Pitting susceptibility of induction-quenched pipeline with microstructural heterogeneity

Abstract

Pitting corrosion was found to be the immature failure mechanism for a slurry pipe that has an internal surface hardened with induction-quenching technique. The metallographic examination shows that the microstructure of surface layer is a banded mixture of martensite, ferrite, and pearlite. The pits initiate preferably in the ferrite bands. Electrochemical measurements are conducted to evaluate the pitting susceptibility of each constituent. When the small amount of chloride is present, the breakdown potential of ferrite is lower than those of martensite and pearlite. The high likelihood of metastable pit initiation in ferrite is also demonstrated by the electrochemical noise records. The development of pits in ferrite in the mixed microstructure is further promoted by the galvanic effect when ferrite is coupled with pearlite or martensite. The passive film of steel is an n-type semiconductor. The Mott–Schottky measurements indicate that the passive film formed on the ferrite specimen contains more oxygen vacancies, suggesting that the lower breakdown resistance is likely due to a highly defective film structure. The high-to-low pitting susceptibility is ferrite > martensite > pearlite. The experimental observations of this study imply that a practical approach to improve the performance of tailing pipes is to eliminate the microstructural heterogeneity in the surface layer by modifying the induction-heating process.