Sulfidation

Environments containing sulfur may rapidly attack high nickel alloys. The problem is more severe under reducing, or low oxygen, environments. The higher the nickel content of the alloy the more sensitive it is to sulfidation attack. This is because at high temperatures nickel reacts chemically with sulfur very readily.

Sulfidation occurs beginning around 1100°F and will continue up to around 1600°F. In this range the metal-metal sulfide eutectic; in this case nickel-nickel sulfide; is molten directly under the protective chromium oxide scale that has formed to protect the base metal from the high temperature environment. When this occurs, the molten nickel sulfide can literally wash the protective chromium oxide layer away exposing unoxidized base metal.

To resist sulfidation, it is important to try and keep the nickel content of the alloy to a minimum. As a maximum, Rolled Alloys would suggest keeping the nickel content of the alloy at 20% or lower. 310 stainless with 20% nickel has shown good resistance to sulfidation attack as it forms a very tightly adhering oxide scale due to the 25% chromium and is still lean enough in nickel content to not suffer severely under oxidizing, sulfur-bearing conditions. Another, possibly more effective alloy is RA 253 MA. With high levels of chromium (21%) and a significantly leaner nickel content (11%) RA 253 MA has help up very well in many sulfur bearing environments.  Under the most extreme sulfur bearing environments, alloys like 446 with no nickel in them, may be the only viable options for extended material life spans.