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Corrosion behavior of steel with different microstructures under various elastic loading conditions
Corrosion Science 75 (2013) 293–299
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behavior of steel with different microstructures under various elastic loading conditions Shuai Zhang, Xiaolu Pang ⇑, Yanbin Wang, Kewei Gao Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 10083, China
a r t i c l e
i n f o
Article history: Received 17 March 2013 Accepted 7 June 2013 Available online 17 June 2013 Keywords: A. Steel B. SEM B. EIS C. Effect of strain
a b s t r a c t The effect of loading conditions on the corrosion behavior of a low carbon and low alloy steel with different microstructures in NaCl solution was investigated. The synergistic effect between the applied elastic stress and chemical attack on the steel was evaluated. The results indicate that the elastic stress accelerated corrosion process of the steel significantly. The steel underwent more serious damage under a dynamic loading condition than that under a static loading condition. The steel with a ferritic and pearlitic dual phase microstructure exhibited higher sensitivity to the mechano-chemical effect compared with the bainitic steel. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Structural materials usually endure mechanical and chemical attack. The mechanical and chemical effects interact with each other leading to a dramatic increase in corrosion rate. This synergistic effect has been defined as the mechano-chemical effect (MCE) [1]. Many previous studies [2–6] focused on the failure of structural components under an applied load condition, such as stress corrosion cracking, corrosion fatigue and hydrogen-induced cracking, and most of them related to plastic deformation processes. Little information on the synergistic effect between elastic stress and electrochemical processes is available in the literature; there is also very little information available on the influence of microstructure on the MCE. Despic et al. [7] noted that the elastic stress only affected the cathodic hydrogen evolution reaction and led to a rise in corrosion potential in H2SO4 solution, whereas in near-neutral pH solution, Cheng [8] did not detect the effect of static elastic stress on corrosion potential of X100 pipeline steel. A study by Huang [9] showed that the polarization resistance of titanium in NaF solution decreased after increasing the applied elastic strain. Melchers and Paik [10] studied the properties of pre-existing rust on plates under high levels of tensile strain and concluded that only strains near or beyond the elastic limit of the steel could induce significant damage to the rust layers. Guo et al. [11] reported that single-phase bainitic steel exhibited higher corrosion resistance than that of ferrite and cementite dual phase steel. An improvement in corrosion
⇑ Corresponding author. Tel.: +86 10 82376048. E-mail address: [email protected] (X. Pang). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.06.012
resistance for HAZ and weld metal in welded API X70 steel after heat treatment was observed and attributed to the formation of a fine and compact corrosion product layer with limited defects [12]. These results indicate that both the applied elastic stress and the microstructure can play a role on the corrosion behavior of steel. Corrosion process of structure materials will cause a decrease in the effective loading area, resulting in an increase in the effective stress applied on the structure. This effective stress rises gradually during the corrosion process, similar to a dynamic tensile or compressive condition at a very low strain rate. In this case, due to the mechano-chemical effect, the increased stress will facilitate the corrosion process, and simultaneously, the corrosion process will lead to further increase in the effective stress. In this paper, the general and localized corrosion behavior of a low carbon and low alloy steel with a bainitic microstructure or ferritic and pearlitic dual phase microstructure was investigated under static and dynamic loading conditions. The degradation of mechanical properties of the steel induced by corrosion was determined, and the mechanism of mechano-chemical effect on the steel was discussed.
2. Experimental procedures The low carbon and low alloy steel was processed in a 25-kg vacuum induction furnace. A plate with a thickness of 6 mm was obtained by controlled rolling, followed by accelerated cooling. The chemical composition of the steel is provided in Table 1. This steel has a bainitic microstructure containing granular bainite and quasi-polygonal ferrite (hereafter known as bainitic steel), as
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Table 1 Chemical composition of the low carbon and low alloy steel (wt.%).
Table 2 Mechanical properties of the low carbon and low alloy steel.
C
Mn
Si
Cu
Cr
Ni
Nb
S
P
Fe
Steel
rs (MPa)
rb (MPa)
d (%)
0.08
1.14
0.33
0.37
0.54
0.38
0.0039
0.0083
0.008
Bal.
