Corrosion Behavior of Annealed Steels with Different Carbon Contents (0.002, 0.17, 0.43 and 0.7% C) in Freely Aerated 3.5% NaCl Solution Prvan Kumar Katiyar, S. Misra, and K. Mondal (Submitted November 10, 2018; in revised form March 14, 2019) Four annealed carbon steels with different carbon contents (0.002, 0.17, 0.43, and 0.7% C), consisting of ferrite, ferrite-pearlite and fully pearlite microstructures, were selected to understand the corrosion behavior of carbon steel as a function of carbon content in freely aerated 3.5% NaCl solution. Dynamic polarization, electrochemical impedance and linear polarization methods were used. The corrosion rate obtained from the different carbon steels was found to increase greatly from ultra-low carbon steel (0.002%) to low carbon steel (0.17%) due to the presence of pearlite in the low carbon steel. However, the increase in corrosion rate was marginal with the increase in carbon content from low carbon (0.17%) to medium (0.43% C) and high carbon steels (0.7% C). The mirostrucural evolution of the steels before and after polarization test without etching as observed by scanning electron microscopy could show that the corrosion behavior of the steels with the presence of pearlite was due to the combined effect from % pearlite, interlamellar spacing and cementite/ferrite area ratio in pearlite. Pearlite morphology also led to the differential corrosion within the pearlite colony in all the steels except the steel with 0.002% C. Catalytic activity of cementite on enhancing oxygen reduction reaction attributes to the higher corrosion rates in case of the steels with the presence of pearlite. Keywords AFM, corrosion, microstructure, SEM, steel 1. Introduction The mechanical and electrochemical properties of steel are directly interconnected to the chemistry, processing route and resulting microstructure of the selected steel. The plain carbon steel has vast ranges of applications too. Interstitial-free (IF) or ultra-low carbon steels are widely used in automobile applica- tions due to their excellent formability (Ref 1, 2). The ferrite- pearlite steels with 0.17 and 0.43% C steels are mostly used as a beam in bridges, buildings, ships and reinforcing bars (Ref 3). Steel with high carbon ( 0.77%) and fully pearlite microstruc- ture is widely used in rail making (Ref 4). Corrosion behavior of the plain carbon steel is also strongly influenced by the composition, especially content of C, Si and Mn (Ref 5). On the other hand, various researchers have tried to develop corrosion-resistant steels at minimum alloying addition. In this regard, they have altered the chemistry of the steel by changing C, Si and Mn contents, which not only offer the excellent strength to the materials, but they also influence the corrosion resistance (Ref 4, 6). The effect produced by Si and Mn is relatively complex, while the increase in C leads to the formation of cementite (Fe 3 C), which results in the development of galvanic couple between the ferrite (anode) and cementite (cathode) phases resulting in higher corrosion rates (Ref 7). Hence, the modification in the microstructures of steel is an effective way to improve the corrosion resistance of the plain carbon steel. Further alloying with Cu, Ni and Cr could effectively increase the corrosion resistance of low alloy plain carbon steel (Ref 8). Moon et al. (Ref 9) have also investigated the passivation behavior of ferrite-pearlite railway axle steels based on composition and microstructure and found better corrosion resistance of the newly developed axle steels due to their finer microstructure and strongly adherent protective rusts. According to the results of previous works (Ref 8), the corrosion resistance of pure iron and interstitial-free (or ultra- low carbon) steel has been reported to be very high because of the minor presence of carbides. On the other hand, in the case of medium and high carbon steels, the relative corrosion resistance has been reported to be low because the corrosion rate increases due to an increase in the cementite-to-ferrite area ratio (Ref 7, 10, 11). In this regard, Ferhat et al. (Ref 12) have reported that the corrosion rate of steel is strongly influenced by the amount of cementite and its morphological distribution on the ferrite phase. Katiyar et al. (Ref 13) have recently reported higher corrosion resistance for an air-cooled fully pearlitic steel due to its fine microstructure and reduction in interlamellar spacing leading to the uniform distribution of the galvanic couple. The corrosion resistance of a fully pearlitic steel can be further modified by altering the interlamellar spacing through the isothermal heat treatment process. However, the corrosion rates of the plain carbon steel having different microstructures Prvan Kumar Katiyar and K. Mondal, Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur 208 016, India; and S. Misra, Department of Civil Engineering, Indian Institute of Technology, Kanpur 208 016, India. Contact e-mail: kallol@iitk.ac.in. JMEPEG ÓASM International https://doi.org/10.1007/s11665-019-04137-5 1059-9495/$19.00 Journal of Materials Engineering and Performance