Corrosion-creep interaction of stainless alloys in acid chloride solutions
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I. INTRODUCTION
ALLOY 22 (UNS N06022), a Ni-Cr-Mo-W based alloy, is a candidate material for fabrication of nuclear waste package (WP) containers with an expected life of 10,000 years. In order to ensure the long-term integrity of the WP containers, environmental degradation mechanisms need to be evaluated and understood. Stress corrosion cracking (SCC) is one of the possible modes of failure of the containers exposed to mineral carrying water that could be dripping through the rock faults of a nuclear repository. Very low SCC growth rates, of the order of 1013 m/s, have been reported for alloy 22 in simulated-concentrated repository water environments[1] using precracked specimens. Cyclic events of rupture of passive film and repassivation have been identified as the essential steps in crack propagation. Andresen and Ford’s model on SCC crack growth[2] considers the rupture of passive film to occur due to the accumulation of creep strain. While investigating SCC behavior of austenitic and ferritic stainless steels in 0.83 M H2SO4 and 0.83 M HCl solutions at 80 °C, Nishimura and coworkers observed a correlation between steady-state creep rate and SCC failure time.[3–6] The deformation of specimens exposed to corrosive environments revealed three regions of creep similar to those observed at high temperature. The three regions observed were (1) primary creep region (controlled by applied stress and considered as the crack incubation period), (2) secondary creep region (which showed a steady-state elongation correlated to the crack induction process), and (3) tertiary creep region (which occurred due to crack propagation K.S. RAJA and S.A. NAMJOSHI, Research Assistant Professors, are with the Department of Metallurgical and Materials Engineering, University of Nevada, Reno, NV 89557. Contact e-mail: [email protected] D.A. JONES, formerly Research Professor, Department of Metallurgical and Materials Engineering, University of Nevada, is deceased. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
and reduction in the net cross section leading to fracture). The SCC failures on a laboratory scale required a minimum steady-state creep rate of 1010 m/s. Many other researchers also have observed the correlation between creep and SCC. Was and co-workers[7,8,9] have also considered creep as a predominant factor controlling SCC of alloy 600 components in pressurized water reactor environments. An increased steadystate creep rate was observed in water compared to an argon (inert) environment at 360 °C.[8] The effect of impressed anodic and cathodic currents on the creep behavior of thin copper wire in deaerated acetate buffer solutions was studied by Revie and Uhlig.[10] In the absence of corrosion (as in the case of ambient a
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