(Corrosion Division Morris Cohen Graduate Student Award) Microscopic Electrochemical Properties of Carbon Steels and Metallurgical Approach for High Corrosion Resistance

Abstract

Carbon steels are widely used in the production of automobiles, bridges, plates, and so on because of their superior mechanical properties and low cost. Carbon steels contain various types of microstructures such as ferrite, pearlite, and martensite, and desired mechanical properties can be obtained by combining these microstructures. While carbon steels exhibit excellent mechanical properties, their pitting corrosion resistance is relatively low in chloride-containing environments. Although the steels are successfully protected from corrosion by coating and/or painting, localized corrosion is readily initiated at cut edges and in coating defect areas in atmospheric environments. To prolong their service life and improve their reliability, it is necessary to elucidate the initiation mechanism of pitting corrosion on carbon steels. While it is widely known that the microstructures of carbon steels affect their corrosion resistance, the detailed mechanism is still unclear. The objective of this research is to clarify the relationship between the microstructure and pitting corrosion resistance of carbon steels in chloride-containing near-neutral pH environments.The pitting corrosion resistance of the typical microstructures of carbon steels was investigated using a micro-electrochemical polarization technique. Various microstructures (ferrite, pearlite, tempered martensite, decarburized martensite, as-quenched martensite etc.) were obtained by conducting different heat-treatments, and micro-scale electrodes (ca. 100 μm × 100 μm) were prepared with only one microstructure included. Small areas without any non-metallic inclusions were chosen as the micro-scale electrodes to eliminate the effect of pit initiation at the inclusion and evaluate the intrinsic corrosion behavior of the microstructure itself. Then, potentiodynamic anodic polarization measurements were carried out in NaCl-containing boric-borate buffers at pH 8.0. It was determined that the pitting corrosion resistance depends on the type of microstructure, with the as-quenched martensite showing the highest pitting corrosion resistance. The as-quenched martensite includes a high amount of interstitial C in the Fe lattice, and it was also clarified that the anodic dissolution of steels was clearly suppressed with increasing interstitial C content. During pitting corrosion, the local breakdown of the passive film is induced by chloride ions, which results in the initiation of active dissolution. It was considered that the low dissolution rate provided by interstitial C results in the superior pitting corrosion resistance of as-quenched martensite.To clarify the reason for the superior active dissolution resistance obtained by interstitial C, first-principles calculations were conducted. At first, the electronic structure of martensite was analyzed using bulk-model supercells with bct Fe crystal structures which included interstitial C. According to first-principles calculations, the electronic density of states (DOS) at and around the Fermi level decreased due to the presence of interstitial C. In addition, a valence electron transfer occurred from Fe to C atoms, resulting in a decrease in the number of valence electrons of Fe. Because the presence of valence electrons at and around the Fermi level affects the kinetics of electron (charge) transfer in oxidation/reduction reactions, including the active dissolution reaction, it was considered possible that the presence of interstitial C suppressed the electronic reactivity of Fe. Furthermore, first-principles calculations were also carried out for slab-model supercells consisting of a metal/vacuum interface, and the effect of interstitial C on the work function of martensite was analyzed. The result of the calculations indicated the work function of martensite linearly increased with increasing interstitial C content. Since the work function and ionization energy of metals are the same, the electrochemical properties of metals, such as dissolution resistance, are considered to be principally associated with their work function. In other words, it is understood that metals with a high work function are associated with a higher dissolution resistance. According to the above calculation results, the change in the electronic structure of Fe and the increase in the work function provided by interstitial C are key factors contributing to the superior dissolution resistance of martensite.