Investigation of Galvanic Corrosion between Graphite Gaskets and Stainless Steel Flanges

At Forsmark, several cases of suspected galvanic corrosion have been detected and believed to be caused by an inappropriate use of graphite gaskets in brackish water. This report studies the likelihood of galvanic corrosion between the graphite gasket and stainless steels and aims to be a reference when investigating galvanic corrosion in the future. To evaluate galvanic corrosion, several electrochemical experiments were studied, such as galvanic series, polarization curves, and galvanic current measurements with a ZRA. The electrochemical results showed that when immersing the stainless steel (254 SMO) and the graphite gasket (Novatec) into brackish water, there was a current flowing between the electrodes. However, the current was very small, which could be explained by the protection from a passive film on the stainless steel. The difference in corrosion potential between the two materials was measured and compared to results from polarization curves. It appeared that 254 SMO would be protected by a passive film at the measured potential. Galvanic current measurements also indicated the presence of a passive film under the investigated conditions. The results indicate that galvanic corrosion itself should not have caused the corrosion attacks at Forsmark. The found corrosion is more likely due to crevice corrosion that was accelerated by the combination of a crevice, a welded flange surface, chlorides in water and possibly a graphite gasket