Sulfur Compounds
Table of Contents
Physical & Chemical Properties

Sulfur compounds in lubricating oils and their chemical activity are directly related to anti-wear and anti-scuff properties. Elemental sulfur was a historical additive used in a gear box to reduce oil temperature. Sulfur content is useful in understanding boundary lubrication performance. The sulfur compounds may be naturally occurring in the base oil, or added as additives. 

Low sulfur content would explain poor boundary lubrication performance. Very high sulfur content would explain corrosion problems where the corrosion product is found to be a metal sulfide, or where contamination by hydrogen sulfide was found. Iron sulfide films are frequently identified on undamaged ferrous surfaces in industrial equipment. The amount of sulfur in oil is reported as percent or ppm total sulfur. Therefore, one must consider the several sources of sulfur, such as in the base oil naturally, additives, ZDDP, and organo sulfur compounds such as sulfurized olefins.

The source of sulfur can be narrowed down by analyzing for the stoichiometric amounts of associated elements. Examples are: analysis of zinc, sulfur and phosphorous for the ZDDP additive, sulfur and phosphorous for a S/P gear oil anti-scuff additive, or molybdenum and sulfur for a black oil possibly containing molybdenum disulfide. Usually, lubricating oil suppliers provide only physical properties and performance data, but little or no additive chemistry or elemental analysis. If the additive chemistry is of interest to a user, a laboratory might perform a series of tests for identification.

For example, if analysis showed the presence of sulfur, and ES analysis showed zinc and phosphorous and infrared analysis showed peak characteristic of ZDDP, then the presence of the ZDDP additive would be indicated.

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