At temperatures less than 235 ºC and with TAN < 0.5 and S>3.0, carbon steel has a corrosion rate of 2 mpy. Increasing temperatures and the TAN corrosion rates will grew up. Regards Luis Marques

Naphthenic acid content. The most common method of indexing crudes according to naphthenic acid content is a titration method to determine total acid number (TAN) which is a measure of the number of mg KOH required to neutralize one gram of oil. Although there are some useful rules of thumb for threshold TAN values where onset of corrosion is to be expected, these guidelines should not be used as absolutes, even at (TAN) less than 0.5 we can have naphthenic corrosion. Temperature. The early literature indicates onset of the corrosion is about 420 F (216 C). It peaks at about 500-550 (260-288 C), and diminishes in the 700-750 F (370-400 C) range. Some recent studies have shown that the onset might be at temperatures lower than indicated in older literature. Nevertheless, it always occurs above the aqueous dew point. At temperatures less than 235 ºC and with a total acid number (TAN ) < 0.5 and Sulphur >3.0, carbon steel has a corrosion rate of 2 mpy. At higher temperatures and higher TAN numbers corrosion rates will be bigger.