The Relationship Between CO2 Corrosion Resistance and Corrosion Products Structure on Carbon and Low Cr Bearing Steels

In order to clarify the effect of microstructure on CO2 corrosion resistance of carbon and low Cr steels (Cr content of 1 to 5 mass %), immersion tests were carried out in CO2 environment at 60 or 80°C by using carbon and low Cr steels with different microstructures. The relationship among corrosion behavior, microstructure and corrosion products structure were investigated. For carbon steel, ferritic pearlitic microstructure had better localized corrosion resistance than martensitic microstructure, because lamellar Fe3C improved denseness and adhesion of the primary corrosion product. On the other hand, for low Cr steel, martensitic microstructure had better CO2 corrosion resistance than ferritic pearlitic microstructure. From the result, the primary corrosion product with good corrosion resistance is considered to be formed on homogeneous microstructure for low Cr steel. Weight loss of the martensitic steels with more than 3%Cr was as low as one forth of carbon steel with ferritic pearlitic microstructure. The primary corrosion product formed on 3%Cr martensitic steel had dark and light gray phases. It is considered that the dark gray phase with higher Cr enrichment content acts as corrosion protective film mainly. CO2 corrosion resistance of 3%Cr martensitic steel was not affected by C content and tempering temperature because Cr enrichment content in the dark gray phase was high enough (more than 10 times of Cr content in mother metal).