Method of surface treating metal and metal surface treated thereby

Abstract

It is an object of the present invention to provide a method of metal surface treatment and a surface treated metal thereby, which has excellent corrosion resistance and can form a coat having high corrosion resistance on metal substrates such as iron, zinc, aluminum and magnesium. A method of metal surface treatment comprising the step of forming a chemical conversion coat on the surface of a metal article to be treated by a chemical conversion treatment reaction by a chemical conversion treatment agent containing a zirconium-containing compound and a fluorine-containing compound, wherein said chemical conversion treatment reaction is conducted through cathodic electrolysis treatment.

Description

[0001] The present invention relates to a method of metal surface treatment and surface treated metal thereby.BACKGROUND TECHNOLOGY[0002] Generally, the surface of a metal is provided with surface treatment for the purpose of enhancing characteristics such as corrosion resistance and the like. As one species of such the surface treatment, there is known surface treatment with a chemical conversion treatment agent containing a zirconium compound. Such a method of surface treatment is performed by an electroless reaction, and an insoluble zirconium salt and a salt of the component metal of the article to be treated, consisting of hydroxides / fluorides of zirconium and fluorides of metal of an article to be treated, are deposited on the metal surface by means of a reaction in which the component metal of the article to be treated is eluted by the treatment solution, the production of fluorides based on a reaction of the eluted metal ions with fluorine ions, the formation of hydrogen thr...

AI Technical Summary

Benefits of technology

[0017] The method of metal surface treatment of the present invention is a method in which a chemical conversion coat is formed by treating the metal surface with a chemical conversion treatment agent containing a zirconium-containing compound and a fluorine-containing compound by a cathodic electrolysis technique. When the chemical conversion treatment reaction is conducted through cathodic electrolysis treatment, coats to be obtained will be dense and excellent in uniformity compared with a chemical conversion coat obtained by using electroless treatment. Therefore, there is formed the chemical conversion coat having high corrosion resistance even when an amount of a coat formed is identical to that of the chemical conversion coat obtained by using the electroless treatment.

[0049] Since the method of metal surface treatment of the present invention is constituted as described above, the corrosion resistance can be more enhanced than the case where the electroless treatment is conducted, or the electrolysis treatment is conducted using a titanic treatment agent or a phosphate treatment agent. Since this method can provide the excellent corrosion resistance for all material such as an iron-based substrate, an aluminum-based substrate, a zinc-based substrate and a magnesium-based substrate, it can also be suitably used to articles to be treated, which consists of a plurality of substrates of an iron-based substrate, an aluminum-based substrate, a zinc-based substrate and a magnesium-based substrate, such as bodies and parts of automobiles. And, the method of the present invention is also a method which places a less burden on the environment and suppresses the formation of sludge.

Problems solved by technology

In such an electroless reaction using a zirconium-based chemical conversion treatment agent, since it is extremely difficult to cause a homogeneous reaction to occur over the entire said surface, it is difficult to form an adequately dense and uniform coat and the resulting coat becomes one containing a high proportion of oxides and fluorides due to etching of substrates, and therefore the corrosion resistance is deteriorated.

Therefore, only a rough and thin chemical conversion coat composed of a substrate metal or an alkali metal can be attained and it is difficult to attain a uniform and dense protection coat.

consequently, it was difficult that a chemical conversion coat obtained by using the electroless treatment with a zirconium-based chemical conversion treatment agent provide a sufficient rust prevention property particularly for an article to be treated such as an iron-based substrate and a zinc-based substrate, having low reactivity with the chemical conversion treatment agent.

However, these methods concern a treatment method of phosphate compounds and titanium-based compounds, but are not methods for forming a uniform and dense zirconium chemical conversion coat.

Particularly, in a method of chemical conversion treatment in which phosphate compounds are used, there is a problem of placing a burden on the environment due to issues of the eutrophication.

Further, in chemical conversion treatment using a titanium-based compound, a high degree of corrosion resistance cannot be attained.

However, in this method, when the chemical conversion treatment is conducted by the electrolysis treatment using a composition for surface treatment, which is formed by dissolving compounds containing fluorine and zirconium, it is difficult to attain an effect of protecting a cathode in applying an electrolysis voltage to a substrate to be treated and a chemical conversion coat, containing a relatively large amount of fluorides and alkali metal compound, is formed since a large amount of and excessive fluorine and alkali metals are present in the solution.

Therefore, the corrosion resistance becomes unsatisfactory.

Further, a corrosion problem of facilities arises due to a large amount of fluorine.

When the concentration is less than 10 ppm, the corrosion resistance may not be achieved since the zirconium compound is not adequately deposited on the metal surface.

When it is more than 10000 ppm, it may be economically disadvantageous since further improvement is not recognized.

Further the corrosion resistance may be deteriorated since a chemical conversion coat having relatively much fluorine content.

When it is more than 1.0, precipitation of metal salt may be occurred since the amount of the total fluorine becomes insufficient.

When the pH is less than 1, the zirconium compound becomes difficult to deposit, and therefore a sufficient amount of the coat cannot be obtained and the corrosion resistance may be deteriorated.

When the voltage is less than 0.1 V, the amount of the coat is insufficient; therefore the corrosion resistance may be deteriorated.

When it is more than 40 V, effect from increase in the amount of the coat becomes saturated and energy disadvantage may occur.

When the current density is less than 0.1 A / dm.sup.2, the amount of the coat is insufficient; therefore the corrosion resistance may be deteriorated.

When it is more than 30 A / dm.sup.2, effect from increase in the amount of the coat becomes saturated and energy disadvantage may occur.

When the treatment time is less than 3 seconds, the amount of the coat is insufficient; therefore the corrosion resistance maybe deteriorated.

When it is more than 180 seconds, effect from increase in the amount of the coat becomes saturated and energy disadvantage may occur.

When the treatment temperature is less than 10.degree. C., the amount of the coat is insufficient; therefore the corrosion resistance may be deteriorated.

When it is more than 70.degree. C., effect from increase in the amount of the coat becomes saturated and energy disadvantage may occur.

In addition, the lower limit of the treatment temperature is not particularly controlled and the cathodic electrolysis treatment can be conducted at room temperature.

When this amount is less than 10 mg / m.sup.2, the corrosion resistance may be insufficient.

When it is more than 300 mg / m.sup.2, it may be economically disadvantageous since further improvement in the corrosion resistance is not recognized.

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