The characteristics of transgranular stress-corrosion cracking

The mechanism of transgranular stress-corrosion cracking in alloys is discussed in some detail from both a physical metallurgical and electrochemical aspect, with particular reference to 18 Cr-8 Ni steels. Plastic deformation occurs at the crack tip. It is enhanced by corrosion attack although in some alloys its main function may be to prevent the formation of a protective film. In all cases the crack follows a soft path through an unyielding matrix. The inability of susceptible alloys to cross-slip may produce very wide steps at the crack tip which alter the aggressiveness of the solution by preferential chemisorption. It is concluded that three conditions are necessary for transgranular cracking susceptibility: ( a ) there must be a localised passive-to-active breakdown or the creation of a noble surface film; ( b ) the alloy must exhibit a high workhardening rate. This may be associated with a co-planar dislocation array but it is also applicable to other alloys; and ( c ) there must be some specific corrosion effect, which is frequently the penetration of a passive film by a chloride ion.