The corrosive wear of cast iron in oil-sulphuric acid mixtures

Abstract

A study has been undertaken into the corrosive wear of marine diesel engine piston ring cast iron pin specimens against cylinder lining grey cast iron disc specimens during reciprocating sliding in mixtures of base lubricating oil and 10% sulphuric acid solution. Wear is low in the base-oil environment but increases progressively with increasing additions of acid solution. The presence of 5% or 10% acid in the oil is sufficient to disrupt partially the hydrodynamic oil film, giving significant periods of metal-metal contact. The dispersed nature of the acid droplets in the oil restricts corrosion to localized sites on the metal surfaces and wear damage is mainly due to mechanical processes. As the volume of acid in oil is increased to 20% and 40%, the hydrodynamic oil film is disrupted almost completely and the aqueous phase becomes almost continuous. This increases the contribution of electrochemical corrosion to the corrosive wear phenomenon as the products of corrosion are removed rapidly from the contacting surfaces by the sliding action. In this situation, both corrosive and mechanical processes contribute to the overall wear damage.