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Acidity
Marine lubricating oils are generally composed of base oils and additives. The purpose of these additives is to enhance the characteristic of the base oil or to give a specific characteristic. The base oils used in quality lubricants are carefully refined and may have a slight acidity because of the presence of small amounts of organic acid but this has no corrosive effects on machinery.
Lubricants in service frequently come into contact with air at high temperatures and become partly oxidised in the process. The rate of oxidation is minimised by the inclusion of a suitable anti-oxidant. Products of oxidation consist partly of organic acid bodies that may cause the lubricant to become corrosive to metals. Hence the degree to which acidity increases in service is a measure of the deterioration of the oil through oxidation or contamination. In many applications such as diesel engines, turbines and hydraulics, resistance of the lubricant to oxidation is important. The acidity of a lubricant is expressed by the neutralisation number, known as the Total Acid Number (TAN). The test for mineral based lubricants is defined in ASTM - D664 and determines the number of milligrams of potassium hydroxide required (KOH) required to neutralise the acid content of 1 gram of oil. The units are mgKOH/g. All oils have a TAN value and what is important is not the initial value, but the change over a period of time.
It should be noted that diesel engine oils which have a level of alkalinity (expressed as Base Number or BN) may also have a TAN value. In the case of a trunk piston engine oil in service the alkalinity reserve is reduced because of the neutralising effect it has on the acidic products of combustion. The predominant one is sulphuric acid as a result of sulphur in the fuel. All trunk piston engine manufacturers recommend a minimum alkalinity value. If the engine was operated at lower values there would be an increasing possibility of the presence of a Strong Acid Number (SAN) (again determined by ASTM-D664). One might expect that this could not happen until the alkalinity of the oil had fallen to 0 mgKOH/g. Because the used oil charge is not totally homogenous, it is possible for an SAN value to exist even though there is a low level of alkalinity expressed as BN.
Strong acids will initiate very rapid corrosion. At no time should a lubricating oil have a Strong Acid Number (SAN). If this is found in service the lubrication system should be flushed through and the oil charge renewed.
Lubricants in service frequently come into contact with air at high temperatures and become partly oxidised in the process. The rate of oxidation is minimised by the inclusion of a suitable anti-oxidant. Products of oxidation consist partly of organic acid bodies that may cause the lubricant to become corrosive to metals. Hence the degree to which acidity increases in service is a measure of the deterioration of the oil through oxidation or contamination. In many applications such as diesel engines, turbines and hydraulics, resistance of the lubricant to oxidation is important. The acidity of a lubricant is expressed by the neutralisation number, known as the Total Acid Number (TAN). The test for mineral based lubricants is defined in ASTM - D664 and determines the number of milligrams of potassium hydroxide required (KOH) required to neutralise the acid content of 1 gram of oil. The units are mgKOH/g. All oils have a TAN value and what is important is not the initial value, but the change over a period of time.
It should be noted that diesel engine oils which have a level of alkalinity (expressed as Base Number or BN) may also have a TAN value. In the case of a trunk piston engine oil in service the alkalinity reserve is reduced because of the neutralising effect it has on the acidic products of combustion. The predominant one is sulphuric acid as a result of sulphur in the fuel. All trunk piston engine manufacturers recommend a minimum alkalinity value. If the engine was operated at lower values there would be an increasing possibility of the presence of a Strong Acid Number (SAN) (again determined by ASTM-D664). One might expect that this could not happen until the alkalinity of the oil had fallen to 0 mgKOH/g. Because the used oil charge is not totally homogenous, it is possible for an SAN value to exist even though there is a low level of alkalinity expressed as BN.
Strong acids will initiate very rapid corrosion. At no time should a lubricating oil have a Strong Acid Number (SAN). If this is found in service the lubrication system should be flushed through and the oil charge renewed.
