Stress corrosion cracking of type 321 stainless steels in simulated petrochemical process environments containing hydrogen sulfide and chloride

Materials Science and Engineering A 407 (2005) 114–126

Stress corrosion cracking of type 321 stainless steels in simulated petrochemical process environments containing hydrogen sulfide and chloride Y.Y. Chen, Y.M. Liou, H.C. Shih ∗ Department of Materials Science and Engineering, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, Taiwan 300, ROC Accepted 12 July 2005

Abstract The susceptibility to stress corrosion cracking (SCC) of type 321 stainless steel (type 321s) in a simulated petrochemical process environment containing hydrogen sulfide and chloride (20 wt.% NaCl + 0.01 M Na2 S2 O3 , pH 2) was assessed using the slow strain rate tensile (SSRT) test and static load (U-bend) tests at the free corrosion potentials. In the SSRT, effects of environmental factors, such as chloride (Cl− ) plus thiosulfate (S2 O3 2− ), Cl− concentration, solution pH, and temperature, on the susceptibility to SCC of type 321s were critically examined. In addition, factorial design experiments using Yates’s algorithm quantitatively estimated the individual and interactive effects of temperature, Cl− concentration, and solution pH on the SCC susceptibility of type 321s. In the U-bend tests, specimens were immersed in an autoclave containing deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) for 1400 h at either 80 or 300 ◦ C. Results of the SSRT tests indicated that the effects of environmental factors on the SCC susceptibility of type 321s decreased in the following order: temperature effect  solution pH effect > Cl− concentration effect. The mechanism of SCC induced by corrosion pits or TiC particles (5–10 ␮m) is discussed. In addition, results of the U-bend tests show that the susceptibility of type 321s to SCC decreases with increasing temperature, which is related to the more compact surface film containing chromic oxide (Cr2 O3 ) and magnetite (Fe3 O4 ) that forms at the higher temperatures. © 2005 Elsevier B.V. All rights reserved. Keywords: Stress corrosion cracking; Type 321 stainless steel; Slow strain rate test; U-bend immersion test; Sodium chloride; Sodium thiosulfate; Factorial experiments; Yates’s algorithm

1. Introduction Corrosion has always been a problem in the petroleum refining and the petrochemical operations. The petrochemical process elements, such as furnace tubes, valves, and pipelines are frequently performed at high temperatures and in severely corrosive environments; therefore, heat- and corrosion-resistant alloys, e.g., austenitic stainless steels, have been used widely in the petrochemical industries because of their excellent mechanical strength and toughness. However, in chloride-containing high temperature environments, pitting, crevice corrosion [1–4], and stress corrosion cracking (SCC) [5–11] is often associated with the operation. ∗ Corresponding author. Tel.: +886 3 571 5131x3845; fax: +886 3 571 0290. E-mail address: [email protected] (H.C. Shih).

0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.07.011

In addition, it was found that the factors most affecting corrosion of structural materials in the petrochemical industry is chloride (Cl− ) [12–16] and hydrogen sulfide (H2 S) [16–24]. H2 S is an important constituent of refinery sour waters and is also formed by the decomposition of organic sulfur compounds that are present at elevated temperatures [25–28]. The type 321s used in this study was a titanium-stabilized austenitic stainless steel. Sensitization (the precipitation of chromium carbide (e.g., Cr23 C6 ) along the grain boundaries) is avoided by reaction of the titanium with the native carbon to form titanium carbide (TiC) precipitates at much higher temperatures. In this study, H2 S was replaced by sodium thiosulfate (Na2 S2 O3 ), the latter being more convenient and less hazardous to the environment. The suggestion that Na2 S2 O3 be a substitute for H2 S was first proposed by Tsujikawa and coworkers [29] of the Japan Society of Corrosion

