The initiation site and morphology during the early stage of pitting on AISI 1045 carbon steel that has a microstructure of primary ferrite and pearlite were investigated in boric-borate buffer solutions with and without NaCl at pH 8.0. The pits initiated by micro-scale polarization were in the pearlite only and not in primary ferrite. In situ real-time observations during the micro-scale polarization of pearlite in a boric-borate buffer solution with 100 mM NaCl indicated that the pits were polygonal or rod-like in shape. In addition, it was found that the pit growth direction was the same as that of the pearlite lamellae that consisted of ferrite and cementite. Field-emission electron probe micro analysis detected segregated points of sulfur in the ferrite lamellae. On the basis of their etching behavior in 3% nital, the corrosion resistance of the cementite was estimated to be higher than that of the ferrite lamellar structure. Thus, pits readily initiated in the ferrite lamellae and proceeded along the ferrite lamellae. Ferrite-pearlite steel is widely used in the production of bars, plates, steel wires, etc.1-3 because of its high strength, ductility, toughness, wear resistance, and low cost.3-5 Pearlite has a lamellar structure that consists of ferrite and cementite (Fe 3 C) phases. A fine pearlite structure improves the mechanical properties of steels in terms of strength and ductility.6-8 While ferrite-pearlite steels exhibit excellent mechanical properties, their pitting corrosion resistance is relatively low. It is widely believed that the boundaries between the different phases act as initiation sites for localized corrosion in chloride environments. 9,10When ferrite-pearlite steel that does not undergo any surface treatment is exposed to chloride environments, the steel readily suffers from pitting corrosion. Although the steel is successfully protected from corrosion by coating and/or painting, to prolong their service life and improve their reliability, it is necessary to elucidate the initiation mechanism of pitting on ferrite-pearlite steel.The pitting corrosion process involves two stages: initiation and propagation.11 In chloride environments, pitting is initiated by a breakdown of the passive film on steel during a local active dissolution process that exposes the bare steel surface to the environment. This film-free dissolution involves the hydrolysis reaction of metal cations and results in acidification. Chloride ions migrate into the pit to maintain electrical neutrality, and the simultaneous accumulation of chloride ions combined with the high level of acidification accelerates the active dissolution inside the pit. There have been many reports about the propagation stage of pitting corrosion in ferrite-pearlite steels. A comparison of the active dissolution rates and the electrochemical properties of ferrite and cementite have been widely discussed. It is commonly believed that cementite has a lower dissolution rate than ferrite. Yumoto et al. synthesized stoichiometric Fe 3 C films with...

The pitting corrosion resistance of AISI 1045 carbon steel with as-quenched, tempered, and low-carbon martensitic microstructures was investigated in boric-borate buffer solutions with and without NaCl. Analysis by micro-scale polarization found that tempering and decarburizing treatments decreased the pitting corrosion resistance of as-quenched martensite. The high corrosion resistance of the as-quenched martensite was likely due to the large amount of interstitial carbon. The pitting corrosion resistances of asquenched martensite, primary ferrite, and pearlite were compared using micro-scale polarization measurements. It was determined that the pitting corrosion resistances of the typical steel structures were ordered as follows: (high) as-quenched martensite > tempered martensite ≈ primary ferrite > pearlite (low). The pitting corrosion resistance of steel was shown to depend on its microstructure. © The Author ( Background.-Quenched and tempered martensitic carbon steels are known for their high strength.1-5 The supersaturation of interstitial carbon and high dislocation density provide martensitic steels with their high strength. 4 However, the toughness of as-quenched martensitic steels is relatively low. To optimize the balance between strength and toughness, the steels are subjected to tempering.1 Tempering involves heating the steels so that the non-equilibrium microstructure can return to near-equilibrium conditions. 4 However, tempering also lowers the dislocation density due to the recovery and recrystallization processes.2-4 It has also been shown that the precipitation of carbides results in a decrease in the amount of interstitial carbon.1 While tempered martensitic carbon steels have excellent mechanical properties, the effect of tempering on the pitting corrosion behavior of carbon steels is unclear. While such steels can be 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 mechanism of pitting corrosion and assess the pitting corrosion resistance of as-quenched, and quenched and tempered martensitic steels.Fundamental research on the corrosion mechanisms of carbon steels started during the 1950s. Stern found that the addition of carbon (0.11 mass% C) increased the corrosion rate of pure iron in 4% NaCl at pH 1 and 2. 6 This increase in the corrosion rate was explained by the decrease in the hydrogen overvoltage due to the addition of carbon. It is known that iron carbides (Fe 3 C) act as cathodic sites.

