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Effects of tensile stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution
Accepted Manuscript Effects of tensile stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution Yongyan Zhu, Liang Li, Chao Wang PII: DOI: Reference:
S0010-938X(15)00098-0 http://dx.doi.org/10.1016/j.corsci.2015.02.023 CS 6229
To appear in:
Corrosion Science
Received Date: Accepted Date:
30 August 2014 19 February 2015
Please cite this article as: Y. Zhu, L. Li, C. Wang, Effects of tensile stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution, Corrosion Science (2015), doi: http://dx.doi.org/10.1016/j.corsci. 2015.02.023
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Effects of tensile stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution a, b
Yongyan Zhua, b, Liang Li
, Chao Wanga, b*
a. Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou 221116, China b. School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, China
Abstract: Effects of elastic and plastic stresses on the oscillatory electrodissolution of X70 carbon steel have been investigated in 0.50 mol/L H2SO4 solution. The elastic stress increases the frequency of the current oscillations, while the plastic stress induces irregular oscillations. This fact suggests that the elastic and plastic stresses degrade the stability and increase the defects of the passive film to different degrees. Microplasticity occurs at the stress below the apparent yield strength and it exerts obvious effects on the oscillations. Finally, a micro-crack model is presented to explain the damage of passive films by the tensile stresses. Keywords: A. elastic deformation; A. plastic deformation; B. microplasticity; C. oscillatory electrodissolution; C. current oscillations
*Corresponding author. Fax: +86 516 83536977. E-mail: [email protected] (C. Wang).
1
1. Introduction Tensile stresses, such as applied loads, residual stresses and thermal stresses exist widely in various metallic structures, and they would cause elastic and/or plastic deformations in the material. The interactions between the deformations and the electrochemical corrosion processes have been investigated by many people for more than 50 years
[1-15]
. However, the mechanisms of the interactions are far from being
fully understood due to their complexity and diversity. Many researchers have studied the active dissolution of iron-based material under various stresses. Tan and Nobe [1] observed that for mild steel in 1 mol/L FeSO4 solution the potential of the stressed electrode was more positive with respect to the unstressed electrode, and it returned to the original no-load value with the removal of the load. Despic et al.
[2]
suggested in the case of iron/acid solution system that the
shift of the corrosion potential was about 1 mV/1000 MPa and the over-all effect of the elastic stress was negligible. Lu et al.
[3]
found that the effect of the elastic
deformation, even the plastic deformation on the active dissolution rate of pipeline steel in near-neutral pH groundwater was very limited. Cheng et al. [4, 5] proposed that for X100 pipeline steel in a neutral pH bicarbonate solution, with an elastic deformation, there was no detectable effect on the electrochemical corrosion activity. In comparison, if the electrode surface is covered by a passive film, which constitutes a protective layer against metal dissolution, the electrochemical corrosion processes will be significantly affected by even elastic stresses. The stresses gave rise to the breakdown of the passive films, leading to an increase in the corrosion rate of
2
X70 carbon steel in concentrated phosphoric acid solution
[6]
. The elastic deformation
accelerated the anodic dissolution of X70 carbon steel in sulfuric acid solution by degrading the stability of the passive film, promoting the breakdown of the film, increasing the surfac
G) of the specimen and causing the effective potential
(Ev) to move negative [7]. Vignal et al. [8] proposed that for a type 316 L stainless steel (SS) in 0.5 mol/L NaCl solution at 50
, an elastic stress above 40% of the elastic
limit decreased the ability of the passive film to protect against localized corrosion. They also proposed that for a duplex SS in 0.5 mol/L NaCl solution at 50
, under an
elastic stress above 54% of the yield strength, the time to pit dropped [10]. Qiao et al. [15]
suggested that for a type 304 SS in 42% MgCl2 solution, the passive film on the
electrode surface generated an additive stress, which assisted the applied stress to enhance the corrosion processes. The additive stress had been also proved by experiments for X70 carbon steel in 0.50 mol/L H2SO4 solution [7]. Current oscillations are usually observed for metallic materials in acidic solutions, such as carbon steel in sulfuric acid solution or phosphate acid solution. They are typical non-linear phenomena resulting from the periodic formation and dissolution of the surface films at the electrode/electrolyte interfaces
[16]
. The
oscillatory electrodissolution processes are mainly determined by the physicochemical microenvironment at the interface, which is very sensitive to outside or inside perturbations. When the physicochemical environment at the interface was changed, artificial oscillations of Fe/H2SO4 system were induced, and the period of the artificial oscillations could be changed as designed
3
[17, 18]
. Since the oscillations are very
sensitive to any perturbation, it is possible to use current oscillations as indicators to investigate the effects of corrosion factors on the formation and the dissolution of passive films. Li et al.
