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.