Physical and Chemical Impact of Sulphuric Acid on Cylinder Lubrication for Large 2-Stroke Marine Diesel Engines

The influence of sulphuric acid, H 2 SO 4 , on the physical and chemical behaviour of marine Diesel engine lubricant base oils was investigated. To understand the basic interaction of H 2 SO 4 with the lubricant film, the saturated hydrocarbon squalane, C 30 H 62 , was chosen as a simple model oil in addition to a fully formulated lubricant and its corresponding API Group I base oil HVI160B. To understand droplet formation in the lubricant, the interfacial tension between aqueous H 2 SO 4 (0–98 %w/w) and C 30 H 62 measured in a previous study (Sautermeister et al., Tribology International, 38th Leeds-Lyon Symposium on Tribology, 2011 ) and is compared with three different API Group I base oils. Interfacial tension decreases with increasing acid concentration, but is much lower for the base oil. Both oils were emulsified with a constant volume fraction of aqueous H 2 SO 4 . The droplet size was smaller and more distinct for the base oil under the same shearing conditions and the emulsions with base oil were are more stable. The concentration of the dispersed phase had no significant influence on the viscosity of the emulsions compared to the viscosity of the bulk oil alone. In addition to the physical measurements, chemical reaction between H 2 SO 4 and the API Group I base oil HVI160B was observed yielding bituminous matter which appears to be a complex emulsion. The critical acid concentration for the formation of bitumen was found to be 40 %w/w. Between 50 and 80 %w/w, bitumen was formed spontaneously at 85 °C and above 85 %w/w, bitumen was formed spontaneously at room temperature. The observed concentrations for bitumen formation correlate well with the first and second dissociation steps of H 2 SO 4 . HSO 4 − ions are likely to be the driving force for bitumen formation. Finally, the viscosity characteristics of two different bitumen were measured. Both were found to exhibit shear thinning and the one formed with highly concentrated H 2 SO 4 created a solid deposit on the measurement equipment above 120 °C.