Stress Corrosion Cracking of Carbon Steel in Carbonate Solutions

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.