Sulfidation Corrosion of Nickel-Base Exhaust Valves 750044
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.