The optimal tempering conditions for a martensitic medium-carbon steel (0.47 mass% C) were investigated in terms of balancing ductility and pitting corrosion resistance. By tempering the as-quenched martensite, its Vickers hardness drastically decreased within 0.1 h, suggesting that ductility was sufficiently recovered by short-time tempering. Based on the results of micro-scale polarization in boric-borate buffer solutions with NaCl (pH 8.0), stable pits were initiated at non-metallic inclusions in the specimens tempered for at least 1 h; however, no stable pit was generated on the 0.1 h tempered and as-quenched specimens. Short-time tempering of martensite was suggested to be a feasible approach to striking an optimal balance between facilitating pitting corrosion resistance and achieving the desired mechanical properties of martensitic carbon steels. To reduce the weight of automobiles and other transportation vehicles, ultra-high strength steels with excellent ductility are necessary. It is desirable that excellent mechanical properties be attained without deteriorating the corrosion resistance. Even if organic or inorganic coatings are used in the actual applications of the steels, bare steel surfaces are exposed at cut edges, scratched surfaces, and other damaged areas of the coatings. Therefore, high corrosion resistance remains a fundamental requirement for ultra-high strength steels.A martensitic structure with fine grains is favored for strengthening steels, and interstitial carbon imparts high strength to martensite. The martensitic structure is formed during shear-dominated transformation via quenching.1 In the phase transformation from austenite to martensite in carbon steels, interstitial carbon atoms in the fcc (facecentered cubic) crystal structure of austenite remain as supersaturated carbon in the bct (body-centered tetragonal) martensitic structure. 1-5Supersaturation of interstitial carbon, a fine structure, and a high dislocation density are introduced by this transformation. 5 To optimize the balance between ductility and strength, the as-quenched martensite is subjected to tempering because the ductility of as-quenched martensitic steels is relatively low.1 During this process, the strength of the steel decreases and the ductility recovers, supplying an optimum balance of strength and ductility. In the case of carbon steel, precipitation of carbides also occurs during tempering. As a result, the concentration of interstitial carbon in martensite decreases, which is associated with changes in both the chemical and mechanical properties. Kadowaki et al. demonstrated that the pitting corrosion resistance of as-quenched martensite (0.47 mass% C) is higher than that of low-carbon martensite prepared by quenching after decarburization treatment.6 Despite the optimum balance between strength and ductility achieved by quenching and tempering treatments, the pitting corrosion resistance decreased during tempering. This seems to be because of the decrease of the interstitial carbon concentration. An...

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The effects of N, C, and B interstitials on the corrosion resistance of Fe were investigated in chloride-free boric-borate solutions at pH 6.0 and 8.0. In potentiodynamic polarization at pH 8.0, the anodic dissolution resistance of Fe-0.3N and Fe-0.3C in the active and passive regions was higher than that of pure Fe. NH 4 + and NO 2 − are considered to be dissolved chemical species that contribute to the higher corrosion resistance of Fe-0.3N. Potentiodynamic polarization measurements in a solution with HCO 3 − indicated that HCO 3 − also decreases the anodic current densities in the active and passive regions, suggesting that the formation of HCO 3 − contributes to the higher corrosion resistance of Fe-0.3C. First-principles calculations showed that the presence of N, C, and B in the Fe-lattice decreases the electronic density of states (DOS) at and near the Fermi level. The consistency between the active dissolution rates and the DOS at and near the Fermi levels of the specimens suggests that the more stable electronic structures occurred by the presence of N and C also result in the suppression of active dissolution of Fe. For Fe-0.3B and Fe-0.006B, the presence of iron boride precipitates promoted localized corrosion.

The anodic polarization behavior of cementite (Fe 3 C), which was prepared by gas carburizing, was investigated in 10 mM NaClcontaining boric-borate buffers with pH values of 6.0, 7.0 and 8.0. The active dissolution current of the Fe 3 C was found to be lower than that of the ferrite. This suggests that the corrosion resistance of the Fe 3 C was clearly higher than that of the ferrite. From the potential-pH diagrams, carbon was predicted to be generated on the Fe 3 C at lower potentials during the anodic polarization, and the existence of carbon was confirmed by XPS. The carbon layer seems to act as a protective film and suppress the active dissolution of the Fe 3 C at lower potentials. AES depth profiles clarified that iron oxide layers existed not only on the ferrite but also on the Fe 3 C. The Volta potential of the Fe 3 C was approximately 40 mV higher than that of the ferrite. The higher Volta potential of the Fe 3 C layer seemed to be associated with the nature of the oxide film formed on the Fe 3 C, and there is a possibility that the passivation of the Fe 3 C at higher potentials is achieved by an oxide film.

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