Corrosion Mechanism of Nickel in Hot, Concentrated H 2 SO 4

Electrochemical techniques, complemented by weight change and ex situ X-ray spectroscopic measurements, were employed to characterize the corrosion of nickel in concentrated H 2 SO 4 solutions. By use of a rotating cylinder electrode, it was found that corrosion is a mass-transport controlled process with the convective diffusion of nickel cations from a saturated NiSO 4 layer as its rate-determining step. The oxidizing nature of the acid solution leads to the formation of additional corrosion products including metastable NiS, and elemental sulfur along with NiSO 4 , none of which is protective. When present on the surface, NiS establishes a galvanic interaction with the uncovered metal, significantly polarizing the anodic metal dissolution reaction. Since corrosion is mass-transport controlled, the resultant corrosion rate of the metal is unaffected during the galvanic-induced polarization.