“The oil is degrading.” An all-too- common phrase uttered during numerous oil analysis case studies courses I’ve taught over the past few years. Typically, this response is given, based on some correct observation that one or more of the measured oil physical or chemical properties, such as viscosity or acid number, have changed significantly. But what does oil degradation really mean?
In discussing what might generically be called “oil degradation”, it is necessary to first understand some fundamentals about lubricant formulation. The majority of lubricating oils are comprised of a base oil - either mineral or synthetic - and additives, used to either enhance an existing base oil property, suppress an undesirable property or add some new property to the finished lubricant. Depending on the lubricant, the additive concentration may be in the range of 0.1 percent to 0.3 percent of the finished oil for a turbine oil, and as much as 30 percent for a heavy-duty diesel engine oil. The rest of the lubricant is comprised of appropriately selected base oil. When addressing the issues surrounding oil breakdown, we must therefore consider both additive degradation and base oil degradation.
The effects of additive degradation and how oil analysis can be used to detect additive depletion rates are discussed in David Wooton’s article on page 24. This article focuses on the physical and chemical changes that occur to base oils when they degrade and how oil analysis can be used as a vital early warning sign of incipient problems.
Understanding the Mechanism of Oil Degradation
Aside from the effects of radiation on base oils (a topic that is beyond the scope of this article), hydrocarbon base oils - both mineral and synthetic - degrade in one of three ways (Figure 1).
Figure 1. The Three Common Methods of Hydrocarbon Oil Degradation.
Although distinctly different, thermal failure, oxidation and the effects of compressive heating all result in a change in the fundamental chemistry of the base oil molecules. However, because the changes that occur are often chemically different, it stands to reason that the observable effect - the used oil analysis test data - will also differ. For this reason, it is vital to assess how likely it is that each mechanism will occur, so that appropriate test slates can be devised to account for all likely scenarios.
Oxidation is perhaps the most common chemical reaction, not just in lubrication chemistry, but also in nature as a whole. Simply stated, oxidation is the chemical reaction of an oil molecule with oxygen, which is present from either ambient or entrained air. (In a strict chemical sense, oxidation does not necessarily need to involve oxygen, although for the purposes of this article, the discussion is confined to oxidation reactions involving oxygen.) Oil oxidation is no different than other commonly encountered oxidation reaction, such as rusting. Just like the effects rusting and other corrosive processes have on metal substrates, oil oxidations results in a catastrophic and permanent chemical change to the base oil molecules.
In the case of oil oxidation, the reaction results in the sequential addition of oxygen to the base oil molecules, to form a number of different chemicals species, including aldehydes, ketones, hydroperoxides and carboxylic acids (Figure 2).
The rate at which base oil molecules react with oxygen depends on a number of factors. Perhaps the most critical is temperature. Like many chemical reactions, oxidation rates increase exponentially with increasing temperature due to the Arrehenius rate rule. For most mineral oils, a general rule of thumb is that the rate of oxidation doubles for every 10°C (18°F) rise in temperature above 75°C (165°F). Because of this, synthetic oils are often required in high temperature applications to prevent rapid oil oxidation. But why are synthetic hydrocarbon oils (SHCs) more oxidatively stable than conventional minerals oils? After all, they’re both comprised of carbon and hydrogen atoms joined together in similar paraffinic chains to refined mineral oils.
The answer to this question is two-fold. First, SHCs, and for that matter highly-refined mineral oils, have very few impurities. Some of the impurities, particularly aromatic compounds found in solvent refined mineral oils, are less stable than the paraffinic molecules that comprise the majority of molecules in SHCs and highly-refined mineral oils.
Aromatic compounds of this type are termed unsaturated, meaning they do not have the full complement of hydrogen atoms surrounding them, due to the double bonds present in the benzene rings (Figure 3).
Unsaturated molecules are by their very nature more reactive than saturated molecules. Anyone who has visited a doctor or dietitian recently will know that switching from saturated fat to unsaturated fat in one’s diet has some health benefits. In this instance, unsaturated molecules are more desirable, because they are more readily broken down by the body - they react faster. In an oil, the reverse is true; everything that can be done should be done to avoid molecules that are prone to reaction to avoid premature oil oxidation. Oxidative degradation of unsaturated impurities not only results in their destruction, but also helps promote oxidation of other more stable oil molecules.
