Effect of structural and phase changes in heat treatment of pipeline steels on the rate of general corrosion

Corrosion is one of key factors limiting the service life of oil and gas pipelines. To increase corrosion resistance steels are alloyed with elements permitting to obtain an oxide film protecting the metal from dissolution. But one may expect that corrosion rate of conventional carbon and low-alloy steels can be partially controlled through changing the structure by means of heat treatment. This research is devoted to studying the role of this factor for pipeline steels 20A and 20FA taken as examples. The steels were subjected to 18 different heat treatment regimes including normalizing at various temperatures, full quench hardening followed with high-temperature tempering, and quenching from intercritical temperature range followed with high-temperature tempering. Mechanical properties of heat treated steels were evaluated, as well as uniform (general) corrosion rate in a model medium containing hydrogen sulphide, with the aim to reveal possible effect of structural changes on steels’ resistance to corrosion. The worst corrosion resistance was provided by regimes including intercritical quench hardening, apparently because redistribution of carbon and alloying elements between α and γ phases during holding in the Ac1…Ac3 range intensifies corrosion. But in general corrosion resistance of steels oscillated irregularly, with no appreciable correlation with grain size, type of structure, tempering temperature and level of impact toughness. It is therefore clearly demonstrated that resistance of pipeline steels to corrosion in hydrogen sulphide media is mainly determined not by heat treatment regimes and structural characteristics, but by the chemistry of metal dissolution process (e.g., formation of protective films) and, possibly, by non-metallic inclusions.