Effect of Different Cooling Rates on the Corrosion Behavior of High-Carbon Pearlitic Steel Prvan Kumar Katiyar, Sudhir Misra, and K. Mondal (Submitted November 10, 2017; in revised form January 18, 2018; published online March 6, 2018) The present work discusses the effect of pearlitic morphology on the corrosion behavior of high-carbon fully pearlitic steel in 3.5% NaCl solution. Four different types of pearlitic steels (furnace-cooled, as- received, air-cooled and forced-air-cooled) consisting of coarse, medium, fine and very fine microstructures, respectively, were tested. Electrochemical behavior of these steels was studied with the help of dynamic and linear polarization and AC impedance spectroscopic tests. The corrosion resistance improved with fineness of the microstructure in general. However, with further reduction in interlamellar spacing beyond a limit, the corrosion resistance reduced slightly. Formation of homogeneous distribution of microgalvanic cells between cementite and ferrite lamellae of fine pearlitic steel improved the corrosion resistance. However, entanglement of the lamellae of pearlite in very fine pearlitic structure as well as breaking of cementite lamellae due to finer pearlitic colonies was attributed to the higher corrosion of the forced-air-cooled steel as compared to the air-cooled steel. Keywords corrosion behavior, heat treatment, microscopy, pearlite 1. Introduction The high-carbon steels of composition (0.7% C, 0.239% Si and 1.12% Mn) with mainly pearlitic microstructure are generally used in rails, high-strength rope for bridge applica- tions, springs, locomotive tires and large forging dies (Ref 1). In order to enhance the mechanical properties of the high- carbon steel, various heat treatment processes are used and these can modify the microstructure of the steel without altering the composition (Ref 2). The mechanical and electrochemical properties of the steel change with microstructure (Ref 3, 4). The strength of the fully pearlitic steel depends on the interlamellar spacing between alternate ferrite (a-Fe) and cementite (Fe 3 C) phases (Ref 5). In order to obtain the fully pearlitic microstructure steel, furnace cooling, air cooling and forced air cooling operations are performed. As the cooling rate of the high-carbon steel increases, the pearlite microstructure becomes finer and inter- lamellar spacing decreases, leading to an increase in strength, toughness, wear resistance and hardness of the steel. Though alloying coupled with higher cooling rate can refine the pearlitic morphology in steel, cooling rate route is much preferred in many applications because of its effectiveness in controlling the microstructure and of its economical nature (Ref 7, 8). In many of the applications, corrosion behavior of the high- carbon pearlitic steel is also an important aspect. In pearlitic steel, two different active (a-Fe) and passive sites (Fe 3 C) are available which can significantly control the corrosion rate of the steel by forming local microgalvanic cells and can increase the dissolution of the ferrite phase (Ref 2, 6-9). Although there are large number of studies on the corrosion behavior of ferritic–pearlitic steels (Ref 10, 11), corrosion behavior of fully pearlitic steels is scarcely available. Numerous studies have been devoted to the corrosion behavior of dual- phase steels, such as ferrite–martensite and ferrite–bainite (Ref 12-14), but very little attention has been paid to the under- standing of corrosion behavior of fully pearlitic steels. Therefore, the corrosion mechanisms and corrosion preven- tion of high-carbon fully pearlitic steel are important aspects. Moon et al. (Ref 8) have reported that the newly developed high-carbon NCC (microalloyed Cr-Cu-Ni steel) normalized steel exhibits similar corrosion resistance as the conventional C-Mn steel. Balasubramaniam et al. (Ref 15) developed the new pearlitic steel by microalloying (Cr-Cu-Ni) and found that the crevice corrosion resistance of the novel microalloyed Cr- Cu-Ni rail steel was superior as compared to the conventional C-Mn pearlitic steel. Panda et al. (Ref 16) also investigated the corrosion behavior of different alloyed (Cr-Cu, Cr-Cu-Ni and Cr-Cu-Ni-Si) rail steels and compared the results with that of the conventional C-Mn, Cu-Mo and Cr-Mn rail steels. They observed that the rusts formed on the Cr-Cu-Ni and Cr-Cu-Ni- Si rail steels after salt fog test were more compact, which resulted in the higher impedance values as compared to other conventional rail steels (Ref 16). Available literature has shown that the corrosion mecha- nisms of the different pearlite microstructures in 3.5% NaCl solution have been broadly recognized (Ref 8, 15-17). But some other aspects, such as the effect of pearlite colony size and interlamellar spacing, on the corrosion behavior are still not clear. The detailed analysis of the influence of different interlamellar spacings on the corrosion mechanism of fully pearlitic steel developed from particular steel with fixed composition could be an interesting study. The present study is an attempt to find out the effect of different cooling rates and subsequent variation in interlamellar spacing for fully pearlitic steels on their corrosion behavior in freely aerated 3.5% NaCl solution. The corrosion mechanism of the different pearlitic Prvan Kumar Katiyar and K. Mondal, Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur 208 016, India; Sudhir Misra, Department of Civil Engineering, Indian Institute of Technology, Kanpur 208 016, India. Contact e-mail: kallol@iitk.ac.in. JMEPEG (2018) 27:1753–1762 ÓASM International https://doi.org/10.1007/s11665-018-3256-3 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 27(4) April 2018—1753