Friday, November 14, 2008

Metal Coating with Chromium

It is convenient to deal next with chromium plating because it is almost always a dirrect sequel to nickel plating, and also because the methods of chromium plating are so radically different from all other types of electroplating.

Chromium is a hard, brittle, silvery-white metal, with a faint blue color, which is not otherwise used unalloyed in industry. It is important constituent of stainless steel, both of the cutlery type (13%), and of the austenitic ductile type (18:8 Nickel : Chromium). Chromium confers passivity to corrosion to these alloys because it automatically form a protective oxide film on their surface. In an alloy with nickel it gives resistance to oxidation at high temperatures, and is thus used for electrical heating elements. It is also an important minor constituent of many special steels and non ferrous alloys.

Although it would appear from the position of chromium in the electrochemical series that this metal is about as reactive as zinc, in practice it automatically covers itself with a protective oxide film, and then behaves like a noble metal, such as gold. It is quite unattacked by atmospheric exposure, by most oxidizing chemicals and by acids and waters.

Attempt were made for years to electroplate chromium could be electroplated satisfactory from chromic acid, if a small, but critical, proportion of chromic sulphate were also prevent. It was soon found that it was the sulphate that was critical, although chromium sulphate was indeed formed during the plating process. The surprising solution composition has reminded the basic of all chromium plating in spite of considerable research to find place that the chromium in chromic acid solution is present in the negatively charged anion, and would not be expected to be discharged at the negatively charged cathode, and in the second place, since electroplating is chemically a reduction process, it is unexpected that it should operated with the strongly oxidizing chromic acid.

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