Understanding Hydrolysis and Hydrolytic Stability

In lubricants, water is the second most destructive contaminant behind particles. It causes issues such as rust and decreased load-carrying capacity (film strength) in oil and also leads to permanent degradation of the lubricant. Similar to oxidation, hydrolysis is the degradation of the base oil’s molecules as a result of water. Not only can a base oil fall prey to this process, but additives are susceptible as well.

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 and phosphate esters, readily pull moisture from the environment.

As a lubricant is contaminated with water, the question then becomes how stable is the fluid in relation to the water. The ability of a lubricant and its additives to resist chemical decomposition in the presence of water is known as the lubricant’s hydrolytic stability. The ASTM standard for hydrolytic stability is D2619-09. It is often referred to as the Coke bottle test, as it employs a pressure-type soda bottle that is capped during the testing process.

The test begins by adding 75 milliliters of test fluid to 25 milliliters of water. Next, a copper strip is added. The bottle is then capped, heated to 200 degrees F and rotated for 48 hours. At the end of the test, the copper strip is removed and the difference in mass is documented, as well as the change in tarnish (as reported by ASTM D130). The test fluid’s acid number (AN) is then determined, along with the water’s acidity level. The results will reveal the fluid’s hydrolytic stability and how well it holds up against acid formation, which coincides with hydrolysis.

Several factors influence the test results, including the water purity, the contamination of the fluid, the viscosity and the additive package. For example, the anti-wear additive zinc dialkyldithiophosphate (ZDDP) is subject to being hydrolyzed and producing acids. When the test results are analyzed, the copper weight loss is measured. Zinc will coat the copper (as expected), but once the copper strip is rinsed (usually with heptane or trichloroethane), the true measure of the copper weight loss is realized.

This table shows results from an ASTM D2619 test. Note the difference
in fluid results and acidities as well as the damage to the copper strip.

Over time even oils with very high hydrolytic stability values will begin to hydrolyze. In lubricating oils, the base stock hydrocarbons and additive compounds break down. The breaking down of these molecules with the addition of water results in a restructuring of the bonds and a modification of the compounds within the fluid.

A change in pH also accompanies this process and can be tracked by monitoring the oil’s acid number. As mentioned previously, ester-based fluids are very susceptible to hydrolysis and should be closely monitored for any signs of this process, especially in equipment with a high risk of moisture ingression.

Hydrolysis within lubricating oils can cause a variety of problems. Not only can it affect the oil’s physical properties (viscosity, color, etc.), but it can also change the chemical properties. One of the most common effects of hydrolysis is the formation of acids, predominately carboxylic acids.

These acids are weak in comparison to sulfuric acids, but they can lead to machine damage. The acids will appear on a Fourier transform infrared (FTIR) spectroscopy scan and can be monitored by routine oil analysis.

As the hydrolysis process continues, the oil’s viscosity will begin to fall. This decrease in viscosity poses a very real threat to a machine’s health. As the viscosity drops, the fluid’s load-carrying capacity will also be diminished, resulting in the machine operating in a boundary lubrication regime and more pronounced wear.

By being proactive and preventing water ingression into your oils, you can mitigate the process of hydrolysis. Vigilance in monitoring the oil’s water content and acid number along with FTIR will serve as the best weapons for determining if hydrolysis is occurring. Keeping your oil dry will save you 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-Based Industrial Hydraulic Oils - Present and Future Developments.” Lubrication Engineering, Jan. 1975: 6-15.

Kajdas, Czeslaw. “Hydrolysis.”

“Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method),” ASTM Standards 2012.