Reaction of Sulfuric Acid in Lube Oil: Implications for Large Two-Stroke Diesel Engines

Abstract

Slow-steaming operation and an increased pressure in the combustion chamber have contributed to increased sulfuric acid (H2SO4) condensation on the cylinder liners in large two-stroke marine diesel engines, thus causing increased corrosion wear. To cope with this, lube oils are formulated with overbased detergent additives present as CaCO3 reverse micelles to neutralize the condensing H2SO4. In this present work, a mixed flow reactor (MFR) setup aims to investigate the neutralization reaction by varying Ca/S molar ratio, stirrer speed, H2SO4 inlet concentration, and residence time. Lube oil samples from the outlet of the MFR were analysed by use of Fourier Transform Infrared Spectroscopy (FTIR) and a titration method. The MFR results indicate that the CaCO3-H2SO4 reaction is very fast in a real engine, if the cylinder liner is well-wetted, the oil-film is well-mixed, and contains excess of CaCO3 compared to the condensed H2SO4. The observed corrosion wear in large two-stroke marine diesel engines could consequently be attributed to local molar excess of H2SO4 compared to CaCO3 reverse micelles on the cylinder liners.