Bainitic steel Dual phase steel
697.6 284.5
802.2 447.5
22.54 41.27
Table 3 Test durations and the corresponding strain rates for the steels. Bainitic steel
Dual phase steel
Test duration (h) Strain rate (s1)
19.1
191
400
1.0 106
1.0 107
4.75 108
Test duration (h) Strain rate (s1)
5
19.1
191
6
1.5 10
7
3.89 10
3.89 108
1.5
Corrosion rate (mm/a)
19.1h 1.0
0.5 191h
400h
(a) 0.0
0
100
200
300
400
Time (h) 2.5 5h
Corrosion rate (mm/a)
shown in Fig. 1a. After heat-treated at 1000 °C for an hour followed by furnace cooling, a ferritic and pearlitic dual phase steel (hereafter known as dual phase steel) was obtained, as shown in Fig. 1b. The mechanical properties of the steel were evaluated using uniaxial tensile tests; the results are displayed in Table 2. To investigate the interaction between mechanical and chemical effects during the elastic deformation process, uniaxial static and dynamic tensile tests were performed in a 3.5 wt.% NaCl solution using a WDML-5 microcontrol slow strain rate tensile machine made in China. Plate tensile specimens with gauge sections with dimensions of 6 mm 2 mm 24 mm were used to allow convenient surface observation after tensile tests. The loading direction was parallel to the rolling direction. The specimen was ground by emery paper to grit 800 and then rinsed with ethanol. Part of the gauge section was exposed to NaCl solution, and the other parts of the section were covered by silicone rubber, which has good impermeability and can protect the steel from corrosion. Applying a static load of 70% of the yield strength (rs) of the steel was defined as r0.7c, while applying a dynamic load at a strain rate to the highest stress at 70% of the yield strength was defined as r0.7d. Testing the steel without loading was defined as r0. It should be pointed out that the steel used in this study has good ductility, as shown in Table 2, so the reproducibility of mechanical test could be ensured. To study the variation of corrosion rate with time, the test duration was identical for each loading condition. For the bainitic steel, it took 19.1 h for the applied stress to increase up to 0.7rs at a strain rate of 1 106/s, so the test durations for r0 and r0.7c conditions were also 19.1 h. Test durations of 191 h and 400 h corresponded to strain rates of 1.0 107/s and 4.75 108/s, respectively. Compared with the bainitic steel, the yield strength of the dual phase steel was lower, and the loading time to reach r0.7d was also shorter when the strain rate was maintained at a similar level, so the test durations for the dual phase steel were 5 h, 19.1 h and 191 h. The test durations and the corresponding strain rates are listed in Table 3. All of the tensile tests were maintained within elastic deformation and performed at room temperature under free corrosion potential. After the tensile tests, the electrochemical properties of the corrosion product formed on the specimen surface were analyzed using a CHI660C electrochemical workstation. All measurements were carried out using a saturated calomel electrode (SCE) as the reference electrode and platinum as the counter electrode. Before each measurement, the potentiostat was calibrated in order to
2.0
1.5 19.1h 1.0
191h
0.5
(b) 0.0
0
50
100 Time (h)
150
200
Fig. 2. Variations of general corrosion rate with time under different loading conditions (a) bainitic steel; (b) dual phase steel.
a
b
Fig. 1. Microstructures of the low carbon and low alloy steel (RD: rolling direction) (a) bainitic steel; (b) dual phase steel.
S. Zhang et al. / Corrosion Science 75 (2013) 293–299
ensure the accuracy of the result. Polarization curve measurements were performed at a sweep rate of 0.5 mV/s. Electrochemical impedance spectra (EIS) measurement was carried out at the open circuit potential with a perturbation amplitude of 10 mV in the frequency range from 100 kHz to 10 mHz. The morphology of the specimen surface was observed using optical microscopy and a Zeiss Evo 18 scanning electron microscope (SEM). After the corrosion product was removed in a solution containing 3.5 g hexamethylenetetramine (C6H12N4) with 500 ml 37%HCl and 500 ml deionized water at ambient temperature for 10 min [13], the mechanical properties of the specimen were evaluated again by a uniaxial tensile test in air at room temperature, and the surface roughness of the working section of the specimen was measured by a Dektak 150 surface contourgraph.
3. Results 3.1. General corrosion rate under various loading conditions Fig. 2 shows the variations of general corrosion rate with time under different loading conditions. The corrosion rate of all specimens decreased with time. The loaded specimen exhibited higher corrosion rate than the loading-free one, but the difference between them decreased with time. This behavior can be attributed
295
to the applied load and deformation of the metal, which increased the surface activity of the metal and reduced the reaction energy [14]. Besides, the corrosion product film became more porous under loaded condition which led to a reduction of diffusion resistance of Cl to the steel [15]. Comparing Fig. 2a and b, the general corrosion rates of the steel with different microstructures under static and dynamic loading conditions were almost identical; these results imply that the influences of the microstructure and the loading mode on the general corrosion rate of the steel were negligible.
3.2. Morphology observations of the steel after the tensile test SEM observations of the surface morphologies of the bainitic steel after removal of the corrosion product are shown in Fig. 3. It can be seen that the extent of the damage on the specimens followed a sequence: r0.7d > r0.7c > r0. Both the diameter and depth of the pits on the specimen under r0.7d were the largest compared with those under r0 and r0.7c. Fig. 4 shows the surface morphologies of the dual phase steel after removal of the corrosion product. In contrast to the bainitic steel, the ferritic and pearlitic phases were etched and could be distinguished at an early period, as shown in Fig. 4a-1, a-2 and a-3. The boundaries between grains and phases gradually became chaotic during the corrosion process.
Fig. 3. SEM morphologies of the bainitic steel after corrosion product removed. The letters a, b and c represent the test duration for 19.1 h, 191 h and 400 h, and the numbers 1, 2, and 3 correspond to r0, r0.7c and r0.7d, respectively.
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Fig. 4. SEM morphologies of the dual phase steel after corrosion product removed. The letters a, b and c represent the test duration for 5 h, 19.1 h and 191 h, and the numbers 1, 2, and 3 correspond to r0, r0.7c and r0.7d, respectively.
