ENVIRONMENTALLY INDUCED FAILURE OF A NICKEL BASED BALL VALVE PLATING AND MODELING THE CORROSION EFFECTS ON PURE NICKEL OF H_2S, CHLORIDES, pH AND PRESSURE

摘要

A failure of a nickel alloy plating of a sour gas ball valve revealed sulfide-and chloride induced local pitting corrosion. With the background of the prevailing field operating conditions, modeling of local corrosion was carried out for pure nickel by coupling of phase precipitation and polarization behavior. The effects of bulk composition on ion migration to the diffusion layer, its sulfide and chloride precipitations affecting anodic polarization are integrated in a time stepwise procedure. As a main parameter the reduction in bulk pH accelerates local corrosion as well as local acidification in pits. The effects of bulk chloride and hydrogen sulfide contents on corrosion rates varies with the respective ranges of those constituents similar to the behavior of pure iron in earlier work. Depending on the hydrogen sulfide to chloride bulk ratio the local environment may either acidify to low pH levels induced by protecting sulfide precipitations leading to reduced corrosion rates or stay at higher pH with precipitation of non protecting chlorides involving higher corrosion rates. Reduced total pressures within the relevant ranges increase the corrosion rates due to acceleration of the cathodic reaction. 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.