834 E. Jafari: J. Mater. Sci. Technol., 2010, 26(9),

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1 J. Mater. Sci. Technol., 2010, 26(9), Corrosion Behaviors of Two Types of Commercial Stainless Steel after Plastic Deformation Esmaeil Jafari Materials Science and Engineering Department, Islamic Azad University Shiraz Branch, Iran [Manuscript received July 20, 2009, in revised form December 4, 2009] The influence of deformation temperature, strain rate and alloying elements such as chromium, nickel, copper, on corrosion resistance of a 304 stainless steel after plastic deformation was investigated. The techniques employed were tensile test at room and elevated temperature, deep drawing tests at various strain rates, X-ray diffraction test and potentiodynamic and cyclic polarization, metallography by optical and scanning electron microscopy. Results showed that chromium, nickel and copper had a significant effect on corrosion resistance of steel after plastic deformation. It was observed that corrosion resistance after plastic deformation was a function of deformation temperature and alloying elements. KEY WORDS: Corrosion resistance; Plastic deformation; Stainless steel; Phase transformation; Polarization test 1. Introduction The effect of plastic deformation on the corrosion behavior of stainless steel has been investigated by numerous researchers [1 5]. The alloying elements of stainless steel play a very important role in controlling corrosion resistance during plastic deformation. Austenitic stainless steels are more resistant to oxidation than binary iron chromium alloys, for example, AISI 304 with 18 wt pct chromium and 9 wt pct nickel resists against oxidation as well as a binary iron-25 wt pct chromium alloy [6]. Commercial austenitic steels are formulated to realize almost completely austenitic structures at minimum cost, consistent with certifiable performance [6]. In practice, the nickel content is minimized because nickel is one of the most expensive alloying elements. Carbon can replace a small but economically significant fraction of the nickel content. It is available free of cost because of the nature of the steelmaking process [6,7]. The maximum carbon content which is convenient to retain in solution is 0.06 wt pct, and the composition for fully austenitic steel with the minimum nickel content is 18 wt pct Tel.: ; Fax: ; address: asjafari@shirazu.ac.ir. chromium, 9 wt pct nickel, and 0.06 wt pct carbon. Corrosion resistance of stainless steel depends on the formation of a passive chromium oxide film on the metal surface which is highly resistant to corrosion [6,7]. The chromium in stainless steels is primarily responsible for the self-passivation mechanism. In contrast to carbon or low alloy steels, stainless steels must have a minimum chromium content of 10.5 wt pct of chromium (and a maximum of 1.2 wt pct carbon) [7]. The corrosion resistance of these chromium-steels can be enhanced by addition of other alloying elements such as nickel, molybdenum, nitrogen and titanium (or niobium). This provides a range of steels with corrosion resistances over a wide range of service conditions as well as enhancing other useful properties such as formability [7,8]. In austenitic stainless steel, martensite may also be formed by deformation above room temperature in the case of unstable steels and below room temperature in the case of stable steels, depending on the martensite deformation temperature (M d ). Generally, the martensitic transformation is believed to be triggered when the austenitic stainless steel is deformed at temperatures below M d, a temperature below which the transformation to martensite readily takes place. Over and above the M d temperature, several other factors are thought to

2 834 E. Jafari: J. Mater. Sci. Technol., 2010, 26(9), Table 1 Chemical composition of two types of commercial 304 stainless steel (wt pct) C Si Mn Cr Ni Cu Mo S W Al V Co Fe Type A Bal. Type B Bal. influence the martensitic transformation. In fact, a number of investigations have been carried out to understand the martensitic transformation in metastable systems and a review of these investigations brings out the fact that the extent of such phase transformation is controlled by the chemistry of the material, rate of deformation, strain, stress state and temperature of deformation. In other words, for a better understanding of the plastic deformation behavior of austenitic stainless steels, it is a priori to have the knowledge on the martensitic transformation characteristics. Although considerable knowledge base is available on this topic for austenitic stainless steels in general, the understanding development for a particular alloy system under certain deformation