IN ORDER TO BE able to properly design and interpret the results from galvanic corrosion tests, it is necessary to have some appreciation of the electrochemical theory behind galvanic corrosion. Metal corrosion consists of at least two reactions. The first is the metal going into solution in the electrolyte M->Mn++ne-.
SOLID PARTICLE and liquid drop erosion are surface removal processes caused by impingement of solid particles or droplets carried in a moving gas or liquid stream against a surface. Erosion can also occur when the surface is the moving body impacting suspended particles or drops, as happens on a helicopter blade moving through rainches.
THERMAL EXPOSURES, such as during welding, fabrication, and faulty heat treatment, can change the composition of grain boundaries of metals and alloys by equilibrium segregation of impurities and/or alloying elements and, most frequently, by the formation of precipitates, such as chromium carbides in stainless steels. These changes can make the grain boundaries susceptible to rapid, preferential attack (sensitization) in many, mostly acid, environments in which the materials are otherwise considered to have an acceptable degree of corrosion resistance for industrial purposes. The materials of construction most widely used for severely corrosive applications and which may also be subject to sensitization are the numerous iron-base austenitic and ferritic stainless steels and certain nickel-rich alloys with chromium or chromium and molybdenum.
HISTORICALLY, exfoliation corrosion problems have been associated principally with high-strength aluminum alloys used in aircraft construction and by the U.S. Department of Defense. This chapter, therefore, will deal mainly with test procedures developed for alloys of aluminum.
TESTING FOR UNIFORM corrosion is one of the most common corrosion tests employed to determine material suitability in a wide variety of applications. The following is intended to be an introduction into the subject of testing for uniform corrosion, its applications, basic methodology, and limitations. The reader is referred to applicable standards fully detailing the procedures.
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