Primary water stress corrosion cracking (PWSCC) mechanism based on ordering reaction in Alloy 600

Abstract

A PWSCC mechanism based on an ordering reaction in Alloy 600 is proposed. An activation energy for the ordering reaction in Alloy 600, Q = ∼46 kcal/mole (∼190 kJ/mole), are determined by a differential scanning calorimeter (DSC). The ordering reaction in Alloy 600 is an indispensable process during reactor operating conditions. The ordering reaction in Alloy 600 causes an anisotropic lattice contraction. This anisotropic contraction produces an additional stress. The stress level would be the maximum value about 70 and 300 MPa according to the lattice planes in Alloy 600 and Weld 182, respectively. In addition, the anisotropic contraction forms the micro cracks in the high angle grain boundary where the difference in lattice contraction is large. The formation of crack induces stress intensification at the crack tip, and this causes crack growth. The initiation and propagation of PWSCC is controlled by the formation, growth, and coalescence of micro cracks due to anisotropic lattice contraction by ordering. These whole processes are governed by the kinetics of the ordering reaction. This is the reason why the activation energy for PWSCC, Q pwscc = 40–50 kcal/mol, is consistent with that for the ordering reaction, Q ordering = 46 kcal/mol. This mechanism can be proved by the comparison of the initiation behavior in the ordered and the disordered specimens.