Secondly, the chemical structure of the paraffinic molecules formed during the production of certain SHCs, such as polyalphaolefins (PAOs), ensures that fewer reactive tertiary hydrogen and carbon atoms are present (Figure 3). Tertiary hydrogen and carbon atoms are more prone to direct reaction with oxygen, resulting in the formation of hydroperoxides and other oxygenated reaction by-products. By carefully controlling the chemical structure of the base oil molecules, synthetic base oil manufacturers can limit the number of reactive hydrogen and carbons atoms, and hence improve the overall oxidative resistance of the base oil.
The Effects of Oxidation - What to Look for on an Oil Analysis Report
While controlling temperature and using higher-quality base oils can help limit the degree and rate of oxidation, the eventual breakdown of the base oil molecules due to oxidative processes is inevitable. When this occurs, a number of oxygenated by-products are formed as illustrated in Figure 2. One common feature of these reaction by-products is the carbon-oxygen double bonds, termed a carbonyl group. Carbonyl groups are noted for their characteristic absorption of infrared light in the 1740 cm-1 region. For this reason, Fourier transform infrared spectroscopy (FTIR), which measures the degree of infrared absorption in different parts of the infrared spectrum, can be an excellent tool for pinpointing base oil oxidation (Figure 4).
Figure 4. FTIR Spectrum Comparing Oxidation and Thermal Degradation.
Perhaps the most noteworthy of the reaction by-products are the carboxylic acids. As the name implies, carboxylic acids are acidic in nature, just like other more common acids such as sulfuric and hydrochloric acids, although they are not nearly as strong. Common household vinegar contains carboxylic acid - an acetic acid. Because oil oxidation results in the formation of carboxylic acids, it stands to reason that the acidity of an oil that has undergone appreciable oxidation will increase. As such, an Acid Number test, which uses a wet chemistry titration method to determine the concentration of acids present in an oil, can be used to determine the degree to which an oil has oxidized. Care must be exercised when using Acid Number data to gauge oil oxidation because a number of additives - both new and degraded - can result in changes in an oil’s Acid Number and can mask the real effects of base oil oxidation. Similarly, depending on the working environment, certain ingressed contaminants may also cause the acid number to change, masking the effects of oil oxidation. For this reason, the presence of a characteristic infra peak at 1740 cm-1 in the FTIR spectrum can be an instructive piece of confirmatory evidence when assessing oil oxidation.
While carboxylic acids by themselves are bad news and can cause acidic corrosion, an increase in acid number is usually a harbinger (forerunner) of an even more damaging chemical process - the formation of sludge and varnish. Sludge and varnish form when oxygenated reaction by-products, such as hydroperoxides and carboxylic acids, combine to form larger molecular species. When a number of such molecules combine, the process is termed polymerization and results in the formation of large molecules of high molecular weight. Because the viscosity of an oil is directly related to the size of the molecules, any degree of polymerization will result in an increase in the measured viscosity. Allowed to progress too far, polymerization continues to such an extent that solid material - sludge and varnish - forms in the oil, as the molecules become too large to remain a liquid. This material is sticky and can cause filter plugging, fouling of critical oil clearances and valve stiction in hydraulics systems.
Thermal and Compressive Base Oil Degradation
Unlike oxidation, the effects of thermal or compressive heating are commonly far less understood than oxidation. Thermal failure typically occurs when the base oil comes in contact with hot surfaces within the oil-wetted path, or due to a sudden and rapid increase in temperature associated with the adiabatic compression of entrained air bubbles in pumps, bearings and other pressurized lubrication environments. When this occurs, the layer of oil that comes in contact with the hot machine surface or compressed air bubble can chemically change. Temperatures in excess of 200°C (400°F) are typically required to initiate a thermal failure event. In the case of compressive heating, as little as 250 psi rise above atmospheric pressure is capable of increasing temperatures from 100°F to 400°F.
