The Effect of Interstitial Carbon on Pitting Corrosion Resistance of Martensitic Carbon Steels
Authors: Kadowaki Mariko , Muto Izumi , Sugawara Yu , Doi Takashi , Kawano Kaori , Hara Nobuyoshi 
Published: February 2020

Ultra-high strength steels with excellent ductility are required to reduce the weight of automobiles and other transportation vehicles. While martensite including high amount of interstitial carbon is favored for strengthening steels, the ductility is relatively low. To optimize the balance between ductility and strength, tempering is conducted.
In the case of carbon steels, the precipitation of carbides occurs, and the amount of interstitial carbon decreases during tempering. Although tempering provides the balance between the strength and the ductility, it was reported that the pitting corrosion resistance tends to decrease by tempering.1 To achieve a balance between pitting corrosion resistance and mechanical properties, it is important to determine the quantitative effect of tempering on the pitting corrosion resistance.
The specimens were AISI 1045 carbon steel (0.47%C, 0.19%Si, 0.85%Mn, 0.015%P, 0.015%S, 0.02%Ni, 0.18%Cr, 0.01%Mo, 0.01%Cu, <0.001%Ti, <0.002%Nb, 0.024%Al, 0.005%N, 0.002%O). The specimens were heat-treated at 1123 K for 3.6 ks and then quenched in water to form a full martensitic structure. To obtain a tempered martensitic structure, the as-quenched specimens were heated to 873 K and kept at this temperature for 0.1, 1, 10, 15, and 20 h in an Ar atmosphere. The metallographic inspection was performed to ascertain the characteristics of the microstructures of the as-quenched and tempered specimens by using FE-SEM. In tempered martensitic specimens, many small white dots were observed. This is evidence of the carbides formed by tempering. The sizes of the carbides increased with tempering time, meaning that the interstitial carbon decreased.
The micro-scale anodic polarization measurements of the as-quenched and tempered specimens were conducted in a boric-borate buffer solution with 500 mM NaCl (pH 8.0) under naturally aerated conditions at 298 K. A small area without non-metallic inclusions was selected for the working electrode. The polarization measurements started at -0.1 V to eliminate the influence of active dissolution on pitting corrosion initiation. For 20 h tempered specimen, a stable pit was initiated immediately after immersion. The pitting potential of 20 h tempered specimen was thought to be less than -0.1 V. In the case of 15 h tempered specimen, the sharp increase in current density was observed at 0.05 V due to the pit initiation. In contrast, no pit was initiated on the as-quenched specimen and the specimens tempered for less than or equal to 10 h. It was found that the pitting corrosion resistance of the martensite tempered for less than or equal to 10 h was higher than that of the martensite tempered for more than or equal to 15 h. Pitting corrosion resistance of martensite decreased with tempering time.
To clarify the role of the interstitial carbon on the pitting corrosion resistance, the macro-scale anodic polarization curves of as-quenched, 10 h tempered, and 20 h tempered specimens in a boric-borate buffer solution with 1 mM NaCl (pH 8.0) were measured. The polarization started at -1.0 V after a cathodic treatment (-1.2 V, 600s). For the macro-scale polarization, the region of active dissolution appeared on all specimens in the potential region from -0.7 to -0.6 V. The dissolution current density for 20 h tempered specimen was certainly higher than that of other two specimens. It is suggested that the higher pitting corrosion resistance of the as-quenched and short-time tempered specimens was due to the low dissolution rate of those specimens. It is proposed that the inhibition of the active dissolution rate of these specimens is because of the existence of interstitial carbon, and interstitial carbon improves the pitting corrosion resistance. Short-time tempering is one of the answers for striking an optimal balance between pitting corrosion resistance.
References;
1. M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).