Sulfidation Corrosion of Nickel-Base Exhaust Valves

Severe corrosion was observed in the neck area of Inconel 751 material exhaust valves operated in an ebulient-cooled natural gas engine. The type of corrosion was identified as sulfidation attack. Deposits removed from the valves consisted mainly of calcium sulfate (CaSO₄) from oil additives and carbon (C) residue from combustion products. Experimentation showed that the presence of C was a necessary pre-requisite for attack. Based on this information, the corrosion mechanism must involve the reduction of the sulfate by C. The reduction of the sulfate results in a high sulfur chemical potential at the alloy surface making transport of sulfur into the substrate a highly favored step.

Aluminizing the Inconel 751 material proved to be an effective deterrent against sulfidation attack.