Environmentally Induced Failure of a Nickel Based Valve Plating and Modeling the Corrosion Effects on Pure Nickel of H2S, Chlorides, pH and Pressure

Environmentally induced failure of a nickel based plate and modeling the corrosion effects on pure nickel of H2S, chlorides ,pH and pressure

Hans Hoffmeister

Institute for Failure Analysis and Failure Prevention ISSV e.V.

c/o Helmut Schmidt University of the Federal Armed Forces

D-22039 Hamburg , Germany

A failure of a nickel alloy plating of a sour gas ball valve revealed sulphide-and chloride induced local pitting corrosion. Based on earlier work, modelling was carried out for pure nickel by coupling of phase and polarization behavior.. The effects of bulk composition on ion migration to the diffusion layer, its sulphide and chloride precipitations affecting anodic polarization are integrated in a time stepwise procedure. As a result an increase in bulk chloride contents, pH2S and reduction in bulk pH accelerate local anodic acidification and corrosion rates. At constant volume contents of H2S increasing total pressure enhance corrosion of nickel Some parameters are identified without significant anodic acidification and corrosion. Closer understanding is provided of the corrosion mechanism based on thermodynamic heterogeneous equilibrium considerations applied to the small reaction volume of the thin diffusion layer.

Keywords: H2S-chloride corrosion modelling, nickel sulphide and nickel chloride layers,polarization coupling principle, anodic acidification