To evaluate the damage quantitatively, after the corrosion product was removed, the surface roughness of the specimens was measured; the results are illustrated in Fig. 5. For both the bainitic and dual phase steels, the surface roughness of the specimens increased with time under each of the loading conditions. The applied load caused more serious damage to the specimens, and the damage under the dynamic loading condition was more serious than that under static condition as time increased. As shown in Fig. 5a and b, the surface roughness of the bainitic steels after 19.1 h and 191 h, respectively, was, in general, lower than that of the dual phase steels, as shown in Table 4. This result implies that the localized corrosion of the loaded specimen with a dual phase microstructure was more serious than that of the bainitic steel. 3.3. Electrochemical measurement results Fig. 6 shows the polarization curves of the bainitic steel with the corrosion product after being tested for 19.1 h. For each test conditions, the measurements were carried out twice and the results were exhibited in Fig. 6 represented by solid and dash lines, respectively. The free corrosion potentials and corrosion current densities as well as the Tafel slopes were measured and listed in Table 5. It can be seen that the applied load influenced only the anodic polarization behavior, the deviations of the current density
and free corrosion potential between the twice measurements were less than 0.0147 lA/cm2 and 4.3 mV, respectively. The free corrosion potentials moved to negative values under the loading conditions, and the specimen under r0.7c exhibited the most negative potential value and the highest current density. This result illustrates that at an early period, the specimen under the static loading condition underwent a more serious attack, which could be attributed to the relatively higher value of the applied load under the static condition. The results of EIS measurement are shown in Fig. 7, and the corresponding values of charge transfer resistance Rct are listed in Table 6. It can be observed that Rct of all specimens under each loading condition increased with time, which should be caused by the growth of corrosion product on the specimen surface. The applied load caused a reduction in Rct, implying that the corrosion product became looser under loading condition. Rct under r0.7d became lower than that under r0.7c after 191 h, indicating that the mechano-chemical effect under the dynamic loading condition exceeded that under the static loading condition with time increasing and became more significant. 3.4. Mechanical properties of the corroded specimen To evaluate the influence of the mechano-chemical effect on the steel, the mechanical properties of the corroded specimens were
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3000
-0.50
-0.55
-0.60 1800 191h
1200 19.1h
600
0
ESCE / V
Surface roughness (nm)
400h 2400
0
100
200
300
-0.65
-0.70
(a)
-0.75
400
-0.80
Time (h) -0.85 -8
1500
-7
-6
-5
-4
-3
-2
-2
Surface roughness (nm)
Log (i / A cm ) Fig. 6. Polarization curves of the bainitic steel with corrosion product after tested for 19.1 h. The solid and dash lines represent twice measurements, respectively.
1000 19.1h Table 5 Parameters of polarization curves of the bainitic steel with corrosion product after tensile tested in NaCl solution for 19.1 h.
191h 500
0
Current density (lA/ cm2)
Free corrosion potential (V vs. SCE)
bc (V/ decade)
ba (V/ decade)
r0
1 2
0.1056 0.1023
0.6870 0.6865
0.1335 0.1272
0.0550 0.0587
r0.7d
3 4
0.1367 0.1223
0.6975 0.6932
0.1354 0.1375
0.0569 0.0562
r0.7c
5 6
0.1403 0.1550
0.7083 0.7072
0.1272 0.1303
0.0627 0.0633
(b)
5h 0
Sample
50
100
150
200
Time (h) Fig. 5. Surface roughness of the specimens after corrosion product removed. (a) bainitic steel; (b) dual phase steel.
Table 4 Surface roughness of specimens under various loading conditions (nm).
r0
r0.7c
r0.7d
Bainitic steel
19.1 191 400
80 324 1928
580 850 2331
452 1172 2542
Dual phase steel
5 19.1 191
215 451 750
287 561 929
253 645 1315
measured again after removal of the corrosion product. Fig. 8 shows that the elongation decreased with time under each loading condition, and the loaded specimens exhibited a lower elongation than the loading-free specimens. The ductility loss / is defined by the following equation, and the results are listed in Table 7:
u¼
d d0 d0
ð1Þ
where d0 is the elongation in air and d is the elongation in NaCl solution. Table 7 shows that, in the early period, the ductility loss for both steels under r0.7c was higher than that under r0.7d. However, the result reversed with time increasing, which indicated that the decline rate of mechanical properties under the dynamic loading condition was faster than that under the static loading condition. The results in Table 7 also reveal that the ductility loss of the bainitic steel at 19.1 h and 191 h was much lower than that of the dual phase steel under each loading condition.
200
150 2
Duration (h)
-Z'' / ohm cm
Steel
100 10 mHz
50 10 mHz 10 mHz 10 mHz
10 mHz
10 mHz
0 0
100
200
300
400
2
Z' / ohm cm
Fig. 7. Nyquist plots of the bainitic steel after tested for 19.1 h and 191 h, respectively.
4. Discussion Normally, a low carbon and low alloy steel with an elongation higher than 20% is considered insensitive to stress-induced crack-
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Table 6 Charge transfer resistance of electrochemical reaction. Immersion time (h) Loading condition Rct/X cm2
19.1
191
r0
r0.7c
r0.7d
r0
r0.7c
r0.7d
347.8
273.2
296.5
438.5
426.4
407.8
20 19.1h 400h
Elongation (%)
15 191h 10
σ0
5
0
σ 0.7c σ 0.7d
(a) 0
kcr ¼
100
200
300
400
Time (h) 35
5h
σ0 σ 0.7c
Elongation (%)
30
σ 0.7d 19.1h
25
191h 20
15
3.5 wt.% NaCl solution, as shown in Figs. 3 and 4. However, localized corrosion was observed on the sample surface, which became more serious with time, as shown in Figs. 5 and 8 and Table 7 show that all corroded sample underwent ductility loss, and the ductility loss of dual phase steel was higher than that of bainitic steel. These results indicate that both the loading mode and the microstructure exhibited influences on the mechano-chemical effect of the steel. Chemical reactions can change the surface morphology by atomic mass transport, which can be triggered by mechanical stress through modifying the surface free energy. Decuzzi and Demelio [16] discussed the influence of the elastic stress field on atomic mobility and determined the critical parameters for a morphological stable condition. Srolovitz [17] established the relation between the critical wavelength kcr of the initial surface roughness for an unstable system and a uniaxial remote loading r21 as follows:
(b) 0
50
100
150
200
Time (h) Fig. 8. Variations of elongation of the tested steel with time (a) bainitic steel; (b) dual phase steel.
