Due to kinetic factors, non-equilibrium sulphides can form and grow under (nominally) oxidizing conditions.  Even if initially formed sulphides are later overgrown by oxide or dissolved, their earlier existence has made the oxide layer less protective.  In existing oxide layers, attacks can occur in pores and cracks.  It is therefore essential that the material is able to form a thin, tough, and adherent oxide layer.  This requires a high chromium content and preferably also additions of silicon, aluminium, and/or reactive elements.

An old rule-of-thumb says that nickel-containing alloys should be avoided in reducing sulphidizing environments, since the formation of low melting point nickel-sulphur compounds may lead to a rapid deterioration of the alloy.  In practice, however, a "reducing" atmosphere can have a sufficiently high oxygen content (e.g. in the form of SO2) to enable the formation of a protective oxide layer, provided the chromium content of an alloy is high enough.

Due to their firmly adhering protective oxides, the MA alloys are better suited for such sulphur-rich environments than materials with similar or higher nickel contents.  However, the recommended maximum service temperature will then somewhat, or even substantially, lower than in air.  In truly reducing sulphurous atmospheres, any layer of oxide will dissolve rapidly, and the bare metal will be exposed to attack.  Under such circumstances, nickel-free alloys should be used.