Galvanic Corrosion of Carbon Steel-Stainless Steel Welds
Abstract
A water leak in one of the Canada Deuterium Uranium (CANDU) nuclear reactors could lead to galvanic corrosion between the materials of its supporting structures: carbon steel (CS) and stainless steel (SS). This project investigates the effects of physical and chemical solution parameters on the corrosion of galvanically coupled dissimilar CS-SS welds, with the aim of developing a corrosion dynamics model that can be used to assess the long-term integrity of the CANDU reactor structural materials with confidence. The studied parameters were solution pH, temperature, the presence or absence of γ-radiation and the cathode:anode surface area ratio. Multiple electrochemical techniques were used to measure the corrosion rates of these steels, including the novel Dual-Electrochemical Cell method. These were augmented with post-test surface and solution analyses to study the oxides formed on the corroded surfaces and to determine the amount of dissolved metal ions in the solution.
This study demonstrated that CS and SS corrosion involve many elementary steps that lead to the dissolution of metal ions as well as the formation and growth of different oxides. Non-linear dynamic behaviours can develop due to the strong coupling between different elementary processes. Therefore, simple linear-dynamic rate models for the corrosion of CS or a CS-SS couple can result in erroneous evaluations and inaccurate predictions of their long-term performance.
The results also showed that the effects of galvanic coupling to SS, higher temperature, higher cathode:anode surface area ratio or gamma-irradiation on the corrosion progression of CS have a strong time-dependence. Initially, they increase the corrosion rate of CS. However, these factors also lead to faster oxide formation and growth on CS, suppressing subsequent Fe dissolution over longer time periods. As a result, the increase in the CS corrosion rate due to galvanic coupling diminishes with time.