conditions cannot be generalized to other metastable systems [9 11]. The chemical composition can be adjusted to increase or reduce the microstructural stability before the imposed deformation. The other parameter commonly used is the M d30 temperature, which represents the lowest temperature where 50% vol. fraction of induced martensite is formed with a true strain of 0.3. In other words, the lower the M d30 temperature is, the greater the stability of plates designed will be [12]. Apart from cobalt, almost all alloying elements depress the martensite transformation start temperature (M s ). Recently a linear equation relating the M s temperature to the composition has been developed for austenitic stainless steels (Eq. 1). This type of relation is important particularly when used to establish the M d30 temperature in assessing the cold formability of austenitic stainless steels [8,10 13]. M s ( C) = (%Cr) 61(%Ni) 33(%Mn) 28(%Si) 1667(%C + %N) M d30 ( C)= (%C+%N) 9.2(%Si) 8.1(%Mn) 13.7(%Cr) 29(%Ni + %Cu) 18.5(%Mo) 68(%Nb) (1) Copper has the same influence as nickel on the M d30 temperature, which decreases with increasing amount of these elements. Since the use of high Ni contents may not be economically viable, austenitic stainless steels designed for higher formability have been developed by replacing some of Ni with Cu in their composition [13]. In some stainless steels, M d30 is higher than room temperature and therefore austenite is transformed to martensite due to plastic deformation and corrosion resistance decreases with martensite formation [8]. In this study, the influence of the alloying element on Fig. 1 Dimension of tensile test specimens M d30 and also the corrosion resistance after plastic deformation are investigated. 2. Experimental The commercially available two types of 304 stainless steel sheet with a thickness of 0.5 mm were used in this study with chemical composition as given in Table 1. The chemical composition was measured by using a quantometer. In addition for this investigation, the following tests were performed. 2.1 Deep drawing test The influence of strain rate on phase transformation during deformation was investigated by deep drawing test. For this test, square shape specimens of two types of stainless steel with dimensions of 40 cm 40 cm 0.5 mm were prepared. Room temperature deep drawing tests were carried out at strain rates of 1, 0.1, 0.01 and s 1 until fracture at room temperature. The microstructures of types A and B before and after the test were investigated. The test was performed by a SA machine (Instron company, Japan) with 12 tf capacity. 2.2 Tensile test An A A Instron Universal testing machine (Japan) with 250 kn capacities was used for tensile tests. In this test, formability and mechanical properties of types A and B were investigated. The test was performed according to ASTM E standard at room temperature and E21-03 standard at elevated temperature. Tensile test was performed at strain rate 0.1 s 1. Figure 1 shows the dimension of tensile test specimens. 2.3 Elevated temperature tensile test The effect of increasing temperature on phase transformation during deformation was investigated by heating specimen during the tensile test. Tensile tests were performed at high strain rate (1 s 1 ) at temperatures of 50, 100, 150 and 200 C. A thermo-

3 couple was used to measure the surface temperature of the specimens, and the measured temperature was considered as the real temperature of the specimen in this work. The specimen were heated up to a specified temperature then loaded until they failed while maintaining them at the same temperature. 2.4 Phase analysis E. Jafari: J. Mater. Sci. Technol., 2010, 26(9), All samples before and after plastic deformation were analyzed by X-ray diffraction (XRD) equipment, Brucker D8 Advance diffractometer (England) with CuKα radiation. The phase analysis and the microstructural studies were characterized by means of XRD, optical and scanning electron microscopy (SEM, Leica Cambridge, Stereoscan S360, UK), respectively. Microstructure analysis of samples was performed by optical microscopy after elevated temperature tensile tests at temperatures of 50, 100, 150 and 200 C. Optical microscope with commercial image analysis software was used to record and quantify the martensite in these specimens. For microstructural studies after mechanical grinding of the specimens with wet silicon carbide abrasive paper, they were polished with 3 µm diamond paste on nylon cloth. Chemical etching was performed at room temperature for 30 s using a solution containing 10 ml oxalic acid and 100 ml distilled water. Fig. 2 Optical microscopy images of type A (a) and type B (b) before deformation 2.5 Electrochemical measurement The