The role of hydrogen and creep in intergranular stress corrosion cracking of Alloy 600 and Alloy 690 in PWR primary water environments — a review

Abstract

This chapter reviews the role of hydrogen and creep in assisting intergranular stress corrosion cracking (IGSCC) of Alloy 600 and Alloy 690 in nuclear steam generator primary side environments. The coarse-grained Alloy 600 is less susceptible to IGSCC in high temperature water as compared to fine-grained alloys. The effect of small grain size is to enhance diffusional creep processes and decrease grain boundary segregation of impurities due to a larger grain boundary area per unit volume. For small grain-sized materials, increasing hydrogen overpressure from 100 to 200 kPa increases the % of intergranular (IG) fracture by 61%, while for large grain-sized materials the % of IG fracture decreases from 13.9 to 6.1%. Hydrogen is more effective in embrittling the material when the carbide size and carbide coverage increases. Aging Alloy 690 increases the grain boundary carbide size and the extent of grain boundary M23C6 carbide coverage. Therefore, aging increases the degree of hydrogen embrittlement of the alloy. The two contributing factors explaining the fact that the increased grain size is responsible for increased hydrogen embrittlement are: increased grain size increases the local stress at the particles, which would result in a lower strain necessary to decohere the grain boundary/carbide interface, and hydrogen is preferentially trapped at the carbide/matrix interface, reducing the strength of the interface more than at the matrix/matrix interface.