The total acid number of a crude oil, or TAN is a measure of the corrosiveness of the crude due to the presence of acids, particularly naphthenic acids.

TAN is measured based on the mg. of potassium hydroxide required to neutralize one gram of crude. Generally an acid number of anything over 1 is consider high.

Refiners have four options for dealing with high acid crude:

Dilution - Often high-acid crudes are mixed with lower-acid grades to reduce the average TAN to a level that does not pose a corrosion risk.

Neutralization - High acid crude can also be neutralized with caustic as it is fed into the distillation tower by injecting the caustic directly into the crude stream.

Corrosion inhibitors - Phosphorus-based corrosion inhibitors can be injected into the crude and allowed to coat metal surfaces reducing the likelihood of corrosion. Inhibitors are also often injected into the parts of the process units where the acidic components can create the highest degree of corrosion

Metallurgy - the metals used for key pieces of equipment can by upgraded to more corrosion-resistant stainless steels containing molybdenum and other corrosion resistant alloys.