Materials Letters 61 (2007) 244 – 247 www.elsevier.com/locate/matlet
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 M23C6 carbide which acts as preferential sites for pitting corrosion. © 2006 Elsevier B.V. All rights reserved. Keywords: Martensitic stainless steel; EIS; Austenitizing treatment; M23C6 carbide; Pitting corrosion
1. Introduction Martensitic stainless steels are commonly used for manufacturing 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 . Care is also required at the austenitizing stage, to avoid δferrite formation, and during the cooling stage, to ensure transformation to martensite rather than ferrite. Depending on the composition and processing history, the microstructure of martensitic steel consists of martensite, undissolved 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.
⁎ Corresponding author. E-mail address: [email protected]
(J.-G. Kim). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.041
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 undissolved carbides are totally dissolved into the matrix. However, grain coarsening, retention of austenite, and formation of δferrite in the as a quenched microstructure cannot be neglected while selecting a suitable austenitizing temperature . 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 . The aim of this work was to analyze the effect of austenitizing treatment temperature on the electrochemical corrosion 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
Y.-S. Choi et al. / Materials Letters 61 (2007) 244–247
Fig. 1. Optical micrographs of austenitized martensitic stainless steel: (a) 970 °C and (b) 1030 °C.
melting and hot-rolled to the thickness of 5 mm. The three austenitizing treatment conditions were 970, 1000, and 1030 °C for 10 min per 1 mm and followed by air-cooling. The microstructures of the heat-treated specimens were examined with an optical microscope by etching in a Vilella's reagent containing 1 g picric acid, 10 ml hydrochloric acid and 100 ml ethanol. Xray diffraction measurements were carried out with the Beamline 10C1 at the Pohang Light Source (PLS) using energy near Fe Kα edge, 7 keV. 2.2. Electrochemical tests Each specimen was mounted in a cured epoxy resin and the epoxy–specimen interface was painted, leaving an exposed area of 1 cm2 on the material surface. The specimen was finished by grinding on 600-grit silicon carbide paper. 0.6 M NaCl solution at 30 °C was used as a corrosion medium. Potentiodynamic polarization test were carried out to evaluate the overall corrosion behavior of the specimens. The specimen was allowed to attain a stable open-circuit potential
Fig. 2. X-ray diffraction patterns of austenitized martensitic stainless steel.
(OCP) before starting the polarization scan. After immersion of the working electrode in the solution for 1 h, the potential of the electrode was swept at a rate of 0.166 mV/s vs. the saturated calomel electrode (SCE). Electrochemical impedance spectroscopy (EIS) measurements and potentiostatic tests were performed to examine the tendency of passivation and breakdown (pit propagation) as a function of heat treatment temperature. EIS measurements were conducted at OCP with a 10 mV (rms) perturbation and 5 points per decade. The frequency range covered was from 10 kHz to 10 mHz. The experimental results were interpreted on the basis of an equivalent circuit determined using a suitable fitting procedure described in ZSimpWin program. 3. Results and discussion 3.1. Microstructure Fig. 1 shows the variations of microstructures according to austenitizing temperatures. It is observed that lath martensite is formed evenly. As the temperature of heat treatment rises from 970 to 1030 °C, grain size increases, which is caused by the dissolution of carbides with increasing temperature. However, δ-ferrite was not formed at the given
Fig. 3. Potentiodynamic polarization curves of austenitized martensitic stainless steels in 0.6 M NaCl at 30 °C.
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Fig. 4. Variation of breakdown potential and passive current density.
temperature ranges. Fig. 2 shows the results of X-ray diffractometry (XRD) for phase analysis after heat treatment. The peak of Cr-rich M23C6 as well as γ peak was observed at 970 and 1000 °C, however, these peaks disappeared at 1030 °C, which can be expected from the increased amount of carbon in the matrix due to the dissolution of undissolved carbides. 3.2. Corrosion behavior The polarization curves of the martensitic stainless steels (MSSs) with heat treatment temperatures are presented in Fig. 3. All the alloys passivated in 0.6 M NaCl solution. However, polarization above the breakdown potential resulted in a marked increase in current density as the result of the initiation of pitting. Fig. 4 shows the effect of heat treatment temperature on the breakdown potential and passive current density. The breakdown potential increased with increasing temperature, indicating increased relative resistance to initiation of localized corrosion. The passive current densities of MSS2 and MSS3 are smaller than that of MSS1, which was the result of more stable passive film on the surface. These results indicated that the temperature of austenitizing treatment can affect both passivation and pitting characteristics.
