Pitting Corrosion Resistance of Martensite of AISI 1045 Steel and the Beneficial Role of Interstitial Carbon

Abstract

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.

Improving Pitting Corrosion Resistance at Inclusions and Ductility of a Martensitic Medium-Carbon Steel: Effectiveness of Short-Time Tempering

“…The procedure used to fabricate the micro-scale electrodes was described elsewhere. 6 Micro-scale electrodes that contained only one non-metallic inclusion were fabricated to determine the localized corrosion resistance at non-metallic inclusions. The potentiodynamic anodic polarization was performed in boric-borate buffer solutions with 1 and 500 mM NaCl under naturally aerated conditions at 298 K. In this study, a system similar to that developed by Chiba et al was used.…”

“…Our previous research reported that the pitting corrosion resistance of as-quenched martensite was higher than that of low-carbon martensite prepared by decarburization because the active dissolution rate of the steel matrix decreases due to the interstitial carbon. 6 It has been shown that the active dissolution of type 304 austenitic stainless steel was suppressed by interstitial carbon in chloride solutions. 37 Therefore, the high pitting corrosion resistance of the as-quenched martensite, 1 h tempered martensite, and 10 h tempered martensite can be attributed to the interstitial carbon in the martensitic structures.…”

“…This finding is consistent with findings from previous studies. 6,37,38 The peak current density in the active region is determined by the dissolution and passivation rates of steel matrix. Low dissolution rate and high passivation rate provide better corrosion resistance, and hence the peak current density is seemed to be related to pitting corrosion resistance.…”

“…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 interesting question is whether it is possible to achieve a balance between corrosion resistance and mechanical properties. Although previous research has reported that a decrease in the amount of interstitial carbon lowers the corrosion resistance, 6 few studies have focused on the optimal tempering condition to strike a balance between the mechanical properties and pitting corrosion resistance. To understand this challenge from an engineering perspective, it is important to determine the quantitative effect of tempering on the pitting corrosion resistance of martensite.…”