influence of deformation on corrosion rate of types A and B was measured before and after tensile test. Electrochemical measurements were performed in a standard three-electrode cell with saturated calomel electrode (SCE) reference electrode and platinum counter electrode by µ-autolab type III instrument (Germany). The electrodes were sealed in epoxy resin (exposed surface area 1 cm 2 ). The electrochemical measurement was performed at the open circuit potential in 3.5% sodium chloride, aerated test solution at room temperature (25 C) with scan rate of 0.2 mv s 1. Corrosion measurements consisted of stabilizing the WE (working electrode) in the corrosion test electrolyte at open circuit potential (OCP) for 1 h, and the following tests were conducted: (1) potentiodynamic polarization measurements; (2) cyclic potentiodynamic polarization measurements. 3. Results and Discussion The results of deep drawing tests showed that formability of type B was higher than that of type A. Rupture was seen at strain rate of 0.01 s 1 for type A, whereas it was observed at strain rate of 1 s 1 for type B. The average thickness of samples after deep drawing test reached 0.3 mm. In the as-received condition, the microstructures of two types A and B are completely austenitic as seen in Figs. 2 and 3. How- Fig. 3 SEM images of type B (a) and type A (b) before deformation ever, microstructures investigation after deformation show that for stainless steel of type A, a partial transformation of austenite to martensite phase occurs at all strain rates, as shown in Figs. 4(a) and 5(b), and for type B, no such transformation is observed at any of the applied strain rate as shown in Figs. 4(b) and 5(a). The results show that before deformation, mi-

4 836 E. Jafari: J. Mater. Sci. Technol., 2010, 26(9), Table 2 Results of tensile tests Yield strength Ultimate tensile /MPa strength/mpa Type A Type B Fig. 4 Optical microscopy images of type A (martensite phase) (a) and type B (austenite phase) (b) after deformation in tensile test at room temperature Fig. 6 Variation of martensite volume fraction with tensile testing temperature in type A Fig. 7 Effect of plastic deformation temperature on corrosion resistance of type A stainless steel Fig. 5 SEM images of type B (austenite phase) (a) and type A (martensite phase) (b) after deformation in tensile test at room temperature crostructures of types A and B are fully austenitic. But after deformation, the austenite phase has partially transformed to martensite in type A while austenite is stable during deformation in type B. The results of tensile tests are given in Table 2 which show that yield and ultimate strength of type B are larger than type A. Figure 6 shows the variation of martensite volume fraction in type A with tensile testing temperature. According to this figure, martensite volume fraction obtained from optical microscope decreases with increasing temperature of tensile test. These results show that no transformation has occurred above 100 C and austenite is stable in both types A and B, but at temperatures lower than 100 C in stainless steel type A, austenite is transformed to martensite. Therefore, plastic deformation has a significant influence on martensite formation due to phase transformation in stainless steel type A. Figure 7 shows the effect of plastic deformation temperature on corrosion resistance of type A stainless steel. The corrosion resistance of type A stainless steel is a function of deformation temperature. In this type, martensite is formed at temperatures lower than 100 C. The corrosion resistance of martensite phase is lower than that of austenitic phase. Therefore, corrosion resistance of type A stainless steel is decreased by plastic deformation temperature. Figures 8 and 9 show the result of XRD test for

5 E. Jafari: J. Mater. Sci. Technol., 2010, 26(9), Table 3 Results of potentiodynamic polarization test I corr/(a/cm 2 ) E corr/v Type A (before deformation) Type A (after deformation) Type B (before deformation) Type B (after deformation) Fig. 8 XRD pattern of type A stainless steel before plastic deformation Fig. 9 XRD pattern of type A stainless steel after plastic deformation Fig. 11 Cyclic polarization curve of two types A and B stainless steel: (a) type B, (b) type A Fig. 10 Potentiodynamic polarization curve of two type A and B stainless steel: (a) type B, (b) type A type A before and after plastic deformation. Figure 9 shows a clear XRD peak of martensite at 2θ of 51 deg. and it is providing an evidence for austenite to martensite transformation during plastic deformation in type A stainless steel. Figure 10 shows potentiodynamic curve of both types A and B stainless steels before and after plastic deformations. The material