Chromium plating method

Abstract

A chromium plating method wherein an article to be plated is immersed in an acidic chromium electroplating bath and electrolysis is carried out using a positive electrode that has an iridium oxide-containing film at least on the surface. The acidic chromium electroplating bath contains a trivalent chromium compound and a hexavalent chromium compound at a ratio such that the total chromium concentration of trivalent chromium and hexavalent chromium is 60-140 g / L, the hexavalent chromium concentration is 5-40 g / L, and the ratio of the hexavalent chromium concentration is 5-35% by mass of the total chromium concentration, while containing 50-400 g / L of organic carboxylate ions and having a lead ion concentration of not more than 2 mg / L. By the method, a chromium plating film that is good and stable over a long time period can be obtained. In addition, the plating bath can be controlled extremely easily in the chromium plating method.

Description

TECHNICAL FIELD[0001]The present invention relates to a chromium plating method that employs a plating bath containing a trivalent chromium compound and a hexavalent chromium compound mixed together.BACKGROUND ART[0002]Among well-known conventional chromium plating baths are the one which contains chromic acid (which is a hexavalent chromium compound) as the major component and the one which contains a trivalent chromium compound. The bath containing chromic acid as the major component is widely used as one of them. Beside, the bath containing a trivalent chromium compound is beginning to be used in the environmental point of view. However, the conventional plating bath containing a trivalent chromium compound causes troubles when it is contaminated with hexavalent chromium ions (Cr6+).[0003]On the other hand, there is also known a chromium plating bath which contains a trivalent chromium compound and a hexavalent chromium compound in combination. (This is referred to as a combined ...

AI Technical Summary

Benefits of technology

[0035]The present invention makes it possible to obtain a good chromium plated film in a stable manner over a long period of time, with very easy maintenance of plating bath.EMBODIMENT FOR CARRYING OUT THE INVENTION

[0036]A chromium plating method according to the present invention employs an acidic combined plating bath which contains a trivalent chromium compound and a hexavalent chromium compound as the chromium source and also contains carboxylate ions and further optionally contains sulfate ions as a stabilizer or a conductive salt.

[0037]The trivalent chromium compound should preferably be a complex of chromium with organic carboxylic acid. Examples of the organic carboxylic acid include oxalic acid, citric acid, formic acid, acetic acid, malonic acid, succinic acid, and lactic acid. Preferable among them are oxalic acid, citric acid, formic acid, and acetic acid. A complex of chromium with oxalic acid is particularly desirable. Incidentally, a preferable example of the complex of trivalent chromium with organic carboxylic acid is one obtained by mixing chromic acid (CrO3) and the above-mentioned organic carboxylic acid in their aqueous solution and reducing the chromic acid with the organic carboxylic acid so that the resulting complex is free of hexavalent chromium. (See Japanese Patent Application No. 2008-294007.)

[0038]Other examples of the trivalent chromium compound include inorganic salts of trivalent chromium, particularly chromium sulfate. In the case where an inorganic chromium salt (such as chromium sulfate) is the only source of chromium, hydrogen occurs during plating due to electrolysis of water. This hydrogen makes the interface of the cathode strongly alkaline and hydrolyzes chromium sulfate, thereby forming chromium hydroxide and basic chromium sulfate. This prevents chromium plating of practical use.

[0039]On the other hand, the organic carboxylic acid forms a complex with trivalent chromium ions, thereby preventing and buffering hydrolysis of trivalent chromium ions, and it also functions as a pH buffer for the plating bath. Therefore, if an inorganic chromium salt (such as chromium sulfate) is to be used, it is preferably used in combination with a complex of chromium with organic carboxylic acid.

[0040]The total amount of trivalent chromium ions in the plating bath is preferably 55 to 135 g / L, particularly 72 to 112 g / L. The chromium complex of organic carboxylic acid is preferably used in such an amount that the ratio (by weight) of the trivalent chromium (as metal) in the complex to the total trivalent chromium (as metal) is from 0.5 to 1, particularly from 0.6 to 1, with the remainder being the inorganic chromium salt. It is desirable to use in combination a chromium complex of organic carboxylic acid and chromium sulfate, because a new initial plating bath forms a plated film having a thickness larger by 20% than that obtained in the case where the chromium complex of organic carboxylic acid is used alone. In the case of combination use, they are preferably used in such an amount that the ratio of trivalent chromium in the complex to trivalent chromium in chromium sulfate (trivalent chromium in the complex: trivalent chromium in chromium sulfate) is from 5:5 to 10:0, particularly from 6:4 to 10:0 (by weight).

Problems solved by technology

However, the conventional plating bath containing a trivalent chromium compound causes troubles when it is contaminated with hexavalent chromium ions (Cr6+).

However, the plating method with the combined chromium plating bath is almost not industrially used at present.

The reason for this is that chromium plating with the conventional combined chromium plating bath proceeds smoothly in its early stage but is liable to cause troubles after a comparatively short period of time, thereby preventing stable plating operation.