Independent thesis Advanced level (professional degree), 20 credits / 30 HE credits
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.