Table 7 Ductility loss of specimens under various loading conditions (%). Steel
Duration (h)
r0
r0.7c
r0.7d
Bainitic steel
19.1 191 400
14.6 28.7 44.5
19.8 32.3 53.3
17.4 37.0 60.5
Dual phase steel
5 19.1 191
17.4 29.8 51.1
22.6 31.8 54.8
20.8 34.5 60.6
ing in a NaCl solution. In the present study, no cracking was observed on the specimen surface, even after tested for 400 h in a
pEc ð1 m2 Þr21
ð2Þ
where c is the surface tension, E and m are Young’s modulus and Poisson’s ratio, respectively. Considering the microstructure morphologies of the bainitic and dual phase steels shown in Fig. 1, if the grain or phase size is considered as the sole effect on the initial surface roughness, assuming a smooth surface inside the grain or phase, it can be concluded from Eq. (2) that the critical stress for an unstable system of the dual phase steel is much lower than that of bainitic steel. This is one reason why the dual phase steel is more sensitive to the mechano-chemical effect than the bainitic steel. Another reason should be attributed to the difference in surface potential between ferrite and pearlite phases, where the preferential dissolution of ferrite will increase the surface roughness. The high surface roughness will lead to inhomogeneous distribution of elastic energy on sample surface because the elastic energy stored at the valleys is larger than that at the crests [16]. Besides, during elastic deformation, the mechanical factors dominate the surface behavior [18]. So the contribution of elastic energy is larger than that of the surface energy which will further increase the surface roughness. The steel immersed in NaCl solution will be gradually covered by corrosion product film, which may protect the steel, depending on the microstructure of the film. EIS measurement results shown in Fig. 7 and Table 6 indicate that the charge transfer resistance under r0.7d was lower than that under r0.7c as the test duration increased, demonstrating that the corrosion product film formed under the dynamic loading condition was more porous and loose. In addition, the outer layer of corrosion product film peeled off more easily and frequently under r0.7d, while the specimen under r0.7c was covered by a compact corrosion product film, as shown in Fig. 9. Previous investigations indicated that a time-dependent flow would cause repetitive film rupturing and reduce the repassivation rate [19,20]. In the present study, a time-dependent strain under the dynamic loading condition led to more serious damage to the steel than a constant strain under the static loading condition by decreasing the compactness of corrosion product film.
Fig. 9. Macro morphologies of the dual phase steel after corroded for 191 h. (a) Under static loading condition, the outer layer of corrosion product with red color can be observed; (b) under dynamic loading condition, the outer layer of corrosion product has peeled off, only the inner layer with a dark brown color left. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S. Zhang et al. / Corrosion Science 75 (2013) 293–299
5. Conclusions The corrosion behavior of a low carbon and low alloy steel in a 3.5 wt.% NaCl solution under an applied elastic stress was investigated. The influence of loading condition and the microstructure of the steel on the mechano-chemical effect of the steel was evaluated. The following results were obtained: (1) The synergistic effect of mechanical and chemical attacks led to a higher ductility loss for the specimen under the loading condition than for the loading-free specimen. (2) The ductility loss of the ferritic and pearlitic dual phase steel was greater than that of the bainitic steel, which could be attributed to higher localized damage on the dual phase steel. (3) The specimen under the dynamic loading condition underwent more serious damage than that under the static loading condition because the time-dependent strain led to a decrease in the protective ability of the corrosion product film on the steel.
Acknowledgement This study was supported by National Natural Science Foundation of China (No. 51071029). References [1] E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific, New Jersey-London, 1994. [2] A. Mustapha, E.A. Charles, D. Hardie, Evaluation of environment-assisted cracking susceptibility of a grade X100 pipeline steel, Corros. Sci. 54 (2012) 5– 9. [3] H.P. Seifert, S. Ritter, Corrosion fatigue crack growth behavior of low-alloy reactor pressure vessel steels under boiling water reactor conditions, Corros. Sci. 50 (2008) 1884–1899.
