Corrosion of Metals and its Prevention

Abstract

WHAT IS CORROSION? Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals. The most widely used metal is iron (usually as steel) and the following discussion is mainly related to its corrosion. The Consequences of Corrosion: The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation-of equipment or structures are often more serious than the simple loss of a mass of metal. Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. Some of the major harmful effects of corrosion can be summarized as follows: 1. Reduction ofinetal thickness, leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localized zones so as to give a cracklike structure, very considerable weakening may result from quite a small amount of metal loss. 2. Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars, aircraft). 3. Loss of time in availability of profile-making industrial equipment. 4. Reduced value of goods due to deterioration of appearance. 5. Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities of heavy metals are released by corrosion). 6. Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings. For example a leaky domestic radiator can cause expensive damage to carpets and decorations, while corrosive sea water may enter the boilers of a power station if the condenser tubes perforate. 7. Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface. 8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products. 9. Added complexity and expense of equipment, which needs to be designed to withstand a certain amount of corrosion, and to allow corroded components to be conveniently replaced. 19