Because thermal degradation typically occurs in the absence of significant amounts of oxygen, the same reaction by-products formed during oil oxidation as illustrated in Figure 2 typically are not seen. In particular, comparatively few oxygenated reaction by-products are formed. Because no molecules containing the carbonyl carbon-oxygen double bond are formed, the 1740 cm-1 peak used to signify oxidative base oil degradation in an FTIR spectrum typically does not show up when thermal failure is the dominant root cause. Likewise, if no oxygenated intermediates are formed, few carboxylic acids show up in the oil, at least initially. For this reason, acid numbers typically do not change when thermal degradation occurs. However, thermal degradation does result in physical and chemical changes to the oil. So, how can oil analysis be used as an early warning tool?
Perhaps one of the earliest indicators is a change in the oil’s color. Like oxidation, thermal failure results in a change in oil color, although typically early thermal failure causes a color change sooner than oxidative failure. The color change is due to the formation of carbon and oxide insolubles - chemical by-products of base oil breakdown, which are suspended in the oil. A number of tests including a simple patch test (using a 0.8mm patch or smaller), or an ultracentrifuge test that uses a high-speed centrifuge to remove suspended materials from the oil for visual examination (Figure 5) can provide an early warning sign of thermal failure.
FTIR is also an effective tool for providing an early warning sign, or for confirming active thermal degradation. However, because few oxygenated by-products are formed in early stage thermal failure, looking at the characteristic 1740 cm-1 region is unlikely to show problems. Instead, the focus should be around the 1600 to 1640 cm-1 region of the spectrum (Figure 4). In this region, the by-products of thermal base oil degradation, specifically nitrogenous molecules, adsorb. Peaks in this region are often referred to as FTIR nitration peaks.
A significant increase in the FTIR nitration region can point toward thermal failure as the dominant mechanism of base oil degradation. In fact, in natural gas and other clean burning engines, the balance of oxidation to nitration is often used to tune the air/fuel ratios - too little fuel resulting in elevated oxidation rates, too little air resulting in nitration.
In some circumstances, where thermal heating is severe and prolonged, such as in heat transfer oils and quench oils, the base oil molecules can experience thermal cracking. Thermal cracking can be thought of as a “chopping up” of the carbon atom backbone of the oil molecules. Because an oil’s viscosity is directly related to the average size (carbon chain length) of the molecules, extreme thermal cracking can result in a drop in viscosity, which can be an effective early warning tool. Perhaps more useful is a slightly more sophisticated test termed gas chromatography (GC). While GC has many and varied uses in oil analysis, GC is perhaps most useful in its ability to separate similar molecules (like hydrocarbon oil molecules) based on their molecular size. A gas chromatogram from a thermally cracked oil would hence show a higher than normal level of light-end fractions (smaller sized molecules) in the degraded oil, compared to a new oil chromatogram. More information about GC can be found in the July-August 2002 issue of Practicing Oil Analysis in an article titled “Gas Chromotography: The Modern Analytical Tool.”
Other Base Oil Failure Mechanisms - Non-hydrocarbon Oils
Beyond mineral oils and SHCs, there are many different types of base oils, from vegetable-based oils such as canola and soybean oils, to phosphate ester, polyol ester, diesters, polyalkylene glycols and silicone fluids. While the chemical structure and mechanism of degradation will vary based on environmental stressing factors, hydrolysis is perhaps the most noteworthy degradation pathway.
Hydrolysis is simply the chemical reaction of the base oil molecule with water. Just like oxidation, base oil hydrolysis results in a permanent chemical change to the base oil molecule, and hence a change in its chemical and/or physical properties. Ester type base oils are perhaps most prone to hydrolysis. Their ester chemical linkages make them potentially susceptible to hydrolysis under certain conditions, resulting in a breakage of the ester bond. Under these circumstances, the typical reaction by-products are acidic in nature, which not only cause an increase in corrosivity, but can also catalyze further reaction. Base oil hydrolysis is best detected using a combination of Acid Number and FTIR.
The health of a lubricant’s base oil plays a significant role in ensuring its suitability for continued service. Whether simply trying to trend the onset of base oil oxidation for a condition based oil change, or looking for signs of thermal failure due to compressive heating, it is vital that the fundamental chemistry of base oil degradation be understood so that the telltale signs can be identified using well-selected oil analysis test slates. Don’t be scared. Learn some basic chemistry, and you’ll be surprised how it can bring into focus many of the problems you deal with on a daily basis!