Electroplating involves the creation of a galvanic cell in which the part to be plated is the cathode and the plating material is the anode. The two metals are placed in an electrolyte bath and a direct current applied from anode to cathode. Ions of the plating material are driven to the plating substrate through the electrolyte and cover the part with a thin coating of the plating material.

Steels, nickel and copper based alloys, as well as other metals are readily electroplated. Two approaches are possible. If a more noble (less active) metal is plated onto the substrate, it can reduce the tendency to oxidize as long as the plating remains intact to protect the substrate from the environment. Tin, nickel, and chromium are often used to electroplate steel for corrosion resistance. Chrome plating also offers an increase in surface hardness to HRC 70, which is above that obtainable from many hardened alloy steels. Unfortunately, any disruptions or pits in the plating can provide nodes for galvanic action if conductive media (such as rainwater) are present. Because the substrate is less noble than the plating, it becomes the sacrificial anode and rapidly corrodes. Electroplating with metals more noble than the substrate is seldom used for parts that will be immersed in water or other electrolytes.

Alternatively, a less noble metal can be plated onto the substrate to serve as a sacrificial anode which will corrode instead of the substrate. The most common example of this is zinc coating of steel, also called galvanizing. The zinc or cadmium coating will gradually corrode and protect the more noble steel substrate until the coating is used up, after which the steel will oxidize. Zinc coating can be applied by a process called "hot dipping" rather than by electroplating, which will result in a thicker and more protective coating recognizable by its "mother-of-pearl" appearance. A caution about electroplated coatings is that hydrogen embrittlement of the substrate can occur, causing significant loss of strength. Electroplated finishes should not be used on parts that are fatigue loaded. Experience has shown that electroplating severely reduces the fatigue strength of metals and can cause early failure.

Hydrogen Embrittlement - Whenever carbon steel is pickled in preparation for plating or during some electroplating processes, hydrogen can become absorbed into the material. While cracks can develop in the pickling or plating bath, more often they appear when the plated springs are in service. The hazard of hydrogen embrittlement becomes more acute when there is (1) high stress concentration, (2) high Rockwell hardness, or (3) high carbon content. Tempered materials are particularly susceptible. To relieve embrittlement, the springs must be baked immediately after plating to drive the hydrogen out of the material.



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