For lubricants, water is the second most destructive pollutant, after the particles. It causes problems such as rust and decreased load bearing capacity (film resistance) in the oils and also leads to a permanent degradation of the oils. Similar to oxidation, hydrolysis is the degradation of the molecules of the lubricating base by the effect of water. But not only the lubricant base is prey to this process, the additives are also susceptible to degrade by the effect of hydrolysis.
Translated by Francisco J. Páez Alfonzo
Posted inMachinery Lubrication (6/2013)
Oils by nature are hygroscopic, which means they absorb moisture from the air. The tendency of an oil to undergo this process is known as hygroscopicity. Ester-type fluids, especially polyol esters and phosphate esters, readily absorb moisture from the environment.
Since the lubricant is contaminated with water, the question we ask is how stable the oil is relative to water. The ability of a lubricant and its additives to withstand chemical decomposition in the presence of water is known as its hydrolytic stability. The ASTM test used to determine the hydrolytic stability of oils is D2619-09. The test is known as the Coca-Cola bottle test, as it uses a soda bottle that is capped during test run.
The test is performed by mixing 75 ml of the oil to be tested with 25 ml of water. Then, a strip of copper is placed inside the sample. The bottle is capped, heated to 93 ° C (200 ° F) and rotated for 48 hours. At the end of the test, the copper strip is removed and its weight loss as well as the change in color is determined (reported as in ASTM D130). The acid number of the oil – AN is determined, as well as the acidity level of the water. The results indicate the hydrolytic stability of the oil and how well it behaves against acid formation, which coincides with the hydrolysis.
Some factors influence test results, including water purity, fluid contamination, viscosity, and the additive package. For example, the zinc dialkyldithiophosphate (ZDDP) antiwear additive produces acids when hydrolyzed. When analyzing the test results, the weight loss of the copper strip is determined. The zinc covers the copper strip (as expected), but once the copper strip is washed (usually with heptane or trichloroethane), it is determined exactly how much was the weight loss of the strip.
This table displays the results of an ASTM D2619 test. Note the difference in fluid and acidity results as well as damage to the copper strip.
Over time, even oils with a high hydrolytic stability will begin to hydrolyze. In oils, mineral lubricants and additives degrade. The degradation of these molecules with the addition of water results in a restructuring of the bonds and a modification of the compounds within the fluid. This process is accompanied by a change in pH and can be monitored by analyzing the acid number of the oil. As previously stated, ester-based fluids are highly susceptible to hydrolysis and should be monitored closely to detect any signs of hydrolysis, especially in equipment with a high risk of moisture ingress.
Hydrolysis in lubricants causes a wide variety of problems. Not only does it affect the physical properties of the oil (viscosity, color, etc.), it also affects its chemical properties. One of the most common effects of hydrolysis is the formation of acids, the predominant being carboxylic acids. These acids are weak compared to sulfuric acid, but still cause damage to the machinery. These acids are detected using Fourier Transform Infrared (FTIR) and can be monitored using a routine oil analysis program.
As the hydrolysis process continues, the viscosity of the oil begins to decrease. This decrease in viscosity poses a real threat to the health of the machine. As the viscosity decreases, its load bearing capacity also decreases, resulting in an operation in a limiting film lubrication regime and a more pronounced wear.
Being proactive and preventing the entry of water into the oil, the hydrolysis process can be mitigated. Monitoring water content and acid number along with FTIR are the best weapons to determine if you are presenting hydrolysis in the oil. Keeping the oil in it will prevent the devastating effects of this chemical process.
References
Forest, M. and Araud, C. «A New Approach for Oil Formulations.»
Papay, Andrew G., and Harstick, Christian S. «Petroleum-B