Fig. 5. Nyquist plots for austenitized martensitic stainless steels at passive state.
For the better understanding of the effects of heat treatment temperature on the passivation and pitting corrosion behaviors, potentiostatic tests and EIS measurements were performed. The first EIS tests were carried out after immersion for 1 h. The potentiostatic tests were performed to break down the passive film. After each potentiostatic test, the second EIS tests were performed to compare the impedance behavior between passivation and breakdown states. Fig. 5 shows Nyquist plots of MSSs in passive state after 1 h immersion. The formation of passive films creates dramatic increase in impedance and show capacitive impedance. Moreover, it means that MSSs at open-circuit potential forms a relatively stable passive layer in this solution. In the case of passive state, the impedance data were analyzed using the Randle's circuit, which consists of a resistance R and a capacitance C in parallel. The values of polarization resistance (shown in Fig. 5) obtained using a semicircular fit indicate that the resistance of passive film for the MSS3 is higher than those of the MSS1 and MSS2. Fig. 6 shows the result of potentiostatic tests performed at an applied potential of + 480 mVSCE. The applied potential was based on the data from polarization curves in Fig. 3, corresponding to the breakdown potential of MSS3. At the potential of +480 mVSCE, localized corrosion initiated on all the specimens. The
Fig. 6. Current density variation with time at a breakdown potential of +480 mVSCE.
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4. Conclusions 1. As austenitizing treatment temperatures increased, the amount of undissolved carbides and retained austenite decreased. 2. In the results of potentiodynamic test, the pitting potential increased and passive current density decreased with increasing austenitizing treatment temperatures. 3. The results of the EIS test indicated that MSS3 (1030 °C) exhibits larger polarization resistance value than MSS1 (970 °C) and MSS2 (1000 °C) at passive and breakdown states. 4. The precipitation of Cr-rich M23C6 carbide reduced the resistances of passive film and pitting corrosion, which can be overcome by the austenitizing treatment at the temperature of 1030 °C. Fig. 7. Nyquist plots for austenitized martensitic stainless steels at breakdown state.
current density for MSS1 increased with time due to the acceleration of the pitting corrosion. For MSS2 and MSS3, the current density first increased rapidly and then decreased continuously with time. A rapid increase at the beginning resulted from pitting initiation and propagation, and the decrease in current density was due to the repassivation of pits. For MSS2 and MSS3, the rate of film repassivation was as high as the rate of film breakdown under this condition. Obviously, the current density of MSS2 was higher than that of MSS3. Fig. 7 shows Nyquist plots of MSSs after potentiostatic test at a constant potential of + 480 mVSCE for 2 h. In breakdown state, the Randle's circuit was also applied to model the EIS data and shown in Fig. 7. Compared with Fig. 5, the results of EIS test just after breakdown of the passive film indicate that the diameter of arc in Nyquist plot decreased due to the pit initiation and propagation [8,9]. Nevertheless, MSS3 exhibits larger polarization resistance value than MSS1 and MSS2. From the results of XRD and electrochemical analyses, the improved pitting corrosion resistance as increasing austenitizing treatment temperature was related to the decrease of the Cr-rich M23C6 carbide. In MSSs, the chromium-depleted regions around carbides precipitated at certain temperatures are thought to act as preferential sites for pitting corrosion [10,11]. Thus, in the present study, the precipitation of Cr-rich M23C6 carbide reduced the resistances of passive film and pitting corrosion, which can be overcome by the austenitizing treatment at the temperature of 1030 °C.
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