The erosion-corrosion of nickel-base diesel engine exhaust valves

Abstract There are many variables that can influence the failure of nickel-base, two-cycle diesel engine exhaust valves, including engine design characteristics and operating conditions, as well as lubricant additives and resulting deposits. The entire system must be considered to understand fully the failure mechanism. However, in this preliminary investigation the emphasis was on modeling exhaust valve failure as an erosion-corrosion wear mode. This study includes (a) the effect of lubricant additives on the formation of exhaust valve deposits and (b) the effect of valve deposit morphology and composition on the erosion-corrosion of valve seat surfaces. In order to study the potential effects of oil ash chemistry, experimental all-calcium and all-magnesium formulations were explored. Testing with a calcium-containing engine oil resulted in the formation of deposits rich in calcium sulfate (CaSO 4 ), and catastrophic failure of several exhaust valves. Although a heavier accumulation of deposits was observed on valves tested with a magnesium-containing oil, erosion-corrosion of valve seat surfaces did not occur. Deposits from the latter test contained magnesium zinc phosphate (MgZn 2 (PO 4 ) 2 ) as the principal constituent. This study provides some initial evidence that the erosion-corrosion of exhaust valves was not exclusively related to the thickness of valve seat deposits. Heat transfer and thermal expansion characteristics of deposits, as influenced by oil additive metals, may have played a role in valve failure. Spalling of calcium-sulfate-containing deposits and accelerated oxidation of the valve alloy resulted in the leakage of hot exhaust gases and subsequently, severe erosion of valve seat surfaces.