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Engineering (JSCE). A series of simple immersion tests in deaerated 20 wt.% sodium chloride (NaCl) solutions at 80 ◦ C, conducted by Tsujikawa et al., indicated a good correlation between the critical concentration of S2 O3 2− and the critical pressure of H2 S for pitting [30]. Tsujikawa et al. then suggested a simulated H2 S-containing chloride environment as follows: deaerated 20 wt.% Cl− + 10−3 –10−2 M S2 O3 2− aqueous solution of pH 4 at 80 ◦ C [29]. In the present study, a more acidified environment (pH 2) was used by adding hydrochloric acid (HCl) to the simulated environment. The effects of chloride concentration (0–20 wt.%), solution pH (1–7), and temperature (25–80 ◦ C) on the susceptibility to SCC of type 321s in the simulated H2 S-containing chloride solutions were investigated using a slow strain rate tensile (SSRT) test [31–34]. In addition, in order to further understand the degree of influence for the three different variables (chloride concentration, solution pH, and temperature) on the SCC susceptibility of type 321s in SSRT tests, it is convenient for us to quantitatively estimate the individual and interactive effects using the Yates’s algorithm [35–40]. The SCC mechanism of type 321s in these simulated petrochemical process environments can be predicted from the morphology of the tested specimen examined in a scanning electron microscope (SEM). The SCC mechanism can also be elucidated from an experimental potential–pH diagram constructed using the electrochemical hysteresis method [41]. Furthermore, the SCC behavior of type 321s under the influence of a static strain (pre-stress) in the same solution was also investigated using U-bend tests [42–45]. Finally, the morphology and the chemistry of the corrosion products were analyzed using SEM, electron probe microanalysis (EPMA), Auger electron spectrometry (AES), and X-ray diffraction (XRD). The results of this study provided insights on how to prevent SCC of type 321s in H2 Scontaining chloride solutions.

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Table 1 Chemical compositions (wt.%) of type 321 austenitic stainless steel analyzed by atomic emission spectrometry Specimen

Type 321s

C Mn P S Si Ni Cr Ti Mo and others Fe

0.069 1.30 0.023 0.006 0.06 10.96 17.89 0.50 0.86 Balance

type 321s used in this study had a typical austenitic structure, as shown in Fig. 1. 2.2. Test solution The test solutions were prepared with deionized water and reagent-grade NaCl and Na2 S2 O3 ·5H2 O. The test solution was adjusted to the appropriate pH by the addition of HCl or sodium hydroxide (NaOH). Deaeration of the test solution was achieved by purging the solution with nitrogen gas for 30 min prior to the test, and then continuing the purging during the test. 2.3. SSRT test Cylindrical specimens, 3.175 mm in diameter and with a gauge length of 12.7 mm according to ASTM G 49 [46], were used for the SCC tests. Prior to testing, the gauge lengths of the specimens were polished with 2000 grit emery paper along the tensile direction to avoid a possible notch effect, degreased with acetone in an ultrasonic cleaner, washed with distilled water, and finally dried in air. The SSRT technique was used in this test, and the assembly was similar

2. Experimental procedures 2.1. Test material The test material used in this study was commercially annealed type 321 (UNS S32100) stainless steel. The chemical composition (wt.%) was determined using an inductively coupled plasma-atomic emission spectrometer (ICP-AES) and is listed in Table 1. In order to observe the surface microstructure of type 321s, a metallographic sample was first mechanically wet ground using a 1500 grit silicon carbide (SiC) paper, then polished with aluminum oxide (Al2 O3 ) powder of 1 ␮m diameter, followed by etching for about 20 min in a mixture of nitric acid and hydrochloric acid (10 ml HNO3 + 30 ml HCl at 40 ◦ C). After etching, the specimen was cleaned with distilled water, and then dried in air. Subsequently, the specimen was examined in an optical microscopy (OM). It was observed that the surface microstructure of the

Fig. 1. Microstructure of type 321s shows an austenitic structure (etchant: 10 ml HNO3 + 30 ml HCl at 40 ◦ C; 500×).

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from the equation as follows: ε=

Fig. 2. Geometry of a U-bend specimen (L = 80, M = 50, W = 20, T = 2.5, D = 10, X = 32, Y = 14, and R = 5; dimensions in mm).

to that used elsewhere [47]. The strain rate was controlled at 10−6 s−1 , which is the critical strain rate for promoting SCC of stainless steels in chloride environments [48]; the load and elongation were monitored continuously by a load cell and electric dial gauge (displacement transducer), the outputs from which were recorded on an X–Y recorder until fracture occurred. SSRT tests were, respectively, conducted in air at room temperature (∼25 ◦ C) and in the simulated H2 S-containing chloride solutions mentioned above to compare the effects of chloride concentration (deaerated X wt.% NaCl + 0.01 M Na2 S2 O3 solution of pH 2 at 25 ◦ C, X = 0–20), solution pH (deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 solution of pH X at 25 ◦ C, X = 1–7), and temperature (deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 solution of pH 2 at X ◦ C, X = 25–80) on the SCC behaviors of type 321s. All experiments were carried out under open-circuit conditions. After each test, the load–elongation relationship recorded was converted into a nominal stress–strain curve. The ultimate tensile strength (UTS) and the uniform elongation (UEL) could be measured directly from the stress–strain curve; the reduction in area (RA) was determined by direct measurement of the fractured specimen. The fracture surfaces were examined by means of a scanning electron microscope.

