Chromium plating is generally carried out in a plating bath containing hexavalent chromium. Recently, since hexavalent chromium has bad effects on the environment, etc., investigations about a trivalent chromium plating bath have proceeded. Chromium plating using a trivalent chromium bath has been proposed for a long time. Although chromium plating using a trivalent chromium plating bath has the feature that the plating adherence to a material is good without causing discoloration of the plated layer and without causing poor adherence of the plated layer to a material, unlike hexavalent chromium, the platable condition is limited. Thus, chromium plating of a material using a trivalent chromium plating bath has not yet been practically used.
A trivalent chromium plating bath has the problem that the stability of the plating liquid deteriorates when hexavalent chromium ions are formed by an anodic oxidation reaction, thereby decreasing the plating quality, etc. Thus, a chromium plating method is proposed wherein, by partitioning the plating bath into an anode chamber and a cathode chamber using an ion-exchange membrane, formation of hexavalent chromium ions is prevented.
In chromium plating, an inexpensive lead or lead alloy is generally used as the anode. However, where a lead-containing electrode is used, a sludge of the lead compound is formed which is difficult to treat, and the lead compound dissolved in the plating bath causes a reduction in the plating quality. Even where an ion-exchange membrane is used, the formation of hexavalent chromium can be avoided, but the formation of a lead compound sludge or dissolved lead compound cannot be prevented.
Thus, use of an electrode formed by coating a titanium substrate with platinum as the anode for chromium plating is described in JP-A-54-134038 (the term "JP-A" as used herein means an "unexamined published Japanese patent publication"), but the electrode does not have sufficient durability and the plating voltage increases in a relatively short period of time.
Also, for preventing the formation of hexavalent chromium at the anode, the use of an electrode comprising an alloy of iron or nickel and the oxide thereof as the anode is described in JP-B-56-43119 (the term :JP-B" as used herein means an "examined published Japanese patent publication"), and the use of a ferrite anode is described in JP-B-61-22037. However, when these electrodes are used as the anode, the formation of a sludge due to dissolution of the catalyst component which constitutes the electrode, reduction in the quality of the plated product by the adherence of the dissolved components on the surface of the plated product, or reduction in the plating efficiency is remarkable.
Also, JP-A-61-23783 and JP-A-61-26797 each describe that a plating bath is partitioned into an anode chamber and a cathode chamber using an ion-exchange membrane, an aqueous solution having dissolved therein a trivalent chromium salt is supplied to the cathode chamber, an acid solution of the same anion as that of the trivalent chromium salt is supplied to the anode chamber, an electrode comprising lead or titanium coated with a noble metal or a noble metal oxide is used as the anode where a sulfuric acid solution is used, and an electrode comprising a graphite or titanium coated with a noble metal or a noble metal oxide is used as the anode where a chloride solution is used. However, in these patent publications, only examples using a graphite electrode are described, and there are no descriptions of an electrode having a coating of a noble metal or a noble metal oxide.