[19]
observed the oscillatory electrodissolution of X70 carbon
steel to study the effects of hydrogen on the electrochemical corrosion processes. By analyzing the variation of the current oscillations, Zhu et al. [7] and Lu et al. [6] studied the effects of elastic stresses on the anodic dissolution of X70 carbon steel in sulfuric acid solution and phosphate acid solution respectively. In this paper, current oscillations are utilized as an indicator to demonstrate the impacts of various tensile stresses on the stability of the passive films. The elastic and plastic stresses are applied during the anodic polarization processes, and then the effects of the deformations on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution are discussed.
2. Experimental The test material was X70 carbon steel. It has the nominal yield strength of 483 MPa and the chemical composition (in wt. %) of 0.22 C, 1.65 Mn, 0.24 Si, 0.015 S, 0.025 P and the balance Fe. The tensile specimens were used as the working electrodes, which were 25.6 mm in gauge length and 2.5 mm in diameter. The details of the specimens can be found in our previous publication
[7]
. In the electrochemical
measurement, only 5 mm of the working segment was exposed to the solution and the rest was carefully sealed with a thin layer of epoxy resin. The specimen was abraded by using a series of emery papers (up to 2000#), with the direction parallel to the applied stress, and then cleaned by alcohol and triply distilled water in an ultrasonic
4
bath before each experiment. The counter electrode was a platinum plate with a size of 10 × 5 mm2 and the reference electrode was a saturated calomel electrode (SCE) with a Luggin capillary tip set at 2 mm from the working electrode. All the potentials reported here are referred to the SCE. To study the effects of hydrogen on the anodic dissolution of X70 steel/0.50 mol/L H2 SO4 system, the working electrode had been cathodically polarized at 5 mA/cm2 for 20 min before the potentiostatic polarization measurement. All the solutions were prepared from reagents of analytical grade and triply distilled water. The electrochemical experiments were carried out by CHI 660 electrochemical analyzer. The scan rate of the i-E curve was 0.001 V/s. The stresses were applied by a Stress Corrosion Testing Machine (Letry, produced by Lichuang Machine Company, The tensile stresses of 200 MPa, 450 Mpa and 580 MPa (the tensile strength was about 640 MPa) used in the electrochemical experiments were about 40%, 90% and 120% of the nominal yield strength of X70 carbon steel. The surface morphologies of X70 carbon steel specimens after exposure to 0.50 mol/L H2SO4 solution in the presence of various potentials and stresses were examined by Hitachi S-3400N scanning electron microscope (SEM, Japan) instrument. All the experiments were carried out at room temperature.
3. Results 3.1. The polarization behavior of X70 carbon steel in sulfuric acid solution The i-E curve of X70 carbon steel in 0.50 mol/L H2SO4 solution, shown in Fig. 1, can be roughly divided into four regions: the active region (
5
), the limiting current
region (
), the oscillatory region (
) and the passive region ( ). In the active region,
the current density increased monotonously with the increase of the applied potential. The limiting current plateau was observed in region
due to the equal rates for the
formation and the dissolution of the surface film on the electrode. In the oscillatory region, a relaxation type of oscillations was observed if the potential was constantly controlled at E < EF (the Flade potential, which is approximately equal to the reactivation potential [20]). The current density decreased to almost zero for a uniform and compact oxide film (the film is composed of Fe3O4 and -Fe2O3
[21]
) was formed
on the electrode in the passive region in which the potential was higher than the passive potential, Ep. The potential regions of interest in this study include the oscillatory region and the initial stage of the passive region in which oscillatory electrodissolution occurs under various stresses. 3.2. Effects of tensile stresses on the i-t curves in the oscillatory region Regular current oscillations were observed for the unstressed specimen when the potential was controlled at E = 0.255 VSCE in the oscillatory region (Fig. 2A), indicating that a balance between the formation and the dissolution of the passive film had been formed. When an elastic stress was applied, the oscillation frequency and the accumulated charge (the integral of current timed the time for the experiments duration of 400 s) increased obviously, the amplitude decreased somewhat, while the oscillation mode was essentially unchanged, as shown in Fig. 2B (200 MPa) or Fig. 2C (450 MPa). Table 1 is the comparison of the parameters of the current oscillations
6
under various stresses. New balance with high frequency had been established under the applied stress meant that the electrodissolution of X70 carbon steel had been accelerated by the elastic stress. However, the application of a tensile stress exceeding the nominal yield strength changed the mode of the oscillations. When the plastic stress of 580 MPa was applied, the oscillations were irregular, as shown in Fig. 2D. 3.3. Effects of tensile stresses on the i-t curves at the initial stage of the passive region When the potential was controlled at E = 0.290 VSCE, the initial stage of the passive region, a small and stable current was detected for the unstressed specimen (Fig. 3A), indicating that the specimen was in a passive state. However, current oscillations appeared when the electrode was applied with an elastic stress, as shown in Fig. 3B (200 MPa) or Fig. 3C (450 MPa). The current oscillations under the small elastic stress of 200 MPa were almost regular, but when the stress was increased to 450 MPa, the current oscillations exhibited a regular pattern after a brief period of irregular behavior within the first 300 s. Similar to Fig. 2D, the plastic stress of 580 MPa induced irregular oscillations, as shown in Fig. 3D.
