Environment Induced Cracking of Turbine Disc Steel: Effect of Organic Acids on Corrosion Fatigue of 3.5NiCrMoV Steels in High Temperature Water

Abstract

Organic acids (acetic and formic acid) are produced by the decomposition of ETA (Ethanol Amine, C2H7NO) used as pH controller of secondary water in nuclear power plants. Corrosion Fatigue (CF) tests (R=0.2, 0.1Hz) were conducted to evaluate the effect of acetic acid on the CF crack growth rate in high temperature water at 150°C. Acetic acid significantly influenced the environmental cracking behavior of turbine disc steels in high temperature water. The CF crack growth rates of turbine disc steels increase as the organic acid concentration increase to a critical saturation pH value (~pH 4). Beyond the saturation value of pH, the CF crack growth rates decrease significantly. The higher CF crack growth rate of the higher pH solution in water of intermediate content range (pH 4~pH 5) of acetic acid is due to the higher content of H+ enhancing the reduction reactions. Crack tip blunting prevents the CF cracks from growing with increasing rate in the solution of organic acid concentrations beyond the critical value.