Co-worker has asked if oil becomes acidic over time.

Anyone have an answer?

Yes, oil can become more acidic as a result of oxidation. This is indicated by an increase in neutralization number.

Thank you much.

Wharfrat,

The answer is a little more complicated than that. When oil oxidizes, it forms weak acidic compounds such as carboxylic acid, ketones and aldehydes. They can be titrated in a test called the "Total Acid Number Test" or TAN. However, many mistakenly equate an increased TAN number in an oil to increased corrosion potential. If the increase in TAN in a used oil is due primarily to oxidation and the formation of weak acids, this will not have a significant impact on the corrosive potential of the oil. In fact, in some oil formulations, new oils have much higher initial TAN numbers due to beneficial additives that are included in the oil. I wrote an article on oil oxidation that is available at Oxidation White Paper.

Hope this is helpful.

Rich Wurzbach
Certified Lubrication Specialist, Oil Monitoring Analyst
MRG Power Labs