The Role of MnS Inclusions in the Localized Corrosion of Carbon Steel

Dear Colleagues,

The corrosion of a material in an aqueous medium is influenced by many factors, including the redox properties and microbial activity of the aqueous medium and the metallurgy of the material. This chapter focuses on the role that the metallurgical preparation of 1018 carbon steel plays in its localized corrosion. In particular, we are interested in the lattice defects immediately surrounding MnS inclusions and the cementite phases that are inherent in carbon steel. When a foreign phase is introduced into bcc iron, it is expected that the ferrite phase in the immediate surroundings of the islands of the foreign phase will be distorted, giving rise to a large number of dislocations that cause plastic strain in these regions. This strain increases the propensity of carbon steel toward corrosion. Almost all the carbon in carbon steel is locked into the cementite phase. Similarly, all the sulfur is locked into MnS stringers (~3000 stringers/mm²). Experiments conducted in marine solutions under sulfidogenic, anaerobic, and suboxic conditions, with anaerobic and/or aerobic bacteria, confirm that localized corrosion initiates and grows from the immediate surroundings of MnS inclusions. Localized defects in the ferrite lattice of the immediate surroundings of MnS inclusions generate anodic sites, which initiate excessive iron dissolution.  The resulting excess Fe⁺²/Fe⁺³ draws excess Cl- into the region, and Fe⁺²/Fe⁺³ hydrolysis acidifies these areas locally, which spurs more corrosion, which in turn dissolves the MnS inclusions. The dissolution of MnS abiotically increases the local H₂S and elemental S; this process in turn increases the cathodic processes. Thus, a snowball effect is created, which promotes deeper and wider pits within localized areas. The spatial distribution of MnS inclusions in low carbon steel shows regions of high concentration, which would most likely to lead to macroscopic pitting at these locations.

Professor Dr. Recep Avci
Guest Editor

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• MnS
• MIC
• SRB
• Pitting
• Sulphide
• Carbon steel
• Pearlite
• Suboxic
• Strain
• Dislocations