Nihal Obeyesekere

9 – Pitting corrosion

This chapter reviews the critical factors that influence pitting corrosion. The factors discussed include temperature, pressure, surface roughness, the environment, potential, metal composition, and the effect of electrolytes, especially the role of anions. Different stages of pit formation, including metastable pits and stable pits due to the passive film breakdown, and factors for pit growth are reviewed. The criteria for stable pit growth are highlighted because only a limited fraction of pits grow to be stable pits because the majority of pits die at metastable state. Pitting corrosion in sour systems is emphasized. The influence of the water chemistry, H2S concentration, iron sulfide polymorphism, system temperature, and pressure in the formation of pits in these systems, and the role of elemental sulfur are discussed. Finally, the importance of metal surface properties to pit growth and pit propagation is presented. Especially, metal surface modifications that can produce a surface to be less or more resistant to both pitting and general corrosion are discussed. The current stage of research toward understanding pitting corrosion in the presence of acid gases such as CO2 and H2S is reviewed. Finally the chapter briefly discusses future research trends in understanding and managing pitting corrosion.