Hot Corrosion Studies of Automotive Exhaust Valves

This study compares the hot corrosion resistance of several internal combustion engine exhaust valve alloys in a simulated diesel engine environment. Hot corrosion of exhaust valves occurs when deposits accumulate on the head and are subjected to temperatures between 650 to 1000°C. Test specimens were subjected to a mixture of CaSO₄, BaSO₄, Na₂SO₄, and carbon at 870°C. The materials tested included head material specimens and valves with cobalt based or nickel based seat facings. The depth of corrosion penetration was measured and the valve base materials and seat face coatings were ranked accordingly. The results indicate that nickel based valve alloys have poorer sulfidation resistance than iron based alloys, and nickel based seat facings accelerate the corrosive attack on the base material in the faced region as compared to the unfaced portion of the valve. Valves with cobalt base Tribaloy seat facings exhibited the best resistance, followed by the iron base, Stellite and nickel base respectively.