Passenger car and diesel engine oils typically exhibit increases in acidity with service, particularly at extended oil drain intervals. The amount of acidity buildup is related to the extent of lubricant oxidation and the extent to which overbased additives are able to neutralize the acidic materials formed.
This paper describes laboratory and field programs examining the relationship between service history, acidity/total acid numbers (TAN), used oil corrosivity, and the ability of lubricant additives to control corrosion. The neutralization of corrosive acids in the oil medium has been monitored by an infrared spectroscopic technique, a method found to be more meaningful than the classical TAN/TBN titration measurements. New insights have been gained into acidity control by metallic detergents, the concept of “reserve TBN”, the types of acids formed in engine oils, and the performance of oils with very low metallic detergent treats.