Is it Thermal Failure or Oxidation?
Quite often, when any type of deposit occurs in a lubrication system, the first assumption is that there is varnish in the oil. Typically, this is followed by the next assumption that oxidation caused this varnish. On the contrary, many types of deposits exist and each nature of deposit can be formed by many different methods of degradation. In this blog, we will take you through the methods of identification for thermal degradation and how these differ from those of Oxidation.
Thermal Failure – what is it exactly?
We are all familiar with the concept that for every 10°C rise in temperature, the life of the lubricant is effectively halved. In thermal degradation, something similar occurs. As the temperature rises, the energy contained in the molecules increases. Eventually, it gets to a point where the energy becomes too great and the bonds become broken. When this occurs the fluid’s properties are altered and the carbon-carbon bonds are broken which propagate through branching reactions. This accelerates the rate of thermal failure.
Some of the first changes which are noticeable in the lubricant are changes to the fluid’s odour and colour. We can also see changes to viscosity of the lubricant as well as depletion of additives. Lastly, we will also notice the formation of by-products which will eventually lead to sludge and varnish. Sludge is often less dense than varnish and often acts as its predecessor although it can have a higher water- moisture content compared to varnish. With elevated temperatures, sludge can be “cured” when its moisture evaporates and its density increases. This leads it to produce varnish.
When the bonds are broken during thermal failure, the lubricant can produce by-products which are of either a higher or lower molecular weight. The by-products of lower molecular weight are usually termed low boilers. These have the tendency to have a low flash point and in small concentrations, they can actually lower the flash point of the entire lubricant in the reservoir. Evaporation is also possible with these low boilers.
On the other hand, the high molecular weight by-products are often the precursors to varnish. These are typically polar in nature and tend to precipitate onto the di-polar metallic surfaces within the system. They can affect several of the lubricants properties including viscosity, foaming and demulsibility characteristics. Additionally, these high molecular weight by-products can cause interference with polar additives such as EP, AW and Corrosion Inhibitors.
When does thermal failure occur?
While some additives are designed to resist thermal failure, base stocks also have a role to play in resisting thermal failure. Traditionally, API Group I base stocks have a lower resistance to thermal failure compared to Group II & III which usually contain higher levels of aromatics and other impurities.
Let’s explore the ways in which thermal failure can occur:
*If there is a hot spot in the system which elevates the lubricant to a very high temperature causing localized thermal failure.
*Pressure Induced Thermal failure which occurs when there is the rapid adiabatic compression of air bubbles in a system that creates excessive high temperatures. This can cause thermal failure.
*Pressure Induced dieseling which occurs as a result of combustion of light end hydrocarbons. In this case, the phenomenon creates both extreme pressures and heat which can rapidly cause thermal failure.
*Static electricity can be generated between oil and mechanical filtration which can create high temperatures and free radical generation leading to thermal failure.
In thermal failure, chain propagation and branching reactions can form organic, soluble compounds. These processes typically repeat until insoluble species are generated forming polymers and other compounds of high molecular weight. These insoluble compounds are usually sticky and easily bond to particles, depleted additives and water.
Varnish will only form when the solubility limit for the high molecular weight insolubles is exceeded. One key factor to solvency of a lubricant is temperature. Some studies have shown that insoluble compounds are more resistant to dropping out of solution at temperatures above 68°C. This is one of the main reasons why varnish will begin to form in the cooler spots in the system first such as strainers. At extreme temperatures, thermal failure can produce black carbon particles which tend to fall out of the solution immediately.
Thermal Failure vs Oxidation
Before talking about the differences between Thermal Failure vs Oxidation, it is worthwhile to examine their similarities. Both mechanisms involve free radical chain proliferation and can effectively disrupt any system. These mechanisms can produce similar by-products although their final deposits can be very characteristic. Some of the analytical methods to identify whether either of these products are present are also the same.
On the other hand, the major differences between these two mechanisms can include;
*Thermal failure occurs in the absence of oxygen while oxidation requires the oxygen molecule as a catalyst for the reaction.
*Thermal failure can occur in the lubricant prior to the antioxidants being depleted.
*Thermal failure can happen before there are noticeable elevations in the acid number.
*Thermal failure can take place in new products which have been improperly stored.
Detecting Thermal Failure and Varnish Potential
One of the main challenges with trending thermal or oxidative degradation at the molecular level is that it is only possible to observe these products after they have been formed. This usually occurs at the end of the process which doesn’t help us much. Routine oil analysis such as measuring the viscosity and acid number can alert users to the increasing levels of degradation. However, once we get to these levels, there are typically a sufficient amount of insolubles to already have the existence of varnish or sludge in the system. Hence we need to design monitoring systems which can identify these changes before the accumulation of varnish.
Ideally, the detection of a fluid’s varnish potential begins when the degradation process initiates the production of soluble and quasi-soluble contaminants. This usually occurs prior to the creation of varnish. When designing a system to monitor or detect these precursors, the following should be considered:
*Trending the increase of insolubles (even at small levels)
*Indication of early detection of thermal failure or oxidation
*Providing a potential root cause for the formation of varnish pre-cursors
*Monitoring the depletion of primary antioxidants
The following tests can help with these objectives:
*Calormetric Analysis – determines the degree of oxidation by-products in the sample.
*Gravimetric Patch – Insolubles are typically larger than 2 microns so their presence is not registered in the ISO particle count. A 0.3 micron or smaller patch can be used in low tolerance varnish applications. For those fluids with higher amounts of contamination a 0.8 micron patch can be used.
*Ultra-centrifuge – A visual rating scale is used to determine the degree of sedimentation and degradation by-products after the sample is spun in excess of 20,000rpm.
*Fourier Transform Infrared (FTIR) – can indicate the presence of oxidation by products through identification of their molecular fingerprint using infrared light.
*Interfacial Tension Test (IFT) – oxidation by-products can immediately affect the surface characteristics of an oil. This allows IFT to detect insoluble oxidation by-products at an early stage.
*RULER™ – this can measure the concentration of antioxidants which can be used to determine the oxidation stability of the fluid and aid in determining the root cause of varnish formation.
These tests can aid in alerting users of the potential for varnish in their system. Whether its oxidation or thermal failure, we should always monitor the condition of the lubricants in service to prevent the onset of varnish which can lead to unplanned equipment downtime.