Selection Factors

The decision to use hard chromium plating on a specific part should take into account the following characteristics:

• The inherent hardness and wear resistance of electrodeposited chromium

• The thickness of chromium required

• The shape, size, and construction of the part to be plated

• The type of metal from which the part is made

• Masking requirements (for parts that are to be selectively plated)

• Dimensional requirements (that is, whether or not mechanical finishing is required and can be accomplished in accordance with desired tolerances)

The hardness of chromium electrodeposits is a function of the type of chemistry selected and the plating conditions. In general, chromium plated in the bright range is optimally hard. Typically bright chromium deposits from conventional plating solutions have hardness values of 850 to 950 HV; those from mixed-catalyst solutions have values of 900 to 1000 hV; and those from fluoride-free chemistries have values of 950 to 1100 HV or higher.

Size. Frequently, a very large part can be plated in sections or can be rotated so that only a portion of the part is immersed in the plating solution at any given time. The latter method has been used to plate large cylinders up to 4 m (12 ft) in diameter and up to 18 m (60 ft) long. When this technique is used, all of the surface to be plated that is exposed to the atmosphere must remain wet with plating solution.

Journal surfaces of the largest diesel crankshafts and bores of large naval guns and M-16 rifles are hard chromium plated on a production basis. Internal surfaces of very large cylinders have been plated by closing off the ends of each cylinder and retaining the plating solution with it.

Base Metal. Most hard chromium deposits are applied to parts made of ferrous alloys; however, numerous aerospace applications require the chromium plating of aluminum and nickel-base alloys. From the standpoint of processing, hard chromium plate may be applied to steels, regardless of their surface hardness or chemical composition, provided that the base metal is hard enough to support the chromium layer in service. Similarly, cast irons can be plated provided that the surface is capable of conducting the required current and is reasonably free of voids, pits, gross silicate inclusions, massive segregation, slivers, and feather edges.

0 0

Post a comment