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Many Types of Chromium Plating

Hard Chromium Plating

The benefit of the hardness of chromium deposits is not effectively obtained unless the coating is deposited on a sufficiently hard basis metal and to a satisfactory thickness. Generally, hardened steel is used for the basis metal. Even a relatively heavy deposit of chromium may be crushed or indented if applied over a soft basis metal such as copper. The best possible adhesion is also important in many uses where the surface may be subjected to severe stress or shock, and any chipping of the deposit would be injurious.

Etch Prevention

The self-regulating baths have a disadvantage common to all baths containing fluorides or complex fluorides, a tendency to corrode or etch recesses which do not cover with chromium. This tendency to etching is especially noted on steel, commercially, the ordinary baths with sulfate catalyst hate a strong tendency to etch copper and brass. This etching tendency is especially marked in over catalyzed bath, and can be overcome to a considerable extent by keeping the bath in balance, or operating at as low a catalyst concentration as possible. In higher ratio of baths, overcome this etching by prefilming the surface to be plated as cathode in a plain chromic acid solution.

Porous Chromium Plating

This name have been given to modified chromium deposits with oil-retaining properties, used on internal combustion engine cylinders and piston rings. Such deposits were used especially during the war on aircraft and diesel engine cylinders for salvage and to make engines last longer. Three main of porous chromium plate have come into common use.

The first is the mechanical type, produce by grit blasting the basis metal, chromium plating, and finally finishing to size by grinding, honing, or polishing. The second and third types of plate are those with pitted and channel porosity. Both of the later are obtained by treating the chromium deposit in an etching solution. The type of porosity obtained depends on careful control and regulation of the condition of deposition. Numerous publications and patents describe the production of all these types of porous chromium plate and the result obtained with them.

Black and Color Chromium Plating

Many attempts have been made to produced "black" chromium plate, and such deposits have been used commercially. They are usually dark grey rather than black, however, and all dull or mat chromium deposits seem to turn bright metallic rather readily if exposed to much abrasion or wear, in spite of the hardness of the metal. Oiling and other final treatments give grey or dull deposits a blacker appearance. An early "black" chromium plate was produced by using a high current density in a cold bath containing principally chromic and acetic acids.

Chromium Alloy Plating

Chromium alloy plating can be considered a subheading under trivalent bath, there is almost no alloy plating possible from hexavalent solutions. There has been a great deal work, and some reviews are available, but nothing of commercial importance seems to have been developed. It appears likely that some fundamental problems of depositing chromium from trivalent baths remain to be solved, and that these are probably not made appreciably easier by introducing the added factor of alloy deposition.