side of the surface of a zinc plated steel sheet is coated with the zinc plated steel sheet rustproofing agent and the rustproofing agent dried to form a coating with a dry coating weight of 0.01 to 3 g / m<2>.

Description

[0001] The present invention relates to a rustproofing agent for zinc plated steel sheet, which contains no hexavalent chromium (a hazardous substance) whatsoever, and has excellent corrosion resistance, fingerprint resistance, and blackening resistance. The invention also relates to a zinc plated steel sheet that has undergone a rustproofing treatment with this agent.DISCUSSION OF THE RELATED ART[0002] Steel sheets used in the fabrication of consumer electrical products and for construction materials are often subjected to a chromating treatment for the purpose of improving the corrosion resistance of the surface of zinc plated steel sheets. However, this chromating involves the use of hexavalent chromium, which is harmful to both the environment and humans. Consequently, there has been an urgent need in recent years for a chemical conversion treatment that does not make use of hexavalent chromium, and even for a chemical conversion treatment that does not involve the use of chromi...

AI Technical Summary

Problems solved by technology

However, this chromating involves the use of hexavalent chromium, which is harmful to both the environment and humans.

Moreover, this level of corrosion resistance can hardly be considered adequate, and it is difficult to achieve a level at which white rust occurs over less than 3% of the surface area after 120 hours of saltwater spray.

With this method, the solution still contains hexavalent chromium, albeit very little, and the resulting corrosion resistance does not even reach the level of temporary rustproofing.

The level of corrosion resistance will be unacceptable if the weight ratio of chromium in component A with respect to a total solids content of 100 wt % in the rustproofing agent is either less than 1 wt % or over 30 wt %.

Examples of metal salts other than a nitric acid metal salt include sulfuric acid metal salts and metal halides, but since it is known that sulfuric acid radicals and halides generally accelerate the corrosion of metals, the use of these metal salts is undesirable.

There will be little improvement in corrosion resistance under 5 wt %, but exceeding 50 wt % is undesirable from a cost standpoint because no significant further improvement in corrosion resistance is achieved.

Generally speaking, it will be preferred to add component C after components A and B have been combined and after any pH adjustment has been performed since otherwise the stability of the water-dispersible silica may be adversely affected.

It is undesirable for the dry coating weight to be less than 0.01 g / m.sup.2 because the corrosion resistance will be inadequate.

On the other hand, a dry coating weight that exceeds 3 g / m.sup.2 is not cost effective because there will be no further improvement in corrosion resistance.