The Effect of Water Contamination on the Thermal Oxidation Stability of Complex Calcium Greases Thickened with Overbased Calcium Sulfonate

Sulfonate greases, which have excellent performance characteristics—high dropping point, good mechanical and structural stability, water resistance, high thermal oxidation stability, and good anticorrosive properties—are widely used in various industries. The greases are exposed to water during operation: moisture in the environment, water-based coolants, operation in the presence of water vapor, etc. Water can affect the properties of the greases during operation. The subject of the study was a commercial complex grease thickened with overbased calcium sulfonate, with a water additive in amounts ranging from 1% to 50% by weight. This paper presents two standardized methods for testing the thermal oxidation stability of lubricants—according to ASTM D942 and ASTM D8206, in standard programs, and with an extension of oxidation duration to 100 h and a temperature increase to 100 °C. The aim of the study was to investigate how the addition of water affected the thermal oxidation stability of this grease. The presentation concludes with an analysis of FTIR differential spectra. The tests showed that as the water content in the grease samples increased, its resistance to oxidation decreased. Water also caused a change in the consistency of the grease at a concentration of just 1% by weight. Mechanical stress affected the thermal oxidation stability of the grease tested. Each method presented separate mechanisms of oxidation initiation, including different sample quantities during the test, the presence of water in the classic method, and different contact with oxygen as a catalyst for this reaction. The work provided a comprehensive presentation of the possibilities for testing the thermal oxidation resistance of greases and a detailed comparison of the two methods.