SYMPOSIA PAPER Published: 01 December 2022
STP163420210022

Root Cause Studies to Investigate Phosphate Varnish Formation by Thermal Degradation Pathways

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Varnish formation is a major problem that leads to costly lubricant-related mechanical failure of machinery equipment. Varnish is typically regarded as the organic insoluble matter within a lubricant that can plug filters to cause flow issues or coat equipment parts, forming a contamination layer. Varnish formation within equipment has been attributed to several issues, including oxidation and thermal decomposition within the fluid, electrostatic spark discharge, excessive operational temperatures, contamination ingress, and additive decomposition. In some recent case studies of gas turbine bearings and servo-valve equipment, the formation of a surface varnish coating having high concentrations of phosphate chemistries contributed to the root cause of the equipment failure. A common source of phosphorus in many lubricants is the phosphate antiwear additives that function by actively forming thin surface films (in some cases monolayer) to supply the antiwear or anticorrosion protection. It has been seen that at exceedingly high surface levels of the additive chemistry, the additive itself will deposit acting as the varnish—leading to the question of why/how it is forming in this manner. The thermal stability of the in-service fluid and its antiwear additive were studied herein to understand the thermal-oxidative reaction pathways that could lead to a root cause of the varnish formation within a lubricant. The thermal stress of new and in-service lubricants was investigated using thermal stability and oxidation standards ASTM E2550 and ASTM E1858 to produce a varnish. Qualitative analysis of the varnish layers was then performed by ASTM E1252 using infrared spectroscopy, and the in-service lubricant chemistry was evaluated using ASTM D7418.

Author Information

Schomburg, K., Cory
PerkinElmer, Inc., Waltham, MA, US
Wooton, David
Wooton-Consulting, Bumpass, VA, US
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Pages: 209–239
DOI: 10.1520/STP163420210022
ISBN-EB: 978-0-8031-7715-4
ISBN-13: 978-0-8031-7714-7