Study on Localized Corrosion Induced by Non-metallic Inclusions in OCTG Steel

The initiation and propagation of localized corrosion induced by typical composite inclusions in OCTG (oil country tubular goods) steels were investigated by continuous immersion test combining SEM-EDS detection and theoretical calculation. Localized corrosion induced by inclusions with spinel covered by calcium aluminate and CaS initiated at the dissolution of calcium aluminate and CaS. Most of spinel parts remained intact but would drop off, and fewer spinel were found dissolved. The reason for localized corrosion induced by inclusions in OCTG was discussed in the view of galvanic corrosion and chemical dissolution. The energy band structure of inclusions and potential difference between the inclusions and the Fe matrix was estimated based on first principles. The chemical dissolution of steel matrix and inclusions were analyzed based on thermodynamic calculation. The calculated results were in good agreement with experimental results. A schematic diagram of mechanism for localized corrosion induced by Mg–Ca–Al–O/S complex inclusions in OCTG steel was established.