That would be an expected damage mechanism, known as Sulphidation.

Sulfidation occurs at elevated temperatures (> 260oC)in the presence of sulphur compounds. Compounds such as H2S, mercaptans, and poly sulphides react with carbon steel surfaces forming metal sulphides. Protection is mostly dependent on the formation of tenacious protective sulphide scales.

Attack can progress in the form of localized attack, uniform thinning, and erosion - corrosion. Sulphur plant equipment and piping operating above 260oC can be susceptible to this attack.

Allonizing the tubes or upgrading metallurgy should mitigate the problem.

Thanks to rustbuster. I expect this is the main mechanism, but I do not yet understand the source of the oxide in the scale, when there should not be any water present. Any insights would be of interest.

Cannot give you an exact reason, my assumption is that the FeS scale convert to oxide s when exposed to the atmosphere. Beyond the assumptions, you would have to research deeper.

Regards  

I understand that dry FeS (pyrophoric) could be ignited when exposed to the atmosphere. You may need precautions to minimize the risk of fires in the exposed equipment.

PAN