Received: 13 October 2017 | Accepted: 17 November 2017 DOI: 10.1002/maco.201709895 ARTICLE Effect of cold working on oxidation behavior and nature of oxide film formed on carbon steel Vivekanand Dubey 1,2 | Supratik Roychowdhury 1,2 | Vivekanand Kain 1,2 1 Materials Processing and Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India 2 Homi Bhabha National Institute, Anushaktinagar, Mumbai 400 094, India Correspondence Vivekanand Dubey, Materials Processing and Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. Email: vdubey@barc.gov.in Carbon steel is extensively used as a structural material in the secondary circuit of nuclear reactors. During fabrication of components such as elbows, T-joint, expander/reducer, etc., the material is subjected to cold working. Aim of this study is to systematically investigate the effect of cold working on the oxidation behavior of carbon steel in alkaline environment at high temperature and pressure. Specimens from cold worked (up to 30%) carbon steel samples were oxidized in an autoclave at 270 °C for durations of up to 240 h in water of pH 25 °C 10–10.2 obtained by adding lithium hydroxide. Oxidation rate, as calculated by weight gain, increased with extent of cold working. Potentiodynamic polarization and electrochemical impedance measurements were done in a borate buffer solution of pH 9.2 at room temperature, to characterize the oxide film formed on cold worked specimens. Impedance measurement followed by Mott–Schottky analysis showed that the defect density in the oxide increased with extent of cold working. Detailed examination established that the type and size of crystallites formed on the outermost oxide layer was the same irrespective of the extent of cold working. KEYWORDS carbon steel, flow accelerated corrosion, magnetite, Mott–Schottky analysis, oxidation 1 | INTRODUCTION Carbon steel is extensively used as a structural material for majority of components in the secondary circuit of pressurized heavy water reactors (PHWRs). Feed water system of Indian PHWRs is made of carbon steel ASTM A 106 Gr B. [1,2] A major mode of degradation of these secondary circuit components/pipelines carrying high velocity water/steam is flow accelerated corrosion (FAC) which causes thinning. The secondary circuit components are exposed to temperatures upto 200 °C, the process fluid being alkaline water (pH adjusted by ethanolamine (ETA) or ammonia). It is known that the mechanism of FAC is electrochemical dissolution and the protective magnetite film formed on the internal surfaces of carbon steel pipelines/components dissolves at a faster rate in a high velocity fluid environment, resulting in an increased rate of wall thinning. [3–6] FAC is a major mode of degradation in carbon steel piping used in fossil, industrial steam, and nuclear power plants. [1–2,6–8] The parameters that affect FAC are environmental, material, and hydrodynamic. [8–9] Environmental parameters include water chemistry parameters such as pH, dissolved oxygen (DO), etc. and temperature of the water. Material parameters include alloying elements in the carbon steel such as chromium, molybdenum and copper. While hydrodynamic parameters affects FAC rate as it determines whether the flow is turbulent or laminar and determines the velocity of process fluid. Water chemistry parameter, especially pH, affects the FAC rate by altering the solubility Materials and Corrosion. 2017;1–9. www.matcorr.com © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim | 1