From observations of the characteristics of nitrate and hydroxide solutions, known to promote stress corrosion cracking (SCC) in carbon steels, and from the form of potentiodynamic polarization curves and the structural dependence of the corrosive attack, it was predicted that carbonate solutions would also produce intergranular stress corrosion in carbon steels. Constant strain rate stress corrosion tests, with some supplementary constant strain and constant load tests, have shown that intergranular cracking can be made to occur in certain ranges of electrode potential in carbonate solutions over a wide range of concentrations and temperatures with NH4, Na, or K as the cation. The range of potentials for cracking, which varies with solution composition and temperature, is shown to coincide with that range in which polarization curves obtained at different sweep rates indicated marked anodic activity and strong passivating tendencies. At more negative potentials than those that promote intergranular cracking, superficial transgranular fissuring is first detected and then, as the potential is moved toward even more negative values, a progressive loss in ductility is observed due to hydrogen entry into the steel.

Annealed C-Mn steel has been shown to fail by transgranular stress corrosion cracking (SCC) in sodium phosphate solutions over a range of potentials and pH's at room temperature. The observed SCC domain is reasonably accurately predicted from fast and slow sweep rate polarization curves. Moreover, the cracking domain boundaries can be calculated with reasonable precision from the thermodynamic properties of the reactants and products involved in the reactions responsible for the various modes of failure at different potentials and pH's.

A study was carried out to understand mechanisms of stress corrosion crack initiation in an X-65 pipeline steel exposed to a near-neutral pH soil environment under a mechanical loading condition typical of a pipeline operating in the field. Microcracks initiated on the polished surface of the X-65 pipeline steel after long-term exposure at open-circuit potential in a near-neutral pH synthetic soil solution. It was found that these microcracks were initiated mostly from pits at metallurgical discontinuities such as grain boundaries, pearlitic colonies, and banded phases in the steel. Strong preferential dissolution was observed along planes of the banded structures in the steel. Selective corrosion at these metallurgical discontinuities is attributed to the anodic nature of those areas relative to the neighboring steel surface. Consistent with previous observations, no increased susceptibility to crack initiation was found at physical discontinuities mechanically introduced into the surface of steel exposed to synthetic soil solution at open-circuit potential.

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