As lubricants degrade from oxidation they form a number of acids. These acids are corrosive to Babbitt, yellow metals, carbon steel, cast iron, and if left uncorrected for a period of time will begin to corrode and possibly result in eventual bearing failure. While small increases in the Total Acid Number (TAN) usually indicate oxidation and lubricant degradation, contaminants with acidic constituents can also be a factor. Monitoring the oil’s Total Acid Number should be an important part of your lubricant maintenance program. Generally when a lubricant’s acid number reaches a condemning limit, replacement or sweetening is your best option.
Total Acid Number (TAN) is the standard neutralization number test for industrial lubricating oils. It is performed by titrating a solution of oil and diluent with an alcohol/potassium hydroxide (KOH) solution, a base, until all the acids present are neutralized. The results are reported as milligrams of potassium-hydroxide per gram of sample.
Strong Acid Number (SAN) is similar to TAN, except the ‘strong’ acids are first extracted from the lubricant. That extract is then titrated with KOH and the SAN reported as mg/gm.
Total Base Number (TBN) is a standard test for engine lubricants. It is a measurement of the amount of protection in the lubricant remaining to neutralize acids formed as a result of combustion. A solution of oil and diluent is titrated with an alcohol/Hydrochloric Acid (HCl) solution until all the alkaline or base constituents in the oil are neutralized. Results are reported as milligrams of HCl per gram of sample, or mg/gm.
Most lubricating oils have a baseline Acid Number as a result of additives. R&O (rust and oxidation) industrial oils generally have a baseline in the 0.03 to 0.06 mg/gm range. AW (anti-wear) and EP (extreme pressure) industrial oils will have much higher baselines because of the additional additives that give them their AW or EP qualities. Baselines for these lubricants can be over 1.0 mg/gm.