T 2.5 = = 25% 2R 2×5

(1)

where T is the specimen thickness and R is the inner radius of the curvature in mm. During the immersion test, the specimens were isolated from the autoclave body. For each test condition, the specimens were tested in batches of six specimens per experimental run. U-bend specimens were immersed in a 1-l Hastelloy C-276 autoclave, which containing deaerated 20 wt.% Cl− + 0.01 M S2 O3 2− aqueous solution (pH 2), for 1400 h at either 80 or 300 ◦ C. In order to ensure that the test environments kept liquid states during the U-bend experiments, the solutions in autoclave were, respectively, pressurized to ∼1 MPa (for 80 ◦ C) and to ∼9 MPa (for 300 ◦ C) before heating. After the tests, the specimens removed from the autoclave were examined by SEM, Auger electron spectrometry, and X-ray diffraction. In order to examine the corroded surfaces, the corrosion products were chemically removed by washing in 10 vol.% HNO3 at 60 ◦ C for 20 min [50].

3. Results 3.1. SSRT tests 3.1.1. Effects of environmental factors on SCC susceptibility 3.1.1.1. Effect of Cl− plus S2 O3 2− . In order to investigate the effect of Cl− plus S2 O3 2− on the SCC behavior of type 321s, SSRT tests were performed in air as well as in deaerated 0.01 M Na2 S2 O3 (pH 2), 20 wt.% NaCl (pH 2), and 20 wt.% NaCl + 0.01 M Na2 S2 O3 (pH 2 or 7) aqueous solutions at 25 ◦ C, as shown in Fig. 3. The corresponding SEM

2.4. U-bend test The static load U-bend tests were performed following ASTM G 30 [49] recommendations. The specimen geometry for U-bend tests is schematically shown in Fig. 2. Prior to bending, the specimen was polished to 500 grit emery paper, then ultrasonically cleaned in acetone, rinsed in deionized water, and finally dried in air. Each U-bend specimen was then single-stage stressed to shape (Fig. 2) and held in this configuration with an isolated type 316Ls nut and bolt, which was tightly screwed within ceramic gaskets. The maximum strain, ε, on the outside of the U-bend was calculated [49]

Fig. 3. Stress–strain curves for type 321s in air as well as in deaerated 0.01 M Na2 S2 O3 , 20 wt.% NaCl, and 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at 25 ◦ C.

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Fig. 4. SEM micrographs of the fracture surfaces and side views of type 321s after the SSRT tests in: (a) air, (b) deaerated 20 wt.% NaCl (pH 2) at 25 ◦ C, (c) deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 (pH 2) at 25 ◦ C, and (d) deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 (pH 7) at 25 ◦ C.

micrographs of the fracture surfaces after the SSRT tests are presented in Fig. 4. Fig. 3 indicates the following results: (1) the UTS and UEL obtained in deaerated 0.01 M Na2 S2 O3 aqueous solution at pH 2 are similar to those obtained in air. (2) The UTS and UEL in deaerated 20 wt.% NaCl at pH 2 are slightly lower than those obtained in air. (3) The UTS and UEL in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 at pH 2 decrease about 11 and 17%, respectively, in comparison with those obtained in air. (4) The UTS and UEL in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 solution at pH 7 remain satisfactory but are slightly lower than those obtained in air. In addition, it is also observed that the influence of the corrosive environment (deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 solution of pH 2 at 25 ◦ C) on yield stress (YS) is to reduce YS from 350 (in air) to 310 MPa. The SEM micrographs shown in Fig. 4 illustrate the following results: (1) the typical cup-and-cone and necking fracture surface observed after performing the SSRT test in air (Fig. 4a) contrasts the semi-cup-and-cone structure after performing the SSRT test in deaerated 20 wt.% NaCl solution of pH 2 at 25 ◦ C. This change in fractographic features is induced by the growth of corrosion pits, as shown in Fig. 4b. (2) Secondary cracks and pits are distributed over the gauge and the RA decreased sharply after performing the SSRT test

