Long-Term Sour Corrosion of Carbon Steel in Anoxic Conditions

Coatings and cathodic protection of pipelines can fail leading to exposure of underlying steel substrate to environmental conditions. In Canada, these conditions are typically alkaline and can contain dissolved sulfides. Sulfides are known to accelerate corrosion of steel, however, the corrosion behavior may differ if a preformed oxide or oxyhydroxide is present on the steel surface. This study investigates the change in corrosion behavior of steel in anoxic alkaline conditions that is directly exposed to sulfide or has a preformed surface oxide. The electrochemical response (corrosion potential and polarization resistance) were monitored for 90 d and 120 d, while simultaneously monitoring the film composition and morphology by Raman spectroscopy and scanning electron microscopy, respectively. In the absence of a preformed oxide the mackinawite film was able to undergo anoxic aging to greigite and pyrite whereas the electrodes with a preformed film had only mackinawite present on the surface. The interconversion of mackinawite to greigite and pyrite lead to a higher relative polarization resistance than the electrode with the preformed oxide.