shows active-passive behavior with distinct passivation and prepassive regions for the two types of stainless steels. Table 3 presents the results of potentiodynamic polarization test. It is clear that after plastic deformation, cor- rosion resistance and passive range of type A are decreased. The anodic currents after plastic deformation for type A are significantly higher than those for type B stainless steel. According to these results, the plastic deformation has a significant influence on polarization resistance of type A stainless steel and the polarization resistance of type A decreases after plastic deformation. Figure 11 shows the cyclic polarization of both types A and B stainless steels before and after plastic deformations. In this figure, width of hystersis loop for type A is increased after plastic deformation. Therefore corrosion resistance of type A decreases due to plastic deformation. The breakdown or critical potential, E b, and protection potential, E p, for each specimen were evaluated by cyclic potentiometers method in 3.5% sodium chloride test solution. Figure 11 shows the change in E p and E b measured in this solution. It is seen that both E p and E b of type B are higher than those of type A. The result indicates that pitting potential for type B is nobler than that for type A. Since the main difference between types A and B is phase transformation during deformation, one may conclude that phase transformation has decreased corrosion resistance of type A. In this respect, the effect of martensite phase on decreasing corrosion resistance of stainless steel has been reported [4,8,14 16]. The potentiodynamic polarization curves of the type A at various strain rates are shown in Fig. 12. A comparison of the curves at different strain rates reveals that at higher strain rates the polarization re-

6 838 E. Jafari: J. Mater. Sci. Technol., 2010, 26(9), Fig. 12 Potentiodynamic polarization curves of type A at various strain rates sistance is different with respect to low strain rate conditions. Moreover, when the strain rate increases, the maximum volume fraction of martensite formed increases up to 0.1 s 1. In the higher value of strain rates (1 s 1 ), the volume fraction of martensite decreases. It may be noted that when the accumulated plastic strain is increased, the dislocation movement will be restricted due to pile-up, causing a local rise in the temperature. This local temperature rise is expected to be high, as the strain rate is increased, as the martensitic transformation at higher strain rates (and at high strains) is likely to be decreased [9]. Therefore, in Fig. 12, the passive range in strain rate (1 s 1 ) is increased with respect to strain rate 0.1 s 1 because the volume fraction of martensite is decreased. Results from Table 1 show that the essential alloying elements in types A and B, for using Eq. 1 are: chromium, nickel, manganese, molybdenum and copper. The copper is lower for type A than for type B. Consequently, M d30 is higher for type A than that for type B. Accordingly, transformation from austenite to martensite is happening above room temperature in type A, and martensite is stable in this material after deformation. By considering this fact that corrosion resistance of martensite is lower than that of austenite, one can conclude that corrosion resistance in type A is decreased due to plastic deformation. The amount of copper in type B is 2.5 wt pct and in type A is 0.28 wt pct, therefore, formability of type B is higher than that of type A. This subject has been confirmed by the result of deep drawing and tensile test. Existence of copper in type B increased its formability [14,17,18]. Formation of carbide phases is another effect of high copper content in type B. The presence of high copper content in austenitic stainless steel promotes the precipitation of carbide phases of M 23 C 6 type in the interdendritic spaces. The precipitation of carbides reduces the Cr and Mo contents on the carbideaustenite boundaries and impairs protective properties of the passive film [19]. Copper dissolved in solid solution austenite stainless steel does not influence the stability of passive film on austenitic stainless steels. High concentration of copper in austenite phase leads to the precipitation of carbide phases in austenitic stainless steels. The small particles of carbide phases dispersed in austenite matrix increase their hardness and diminish corrosion resistance of stainless steel. Accordingly, before plastic deformation, corrosion resistance of type B is lower than that of type A. But after plastic deformations corrosion resistance of type A is decreased due to martensite formation. 4. 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