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[4] T. Michler, C. San Marchi, J. Naumann, S. Weber, M. Martin, Hydrogen environment embrittlement of stable austenitic Steels, Int. J. Hydrogen Energy 37 (2012) 16231–16246. [5] T. Neeraj, R. Srinivasan, J. Li, Hydrogen embrittlement of ferritic steels: observations on deformation microstructure, nanoscale dimples and failure by nanovoiding, Acta Mater. 60 (2012) 5160–5171. [6] F.J. Meng, J.Q. Wang, E.H. Han, W. Ke, Effects of scratching on corrosion and stress corrosion cracking of alloy 690TT at 58 °C and 330 °C, Corros. Sci. 51 (2009) 2761–2769. [7] A.R. Despic, R.G. Raicheff, J.O’M. Bockris, Effect of elastic strains on the electrochemical behavior of metals in sulfuric acid solution, J. Chem. Phys. 48 (1968) 49–51. [8] L.Y. Xu, Y.F. Cheng, An experimental investigation of corrosion of X100 pipeline steel under uniaxial elastic stress in a near-neutral pH solution, Corros. Sci. 59 (2012) 103–109. [9] H.H. Huang, Effects of fluoride concentration and elastic tensile strain on the corrosion resistance of commercially pure titanium, Biomaterials 23 (2002) 59–63. [10] R.E. Melchers, J.K. Paik, Effect of tensile strain on the rate of marine corrosion of steel plates, Corros. Sci. 51 (2009) 2298–2303. [11] J. Guo, S.W. Yang, C.J. Shang, Y. Wang, X.L. He, Effect of carbon content and microstructure on corrosion resistance of low alloy steels, Iron Steel 43 (2008) 58–62. [12] M. Alizadeh, S. Bordbar, The influence of microstructure on the protective properties of the corrosion product layer generated on the welded API X70 steel in chloride solution, Corros. Sci. 70 (2013) 170–179. [13] G1-03, Annual Book of ASTM Standards, vol. 03.02, ASTM, Philadelphia, PA, 2011. [14] X.D. Liu, G.S. Frankel, Effects of compressive stress on localized corrosion in AA2024-T3, Corros. Sci. 48 (2006) 3309–3329. [15] K. Gao, D. Li, X. Pang, S. Yang, Corrosion behaviour of low-carbon bainitic steel under a constant elastic load, Corros. Sci. 52 (2010) 3428–3434. [16] P. Decuzzi, G.P. Demelio, Stress-driven morphological instability and catastrophic failure of microdevices, Int. J. Solids Struct. 40 (2003) 729–745. [17] D.J. Srolovitz, On the stability of surfaces of stressed solids, Acta Metall. 37 (1989) 621–625. [18] J. Orlikowski, K. Darowicki, A. Arutunow, W. Jurczak, The effect of strain rate on the passive layer cracking of 304L stainless steel in chloride solutions based on the differential analysis of electrochemical parameters obtained by means of DEIS, J. Electroanal. Chem. 576 (2005) 277–285. [19] D.A. Vermilyea, Concerning strain-enhanced corrosion mechanisms of SCC, Corrosion 32 (1976) 26–29. [20] M.M. Hall Jr., Film rupture model for aqueous stress corrosion cracking under constant and variable stress intensity factor, Corros. Sci. 51 (2009) 225–233.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behavior of steel with different microstructures under various elastic loading conditions Shuai Zhang, Xiaolu Pang ⇑, Yanbin Wang, Kewei Gao Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 10083, China
a r t i c l e
i n f o
Article history: Received 17 March 2013 Accepted 7 June 2013 Available online 17 June 2013 Keywords: A. Steel B. SEM B. EIS C. Effect of strain
a b s t r a c t The effect of loading conditions on the corrosion behavior of a low carbon and low alloy steel with different microstructures in NaCl solution was investigated. The synergistic effect between the applied elastic stress and chemical attack on the steel was evaluated. The results indicate that the elastic stress accelerated corrosion process of the steel significantly. The steel underwent more serious damage under a dynamic loading condition than that under a static loading condition. The steel with a ferritic and pearlitic dual phase microstructure exhibited higher sensitivity to the mechano-chemical effect compared with the bainitic steel. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Structural materials usually endure mechanical and chemical attack. The mechanical and chemical effects interact with each other leading to a dramatic increase in corrosion rate. This synergistic effect has been defined as the mechano-chemical effect (MCE) [1]. Many previous studies [2–6] focused on the failure of structural components under an applied load condition, such as stress corrosion cracking, corrosion fatigue and hydrogen-induced cracking, and most of them related to plastic deformation processes. Little information on the synergistic effect between elastic stress and electrochemical processes is available in the literature; there is also very little information available on the influence of microstructure on the MCE. Despic et al. [7] noted that the elastic stress only affected the cathodic hydrogen evolution reaction and led to a rise in corrosion potential in H2SO4 solution, whereas in near-neutral pH solution, Cheng [8] did not detect the effect of static elastic stress on corrosion potential of X100 pipeline steel. A study by Huang [9] showed that the polarization resistance of titanium in NaF solution decreased after increasing the applied elastic strain. Melchers and Paik [10] studied the properties of pre-existing rust on plates under high levels of tensile strain and concluded that only strains near or beyond the elastic limit of the steel could induce significant damage to the rust layers. Guo et al. [11] reported that single-phase bainitic steel exhibited higher corrosion resistance than that of ferrite and cementite dual phase steel. An improvement in corrosion
⇑ Corresponding author. Tel.: +86 10 82376048. E-mail address: [email protected] (X. Pang). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.06.012
resistance for HAZ and weld metal in welded API X70 steel after heat treatment was observed and attributed to the formation of a fine and compact corrosion product layer with limited defects [12]. These results indicate that both the applied elastic stress and the microstructure can play a role on the corrosion behavior of steel. Corrosion process of structure materials will cause a decrease in the effective loading area, resulting in an increase in the effective stress applied on the structure. This effective stress rises gradually during the corrosion process, similar to a dynamic tensile or compressive condition at a very low strain rate. In this case, due to the mechano-chemical effect, the increased stress will facilitate the corrosion process, and simultaneously, the corrosion process will lead to further increase in the effective stress. In this paper, the general and localized corrosion behavior of a low carbon and low alloy steel with a bainitic microstructure or ferritic and pearlitic dual phase microstructure was investigated under static and dynamic loading conditions. The degradation of mechanical properties of the steel induced by corrosion was determined, and the mechanism of mechano-chemical effect on the steel was discussed.
2. Experimental procedures The low carbon and low alloy steel was processed in a 25-kg vacuum induction furnace. A plate with a thickness of 6 mm was obtained by controlled rolling, followed by accelerated cooling. The chemical composition of the steel is provided in Table 1. This steel has a bainitic microstructure containing granular bainite and quasi-polygonal ferrite (hereafter known as bainitic steel), as
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Table 1 Chemical composition of the low carbon and low alloy steel (wt.%).
