Role of corrosion product on the protectiveness of corrosion layers for microalloyed pipeline steel

Corrosion behavior of API grade microalloyed pipeline steel was studied in CO₂ saturated distilled water using weight loss technique. The protectiveness of corrosion layers varies with temperature. The experimental results show that corrosion layers are composed of an insoluble corrosion product of iron carbonate (FeCO₃) which is partially protective (30–60°C), or highly protective (90–120°C) and very severe corrosive between (60–90°C). At the lower temperatures, the metal is partially protected but as the temperature increases, the protectiveness completely diminishes due to non-uniform and porous nature of the corrosion layer. Beyond 90°C, the formation of FeCO₃ film predominates on the steel surface, which is dense and more adherent and hence the metal surface is completely protected from further corrosion. The corrosion products formed were analyzed by scanning electron microscope (SEM) and X-ray diffraction analysis.