Oxidation occurs when oil chemically reacts with oxygen. This reaction alters the structure and properties of oil – replacing oil molecules with chemicals like acids, ketones, and aldehydes. This, in turn, increases the oil’s viscosity and facilitates the appearance of varnish and sludge, all of which slow machine operations and increase wear.
The results of oxidation can be particularly troublesome to hydraulic systems. The valves that regulate the processes of hydraulic systems can be plugged by sludge, limiting their functionality or entirely disabling them.
The oxidation process will happen in hydraulic oil — it can’t be avoided. You can, however, combat oxidation and delay its progression. Lubricants are designed to control oxidation, typically through the implementation of antioxidants. These antioxidants, however, are sacrificial, meaning that they are depleted during the process of slowing oxidation. To successfully manage oxidation, you must understand the oxidation process.
Varnish formation visible on a valve.
The oxidation process
There are three basic steps in the oxidation process: initiation, propagation, and termination. Controlling these steps helps manage oxidation.
1. Initiation
In this stage, the components of an oil combine with a catalyst to create molecules called free radicals. Free radicals are reactive molecules — they locate and combine with other molecules to create entirely new substances. You can quell the initiation phase of oxidation by limiting the amount of oxygen entering your oil.
2. Propagation
In this, the second stage of oxidation, free radicals continue to increase, leading to a sequence of chemical reactions. These reactions, in turn, create more free radicals — this is a reaction cycle. Limiting these reaction cycles is the best way to combat oxidation at this stage.
3. Termination
The final stage of the oxidation process can be either negative or positive. A negative termination would mean that an oil’s antioxidants have been depleted, allowing for unfettered oxidation. A positive termination would mean that antioxidants have been successfully implemented and have halted oxidation’s progress.
There are a few factors that contribute to oxidation in hydraulic oil.
1.Temperature
The rate at which oxidation occurs increases as temperature increases. Even high temperatures localized to a small portion of the hydraulic oil can cause a chain reaction, affecting the entire oil system.
2.Pressure
Pressure increases the viscosity of a fluid, which in turn facilitates heat generation and friction. Increased pressure also increases the amount of entrained air and oxygen. High-pressure conditions, because they provide more oxygen, increase the rate of oxidation.
3.Water and Metal Contamination
Some metals encourage oxidation, especially when combined with water. The initial stages of oxidization see an increased production of acids, leading to an increase of viscosity and neutralization numbers. Neutralization numbers are a measure of acidity and alkalinity in a fluid.
How can you tell if oil is oxidized?
The presence of sludge and varnish is an obvious indicator that oil has oxidized. Additionally, oxidized hydraulic oil may appear dark in color and exude an unpleasant odor.
Oil in sample bottles displaying darkening that may be an indicator of oxidation.
You can also perform a sample analysis on oil to determine its level of oxidization. A Total Acid Number (TAN) test measures the amount of acid present in the oil. The amount of acids present indicates how far along the oxidation process is. Infrared spectroscopy can also be used to measure the presence of oxidation-related molecules.
The ASTM D943 “Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils” is aimed at testing oxidation levels of lubricants. It is especially useful for hydraulic oil, which is prone to water contamination. This test reveals a lubricant’s oxidation stability but does not reflect other forms of oil deterioration, such as the sludge formation.
The ASTM D4310 “Standard Test Method for Determination of Sludging and Corrosion Tendencies of Inhibited Mineral Oils” aims to determine if significant amounts of insoluble and metallic corrosive products will be created during an oil’s service.