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

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.