Electrochemical characterization of oxide formed on chromium containing mild steel alloys in LiOH medium

• Published in: Materials chemistry and physics Vol. 145; no. 3; pp. 499 - 509
• Main Authors: Subramanian, Veena, Chandran, Sinu, Subramanian, H., Chandramohan, P., Bera, S., Rangarajan, S., Narasimhan, S.V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 16.06.2014
• Subjects: A. Alloys; C. Electrochemical techniques; C. X-ray photo-emission spectroscopy (XPS); D. Corrosion; D. Corrosion; C. Electrochemical techniques; A. Alloys; C. X-ray photo-emission spectroscopy (XPS)
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2014.03.003

Flow accelerated corrosion leads to wall thinning of outlet-feeder pipes in the primary heat transport system of pressurized heavy water reactors and can even necessitate enmasse feeder replacement. Replacement of carbon steel 106-grade-B (CS) with chromium containing carbon steel reduces the risk of this failure. This paper discusses the role of small additions of chromium in modifying the properties of the oxide film. CS and chromium containing mild steels viz., A333, 2.25Cr–1Mo and modified 9Cr–1Mo alloy were exposed to primary heat transport (PHT) system chemistry conditions. The oxide films formed were characterized by electrochemical and surface characterization techniques. Mott–Schottky analysis showed donor type of defects. The densities of defects in the oxides of chromium containing alloys were 3–15 times less than that in CS. In presence of ∼200 ppb of dissolved oxygen, the oxides formed were hematite with two orders of magnitude smaller concentration of defects as compared to that formed under reducing conditions. These results suggest that the presence of chromium lowers the defect density of the oxide film and thus ensures a reduced corrosion rate. [Display omitted] •High temperature oxides formed on Cr containing mild steels are less defective.•Defect densities of oxides decrease with increase in Cr content in the alloy.•O2 in solution greatly influences the nature and defect chemistry of oxides.