Write a short note on:

(a) Stress corrosion

(b) Metal cladding

(c) Intergranular corrosion

1. Stress Corrosion: Stress cracking it is combine effect of static tensile stress and the corrosion environment on the metal. Stress corrosion is characterize by a highly localized attack occurring when over all corrosion is negligible. For stress corrosion to occur: The presence of tensile stress Specific corrosive environment are necessary The corrosive agents are highly: Caustic alkalis and storing nitrate for mild steel. Traces of ammonia for brass. Acid chloride selection for stainless steel This type of corrosion is seen in fabricated articles of certain alloys like high zinc brasses and nickel brasses due to the presence of stresses caused by heavy working like rolling drawing or insufficient annealing pure metal are relatively immure to stress corrosion. It is generally believed that stress corrosion involves in localized electrochemical corrosion occurring along the narrow path, forming anodic area with respect to larger cathodic areas of the metal surface. Presence of stress produces strain which result in localized zone of higher electrical potential. Theses become so chemically active that they are attacked even by mild corrosive environment resulting the formation of crack. Under sufficiently high tensile stress and specific environment nearly alloys are susceptible to corrosion.

SEASON CRACKING: This is a term applied to stress corrosion of copper alloy mainly brakes pure copper is immune to stress corrosion but presence of small amount of alloying element like P, As, Sb, Zn, Al, Sn result in marked sensitivity. The best known examples are brass which when highly stressed undergo intergranular cracking in a atmosphere containing traces of ammonia, amines. The attack occurs along the grains boundaries which become more anodic with respect to grain themselves due discard of atom caused by different orientation of adjacent grain both Cu and Zn are electrochemical very achieve in ammonia solution due to formation stable complex ion Cu(NH3) Zn(NH3). This is real cause of brass initiating propagates in the formation presence of high tensile stress. Metal cladding or sand witching: Thin sheet is electroplated with the help of molten metal by passing it through series of rollers. Advantage is both sides are protected

Intergranular corrosion: This type of corrosion occurs along the grain boundaries and only where the material is especially sensitive to corrosive attack moreover corrosive liquid posses a selective character of attacking only at the grain boundaries but leaving the interior untouched or slightly attacked. This type of corrosion is believed to be due to fact that the grain boundaries contain material which shows electrode potential grain anodic than that of the grain centre in the particular corroding medium. This may result precipitation of certain compounds at the grain boundaries there by leaving solid solution just beneath to the grown boundary. This solid solution is anodic with respect to grain centre as well as to the precipitated compound so that it will be preferentially attacked by corrosive environment. EXAMPLE---The grain boundary corrosion is generally encountered in alloy. For example during the welding of stainless steel (fe, c, cr) chromium carbide is precipitated at grain boundaries. Thus the region just adjacent to grain boundaries become depleted chromium in composition is more anodic with respect to solid solution with in the grain which is richer in chromium. For the same reason it is also anodic to the particle of chromium carbide is precipitated. Usually corrosion in welded stainless steel is not serious problem which can be restored by heat treatment method which dissolves chromium carbide precipitated during the welding. However when stainless steel assembles are to large to be heat treated it is then necessary to prevent the depletion of chromium by special means use of law carbon 0.03 is one remedy Intergranular corrosion occur on microscopic scale with out any external sign of intensive attack on account of this sudden failure of the material occurs (without pre warning ) due to loss of cohesion between grains. The remedy for such corrosion of alloy is rapid heat treatment followed by rapid quenching to avoid the heterogeneous precipitation that usually occurs due to slow cooling.