The Arrhenius Equation is simply a way to calculate the rate of reaction based on factors of temperature, energy present and the molecular composition of the material. Simply stated it says that for every 10 degree C (approx. 18 deg F) increase in temperature, the rate of oxidation doubles. This is an exponential increase and so as temperatures rise, oxidation increases dramatically–it is a compound increase.
Our concern in the rotating equipment industry is that of reliability. Reliability is almost always related to the service life of bearings in our rotating equipment. According to most experts, bearing failure is directly attributable to lubrication issues approximately 70% of the time. So the question becomes, why do lubricants fail? According to Noria Corporation there are three reasons oils fail: contamination, oil degradation and additive depletion. Oil degradation is merely oxidation. “Oxidation occurs when atmospheric oxygen combines with hydrocarbon molecules and undergoes a chemical change. This chemical change results in the catastrophic and permanent change to a different chemical make-up for the oil molecule. The rate at which the oil molecules react with the oxygen depends on a number of factors, but the most prevalent is temperature.“ (source)
Heat increases both the collision rate of molecules and the activation energy of the reaction. The higher activation energy helps overcome the barrier (or natural resistance) molecules have to chemical reactions. In fact, there are a range of problems associated with too much heat:
- Additive and base oil decomposition is accelerated.
- Some additives will volatilize and escape into the atmosphere.
- Viscosity-index improvers shear down more rapidly.
- Microbial contaminants prefer warmer temperatures (but not scalding).
- Heat collapses oil films, causing accelerated abrasion and scuffing conditions.
- Hot oil shortens the life of filters and seals and accelerates corrosion.
- Both oil and grease are more prone to leakage.
- Grease separates faster (oil from thickener) at elevated temperatures.
- High surface temperatures can form carbonaceous gum and resins.
So it becomes very important to maintain temperatures below 160 deg F if possible. If high temperatures are present, monitor the oil carefully and plan on changing oil at least twice as frequently.
The Arrhenius Equation and Corrosion
Corrosion costs many billions of dollars a year. NACE (the National Association of Corrosion Engineers) states that in 2013 the direct cost of corrosion was 3.1% of the 15.1 Trillion U.S. GDP, which in June 2013 is estimated to equal to $500.7 Billion. This is a staggering number. A fundamental lack of understanding of how important this is seems to be lacking. Much of the corrosion visible is due to focusing on short-term issues while our infrastructure crumbles around us. Here is a graph showing the corrosion rate of carbon steel as a function of temperature and acid concentration.
Recent developments in protective barrier coatings enable us to do a better job of protecting assets from the effects of corrosion.