Table 2 Mechanical properties of the low carbon and low alloy steel.
C
Mn
Si
Cu
Cr
Ni
Nb
S
P
Fe
Steel
rs (MPa)
rb (MPa)
d (%)
0.08
1.14
0.33
0.37
0.54
0.38
0.0039
0.0083
0.008
Bal.
Bainitic steel Dual phase steel
697.6 284.5
802.2 447.5
22.54 41.27
Table 3 Test durations and the corresponding strain rates for the steels. Bainitic steel
Dual phase steel
Test duration (h) Strain rate (s1)
19.1
191
400
1.0 106
1.0 107
4.75 108
Test duration (h) Strain rate (s1)
5
19.1
191
6
1.5 10
7
3.89 10
3.89 108
1.5
Corrosion rate (mm/a)
19.1h 1.0
0.5 191h
400h
(a) 0.0
0
100
200
300
400
Time (h) 2.5 5h
Corrosion rate (mm/a)
shown in Fig. 1a. After heat-treated at 1000 °C for an hour followed by furnace cooling, a ferritic and pearlitic dual phase steel (hereafter known as dual phase steel) was obtained, as shown in Fig. 1b. The mechanical properties of the steel were evaluated using uniaxial tensile tests; the results are displayed in Table 2. To investigate the interaction between mechanical and chemical effects during the elastic deformation process, uniaxial static and dynamic tensile tests were performed in a 3.5 wt.% NaCl solution using a WDML-5 microcontrol slow strain rate tensile machine made in China. Plate tensile specimens with gauge sections with dimensions of 6 mm 2 mm 24 mm were used to allow convenient surface observation after tensile tests. The loading direction was parallel to the rolling direction. The specimen was ground by emery paper to grit 800 and then rinsed with ethanol. Part of the gauge section was exposed to NaCl solution, and the other parts of the section were covered by silicone rubber, which has good impermeability and can protect the steel from corrosion. Applying a static load of 70% of the yield strength (rs) of the steel was defined as r0.7c, while applying a dynamic load at a strain rate to the highest stress at 70% of the yield strength was defined as r0.7d. Testing the steel without loading was defined as r0. It should be pointed out that the steel used in this study has good ductility, as shown in Table 2, so the reproducibility of mechanical test could be ensured. To study the variation of corrosion rate with time, the test duration was identical for each loading condition. For the bainitic steel, it took 19.1 h for the applied stress to increase up to 0.7rs at a strain rate of 1 106/s, so the test durations for r0 and r0.7c conditions were also 19.1 h. Test durations of 191 h and 400 h corresponded to strain rates of 1.0 107/s and 4.75 108/s, respectively. Compared with the bainitic steel, the yield strength of the dual phase steel was lower, and the loading time to reach r0.7d was also shorter when the strain rate was maintained at a similar level, so the test durations for the dual phase steel were 5 h, 19.1 h and 191 h. The test durations and the corresponding strain rates are listed in Table 3. All of the tensile tests were maintained within elastic deformation and performed at room temperature under free corrosion potential. After the tensile tests, the electrochemical properties of the corrosion product formed on the specimen surface were analyzed using a CHI660C electrochemical workstation. All measurements were carried out using a saturated calomel electrode (SCE) as the reference electrode and platinum as the counter electrode. Before each measurement, the potentiostat was calibrated in order to
2.0
1.5 19.1h 1.0
191h
0.5
(b) 0.0
0
50
100 Time (h)
150
200
Fig. 2. Variations of general corrosion rate with time under different loading conditions (a) bainitic steel; (b) dual phase steel.
a
b
Fig. 1. Microstructures of the low carbon and low alloy steel (RD: rolling direction) (a) bainitic steel; (b) dual phase steel.
S. Zhang et al. / Corrosion Science 75 (2013) 293–299
ensure the accuracy of the result. Polarization curve measurements were performed at a sweep rate of 0.5 mV/s. Electrochemical impedance spectra (EIS) measurement was carried out at the open circuit potential with a perturbation amplitude of 10 mV in the frequency range from 100 kHz to 10 mHz. The morphology of the specimen surface was observed using optical microscopy and a Zeiss Evo 18 scanning electron microscope (SEM). After the corrosion product was removed in a solution containing 3.5 g hexamethylenetetramine (C6H12N4) with 500 ml 37%HCl and 500 ml deionized water at ambient temperature for 10 min [13], the mechanical properties of the specimen were evaluated again by a uniaxial tensile test in air at room temperature, and the surface roughness of the working section of the specimen was measured by a Dektak 150 surface contourgraph.
3. Results 3.1. General corrosion rate under various loading conditions Fig. 2 shows the variations of general corrosion rate with time under different loading conditions. The corrosion rate of all specimens decreased with time. The loaded specimen exhibited higher corrosion rate than the loading-free one, but the difference between them decreased with time. This behavior can be attributed
295
to the applied load and deformation of the metal, which increased the surface activity of the metal and reduced the reaction energy [14]. Besides, the corrosion product film became more porous under loaded condition which led to a reduction of diffusion resistance of Cl to the steel [15]. Comparing Fig. 2a and b, the general corrosion rates of the steel with different microstructures under static and dynamic loading conditions were almost identical; these results imply that the influences of the microstructure and the loading mode on the general corrosion rate of the steel were negligible.
