Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/520842
44 May- June 2015 | www.machinerylubrication.com LESSONS IN LUBRICATION low, the chain reactions are generally slow but will increase at moderate rates above 100 degrees C. Oxygen (g) is the major catalyst of oxida- tion. Whether it exists in the air or in other molecules such as water, oxygen is integral in the oxidation process at almost every stage. Free radicals (h) are among the unwanted results of the intermediate reactions within oxidation. These highly reactive molecular fragments contain one or more unpaired electrons that are prone to reactions with hydrocarbons and other molecules. Some free radicals may exist as molecular frag- ments of alkyl, hydroperoxide, alkyloxy, hydroxy, alkylperoxy, etc. Once free radicals exist, they quickly will react again — propagating (i) the process — with the hydrocarbons and dissolved or free oxygen to form more free radicals and oxygen- ated compounds. High temperatures also play a role in furthering this reaction at increased levels. Another example of oxidation propaga- tion would be peroxy radicals reacting with additional hydrocarbons to produce hydrop- eroxides and more alkyl radicals. As a product of several chain reactions during the propagation stage, oxygenated compounds (j) are formed such as alde- hydes, ketones, alcohols and water. These products are the result of a reaction between the alkyloxy radicals and hydrocarbon. Alkyl hydroperoxide and alkylperoxy radicals can be neutralized and removed from the propagation of oxidation when they come in contact with antioxidants (k). The most common antioxidants will work as chain breakers and decomposers. Oxidation can move past the propaga- tion stages unfavorably (l) when the oxygenated compounds continue to react with the hydrocarbons and oxygen. On the other hand, oxidation may terminate favor- ably (m) when stabilization of free radicals occurs. Chain-breaking antioxidants, such as phenolic or aromatic amines, react with the free radicals to form stable radicals and inert byproducts. Peroxide decomposer types of antioxidants like phosphates and sulfi des react with peroxides to produce alcohols and water. Oxygenated compounds (n) will react further with oxygen resulting in carboxylic acid, esters and water. This process is again increased when subjected to high tempera- tures. Carboxylic acids are largely formed by the oxidation of aldehydes and ketones, which continues through an abstraction of an alpha hydrogen or aldehydic hydrogen from the hydrocarbon molecules. Polycondensation and polymerization (o) of the oxygenated compounds describe the effects of the fi nal stages of oxidation. The increase in weight of the molecular compounds translates to the formation of sludge, varnish and deposits. Not only do these insoluble products form, but organic acids corrosively attack the surfaces along with water. Most of the additives are also negatively affected and in turn become part of the deposit agglomeration. As aldehydes and ketones increase in the lubricant due to the condensation reactions, the apparent viscosity will begin to increase. Insoluble, oxidized oil products (p) like sludge and deposits are signifi cantly different from the oil molecules that make up the majority of the lubricant. Sludge is the buildup of insolubles, while deposits are insolubles that attach tightly to metal surfaces. Corrosion will be promoted as water and acids exist on metal surfaces if the corrosion inhibitor additives are depleted, which may also be the byproduct of oxidation. In conclusion, contaminants like water, oxygen and wear metals can have serious consequences on lubricants and machines. Additives such as antioxidants can help substantially, but they eventually will become depleted during the induction period. For your lubricants to last longer and your machines to run longer, you must understand why oxidation occurs. Note: For further reading on the oxida- tion process, see Dave Wooton's article at ht tp://w w w.machiner ylubrication.com/ Read/999/lubricants-oxidation. About the Author Bennett Fitch is a technical consultant with Noria Corporation. He is a mechanical engi- neer who holds a Machine Lubricant Analyst (ML A) Level III certifi cation and a Machinery Lubrication Technician (MLT) Level II certi- fi cation through the International Council for Machinery Lubrication (ICML). Contact Bennett at bfi tch@noria.com.