Surface preparation agent and surface preparation method

Abstract

An agent suitable for use in preparing metal surfaces, especially surfaces containing aluminum, is provided which contains at least one water-soluble vanadium compound, at least one titanium-containing or zirconium-containing water-soluble complex fluoride, and at least one resin. The agent is capable of forming a strongly rust preventive base coating that is highly adherent for top coats and bonded films.

Description

[0001] This invention relates to a surface preparation agent and a surface preparation method. More particularly, this invention relates to a chromium compound-free dry-in-place rust preventive surface preparation agent that can form an organic-inorganic composite coating on the aluminum sheet and aluminum alloy sheet used in the automotive sector and for household electrical appliances, construction materials, and food containers. This organic-inorganic composite coating can function as an underpaint base coating or as a base coating for bonded films and improves the antirusting performance and is highly adherent for top coats and films. The invention additionally relates to a rust preventive surface preparation method that uses said surface preparation agent.BACKGROUND OF THE RELATED ART[0002] Chromate treatments that use a treatment agent containing chromic acid, dichromic acid, or a chromate salt are known as a rust preventive surface preparation method for the aluminum and alum...

AI Technical Summary

Benefits of technology

[0015] As a result of intensive investigations into means that would remedy the foregoing problems, the inventors found that a composition containing a vanadium compound and a particular complex fluoride could impart an excellent rustproofing performance to aluminum alloy surfaces. The inventors also discovered that the admixture with, for example, waterborne polymer, made it possible to impart an excellent rustproofing performance of the dry-in-place type to aluminum alloy surfaces.

[0033] With regard to the titanium-containing or zirconium-containing water-soluble complex fluoride, it is thought that in aqueous solution this component liberates small quantities of fluoride ion, which etches the surface of the aluminum alloy substrate and thereby secures adhesion between the substrate and the rust preventive base coating through an anchoring effect. In addition, upon treatment and drying, the complex fluoride is believed to precipitate metal salts on the aluminum surface, in addition to zirconium oxide or titanium oxide, and this phenomenon is thought to function as a rust preventive coating that inhibits aluminum corrosion.

[0035] Treatment with an agent that contains the foregoing compounds in combination is thought to accrue the following effects. First, an organic-inorganic composite coating is formed from the inorganic solids (e.g., oxides and fluorides of Zr, Ti) and the waterborne organic polymer, and the presence of this composite coating improves the barrier performance of the coating against corrosive conditions. Second, with regard to coating defect regions and damaged regions produced after coating formation, the aluminum alloy substrate surface exposed in such regions is successively oxidized and stabilized, with the formation of nonconducting material, by the action of the water-soluble V-containing ion (e.g., vanadyl ion) produced by the vanadium compound. This again results in the manifestation of a corrosion inhibiting activity.

Problems solved by technology

At the same time, however, the human-toxic hexavalent chromium present in the treatment bath is eluted by contact with water, and as a result dry-in-place chromate treatment suffers from problems with regard to environmental pollution and toxic effects for the human body.

The coatings formed by reactive chromate or electrolytic chromate treatment do not, however, have the self-repairing function described above and may exhibit an inadequate corrosion resistance depending on the particular service.

These technologies also impose heavy wastewater treatment burdens since they cannot avoid the production of, for example, rinse water that contains hexavalent chromium.

This treatment bath, however, cannot be considered a fully rationalized surface treatment for metal sheet because it is used in a treatment methodology that entails the immersion of aluminum alloy in the bath for 1 to 20 minutes and preferably 3 to 5 minutes.

However, since this is a conversion treatment method, it requires a post-treatment water rinse, there are limitations on the weight of the obtained coating, and the corrosion resistance may be unsatisfactory depending on the particular application.

This is not, however, a technology contemplated for use for surface preparation prior to painting or film lamination, and its application within this sphere has been highly problematic.

The adherence of the ultimately obtained base coating for aluminum alloy surfaces has a tendency to deteriorate at V: (Zr+Ti)=5000:less than 1), i.e., when there is less than 1 part by weight total Zr+Ti per 5000 parts by weight V. At {V+(Zr+Ti)}:(C)=10:(les-s than 1) there is a tendency for coating defects to occur when the surface preparation agent is applied and / or for the adherence of the ultimately obtained base coating for aluminum alloy surfaces to deteriorate.

At {V+(Zr+Ti)}:(C)=(less than 1):100 there is a tendency for the rustproofing performance of the ultimately obtained base coating to be unsatisfactory and / or for the adherence to top coats and bonded films to deteriorate.

This again results in the manifestation of a corrosion inhibiting activity.

The rustproofing performance of the ultimately obtained base coating will be unsatisfactory when the total deposition of V plus Zr plus Ti is less than 0.05 mg / m.sup.2.

A total deposition in excess of 100 mg / m.sup.2 accrues no additional enhancements in rust prevention and for this reason is uneconomical.