in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 solution of pH 2 at 25 ◦ C, revealing the occurrence of SCC (Fig. 4c). The fracture mode of SCC was ductile dimpled tearing fracture [51]. (3) Fig. 4d shows that the fracture surface after performing the SSRT test in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 solution of pH 7 at 25 ◦ C still has some similarity with the cup-and-cone structure obtained in air (Fig. 4a). The above results lead to the conclusion that chloride ion (Cl− ) promotes pitting and S2 O3 2− accelerates the occurrence of SCC, although only in the acid-containing Cl− aqueous solution. 3.1.1.2. Effect of Cl− concentration. The effects of Cl− concentration on the SSRT results in deaerated X wt.% NaCl + 0.01 M Na2 S2 O3 solution (X = 0–20) of pH 2 at 25 ◦ C are shown in Fig. 5 and the corresponding SEM micrographs of the fracture surfaces and side views of the specimens after the SSRT tests are presented in Fig. 6. Fig. 5 indicates that the main change that occurred in the stress-strain curves as the Cl− concentration increased was the decrease in UTS, while the UEL was essentially unaffected by Cl− concentration and remained at about 44%, as shown in Fig. 5b. It is reasonable to have a higher RA than UEL, since the localized plasticity is always more serious than the overall or averaged ductility.

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Fig. 5. Effect of Cl− concentration on the SSRT results of type 321s in deaerated X wt.% NaCl + 0.01 M Na2 S2 O3 (pH 2) at 25 ◦ C: (a) stress–strain curves and (b) effect of Cl− concentration on UTS, UEL, and RA.

Fig. 6. SEM micrographs of the fracture surfaces and side views of type 321s after SSRT tests in: (a) 0.1 wt.% NaCl + 0.01 M Na2 S2 O3 (pH 2) at 25 ◦ C and (b) 1 wt.% NaCl + 0.01 M Na2 S2 O3 (pH 2) at 25 ◦ C.

From the SEM micrographs shown in Fig. 6a (0.1 wt.% NaCl), Fig. 6b (1 wt.% NaCl), and Fig. 4c (20 wt.% NaCl), it appears that, although pitting did not seem to have played a significant role in increasing SCC susceptibility, the distribution of secondary cracks gradually increased and the secondary cracks became broader and longer with increasing Cl− concentration. In addition, the observed decrease in the necked area suggests that SCC has occurred.

of 1, a clearly visible necking can be observed in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at 25 ◦ C, as exhibited in Fig. 8b. The effect of lowering the solution pH on the SCC susceptibility is partly to provide an acidified environment for the pit growth and partly to accelerate the following S2 O3 2− decomposition reactions [29,52]:

3.1.1.3. Effect of pH. The effects of solution pH on the SSRT results in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at 25 ◦ C are shown in Fig. 7. The corresponding SEM micrographs of the fracture surfaces after the SSRT tests are presented in Fig. 8. In Fig. 7, it is clear that the UTS value decreases with a decrease of solution pH. On the other hand, it can be found that the effect of pH on UEL and RA is not large except for a solution pH of 2. Some small pits and cracks appear on the gauge after testing in the pH 4 solution, as shown in Fig. 8a. However, secondary cracks are distributed over the gauge in the pH 2 solution (Fig. 4c). As the solution pH decreases to the value

Formation of S (pH < 5): S2 O3 2− + H+ ⇔ S + HSO3 −

(2)

S2 O3 2− + 6H+ + 4e− ⇔ 2S + 3H2 O

(3)

Formation of H2 S: S + 2H+ + 2e− ⇔ H2 S

(4)

4S + 4H2 O ⇔ 3H2 S + H2 SO4

(5)

Then, H2 S reacts with the base metal to form FeS: Fe + H2 S ⇔ FeS + H2

(6)

this reaction causes the pits to become broader and deeper. In other words, once Cl− breaks down the passive film to form

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Fig. 7. Effect of solution pH on the SSRT results of type 321s in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at 25 ◦ C: (a) stress–strain curves and (b) effect of solution pH on UTS, UEL, and RA.