4. Discussion It has long been accepted that the accelerated dissolution caused by tensile stresses is due to the strain energy stored in metals during the deformations. Through thermodynamic analysis, Lu et al. [3] suggested that for pipeline steel in a near-neutral pH groundwater, the increment of the free energy due to the elastic stress or the plastic stress was not insufficient to alter the active dissolution kinetics substantially.
7
It had been also confirmed by our experimental results. At the potential of E = - 0.350 VSCE (in the initial of the active region), the current response to the tensile stress of 450 MPa was not remarkable (Fig. 4), which indicated that the impacts of the tensile stress on the active dissolution kinetics were very limited. According to Yuan et al, when the applied potential was in the initial of the active region, there was no passive film on the surface of the specimen in 0.50 mol/L H2SO4 solution
[23]
. So the impacts
of the tensile stresses on the oscillatory electrodissolution should be related to the changes in the passive films, as have been reported in the literatures [6, 7]. Vignal et al.
[8]
used the Mott-Schottky analysis to study the effects of
mechanical stresses on the conductivity of the passive films. The obtained results indicated that the semiconducting properties of the passive film on the surface of 316 L SS reflected a higher vacancies concentration than those formed in a stress-free state in the same sodium chloride solution. However, the semi-conductive properties of the passive films were not markedly modified by the stresses, and the kinetics of the charge transfer remained unchanged. So it could be concluded that the applied stresses increased the defect density in the passive film and decreased its ability to protect the substrate against localized corrosion. Generally, current oscillations have been attributed to the periodic formation and dissolution of the passive film formed on the surface of the electrode oscillations
will
be
modified
by
the
changes
in
the
[16]
. The
physicochemical
microenvironment at the electrode/electrolyte interface, such as charging hydrogen into the electrode and adding chloride ions in the electrolyte. The hydrogen charged
8
into the electrode caused some changes in the current oscillations (Fig. 5A) by leading to highly defective passive films with low stability on the electrode
[19]
. The chloride
ions in the sulfuric acid solution led to significant changes in the frequency and the mode of the oscillations (Fig. 5B) by changing both the electronic and ionic properties of the passive film on the iron electrode
[20-22]
. However, at the potential of 0.290 V SCE
without hydrogen, chloride ions or stress, as shown in Fig. 3A, the electrode is in a passive state. The effects of the elastic stress and the plastic stress on the current oscillations (Fig. 3B and D) resemble the impacts caused by the hydrogen charged into the electrode and the chloride ions in the electrolyte respectively (Fig. 5 A and B). So the tensile stresses may affect the oscillatory electrodissolution of X70 carbon steel by degrading the stability and increasing the defects of the passive film. At the potential of E = 0.255 VSCE, the frequency increased obviously (Fig. 2 and Table 1) when the elastic stresses were applied. The increase in the frequency of the current oscillations suggests that the elastic deformations accelerate the corrosion rate and reduce the stability of the passive film on the electrode surface, leading to the establishment of a new balance between the dissolution and the formation of the film. With the plastic stress of 580 MPa, the slip due to the plastic deformation gave rise to severe damage of the passive film, leading to irregular oscillations (Fig. 2D). The current density was small and stable in the absence of tensile stress at the potential of E = 0.290 VSCE, as shown in Fig. 3A, for a passive film had been formed on the surface of the electrode. However, current oscillations were induced by the stresses, as shown in Fig. 3B, C and D, which indicates that the deformations degrade
9
the stability of the passive films or lead to the formation of the films with many defects. The elastic stress makes it more difficult to keep the passive state all the time, so the electrode will transit from the passive region to the oscillatory region at the potential of E = 0.290 VSCE. Thus regular and periodic current oscillations were observed with the elastic stresses (Fig. 3B and C). Similar to the results at E = 0.255 VSCE, the plastic stress of 580 MPa induced irregular oscillations for the tremendous destruction in the passive film (Fig. 3D). From Fig. 2C, we found that the current oscillations with the stress of 450 MPa were different from those with the stress of 200 MPa (Fig. 2B). Although the mode of the oscillations remained essentially the same, irregular phenomena were observed in Fig. 2C at 170 s, 220 s and 380 s, indicating that more severe defects appeared in the passive films under the stress of 450 MPa. Irregular phenomena were also observed when the potential was controlled at E = 0.290 VSCE (the first 200 s in Fig. 3C). Sazou et al. [21] studied the effects of halides on the passive film of iron/sulfuric acid system and suggested that mono-periodic (regular) oscillations were associated with a uniform dissolution (general corrosion), while complex (irregular) oscillations were associated with a localized attack of the film. So it is deduced that the irregular phenomena in Fig. 2C and Fig. 3C are probably produced by the localized severe attack in the films. In this study, microplasticity is utilized to explain the irregular phenomena in the regular oscillations. Several studies have shown that microplasticity occurs in metallic alloys under the stress below the apparent yield strength
[8-10]
. The
term microplasticity is employed when applying a mechanical stress below the
10
apparent yield strength some grains of a polycrystalline material are plastically deformed
[9]
. Microplasticity is often neglected and ignored since it could not affect
the metallic structures obviously at the early stage after its appearance, but its accumulation does lead to irreversible structural changes even the final failure Vignal et al.
