OXIDATION:


Oxidation occurs when the hydrocarbon constituents (and other products) of lubricating oil combine chemically with oxygen. Lubricating oil in engines will combine with available oxygen under certain conditions to form a wide variety of oxidation products. Many of these direct or primary oxidation products combine with other materials such as wear metals, solid contamination and moisture to form second and third derivative products. As with most chemical reactions, oil oxidation is accelerated by heat and pressure. Heat in particular will speed up the oxidation process. Various studies have shown that lubricating oxidation (with many variables such as the type lubricant and additive package in the lubricant) that the oxidation rate can be doubled for every 15 to 20 degrees increase over 180 degrees F. Also, engine load, which will dictate the levels of oxygen and pressure within the engine, can be seen in the form of accelerated acid formation, corrosion, oil thickening, deposit formation and accelerated wear.


As oil oxidizes, its ability to lubricate diminishes and, in cases of severe oxidation, noticeable changes occur as the oil becomes darker and emits odour; varnishes, lacquers and resins are formed; and in the advanced stages viscosity increases, often rapidly. Fortunately, the chemical reaction between oxygen and lubricant molecules at room temperature is slow and oxidative degradation is not an issue under these conditions. The situation changes when reaction conditions are altered to favour a more rapid reaction rate. Engine lubricants are formulated with a hostile environment in mind. Many conditions promoting accelerated oxidation co-exist in an engine, such as high temperatures, high pressures, a good air supply, agitation, the presence of metal catalysts and thin film exposure. The most significant of these conditions is the operating temperature because


40 | The Report • June 2017 • Issue 80


rate of oxidation doubles for every 10°C increase in temperature. Excessively high operating temperature (overheating) is generally accompanied by increased wear (lead, copper, tin and iron) and increased baseline viscosity. Sometimes overheating leads to the evaporation of volatile fractions in the oil, making regular top-ups necessary.


In such case,


the sump oil will exhibit increased additive levels (concentration of non-volatile components) and an increased viscosity as a direct result of light end loss. As the lost oil is replaced with fresh oil, the antioxidants are replaced and oxidation is often not immediately evident. Results should be compared with acid number and possibly viscosity for confirmation.


All top quality lubricating oils have an additive package that contains oxidation inhibitors to slow the oxidation process and alkaline detergents that will neutralize acids formed by oxidation. Normally these additives will only last a certain length of time before they are depleted and the oil must be drained. Oxidation is greatly stimulated by the contamination solids and moisture. Solids tend to hold heat, thereby increasing the lubricating oil temperature around the solid contamination. This condition acts to accelerate oxidation. Combine this effect with the presence of moisture (H2


O)


from normal condensation and the oxidation process accelerates even faster. When moisture is present in the lubrication system, the level of oxygen available to mix with hydrocarbons in the lubricating oil is raised dramatically. The presence of normal solid and moisture contamination, combined with maximum operating load of the equipment, will produce high oil oxidation rates, even with normal oil temperatures. By-pass filtration products that can control the levels of moisture, wear metals and other solid contamination are recommended. By removing this contamination, the oil will offer a better seal between the rings and liners and therefore reduce


the amount of blow by during the combustion process. Blow by contributes to the amount of oxygen and moisture within the engine.


Once the contamination which acts as catalyst to accelerate the oxidation process has been removed and a cleaner oil offered to seal the engine, then only minimal oxidation is left for the additive package of the oil to contend with. The engine will use a certain amount of oil each operating day. Combine this amount of new oil with the amount added at the time the By-Pass Filter is serviced and the engine will maintain a sufficient amount of active additives to keep oxidation in check indefinitely.


NITRATION:


The combustion chambers of engines provide one of the few environments where there is sufficient heat and pressure to break the atmospheric nitrogen molecule down to two atoms that can react with oxygen to form nitrous oxides (NOx). When nitrogen oxide products enter the lubricating oil through normal blow by, they react with moisture present in the lubricating and become very acidic and rapidly accelerate the oxidation rate of the oil. Proper By- pass filters can control the effects of nitration in the same ways it controls oxidation. By delivering cleaner oil to offer as a seal between the ring and liner, blow by of NOX


components are kept to


a minimum. Also, the filter keeps the oil chemically dry and prevents the mixing of NOX


and moisture and


this controls NOx acid formation and accelerated oxidation of the oil.


As with sulphation, nitration is the reaction of the oil with combustion by products of nitrogen. These reactions tend to become more pronounced at higher temperatures. Therefore, increased nitration is often an indication of increased blow by as the hot combustion gases react with the oil. Nitration is rarely mentioned


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