High Temperature Mercaptan Corrosion of Steels

Mercaptan sulfur content is a more reliable indicator of a petroleum naphtha's corrosivity at elevated temperatures than is total sulfur content.

Corrosion of steels by mercaptan vapors at 650–850 F was found to be essentially linear with time and the double-sulfide scale typical of H₂S corrosion was formed. The corrosion rates were proportional to the mercaptan concentration and gave Arrhenius activation energies of 11–16 k cal/mole. Hydrogen accelerated mercaptan corrosion and voided the corrosion resistance obtained by adding up to 9 percent chromium to the steels. Corrosion rates were the same in primary, secondary and tertiary amyl mercaptans.

At 750 F, the amyl mercaptans were thermally stable in the test apparatus. About 10 percent of the n-amyl mercaptan decomposed thermally to H^S at 850 F. However, at 750 F, some corroding metals and some corrosion scales caused the mercaptans to decompose. The effect of exposure time on the decomposition can be correlated with surface changes caused by corrosion reactions. The stability of the mercaptans in the presence of corroding steels was primary>secondary>tertiary.

Corrosion and mercaptan decomposition both were reduced by the presence of halides. Added H₂S in the corroding gases reduced mercaptan decomposition without affecting the corrosion rate.

An assumed free-radical mechanism can be used to explain qualitatively both the corrosion reactions and the mercaptan decomposition.