In the engine chamber by-products formed during combustion by burning diesel fuel, water and sulphur oxides like SO2 and SO3, who readily combine to form sulphur based acids. Thus as engine oils continue to be used for elogated periods, combustion of gases result which combine with the oil, with the result that they form acids which are corrosive in nature and affect the surfaces with which they come in contact. Most of these corrosive acids are removed during combustion process as part of the engines exhaust, but a few remain and escape into the cavity of the engine who proceed to attack the oil which provides the lubrication around piston rings and seals. They also react with the water formed during combustion to produce powerful inorganic acids such as sulphuric acid (H2SO4).
High Sulfation is responsible for the degradation of the lubricant which causes numerous problems like :
- Increase in viscosity
- Varnish, sludge and sedimentation formation
- Base oil breakdown
- Additive depletion and depletion of basic reserve
- Loss of lubricity
- Decrease in Total Base Number (TBN)
The amount of sulfation is the measure of sulphate index (SOx) measured by infared analysis which is the amount of sulphur based acids which have been absorbed and reacted with the oil.
The co-relation between high Sulphation and high Total Base Number (TBN) can be easily understood as such, Sulphation reacts with the oil and the reaction by-products like sulphuric acid. Thus Sulfation correlates inversely with TBN. Thus there is a depletion of basicity which will lower the Base Number of the oil.
Testing of Sulphation
The testing of Sulphation is done in our laboratory by using (FTIR) Fourier transform infrared spectrometer, which can be coupled with Base Number Test and Viscosity to determine the extent of oxidation and degradation of the oil.
Method of Testing
As per ASTM E2412 & ASTM D7415 : Test Method for Condition Monitoring of Sulfate By-Products in In-Service Petroleum and Hydrocarbon Based Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry.
Testing Equipment used by us in our laboratory : PerkinElmer make FT-IR Spectrometer model Spectrum 65.
A typical graph in which Sulpahation is detected and determined