When 9:00 AM - 11:00 AM Nov 11, 2014
Where 2906 Cooley, Baer Room
Add event to calendar vCal
iCal

Stress Corrosion Crack Initiation of Alloy 600 and Alloy 690 in Hydrogenated Supercritical Water


Tyler Moss
Thesis Defense

Gary Was, Advisor



Stress corrosion crack (SCC) initiation of highly resistant materials can be studied by conducting accelerated testing as long as there is no change in the cracking mechanism. The objective of this dissertation is to determine if accelerated SCC initiation testing of Alloy 600 and Alloy 690 can be conducted without changing the mechanism of crack initiation between subcritical and supercritical water. Unfortunately, the mechanism of crack initiation of these alloys is not known. This makes demonstrating whether there is a change in the SCC mechanism dependent on determining if there is a change in the oxidation, stress corrosion crack initiation morphology, and the temperature dependence of crack initiation between subcritical and supercritical water.


The corrosion environment was maintained at a fixed electrochemical potential above the Ni/NiO phase transition in the NiO stable regime by controlling the dissolved hydrogen concentration, with the location of the boundary determined by exposures of pure nickel. Exposures of unstressed corrosion coupons of Alloy 600 and 690 were conducted in hydrogenated subcritical and supercritical water for characterization of the oxide morphology, structure, and composition. Tensile bars of Alloy 600 and Alloy 690 were strained in constant extension rate tensile experiments in both environments to characterize the crack initiation morphology and to determine the temperature dependence of crack initiation.


The oxidation for both alloys was consistent between subcritical and supercritical water, composed of a multi-layer oxide structure composed of particles of NiO and NiFe2O4 formed by precipitation on the outer surface and a chromium rich inner oxide layer formed by diffusion of oxygen to the metal-oxide interface. The crack initiation morphology of Alloy 690 was consistent between subcritical and supercritical water, and a mechanism of crack initiation was developed. The SCC initiation temperature dependence of both alloys shows no discontinuity or change in slope in the activation energy at the critical point. All available evidence supports a consistent mechanism of stress corrosion crack initiation in both hydrogenated subcritical and supercritical water for Alloy 600 and Alloy 690.