pits; the lower the solution pH, the easier the occurrence of the SCC is. 3.1.1.4. Effect of temperature. The effects of temperature on the SSRT results in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at pH 7 and 2 are shown in Fig. 9. In addition, the corresponding SEM micrographs of the fracture surfaces after the SSRT tests are exhibited in Figs. 10–12. In Fig. 9, it is clear that the UTS, UEL, and RA decrease with the increasing temperature in the solutions of both pH 7 and 2. In addition, it was found that the effect of solution pH on the SCC susceptibility of type 321s in SSRT tests, measured by such parameters as UTS and UEL, becomes more marked with increasing temperature. For instance, when the temperature increases from 25 to 80 ◦ C, the decrease in UTS at pH 2 (from 550 to 370 MPa) is larger than that at pH 7 (from 610 to 520 MPa). It is clear that the area of ductile fracture surface decreases with increasing temperature, as shown in Fig. 10. Furthermore, for specimens tested in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution at pH 7, symp-

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Fig. 8. SEM micrographs of the fracture surfaces and side views of type 321s after the SSRT tests in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at 25 ◦ C at: (a) pH 4 and (b) pH 1.

toms of SCC appear at 50 ◦ C in that there is a significant decrease in the size of the necked region. At 80 ◦ C, the brittle zone is the main feature of the fracture surface, RA decreases substantially, and secondary cracks become more severe than those observed at 50 ◦ C. Similarly, specimens tested in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at pH 2 (shown in Figs. 11 and 12) indicate a similar corrosion tendency (i.e., the higher the temperature, the higher is the susceptibility to SCC), and a tear zone perpendicular to the fracture surface is also observed. As shown in Fig. 12, the fracture mode of region A owing to SCC is ductile dimpled rupture; B is brittle fracture region showing a cleavage mode, and in region C, the crack appears to have initiated at a stress-concentrating pit. Furthermore, the fractography and side view observations of the tested specimens are consistent with the SSRT results (Fig. 9a) showing that, at each temperature, the SCC susceptibility of type 321s is more significant at pH 2 than at pH 7. The above results lead to the conclusion that increasing temperature results in an increase of the Cl− activity as well

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Fig. 10. SEM micrographs of the fracture surfaces and side views of type 321s after SSRT tests in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions (pH 7) at: (a) 25 ◦ C, (b) 50 ◦ C, and (c) 80 ◦ C. Fig. 9. Effect of temperature on the SSRT results of type 321s in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions at 25 ◦ C: (a) stress–strain curves; (b) effect of temperature on UTS, UEL, and RA at pH 7; (c) effect of temperature on UTS, UEL, and RA at pH 2.

as the acceleration of the S2 O3 2− decomposition reactions, causing the passive oxide film to become unstable with an associated decrease in the pitting resistance; in the low-pH solution, susceptibility to pitting and SCC increase to an even greater extent.

3.2. U-bend tests SEM micrographs and EPMA analysis of the corroded surfaces in the most highly stressed areas of the U-bend specimens after the 1400 h immersion in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at both 80 and 300 ◦ C are presented in Figs. 13 and 14, respectively. In addition, the corrosion products formed on the most highly stressed site of the U-bend specimens were analyzed by AES and XRD; these results are shown in Figs. 15 and 16.

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Fig. 11. SEM micrographs of the fracture surfaces and side views of type 321s after SSRT tests in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions (pH 2) at: (a) 50 ◦ C and (b) 80 ◦ C.

Figs. 13 and 14 indicate the following results. (1) The surface of the U-bend specimen tested at 80 ◦ C appears to contain some cracks, and the pits are also broader and deeper than those observed on the specimen tested at 300 ◦ C. (2) EPMA analysis of the specimen surfaces tested at 300 ◦ C demonstrated that S is evenly distributed while the O concentration is low in pitted areas. On the other hand, EPMA analysis of the specimen surfaces tested at 80 ◦ C showed that S is more concentrated than on the specimens tested at 300 ◦ C. Fig. 15 illustrates that the corrosion products at 300 ◦ C contain higher concentrations of O and Cr, but lower concentrations of Fe and S than at 80 ◦ C. The results of AES are consistent with those obtained from EPMA. EPMA analyses of the specimen surfaces also show that the corrosion products at 80 ◦ C consist of lower concentrations of O and Cr, but higher concentration S than at 300 ◦ C. Furthermore, XRD analysis showed that the corrosion product is mainly ferrous sulfide (FeS) at 80 ◦ C but mainly magnetite (Fe3 O4 ) at 300 ◦ C, as shown in Fig. 16. The reason that corrosion damage of the U-bend specimen at 80 ◦ C is more serious than at 300 ◦ C can be explained by the fact that the Fe3 O4 corrosion product formed at 300 ◦ C is more compact than the

Fig. 12. High magnification SEM micrographs of the three regions marked in Fig. 11b (a) region A; (b) region B; (c) region C.

