Corrosion and deposition during the exposure of carbon steel to hydrogen sulphide-water solutions

Abstract

Complex multi-phase corrosion films develop on rotating carbon steel discs exposed to aqueous hydrogen sulphide solutions; their structure and morphology can have a profound effect on the corrosion process. Iron sulphide corrosion products formed on corroding carbon steel discs in titanium autoclaves have been characterized after exposure periods ranging from 1 to 840 h at temperatures of 308, 373 and 433 K and a total initial pressure of 1.5 MPa. These reaction conditions pertain to the Girdler-Sulphide process for separating heavy water. In oxygen-free solutions, the evolution of corrosion products on the discs progresses from iron-rich to sulphur-rich phases according to the sequence, mackinawite (tetragonal FeS 1−x) → ferrous sulphide (cubic FeS) → troilite (hexagonal FeS) → pyrrhotite (hexagonal Fe 1−xS) → pyrite (cubic FeS 2), the latter phase being thermodynamically favoured. All phases except mackinawite appear as characteristic microcrystals of regular geometry, indicating relatively slow solution growth at low supersaturation. Higher temperatures accelerate the sequential transformations while higher speeds of rotation of the disc retard it. Edge turbulence induced at high rotation frequencies prevents the formation of solution-grown phases. Added oxidants promote the formation of the disulphide ion required for FeS 2 formation. Fe 2+ ions released to the bulk solution by dissolution of the base metal and metastable sulphides are deposited as pyrrhotite or pyrite on the titanium vessel. The S 2 2− ion required for pyrite deposition is thought to arise by electro-oxidation of a sulphide species at the titanium surface.