3.2. Morphology observations of the steel after the tensile test SEM observations of the surface morphologies of the bainitic steel after removal of the corrosion product are shown in Fig. 3. It can be seen that the extent of the damage on the specimens followed a sequence: r0.7d > r0.7c > r0. Both the diameter and depth of the pits on the specimen under r0.7d were the largest compared with those under r0 and r0.7c. Fig. 4 shows the surface morphologies of the dual phase steel after removal of the corrosion product. In contrast to the bainitic steel, the ferritic and pearlitic phases were etched and could be distinguished at an early period, as shown in Fig. 4a-1, a-2 and a-3. The boundaries between grains and phases gradually became chaotic during the corrosion process.
Fig. 3. SEM morphologies of the bainitic steel after corrosion product removed. The letters a, b and c represent the test duration for 19.1 h, 191 h and 400 h, and the numbers 1, 2, and 3 correspond to r0, r0.7c and r0.7d, respectively.
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Fig. 4. SEM morphologies of the dual phase steel after corrosion product removed. The letters a, b and c represent the test duration for 5 h, 19.1 h and 191 h, and the numbers 1, 2, and 3 correspond to r0, r0.7c and r0.7d, respectively.
To evaluate the damage quantitatively, after the corrosion product was removed, the surface roughness of the specimens was measured; the results are illustrated in Fig. 5. For both the bainitic and dual phase steels, the surface roughness of the specimens increased with time under each of the loading conditions. The applied load caused more serious damage to the specimens, and the damage under the dynamic loading condition was more serious than that under static condition as time increased. As shown in Fig. 5a and b, the surface roughness of the bainitic steels after 19.1 h and 191 h, respectively, was, in general, lower than that of the dual phase steels, as shown in Table 4. This result implies that the localized corrosion of the loaded specimen with a dual phase microstructure was more serious than that of the bainitic steel. 3.3. Electrochemical measurement results Fig. 6 shows the polarization curves of the bainitic steel with the corrosion product after being tested for 19.1 h. For each test conditions, the measurements were carried out twice and the results were exhibited in Fig. 6 represented by solid and dash lines, respectively. The free corrosion potentials and corrosion current densities as well as the Tafel slopes were measured and listed in Table 5. It can be seen that the applied load influenced only the anodic polarization behavior, the deviations of the current density
and free corrosion potential between the twice measurements were less than 0.0147 lA/cm2 and 4.3 mV, respectively. The free corrosion potentials moved to negative values under the loading conditions, and the specimen under r0.7c exhibited the most negative potential value and the highest current density. This result illustrates that at an early period, the specimen under the static loading condition underwent a more serious attack, which could be attributed to the relatively higher value of the applied load under the static condition. The results of EIS measurement are shown in Fig. 7, and the corresponding values of charge transfer resistance Rct are listed in Table 6. It can be observed that Rct of all specimens under each loading condition increased with time, which should be caused by the growth of corrosion product on the specimen surface. The applied load caused a reduction in Rct, implying that the corrosion product became looser under loading condition. Rct under r0.7d became lower than that under r0.7c after 191 h, indicating that the mechano-chemical effect under the dynamic loading condition exceeded that under the static loading condition with time increasing and became more significant. 3.4. Mechanical properties of the corroded specimen To evaluate the influence of the mechano-chemical effect on the steel, the mechanical properties of the corroded specimens were
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3000
-0.50
-0.55
-0.60 1800 191h
1200 19.1h
600
0
ESCE / V
Surface roughness (nm)
400h 2400
0
100
200
300
-0.65
-0.70
(a)
-0.75
400
-0.80
Time (h) -0.85 -8
1500
-7
-6
-5
-4
-3
-2
-2
Surface roughness (nm)
Log (i / A cm ) Fig. 6. Polarization curves of the bainitic steel with corrosion product after tested for 19.1 h. The solid and dash lines represent twice measurements, respectively.
1000 19.1h Table 5 Parameters of polarization curves of the bainitic steel with corrosion product after tensile tested in NaCl solution for 19.1 h.
191h 500
0
Current density (lA/ cm2)
Free corrosion potential (V vs. SCE)
bc (V/ decade)
ba (V/ decade)
r0
1 2
0.1056 0.1023
0.6870 0.6865
0.1335 0.1272
0.0550 0.0587
r0.7d
3 4
0.1367 0.1223
0.6975 0.6932
0.1354 0.1375
0.0569 0.0562
r0.7c
5 6
0.1403 0.1550
0.7083 0.7072
0.1272 0.1303
0.0627 0.0633
(b)
5h 0
Sample
50
100
150
200
Time (h) Fig. 5. Surface roughness of the specimens after corrosion product removed. (a) bainitic steel; (b) dual phase steel.
Table 4 Surface roughness of specimens under various loading conditions (nm).
r0
r0.7c
r0.7d
Bainitic steel
19.1 191 400
80 324 1928
580 850 2331
452 1172 2542
Dual phase steel
5 19.1 191
215 451 750
287 561 929
253 645 1315
measured again after removal of the corrosion product. Fig. 8 shows that the elongation decreased with time under each loading condition, and the loaded specimens exhibited a lower elongation than the loading-free specimens. The ductility loss / is defined by the following equation, and the results are listed in Table 7:
u¼
d d0 d0
ð1Þ
where d0 is the elongation in air and d is the elongation in NaCl solution. Table 7 shows that, in the early period, the ductility loss for both steels under r0.7c was higher than that under r0.7d. However, the result reversed with time increasing, which indicated that the decline rate of mechanical properties under the dynamic loading condition was faster than that under the static loading condition. The results in Table 7 also reveal that the ductility loss of the bainitic steel at 19.1 h and 191 h was much lower than that of the dual phase steel under each loading condition.
