In the present work, the environmental conditions under which localized corrosion of carbon steel could take place were examined. Using creviced specimens, it was determined that at potentials above the repassivation potential the ratio of the chloride concentration to the carbonate concentration and the solution temperature played critical roles in establishing the conditions that promote localized corrosion as well as influencing the rate of propagation. The pH was also found to be key, with localized corrosion possible at pH values> 9.6. Preoxidation in air at 200 Cfor short times was found to improve resistance to localized corrosion, but at longer times was found to be detrimental. Examination of the open circuit potential of creviced samples in air-saturated, pH 11solutions revealed that the repassivation potential can be exceeded and localized corrosion can occur under free corrosion conditions.
The safety strategy adopted by the U.S. Department of Energy (DOE) for the proposed high-level nuclear waste (HLW) repository at Yucca Mountain (YM) relies on four key attributes for the unsaturated repository system, one of which is long waste package (VW)lifetimes?. The DOE design of Wps continues to evolve, but for the viability assessmentanA516 carbon steel outer over pack and Ni-base alloy [Alloy C-22a(UNSN06022)] inner over pack are being considered. The principal desired attribute of carbon steel is its expected behavior a satypical corrosion allowance material undergoing relatively predictable, uniform corrosion. Under some environmental conditions, however, carbon steel mayform a protective surface film. Depending on the environmental conditions, this protective surface film may subsequently breakdown and carbon steel may exhibit localized predicted from general corrosion.
The factors influencing the performance of carbon steel in the repository environment have been 3-5Even though a number of investigations have been conducted examining the reviewed in previous works . localized corrosion behavior of carbon steels, one of the principal uncertainties with respect to the performance of carbon steel in the proposed repository is the sensitivity of the aqueous corrosion mode (general corrosion vs. localized corrosion) to the environmental conditions (pH, Cl- concentration, temperature, redox potential). Because localized corrosion rates are typically more rapid than general corrosion and can in many cases lead to catastrophic failure, it is critically important to map out the conditions under which localized corrosion takes place.
For localized corrosion, the commonly measured pit initiation potential, ,is the potential above which stable pit growth is detected. Even though pits may be initiated well below this potential, spontaneous repassivation occurs and the pits fail to continue propagating, thus leading to their classification as metastable pits. Metastable pitting, though of fundamental importance to the understanding of localized corrosion dynamics, is generally not important for long-term life prediction as these pits cannot lead to perforation of the structure. The potential to repassivate stably growing pits (EJ is approximately equal to the pit initiation potential, provided the former is measured for deep pits under slow backward scan rates and the latter is measured under extremely long exposure conditions. Further, the utility of using the repassivation potential as a critical parameter for enabling pitting, crevice corrosion and stress corrosion cracking has been demonstrated by Akashi and co-workers for carbon steelsg-10,and has been shown to be effective for other materials as we113?7?11.
Thus, the objective of this work