FeS that forms at 80 ◦ C because the average valence of Fe in Fe3 O4 (8/3) is larger than that in FeS (2). Consequently, Fe3 O4 is more resistant to the diffusion of O, S, and Cl− into the base metal than FeS. In addition, the passive oxide film of chromic oxide (Cr2 O3 ) formed at 300 ◦ C is more compact than the oxide forming at 80 ◦ C because the Cr and O concentrations at 300 ◦ C are higher than at 80 ◦ C, as proven by the results of AES analysis. Therefore, the former (Cr2 O3 at 300 ◦ C) is more resistant to the diffusion of O, S, and Cl− into the base metal than the latter (Cr2 O3 at 80 ◦ C).

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Fig. 13. SEM micrograph and EPMA analysis of the corroded surface in the most highly stressed areas of the U-bend specimen of type 321s after 1400 h immersion in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 300 ◦ C: (a) surface morphology showing shallow pits, (b) X-ray mapping of S, (c) X-ray mapping of O, and (d) X-ray mapping of Cr. Magnification 800×.

Fig. 14. SEM micrograph and EPMA analysis of the corroded surface in the most highly stressed areas of the U-bend specimen of type 321s after 1400 h immersion in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 80 ◦ C: (a) pit morphology; (b) X-ray mapping of S; (c) X-ray mapping of O; (d) X-ray mapping of Cr; (e) X-ray mapping of Ti. Magnification 800×.

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Fig. 15. AES quantitative analysis of the corrosion products formed in the most highly stressed areas of U-bend specimens of type 321s after 1400 h immersion in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 80 and 300 ◦ C.

Fig. 16. X-ray analysis of the corrosion products formed in the most highly stressed areas of U-bend specimens of type 321s after 1400 h immersion in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 80 and 300 ◦ C: (a) after immersion at 80 ◦ C and (b) after immersion at 300 ◦ C.

4. Discussion 4.1. Comparison of environmental factors affecting the SCC susceptibility of type 321s The above experimental results suggest that environmental factors such as Cl− concentration, solution pH, and temperature have certain degree of influence on the SCC suscep-

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tibility of type 321s. However, it is difficult to know which factor most affects the susceptibility to SCC and whether there are interactive effects among these factors based solely on the results of individual SSRT tests. In order to quantitatively estimate the individual and interactive effects of temperature, Cl− concentration, and solution pH on the SCC susceptibility of type 321s in SSRT tests, eight runs were performed in deaerated solutions according to Yates’s algorithm [35–40]. Yates’s algorithm is applied to the observations after they have been rearranged in what is called standard order. A 2k factorial design is in standard order when, as listed in Table 2, the first column of the design matrix consists of alternating minus and plus signs, the second column consists of alternating pairs of minus and plus signs, the third column consists of four minus signs followed by four plus signs, and so forth. In general, the kth column consists of 2k − 1 minus signs followed by 2k − 1 plus signs. The effect of a factor is defined as the change in the response of that factor when the experimental condition moves from the minus (low) to the plus (high) level. The value of k represents the number of experimental variables. In this study, there are three variables (chloride concentration, solution pH, and temperature), which affect the SCC susceptibility of type 321s in the simulated H2 S-containing chloride solutions. Therefore, a 23 factorial experiment was performed. The Yates’s calculations for UTS, UEL, and RA are shown in Tables 3a–3c, respectively. Taking UTS (Table 3a) as an example, column “UTS” contains the corresponding UTS for each run, and these values are then considered in successive pairs, as follows. The first four entries in column (1) are obtained by adding the pairs together. Thus, 610 + 540 = 1150, 606 + 515 = 1121, and so on. The second four entries in column (1) are obtained by subtracting the first number from the second number of each pair. Thus, 540 − 610 = −70, 515 − 606 = −91, and so on. In just the same way that column (1) is obtained from column “UTS”, column (2) is obtained from column (1). Finally, column (3) is obtained from column (2) in the same manner. The variance analysis results, listed in Table 4, indicate that the results for UTS, UEL, and RA are similar in that the influence of test parameters on SCC susceptibility decreases in the order, temperature effect  solution pH effect > Cl− concentration effect. This conveys the important message that a decrease in temperature can effectively prevent SCC when type 321s is exposed to H2 S-containing chloride solutions. In addition, the interactive effects of test variables for UTS, UEL, and RA are synergistic. In comparison with the results of SSRT tests and visual analysis of fracture surfaces, it can be found that the results of the Yates’s algorithm are consistent with those observed from SSRT tests (σ–ε curves) and visual analysis of fracture surfaces. That is, the SCC behaviors of type 321s in simulated petrochemical process environments containing H2 S and chloride is very susceptible to the variation of temperature.

