Inverse Crevice Corrosion of Carbon Steel: Effect of Solution Volume to Surface Area

Abstract

Crevice corrosion of carbon steel was investigated in different exposure environments by performing coupon exposure and electrochemical tests. The extent of corrosion on the bold surface of a carbon steel crevice coupon was more severe at 80 • C than at 21 • C, in aerated rather than dearated solutions, and with γ-radiation present. In contrast to normal crevice corrosion, we observed 'inverse crevice corrosion' behavior, the phenomenon where it is the corrosion on the bold surface that is accelerated when coupled, rather than that on the crevice surface. The coupling current measured between a crevice and a bold electrode in an electrochemical cell was also negative. This inverse crevice corrosion behavior is attributed to a significantly lower metal cation dissolution capacity of the small occluded water volume in the crevice, compared to that of the bulk water volume over the bold surface. The reduction in dissolution capacity results in faster and earlier formation of a protective oxide layer. Corrosion of the bold and crevice surfaces evolves at different rates, leading to galvanically accelerated corrosion of the bold surface. The effect of γ-radiation on corrosion evolution in different solution environments leading to inverse crevice corrosion is discussed. Many countries, including Canada, are exploring long-term disposal of used nuclear fuel in a deep geological repository (DGR) using a multiple-barrier system, with a key barrier being the used fuel container (UFC).1-7 Certain UFC designs considered for disposal in a DGR consist of an inner vessel made of carbon steel (CS) or cast iron for structural strength, with an outer Cu shell or coating as an external corrosion barrier (e.g. the Swedish and Canadian UFC designs).3,8 The current Canadian UFC design consists of a pressure-vessel-grade CS pipe pre-coated with Cu. The vessel would be sealed by laser welding on site, in air, by attaching a pre-Cu-coated hemispherical CS head at one end. This would be followed by applying a Cu coating over the welded regions.9,10 A concern regarding the structural integrity of the inner CS vessels of this UFC design is localized corrosion.11 Moisture trapped inside a UFC could condense on the stressed regions near the welds and, for the Canadian design, within the gap between the hemispherical head and the body. This could lead to localized corrosion, a potential failure mechanism of the container.The environment inside the UFC will be initially humid and it will include a flux of ionizing radiation (and particularly γ-radiation), emitted from the decay of radionuclides in the used fuel. For the current Canadian UFC design the dose rate at the inner surface of a UFC is anticipated to be less than 100 Gy·h -1 (1 Gy = 1 J·kg -1 ). The dose rate is calculated to be 51 Gy·h -1 for 10-year old fuel and the dose rate will steadily decreases to 4.9 Gy·h -1 after the used fuel ages for 100 years. 12Interaction of matter with high energy photons or particles usually results in the absorbed energy being dissipated predominantly...

A New Method for Corrosion Current Measurement: the Dual-Electrochemical Cell (DEC)

“…26 Multiple electron transfer steps are more common for metal redox halfreactions because transition metal cations have more than one stable oxidation state in solution. [27][28][29][30][31][32][33] Thus, depending on E corr and mass transport conditions, polarizing E elec away from E corr can change the rate determining step, resulting in a current-potential relationship very different from that at E . corr The perturbation of E elec away from E corr and the uncertainties in the analysis of the observed dependence of i meas on E elec can result in incorrect determination of i ox at E corr (i.e., i corr ), consequently leading to the development of an inaccurate corrosion model.…”

Chemistry conditions and corrosion behaviour in simulated crevices of alloy steel

“…Such environments include water with strongly alkaline pHs and solutions containing certain anions such as carbonates/bicarbonates, chromates and halides. Similar anoxic crevice conditions are also expected to be developed during the long term storage of high-level radioactive waste in the deep repository disposal site boreholes lined with metallic canisters of stainless steel or low alloy carbon steel) embedded in clay [22 24].…”

Investigation of the evolution of an iron carbonate layer and its effect on localized corrosion of X65 carbon steel in CO2 corrosion environments

“…Crevice corrosion of carbon and low-alloy steels has been discussed by several researchers. Wu et al [15] observed higher rates of corrosion on a carbon steel surface exposed to a bulk electrolyte compared to a surface within a crevice. The term inverse crevice corrosion was introduced due to more aggressive corrosion being observed on a bare sample after galvanically coupling with a crevice sample.…”

“…The term inverse crevice corrosion was introduced due to more aggressive corrosion being observed on a bare sample after galvanically coupling with a crevice sample. Wu et al [15] proposed that carbon steel corrosion in a crevice involves many oxidation steps that lead to metal dissolution and formation of different oxides. Therefore, the solution volume to surface area ratio and solution chemistry have a significant effect on crevice corrosion.…”