Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/136253
Water in Oil BACK PAGE BASICS WES CASH NORIA CORPORATION 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 54 | May - June 2013 | www.machinerylubrication.com 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. HYDROLYTIC STABILITY OIL A OIL B OIL C OIL D Copper Appearance 1A 4C 4B 2C Copper Weight Loss (mg cm^2) 0.3 2 12 0.014 Acidity 1.70 160 15 0.9 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- 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. 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.