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Table 2 Factorial experiments for data tested in deaerated 10−2 M Na2 S2 O3 Test number

Experimental conditions Temperature

(◦ C)

−25 +80 −25 +80 −25 +80 −25 +80

1 2 3 4 5 6 7 8

Mechanical properties Cl−

concentration (wt.%)

−1 −1 +20 +20 −1 −1 +20 +20

pH value

UTS (MPa)

UEL (%)

RA (%)

−7 −7 −7 −7 +2 +2 +2 +2

610 540 606 515 581 467 545 365

48.6 36.4 46.5 32.4 43.6 28.3 43.5 8.3

80.1 55.3 78.8 29.1 63.4 44.2 47.4 21.5

Table 3a Effects of test variables on UTS Test number

1 2 3 4 5 6 7 8

Design matrix variable

Algorithm

Identification

T

Cl−

pH

UTS (MPa)

1

2

3

Divisor

Estimate

− + − + − + − +

− − + + − − + +

− − − − + + + +

610 540 606 515 581 467 545 365

1150 1121 1048 910 −70 −91 −114 −180

2271 1958 −161 −294 −29 −138 −21 −66

4229 −455 −167 −87 −313 −133 −109 −45

8 4 4 4 4 4 4 4

528.63 −113.75 −41.75 −21.75 −78.75 −33.25 −27.25 −11.25

Average T Cl− T × Cl− pH T × pH Cl− × pH T × Cl− × pH

Table 3b Effects of test variables on UEL Test number

1 2 3 4 5 6 7 8

Design matrix variable

Algorithm

Identification

T

Cl−

pH

UEL (%)

1

2

3

Divisor

Estimate

− + − + − + − +

− − + + − − + +

− − − − + + + +

48.6 36.4 46.5 32.4 43.6 28.3 43.5 8.3

85.0 78.9 71.9 51.8 −12.2 −14.1 −15.3 −35.2

163.9 123.7 −26.3 −50.5 −6.1 −20.1 −1.9 −19.9

287.6 −76.8 −26.2 −21.8 −4.02 −24.2 −14.0 −18.0

8 4 4 4 4 4 4 4

35.95 −19.20 −6.55 −5.45 −10.05 −6.05 −3.50 −4.50

Average T Cl− T × Cl− pH T × pH Cl− × pH T × Cl− × pH

Table 3c Effects of test variables on RA Test number

1 2 3 4 5 6 7 8

Design matrix variable

Algorithm

Identification

T

Cl−

pH

RA (%)

1

2

3

Divisor

Estimate

− + − + − + − +

− − + + − − + +

− − − − + + + +

80.1 55.3 78.8 29.1 63.4 44.2 47.4 21.5

135.4 107.9 107.6 68.9 −24.8 −49.7 −19.2 −25.9

243.3 176.5 −74.5 −45.1 −27.5 −38.7 −24.9 −6.7

419.8 −119.6 −66.2 −31.6 −66.8 −29.4 −11.2 18.2

8 4 4 4 4 4 4 4

52.48 −29.90 −16.55 −7.90 −16.70 7.35 −2.80 −4.55

Average T Cl− T × Cl− pH T × pH Cl− × pH T × Cl− × pH

Table 4 Effects of test variables on UTS, UEL, and RA Test item

UTS UEL RA

Identification T

Cl−

pH

T × Cl−

T × pH

Cl− × pH

T × Cl− × pH

Mean

−113.75 −19.20 −29.90

−41.75 −6.55 −16.65

−78.25 −10.05 −16.70

−21.75 −5.45 −7.90

−33.25 −6.05 7.35

−27.25 −3.50 −2.80

−11.25 −4.50 4.55

528.63 35.95 52.84

Y.Y. Chen et al. / Materials Science and Engineering A 407 (2005) 114–126

4.2. SCC mechanism of type 321s in test solutions Results from this study suggest that there are two possible explanations of why type 321s is susceptible to SCC in H2 Scontaining chloride solutions. One explanation is that the TiC

