Sulphidation resistance of stainless steels

Introduction

For practical considerations, sulphidation in relation to high temperature attack of stainless steels, is more a consideration of the effects of sulphur containing atmospheres, rather than a specific attack mechanism. Both oxides and sulphides may be formed depending on the atmosphere, but generally the resistance to sulphur containing gasses is related to chromium content, in the same way as oxidation.

The combustion of high sulphur level fuel oils is not normally considered hazardous to stainless steels, provided there is sufficient oxygen present to make conditions sufficiently oxidising.

Affect of sulphur containing gas conditions-sulphur dioxide

Dry sulphur dioxide is normally not considered a hazard to stainless steels, although the scaling rates are higher for a given temperature than in dry air.
Damp sulphur dioxide is more aggressive, which may be similar to the comparison of dry and damp air oxidation.
Higher levels of chromium are beneficial in resisting attack. Although there may be little difference between grades like 1.4301 (304), 1.4401 (316) and 1.4845 (310) at around 700 C in damp sulphur dioxide, these conditions can be expected to compromise the resistance of 1.4301 (304) at 850C. Generally grades 1.4016 (430), 1.4301(304) and 1.4541 (321) can be considered up to 800C in sulphur dioxide. Ferritic 1.4749 (446) and austenitic 1.4845 (310) heat resisting grades can be considered up to 1025C and 1050C, respectively.

Affect of sulphur containing gas conditions-hydrogen and hydrogen sulphide

Stainless steels with less than 20% chromium may form less stable chromium sulphide 'scales'. The 1.4845(310) grade (24-26% Cr in BS EN 10095) can be considered for hydrogen sulphide environments.

Combustion atmospheres with sulphur

The sulphur levels in fuels such as natural gas are low and so the combustion products of these fuels are not a hazard to stainless steels, from the point of view of sulphur content. High sulphur oil combustion products may be a hazard and it is not sensible to generalize on these.

Liquid sulphur and sulphur vapour

Liquid sulphur at up to 240C should not attack stainless steels, but at its boiling point (445C), liquid sulphur is very aggressive to 1.4301 (304) type. Grades such as 1.4401(316) have some resistance and may be useful under certain conditions.
Sulphur vapour is very aggressive to stainless steels but the higher chromium 1.4845(310) grade may be worth considering.

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