Elemental Sulfur Corrosion

Elemental sulfur is assumed to have an effect on both active and passive dissolution. In the case of active dissolution, we assume the same mechanism as for carbon steel. We assume the additional cathodic process

FeS.S = FeS + S^2- – 2e

The symbol FeS.S denotes elemental sulfur attached to the surface covered (usually partially) by FeS.

The intermediate formation of FeS in the active state is a very reasonable assumption because elemental sulfur readily disproportionates and can readily induce the formation of sulfides (which form very easily on Fe and even easier on Ni). Then how is the sulfur attached to the surface? The particle size dependence enters here. Experimental studies indicate that S⁰ can be a powerful oxidizing agent if it is contact with the metal. On the other hand, it cannot do much if it just lies at the bottom of an apparatus and is not in contact with the metal. Consequently, small fine particles are much more likely to get attached to the surface than big particles. At the extreme, a clump of sulfur that you can hold in a hand cannot be in close contact with the surface except at one point for geometrical reasons. Therefore, we introduced an empirical function for the effect of sulfur particle size:

F = exp (d * size)

The parameter d is of course negative and F has a maximum at zero particle size (i.e., for extremely fine particles). For big clumps, it rapidly decays to zero. This function multiplies the exchange current density for the reaction

FeS.S = FeS + S^2- – 2e

The above mechanism of S⁰ contribution is sufficient for carbon steel. However, it cannot be the only phenomenon that is operative on stainless steels. If S⁰ did not affect passive dissolution, there would be no increase in corrosion rate at pH conditions when stainless steels are usually passive. Therefore, we assume that elemental sulfur acts as an aggressive species according to our formalism of aggressive and inhibitive species (which is described in our older papers). This is a reasonable assumption because various experimental studies have shown that sulfur (no matter in what oxidation state) can be incorporated in the passive layer. Again, we have to assume a certain function of particle size and we use the same function F as above. This function multiplies the dissolution rate for the aggressive species.

So, the net effect on the corrosion rate and corrosion potential results from the above two contributions. The S⁰ effect on passive dissolution increases the dissolution rate and decreases the corrosion potential. The partial cathodic reaction

FeS.S = FeS + S^2- – 2e

increases the corrosion potential. The net effect is a balance between these effects.

It needs to emphasized that our knowledge of the mechanism of elemental sulfur corrosion is still completely insufficient. The above mechanism should be treated only as a hypothesis. While each assumption is qualitatively reasonable, it should be verified. The parameters for the effect of particle size are essentially a conjecture and would need to be calibrated based on well-defined experimental data. There is a strong need for a systematic experimental study of the effect of elemental sulfur that would elucidate the mechanism.

Editor: James Berthold Author: Andre Anderko