Environmental effects in the stress corrosion cracking of turbine disc steels

The effects of environmental parameters, such as the impurity concentration, oxygen content and pH value on the stress corrosion cracking (SCC) of 3.5Ni-Cr-Mo-V and 2Cr-Ni-Mo steels were studied. SCC of these two alloys only occurs in steams containing both oxygen and contaminating ions. A concentration of 40 ppb of Cl⁻ or around 5 ppm of SO₄²⁻ in steam is found to be enough to induce SCC on both alloys when oxygen exists in the steam. Sulfate ions suppress the severity of chloride ions in causing SCC of both materials in a Cl⁻ and SO₄²⁻ containing steam. This effect was proposed to result from the iron sulfate salt deposition. SCC of these steels in steam is accomplished by corrosion of active path, while the reaction rate is determined by the rate of cathodic reaction of the SCC system. Unless the pH value is 3 or less, the change in pH of de-aerated environments induces no SCC on both materials. SCC in the pH 3 environment is a result of the change of the cathodic reaction from water decomposition to proton reduction in this SCC system.