Oil Varnishing, Part 1: Causes

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Oils are widely used in hydraulic systems, both as lubricants to reduce friction and to increase cooling capacities. An increase in the demands on the properties of the oil, and a move to more advanced base stock, which requires new additives, has led to an increase in varnishing problems associated with oil degradation.

Most of these fluids are hydrocarbons with additives to enhance the properties and performance of the oil. During use, the oil may become contaminated or deteriorate due to oxidation and thermal stresses. Oxidation causes a breakdown of the oils due to a reaction between the oxygen and hydrocarbons to form radicals and peroxides, which are scavenged by antioxidant additives in the oil. Thermal stresses cause the depletion of the additives as well as the degradation of the base oil and its associated physical and chemical properties. Depletion of the additives and degradation of the base oil leads to a higher degree of wear and hence wear particles in the oil.

Oxidation typically increases with greater exposure to oxygen, high temperatures, as well as the presence of wear metal and water contaminants in the oil. When the oil degrades, oxidation products accumulate and once the oil is saturated with these oxidation products sludge, resins, acids and carbonaceous deposits begin to precipitate. These polar oxidation products/resins create a thin, relatively hard, oil-insoluble layer made up mostly of organic residues that have a varnish-like appearance when deposited on a surface. The varnish is typically sticky and adheres to the metal surfaces where it traps dirt and wear particles from the system and eventually becomes darker and harder. Varnish differs from sludge in that it is not suspended in the fluid and is not easy to remove from surfaces. Lacquer is similar to varnish but is strongly bonded to the metal surface, is insoluble to many solvents and may require acid to remove.

Varnish is typically present in both soluble and insoluble forms. During operation, lubricants will deteriorate and oxidize in an irreversible process often related to factors such as heat, air and moisture. This oxidation process will produce soluble products/varnish precursors that increase in the solution with ongoing oxidation. As more varnish precursors accumulate in the oil, the oil becomes saturated and absorption of these varnish precursors ceases. Further degradation of the oil results in the precipitation of the varnish onto surfaces. The point at which saturation occurs and varnish starts being deposited is affected by temperature, molecular polarity and the levels of contamination of the oil. Increasing the temperature increases the solvency of the oil, but high temperatures change the properties of the oil and increase the rate of oxidation. Cooling of the saturated lubricant results in precipitation and deposition of the varnish until the levels reduce to below the saturation level for the particular temperature. These temperature affects result in i) oxidation in areas of high temperature such as those that occur during electrical discharge, adiabatic compression of bubbles and micro dieseling, and ii) deposition of varnish in cooler regions of the system such as coolers and oil tanks.

Varnishing is an indication of the degree of degradation of the oil and can cause sticking and the faulty operation of control valves, blockages of filters, plugging of oil inlets and strainers, and poor heat transfer.

There are several methods to measure the varnish potential of a lubricant, including oxidation stability, deposit measurement, and measurement of the contaminants. The membrane patch colourimetry (MPC) test is the industry standard for measuring the oil’s potential to form harmful varnish. The MPC test involves passing a sample through a fine filter patch and quantifying the colour of the remaining residue, with a darker colour highlighting a greater degree of degradation.

Other tests can determine other factors that relate to varnishing, such as the rotating pressure vessel oxidation test (RPVOT) and RULER (voltammetric techniques) tests, which are used to determine the remaining life of its antioxidants. The turbine oil stability test (TOST) measures the amount of antioxidant activity in turbine oil and involves exposing the oils to environments that promote oxidation and deposition of oxygen, water, high temperate and metallic components. Fourier Transform Infrared (FTIR) spectroscopy measures the additive depletion, contaminants, and base-stock degradation in the lubricants, while also measuring the varnishing potential of the oil. Differential scanning calorimetry (DSC) and the Acid Number (AN), previously referred to as the Total Acid Number, also estimates the varnish potential via different means.

The message is simple – oil varnishing is a by-product of lubricant degradation and should not be ignored. Our next Technical Tip will address methods to i) evaluate the varnish potential of the lubricant, and ii) remove varnish from the system.