Austenitizing treatment influence on the electrochemical corrosion behavior of 0.3C–14Cr–3Mo martensitic stainless steel Yoon-Seok Choi a , Jung-Gu Kim a, ⁎ , Yong-Soo Park b , Jee-Yong Park b a Department of Advanced Materials Engineering, Sungkyunkwan University, 300, Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea b Department of Metallurgical System Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120-749, South Korea Received 2 August 2005; accepted 6 April 2006 Available online 5 May 2006 Abstract Effects of austenitizing treatment temperatures on aqueous corrosion properties of martensitic stainless steels were investigated by electrochemical tests (potentiodynamic test, potentiostatic test and electrochemical impedance spectroscopy), and surface analyses (optical microscopy and XRD). The results of potentiodynamic test revealed that the breakdown potential increased with the increased austenitizing temperature, indicating increased relative resistance to initiation of localized corrosion. EIS measurements showed that MSS3 (1030 °C) exhibits larger polarization resistance value than MSS1 (970 °C) and MSS2 (1000 °C) at passive and breakdown states. This was caused by decreasing the amount of Cr-rich M 23 C 6 carbide which acts as preferential sites for pitting corrosion. © 2006 Elsevier B.V. All rights reserved. Keywords: Martensitic stainless steel; EIS; Austenitizing treatment; M 23 C 6 carbide; Pitting corrosion 1. Introduction Martensitic stainless steels are commonly used for manu- facturing components with excellent mechanical properties and moderate corrosion resistance, operating under conditions of either high or low temperature. As their properties can be changed by heat treatment, these steels are suitable for a wide range of applications such as steam generators, pressure vessels, cutting tools, and offshore platforms for oil extraction [1–4]. The material is complex metallurgically, requiring careful control of heat treatment to ensure a fully martensitic structure [5]. Care is also required at the austenitizing stage, to avoid δ- ferrite formation, and during the cooling stage, to ensure trans- formation to martensite rather than ferrite. Depending on the composition and processing history, the microstructure of martensitic steel consists of martensite, undis- solved as well as reprecipitated carbides and δ-ferrite. Therefore, for the given composition of the steel, the strength and the corrosion resistance mainly depend on the extent of solution of carbides and alloying elements in the austenite. In general, medium carbon-contained martensitic stainless steel which contains more than 0.2 wt.% of carbon should be heat-treated and quenched at the temperatures where undis- solved carbides are totally dissolved into the matrix. However, grain coarsening, retention of austenite, and formation of δ- ferrite in the as a quenched microstructure cannot be neglected while selecting a suitable austenitizing temperature [6]. Thus, it is well known that the properties obtained in martensitic stainless steels are strongly influenced by the temperature of heat treatments. The amount of carbides in the quenched microstructures exerts an important influence on the hardness, corrosion resistance, abrasion, and wear characteristics [7]. The aim of this work was to analyze the effect of aus- tenitizing treatment temperature on the electrochemical corro- sion resistance of martensitic stainless steel, from the point of view of the changes of the microstructure. 2. Experimental 2.1. Material and heat treatment Alloy specimen was martensitic stainless steel which has 0.3C–14Cr–3Mo–1.5Mn–0.5Si –0.2V–1.5Ni –0.12N as a basic chemical composition and produced by vacuum induction Materials Letters 61 (2007) 244 – 247 www.elsevier.com/locate/matlet ⁎ Corresponding author. E-mail address: kimjg@skku.ac.kr (J.-G. Kim). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.041