A sensitive technique for evaluating susceptibility to IGSCC of alloy 600 in high temperature water

Abstract

In laboratory tests conducted to simulate conditions leading to intergranular stress corrosion cracking (IGSCC) of Alloy 600 steam generators tubings in pressurized water reactors (PWR), both the slow strain rate (SSR) technique and the reverse U-bend (RUB) technique are mostly used. The former technique is more accurate, permitting, for example, application of a constant potential on the test specimen and measurement of the elongation to fracture but is much more time consuming than the latter technique. Under conditions of uniaxial tensile stress a strain rate of 1.0x10/sup -7/s/sup -1/ is required to obtain a noticeable area of IGSCC for Alloy 600 tube specimens on exposure to pure water at 330/sup 0/C, and heats of alloy 600 which show susceptibility to IGSCC in RUB experiments do not crack in SSR experiments even at a strain rate of 1.0x10/sup -8/s/sup -1/, suggesting that a complex state of stress (biaxial or triaxial) is necessary to induce IGSCC of Alloy 600. In the service of PWR steam generators, a definite tube cracking preference is experienced at sites where high residual stresses exist. These stresses result from fabrication, bending and tube installation. Cracking mostly occurs at points of the highest stresses and strains. The purposemore » of the present work is to compare the practical utility of both the above techniques and to propose an accelerated, sensitive technique enabling accurate evaluation of the IGSCC susceptibility and life expectancy of Alloy 600 tubing in high temperature water with and without hydrogen gas. Hydrogen is commonly used as an addition to the primary environment of PWRs to scavange oxygen which is produced by the radiolytic decomposition of water. However, it has been found recently that H/sub 2/ promotes IGSCC of Alloy 600 in high temperature water.« less