200
150 2
Duration (h)
-Z'' / ohm cm
Steel
100 10 mHz
50 10 mHz 10 mHz 10 mHz
10 mHz
10 mHz
0 0
100
200
300
400
2
Z' / ohm cm
Fig. 7. Nyquist plots of the bainitic steel after tested for 19.1 h and 191 h, respectively.
4. Discussion Normally, a low carbon and low alloy steel with an elongation higher than 20% is considered insensitive to stress-induced crack-
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Table 6 Charge transfer resistance of electrochemical reaction. Immersion time (h) Loading condition Rct/X cm2
19.1
191
r0
r0.7c
r0.7d
r0
r0.7c
r0.7d
347.8
273.2
296.5
438.5
426.4
407.8
20 19.1h 400h
Elongation (%)
15 191h 10
σ0
5
0
σ 0.7c σ 0.7d
(a) 0
kcr ¼
100
200
300
400
Time (h) 35
5h
σ0 σ 0.7c
Elongation (%)
30
σ 0.7d 19.1h
25
191h 20
15
3.5 wt.% NaCl solution, as shown in Figs. 3 and 4. However, localized corrosion was observed on the sample surface, which became more serious with time, as shown in Figs. 5 and 8 and Table 7 show that all corroded sample underwent ductility loss, and the ductility loss of dual phase steel was higher than that of bainitic steel. These results indicate that both the loading mode and the microstructure exhibited influences on the mechano-chemical effect of the steel. Chemical reactions can change the surface morphology by atomic mass transport, which can be triggered by mechanical stress through modifying the surface free energy. Decuzzi and Demelio [16] discussed the influence of the elastic stress field on atomic mobility and determined the critical parameters for a morphological stable condition. Srolovitz [17] established the relation between the critical wavelength kcr of the initial surface roughness for an unstable system and a uniaxial remote loading r21 as follows:
(b) 0
50
100
150
200
Time (h) Fig. 8. Variations of elongation of the tested steel with time (a) bainitic steel; (b) dual phase steel.
Table 7 Ductility loss of specimens under various loading conditions (%). Steel
Duration (h)
r0
r0.7c
r0.7d
Bainitic steel
19.1 191 400
14.6 28.7 44.5
19.8 32.3 53.3
17.4 37.0 60.5
Dual phase steel
5 19.1 191
17.4 29.8 51.1
22.6 31.8 54.8
20.8 34.5 60.6
ing in a NaCl solution. In the present study, no cracking was observed on the specimen surface, even after tested for 400 h in a
pEc ð1 m2 Þr21
ð2Þ
where c is the surface tension, E and m are Young’s modulus and Poisson’s ratio, respectively. Considering the microstructure morphologies of the bainitic and dual phase steels shown in Fig. 1, if the grain or phase size is considered as the sole effect on the initial surface roughness, assuming a smooth surface inside the grain or phase, it can be concluded from Eq. (2) that the critical stress for an unstable system of the dual phase steel is much lower than that of bainitic steel. This is one reason why the dual phase steel is more sensitive to the mechano-chemical effect than the bainitic steel. Another reason should be attributed to the difference in surface potential between ferrite and pearlite phases, where the preferential dissolution of ferrite will increase the surface roughness. The high surface roughness will lead to inhomogeneous distribution of elastic energy on sample surface because the elastic energy stored at the valleys is larger than that at the crests [16]. Besides, during elastic deformation, the mechanical factors dominate the surface behavior [18]. So the contribution of elastic energy is larger than that of the surface energy which will further increase the surface roughness. The steel immersed in NaCl solution will be gradually covered by corrosion product film, which may protect the steel, depending on the microstructure of the film. EIS measurement results shown in Fig. 7 and Table 6 indicate that the charge transfer resistance under r0.7d was lower than that under r0.7c as the test duration increased, demonstrating that the corrosion product film formed under the dynamic loading condition was more porous and loose. In addition, the outer layer of corrosion product film peeled off more easily and frequently under r0.7d, while the specimen under r0.7c was covered by a compact corrosion product film, as shown in Fig. 9. Previous investigations indicated that a time-dependent flow would cause repetitive film rupturing and reduce the repassivation rate [19,20]. In the present study, a time-dependent strain under the dynamic loading condition led to more serious damage to the steel than a constant strain under the static loading condition by decreasing the compactness of corrosion product film.
Fig. 9. Macro morphologies of the dual phase steel after corroded for 191 h. (a) Under static loading condition, the outer layer of corrosion product with red color can be observed; (b) under dynamic loading condition, the outer layer of corrosion product has peeled off, only the inner layer with a dark brown color left. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S. Zhang et al. / Corrosion Science 75 (2013) 293–299
5. Conclusions The corrosion behavior of a low carbon and low alloy steel in a 3.5 wt.% NaCl solution under an applied elastic stress was investigated. The influence of loading condition and the microstructure of the steel on the mechano-chemical effect of the steel was evaluated. The following results were obtained: (1) The synergistic effect of mechanical and chemical attacks led to a higher ductility loss for the specimen under the loading condition than for the loading-free specimen. (2) The ductility loss of the ferritic and pearlitic dual phase steel was greater than that of the bainitic steel, which could be attributed to higher localized damage on the dual phase steel. (3) The specimen under the dynamic loading condition underwent more serious damage than that under the static loading condition because the time-dependent strain led to a decrease in the protective ability of the corrosion product film on the steel.
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