125

particles within type 321s result in the nucleation of sites for SCC, and the other is that SCC is induced by pitting. The sizes of the TiC particles in titanium-stabilized type 321 austenitic stainless steel range from several micometers to 20 ␮m. These TiC particles, which are hard to eliminate by means of heat treatment, may result in the initiation of corrosion pits or the nucleation of cracks [53,54]. Therefore, it is reasonable to assume that the TiC particles may also become the preferred nucleation sites for SCC and the favorable path for the growth for SCC. For instance, SEM micrographs and results of the EPMA analysis on the fracture surface of type 321s after the SSRT test in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 25 ◦ C are shown in Fig. 17. Fig. 17b illustrates that the deep holes observed in the fracture surfaces do not have the appearance of ductile dimples. The diameter of the holes range from 5 to 10 ␮m, similar to the diameters of the TiC particles, which suggests the holes may have been the sites of the TiC particles. Additionally, EPMA analysis results of the fracture surfaces (Fig. 17c) demonstrate that Ti is concentrated in the deep-hole sites; moreover, cracks can be observed near the deep holes in Fig. 17b, leading to the conclusion that TiC particles may be the crack nucleation sites. On the other hand, an examination of the side view and the fracture surface of type 321s after the SSRT test in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 25 ◦ C (Fig. 4c) reveals that secondary cracks caused by SCC are distributed over the gauge. Furthermore, according to the experimental potential–pH diagram (constructed by using the electrochemical hysteresis method) for type 321s in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solutions of various pH values at 25 ◦ C [41,51], the open circuit potential (OCP) at pH 2 falls in the pitting region. This leads to the conclusion that pitting is a precursor to SCC of type 321s. That is to say, as long as the synergistic effects of temperature, Cl− concentration, and the solution pH result in pitting, these three factors will also promote SCC in type 321s.

5. Conclusions Based on the above results and discussion, the following conclusions can be drawn:

Fig. 17. SEM micrographs and EPMA analysis result of the fracture surface of type 321s after the SSRT test in deaerated 20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution (pH 2) at 25 ◦ C: (a) top view; (b) high magnification view of (a); (c) X-ray mapping of Ti in (b).

1. Only in acid aqueous solution containing Cl− does S2 O3 2− accelerate SCC, which, in turn, is induced by pitting owing to the presence of Cl− . 2. An increase in Cl− concentration increases SCC susceptibility, which is reflected primarily by a decrease in the UTS. 3. According to the T × Cl− and T × pH values obtained from the factorial experiments for UTS, UEL, and RA, an increase in temperature in the range 25–80 ◦ C will cause an increase in the SCC susceptibility and enhance the effects of Cl− concentration and solution pH.

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4. The individual effects of the environmental factors on SCC susceptibility decrease in the order: temperature effect  solution pH effect > Cl− concentration effect. In addition, the interactive effects (T × Cl− , T × pH, Cl− × pH, and T × Cl− × pH) are synergistic. This conveys the important message that decreasing the temperature can effectively prevent SCC of type 321s in H2 Scontaining chloride solutions. 5. The results of the Yates’s algorithm are consistent with those observed from SSRT tests (σ–ε curves) and visual analysis of fracture surfaces, indicating that the SCC behaviors of type 321s in simulated petrochemical process environments containing H2 S and chloride is very susceptible to the variation of temperature. 6. Results of the U-bend tests indicate that the susceptibility to SCC decreases with an increase in temperature from 80 to 300 ◦ C, which is related to the more compact Cr2 O3 oxide film and the Fe3 O4 corrosion product formed at 300 ◦ C. 7. The TiC particles with diameters ranging from 5 to 10 ␮m may result in the nucleation of stress corrosion cracks. 8. SCC of type 321s in a simulated petrochemical process environment containing H2 S and chloride (20 wt.% NaCl + 0.01 M Na2 S2 O3 aqueous solution, pH 2) is also found to be induced by pitting. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

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