Introduction

Most metals are thermodynamically unstable with respect to many of their compounds, particularly metal halides and sulphides, in addition to oxides. This is reflected in the extensive processing that is required to extract metals from their various ores. This chapter is concerned with the corrosion reactions by which metals return to their stable oxidized state, by reaction with sulphur, carbon, nitrogen, and the halogens. Often the corrosive gases encountered in practice contain oxygen, as well as one of these components. This problem of ‘mixed-oxidant corrosion’ will be addressed in the next chapter.

Reactions with sulphur

Metals react readily with sulphur, by mechanisms that parallel those involved in reactions with oxygen. The sulphides of iron and nickel show metallic conduction, i.e., they have high electrical conductivities, which decrease with increasing temperature and are nearly independent of sulphur partial pressure and stoichiometry (H. J. Grabke, personal communication). The departure from stoichiometry in sulphides is generally greater than in the case of oxides, diffusion is faster in sulphides, which are also more plastic than the corresponding oxides and generally have lower melting points. In particular, low-melting metal-sulphide eutectics are common.

Compared with oxidation, the sulphidation of a metal proceeds more rapidly and can be studied at lower temperatures and over shorter times. Sulphide scales tend to be quite plastic but their adherence is not good because most grow by outward cation diffusion and form voids at the scale–metal interface.