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 H2SO4 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 NiSO4 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 NiSO4 , 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. © 2000 The Electrochemical Society. All rights reserved.