Grain size and other structural features are known to influence the mechanical properties of mild steels, and it is shown that their effects in stress-corrosion crack propagation are essentially the same as their effects upon plastic-flow characteristics. Preferential corrosion in the grain-boundary regions of mild steel immersed in a boiling nitrate solution is considered to result from the presence of boundary carbide particles or interstitial segregates acting as points of intense cathodic activity. This boundary region penetration may be sustained by anodic polarisation or by the imposition of tensile stress, the latter resulting In the familiar stress-corrosion cracking.
