Effects of Pb on SCC of Alloy 600 and Alloy 690 in Prototypical Steam Generator Chemistries

Abstract

Intergranular attack/stress corrosion cracking of Alloy 600 continues to be an issue in the tube/tube support plate crevices and top of tubesheet locations of recirculating steam generators and in the upper bundle of free span superheated regions of once through steam generators (OTSG). Recent examinations of degraded pulled tubes from several plants suggest possible lead involvement in the degradation. Laboratory investigations have been performed to determine the factors influencing lead cracking in Alloy 600 and Alloy 690 steam generator tubes. The test environment is believed to be prototypical, with the addition of lead oxide, of a concentrated liquid phase existing in the pores of thin deposits on upper bundle tubes of an OTSG. Highly strained reverse U-bend specimens were tested at controlled electrochemical potentials. Maximum susceptibility was at open circuit potential, unlike cracking of Alloy 600 in caustic and acid sulfate environments where maximum susceptibility occurs when specimens are polarized above the open circuit potential. Transgranular, intergranular and mixed mode cracking was observed and in all Alloy 600 conditions tested (mill annealed, sensitized, thermally treated) while thermally treated Alloy 690 has so far resisted cracking. A film rupture/anodic dissolution model with displacement plating of Pb preceding passive film formation is consistent with the experimental observations

Investigation of passive films on nickel Alloy 690 in lead-containing environments

“…It has been reported that the presence of lead contamination in the feedwater on the secondary side can increase the SCC susceptibility of the SG tubing materials, especially nickel-based alloys [1,2]. In order to improve lead-induced SCC (PbSCC) resistance, nickel Alloy 690 has been developed to replace Alloy 600 as SG tubing materials by means of optimizing the alloy composition.…”

“…This is to some extent contradictive to the common knowledge that stainless steels and nickel alloys are the best choices in alkaline aggressive conditions. Several PbSCC mechanisms, such as liquid metal embrittlement [1], selective grain boundary oxidation [2] and hydrogen embrittlement [5], have been proposed, but none of them gives a completely interpretation to the PbSCC phenomena. In recent decades, experimental evidences revealed that the susceptibility of PbSCC is related to the instability of passive films [1,6,7].…”