The H2S present in the crude oil can form thin layer of iron sulfide scale inside distillation towers. This can cause serious risk of fire when the vessel is open to air for maintenance.
Fe2O3 (rust) + 3H2S ⇒ 2FeS + 3H2O
4FeS + 3O2 ⇒ 2Fe2O3 + 4S + Heat
4FeS + 7O2 ⇒ 2Fe2O3 + 4SO2 + Heat
Packed distillation towers are more prone to pyrophoric scale specifically the structured packings. Because the structure packing has relatively small thermal mass which can achieve high local temperatures. The solution is to keep the structured packing wet during the shutdowns and before entry to the vessels. Special chemicals can also be used to neutralize the iron sulfide scale .