Saturday, April 5, 2008

Corrosion and Wear Resistance

Thick nickel coating are used on process equipment to resist corrosion or combined corrosion, wear and fatigue. Nickel-lined steel pipe is available commercially and is in service in corrosive oil wells and in the chemical industry. An old application is the coating of huge, highly polished, cast iron drums used in this service is so high that it cannot be attained with wrought metals because they contain non metallic inclusions which cause tiny pits on buffing. The coating now used on these drums are deposited from a high chloride Watts bath under conditions giving nickel of moderate hardness. Nickel lined food processing kettles, nickel coated paper mill rolls, and nickel coated filter pressed for viscose rayon solutions are outstanding examples of heavy nickel plating on large units.

In applications where steel trends to fail by corrosion fatigue, a nickel coating has a great beneficial effect, for example, the life of sucker rods in corrosive oil wells is lengthened many times by application of 150 µm of nickel from the Watt bath. Nickel coating seem to be particularly effective in preventing "fretting corrosion." Nickel plating, like chromium plating, can reduce the fatigue strength of plated steel components subject to alternating stressing or vibration. A compressive stress in the nickel favor a minimum loss of fatigue strength.

Hard nickel is generally specified for application involving wear. The performance of nickel from the other baths, however, is better than might be expected, because this metal work-hardness rapidly under sliding action. Thus the life of hardened steel gasoline pump gears, which failed by seizure and dragging, was multiplied more than fivefold by application of 5 to 7.5 µm of nickel. For the most severe conditions of contact with steel under high pressure, it is preferable to use a composite coating consisting of nickel plated with a few micrometers of chromium.

No comments: