Effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels

Effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels

Corrosion Science 52 (2010) 1847–1852 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci ...

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Corrosion Science 52 (2010) 1847–1852

Contents lists available at ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels Jeong Kil Kim a,b,*, Yeong Ho Kim b, Jong Sub Lee b, Kyoo Young Kim a a b

Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, San 31, Pohang 790-784, Republic of Korea POSCO Technical Research Laboratories, Pohang 790-704, Republic of Korea

a r t i c l e

i n f o

Article history: Received 3 December 2009 Accepted 28 January 2010 Available online 2 February 2010 Keywords: A. Stainless steel B. TEM C. Intergranular corrosion C. Segregation A. Chromium

a b s t r a c t Intergranular corrosion (IGC) and precipitation of ferritic stainless steels (FSS) were investigated with change in Cr content from 11 wt.% to 17 wt.%. The increase in Cr content improved IGC resistance as temperature and time for the sensitization became higher and longer, respectively, but it did not prevent IGC. The analysis on the intergranular precipitates revealed that Cr segregation around fine intergranular TiC in developed all FSS regardless of Cr content. This Cr segregation is proposed to explain the Cr depletion for the cause of IGC in Ti-stabilized Cr FSS. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Type 409L, Ti-stabilized 11 wt.% Cr ferritic stainless steel (FSS), is extensively used for cold end parts such as a muffler and a tail pipe in automotive exhaust systems which are categorized into the cold end parts (below 600 °C) and the hot end parts (above 600 °C). The environment in the cold end part is quite corrosive due to the exhaust gas and the condensed water with Cl , SO42 , SO32 , CH3COOH, CO32 and HCO3 [1–5]. Since these corrosive species induce the intergranular corrosion (IGC), the interstitials of C and N are reduced and the stabilizers are also added to improve IGC resistance of this type of FSS. It is well known that IGC is induced by electrochemical potential difference between the matrix and the Cr depletion around intergranular Cr compounds such as Cr-carbide and sigma phase [6–14]. However, the previous studies suggested that type 409L, which contained C and N of around 0.01 wt.% and Ti (as the stabilizer) of 0.23 wt.%, still suffered from IGC and moreover IGC was developed by Cr segregation around fine intergranular TiC [8,10]. Therefore, the authors in the study consider increasing Cr content as the method to prevent IGC in FSS. Although it is generally accepted that the increase in Cr content improves IGC resistance of FSS, there lack of study to investigate the effect of Cr content

* Corresponding author. Address: Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, San 31, Pohang 790-784, Republic of Korea. Tel.: +82 54 220 6371; fax: +82 54 220 6000. E-mail address: [email protected] (J.K. Kim).

on IGC and to propose the optimum Cr content in low Cr FSS with 11–13 wt.% Cr, which have been commercially used in cold end parts for a quite long time [2,3,13]. In this article, the effect of Cr content on IGC and precipitation of FSS with Cr content from 11 wt.% to 13 wt.% was investigated. For comparison of precipitation characteristics, 17 wt.% Cr FSS was employed although it was not a type of low Cr FSS. A free-exposure corrosion test was conducted to examine IGC resistance of the steels. A transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) was used to analyze precipitates. 2. Experimental procedures The FSS ingots with various Cr contents were prepared by vacuum-melting. And they were rolled to 1.2 mm thick sheet and homogenized at 950 °C for 5 min. These steels have a fully ferritic structure as shown in Fig. 1. The chemical compositions of the experimental FSS are given in Table 1. The contents of C and N are about 0.01 wt.% and also the contents of Ti are around 20 times those of C and N in all of steels. This stabilizing ratio (Ti/(C + N)) well matches to the guide lines suggested by the previous studies as well as the present standard specification (ASTM A 240/a 240M08) [2–4,11,15,16]. The specimens were solution-treated at 1300 °C for 10 min, quenched by water. Then, the low Cr FSS were aged at 400– 700 °C for 0.25–2 h and 17 wt.% Cr FSS was done at 500 °C for 4 h for comparison of precipitation characteristics. Intergranular corrosion resistance of 11–13 wt.% Cr FSS was examined by the free-exposure corrosion test using a mod-

0010-938X/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.037

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Fig. 1. Microstructures of the homogenized FSS specimens (a) 11 wt.% Cr, (b) 12 wt.% Cr, (c) 13 wt.% Cr, (d) 17 wt.% Cr.

Table 1 Chemical composition of experimental FSS (wt.%). Elements (wt.% Cr)

C

Cr

Ti

N

Ti/(C + N)

11 12 13 17

0.0054 0.0060 0.0040 0.0060

110.10 12.15 13.17 17.64

0.246 0.232 0.238 0.290

0.0047 0.0051 0.0046 0.0064

24.35 20.9 27.67 23.38

ified copper acid sulfate solution (0.5% H2SO4 + 6% CuSO4) introduced by Devine for low Cr FSS (10–13 wt.% Cr) [4,17,18]. Rect-

angular test coupons (12  12  1.2 mm) after surface grinded with abrasive paper #2000 were immersed and contacted electrically with copper balls in the boiling solution of 300 ml for 20 h. Metallographic observation was performed with the homogenized specimens which was polished up to 1 lm surface finish and etched using Vilella‘s reagent (1 g picric acid + 100 ml ethanol + 5 ml HCl) [19], and with the no-etched specimens after corrosion test. The precipitates on carbon replica of 11–17 wt.% Cr FSS were identified by TEM and EDS.

Fig. 2. No-etched OM micrographs of 11, 12 and 13 wt.% Cr FSS aged at 500 °C for various aging times after the free-exposure corrosion test.

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Fig. 3. No-etched OM micrographs of 11, 12 and 13 wt.% Cr FSS aged for 1 h at various temperatures after the free-exposure corrosion test.

Table 2 Summary of TEM and EDS analysis results on each FSS.

Fig. 4. Time–temperature-sensitization curves of 11, 12 and 13 wt.% Cr FSS determined from the chemical corrosion test results.

Steels (wt.% Cr)

Matrix

Grain boundaries

11 12 13 17

TiN, TiN, TiN, TiN,

No No No No

TiC TiC TiC TiC

precipitation precipitation precipitation precipitation

Fig. 4 depicts the time–temperature-sensitization (TTS) curves of FSS plotted according to the free-exposure corrosion test results. These curves imply that the increase in Cr content requires higher temperature and longer time for FSS to be sensitized, but it can not prevent IGC completely. In addition, the sensitization noses of these FSS are located around 600 °C and IGC does not occur at 700 °C. 3.2. Analysis on the precipitates

3. Result and discussion 3.1. Free-exposure corrosion test Fig. 2 presents the surface micrographs of the no-etched 11–13 wt.% Cr ferritic stainless steels aged at 500 °C for various aging times after the free-exposure corrosion test to investigate the effect of aging time on IGC. In 11 wt.% Cr FSS, there was no evidence of IGC in the homogenized and the solution-treated specimens, while the grain dropouts (indicated as ‘A’ in 11 wt.% Cr FSS aged for 0.5 h) as an evidence of IGC were observed in specimens aged for 0.5 and 1 h [8,20,21]. The grain dropouts were also observed in 1 h-aged 12 wt.% Cr FSS and 2 h-aged 13 wt.% Cr FSS. IGC did not develop in all of the homogenized and the solution-treated specimens. Fig. 3 compares the surface micrographs of the no-etched FSS aged for 1 h at various aging temperatures after the free-exposure test. This result reveals that IGC occurred in 11 and 12 wt.% Cr FSS aged at 500 °C and all FSS aged at 600 °C, but not in all FSS aged at 400 and 700 °C.

TEM and EDS analysis on precipitates was carried out to investigate the reason for IGC development. Firstly, in the homogenized 11 wt.% FSS, the precipitates observed in the matrix were mostly TiC and TiN, but no precipitation was observed along the grain boundaries as indicated in Fig. 5. The analysis on the homogenized 12–17 wt.% Cr FSS also revealed that TiC and TiN precipitated in the matrix, but there was no intergranular precipitates as summarized in Table 2. On the other hand, in 0.5-h aged 11 wt.% Cr FSS where IGC developed lot of fine precipitates were formed along the grain boundary (in Fig. 6). The precipitates were identified as TiC with an fcc structure having lattice parameter of 0.433 nm, and Cr peak was also detected by EDS analysis on the precipitate. No second phase such as carbide or intermetallic phase containing Cr was detected in the analysis. These TiC precipitates with Cr were reported in the previous studies on Ti-stabilized type 409L to suggest IGC mechanism by Cr segregation around intergranular TiC [8,10]. The authors of the studies explained IGC mechanism with experimental data as fol-

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Fig. 5. TEM characterization of the homogenized 11 wt.% Cr FSS. (a) Carbon replica for precipitates in the matrix. (b) EDS spectra of ‘A’ marked in (a). (c) EDS spectra ‘B’ marked in (a). (d) Carbon replica around the grain boundaries.

lows. During aging the solution treated FSS, the soluble C segregates to the grain boundaries and the C segregation leads to the segregation of Cr and Ti both of which have a strong affinity for carbon. Since Ti atoms have a stronger affinity for C than Cr, TiC precipitates are preferentially formed leaving the un-reacted Cr segregated around the TiC precipitates. Eventually the segregation of un-reacted Cr causes the Cr depletion adjacent to the segregation and IGC. To examine the effect of Cr content on intergranular precipitation, the further analysis were conducted on 1-h aged 12 wt.% Cr FSS and 2-h aged 13 wt.% Cr FSS suffered from IGC in the corrosion test. As shown in Fig. 7, intergranular precipitates found in these specimens are the same TiC precipitates as those observed in 11 wt.% Cr specimen. What is clarified through TEM and EDS analysis is that TiC carbides with un-reacted Cr are precipitated along the grain boundaries of 11–13 wt.% Cr FSS regardless of Cr contents. Therefore, it can be concluded that IGC in low Cr FSS is induced by the un-reacted Cr segregation around intergranular TiC. However, it is still not clear whether intergranular precipitation observed in low Cr FSS develops in Ti-stabilized high Cr FSS with around 17 wt.% Cr. In several studies, intergranular precipitation of stabilized high Cr stainless steels with 16–18 wt.% Cr was already investigated, although some stainless steels among them were austenitic and they had a different stabilizers such as Nb. The studies reported that various Cr carbides, sigma phase and even laves phase were found along the grain boundaries, and these phases were the reasons for IGC [14,20,22,23]. But no one suggested that IGC was

induced by the segregation of un-reacted Cr around the intergranular TiC carbides. Therefore, in this study TEM and EDS analysis were carried out on Ti-stabilized 17 wt.% Cr FSS aged at 500 °C for 4 h to understand the intergranular precipitation of high Cr FSS. And the result is presented in Fig. 8. Fine precipitates were formed along grain boundaries of this material. These precipitates were identified as the same type of TiC carbides with Cr as those observed in the low Cr FSS. Base on the TEM and EDS result, it can be concluded finally that in Ti-stabilized ferritic stainless steels with 11–17 wt.% Cr, IGC is induced by the Cr depletion in the vicinity of Cr segregation around TiC along the grain boundaries regardless of Cr content. The corrosion test using the proper solution for high Cr FSS will be performed in the future to verify the effect of Cr segregation on IGC resistance.

4. Conclusion The results of this study with 11–17 wt.% Cr FSS suggest the following conclusion. In free-exposure corrosion test with 11–13 wt.% Cr FSS, IGC developed in 11 and 12 wt.% Cr FSS aged 400–600 °C, but not in 13 wt.% FSS aged at 400 °C. In addition, sensitization noses of these FSS are located around 600 °C and IGC does not occur at 700 °C in all low Cr FSS. The increase in Cr content improves IGC resistance of FSS but can not prevent IGC completely. In Ti-stabilized 11–17 wt.% Cr FSS containing low C and N, the cause of IGC is the Cr

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Fig. 6. TEM characterization of the intergranular precipitates along the grain boundary in 11 wt.% FSS aged at 500 °C for 0.5 h. (a) Carbon replica for the intergranular precipitates (Bright field image). (b) EDS spectra of the precipitate. (c) Electron diffraction pattern of the precipitate with a zone axis of [0 0 1]. (d) Dark field image of the precipitates.

Fig. 7. TEM characterization of the intergranular precipitates along the grain boundaries of 1-h aged 12 wt.% Cr and 2-h aged 13 wt.% Cr FSS.

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Fig. 8. TEM characterization of the intergranular precipitates along the grain boundaries of 17 wt.% Cr FSS aged at 500 °C for 4 h. (a) Carbon replica for the intergranular precipitates. (b) Higher magnification of ‘A’ marked in (a). (c) EDS spectra of the precipitate. (d) Electron diffraction pattern of the precipitate with a zone axis of [0 0 1]. (e) Dark field image of the precipitates.

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