Mechanisms of Environmental Cracking in Systems Peculiar to the Power Generation Industry

摘要

Environmentally-controlled cracking (i.e. stress corrosion, corrosion-fatigue, hydrogen-embrittlement) continues to be a significant issue to the power generation industry. In this investi¬gation the mechanisms of cracking have been quantified in an attempt to provide a sounder plan¬ning basis for other design-oriented programs and to indicate, in a more efficient manner, possible causes of, and remedies against, environmentally-controlled cracking. Attention has been focused primarily on ductile alloys in aqueous environments, with major attention to the stainless steel/water and turbine disc steel/caustic systems.nThe slip-dissolution mechanism of subcritical crack propagation has been shown to be valid quanti¬tatively at temperatures < 100℃ for sensitized 304 in dilute sulphate solutions and for a 3CrMo turbine disc steel in a variety of neutral and alkaline environments. The model also explains quali¬tatively the cracking response of stainless steel and carbon steel at temperatures up to 288℃. The rate determining steps in this propagation mechanism have been identified by a transient electro¬chemical technique which simulates the conditions at the crack tip by rapidly straining a wire elec¬trode under potentiostatic control. These rate-controlling reactions are the bare surface dissolution rate, passivation rate, oxide rupture rate, and liquid diffusion rates in the crack tip region.