[8]
[8]
.
studied the electronic properties of the passive film formed on 316 L
SS in the acidic NaCl solution in the presence of the elastic stress and found that above 70% of the nominal yield strength, microplasticity was observed. Between 52% and 100% of the yield strength of a type 316 L SS in sodium chloride media, microplasticity was detected corresponding to the emergence of slip steps in some grains of the substrate
[9]
. They also studied the effects of an elastic stress on the time
to pit of a duplex SS immersed under potentiostatic control in 0.5 mol/L NaCl, pH 6.5 at 50
and proposed that above 54% of the nominal yield strength, irreversible
damage (microplascity) occurred [10]. Other experiments have been also performed in this study to verify the existence of the microplasticity under the stress of 450 MPa. Fig. 6 shows the elastic extensions of the specimens after applying each tensile stress for 20 min at the potential of E = 0.255 VSCE. It can be seen from the plot that the amount of the elastic extensions corresponding to the elastic stresses were very small and appeared linear. However, the extension corresponding to the stress of 450 MPa deviated from the linear relationship slightly (point a in Fig. 6). The SEM images with the stresses of 300 MPa and 450 MPa have been taken after anodic polarization at the potential of E = 0.275 VSCE (Fig. 7). No obvious
11
micro-fracture on the specimen was found at the stress of 300 MPa even after 3600 s, as shown in Fig. 7C and D. However, micro-fractures were observed at the stress of 450 MPa (Fig. 7E and F). Therefore, it can be concluded that microplasticity does occur at the stress of 450 MPa, which is about 90% of the nominal yield strength of X70 carbon steel. In the presence of the elastic stress of 450 MPa, where the microplasticity is observed, some defects in the passive film, even penetrate through the film to the substrate, could not be completely repaired after unloading, resulting in irreversible damage in the film. The new balance under the stress of 450 MPa was significantly affected by the damage, thus the rapid increase of the oscillation frequencies and the accumulated charge (Table 1), and the deviation from the linear relationship in the curve of the elastic extension vs. the elastic stresses (point a in Fig. 6), the irregular phenomena in the regular oscillations (Fig. 2C and Fig. 3C) and the micro-fractures in the SEM images (Fig. 7E and F) were observed. Gao et al. and Zuo et al.
[24-26]
used wedge models to discuss the diffusion of
micro-cracks in metal films. A similar model, shown in Fig. 8, has been proposed in this paper to explain the damage of the passive films by the elastic stresses and the plastic stress. The passive film is almost compact and complete for the unstressed electrode (Fig. 8A). Under the elastic stress of 200 MPa, there is only a small elastic deformation in the X70 carbon steel electrode. The small deformation may cause some perpendicular micro-cracks in the passive film (Fig. 8B). However, these cracks do not penetrate to the substrate and will be almost completely re-passivated after
12
unloading. As the stress magnitude increases to 450 MPa, more micro-cracks are formed in the passive film, and some of these cracks may penetrate through the film to the substrate (Fig. 8C). In such a case, it is difficult to reestablish the passive state after the stress has been removed, resulting in non-ignorable damage in the film, even in the substrate (microplasticity). An even more serious situation occurs when the plastic deformation takes place in the substrate as the passive film is subjected to the higher stress of 580 MPa. More micro-cracks, even macro-cracks penetrating through the film to the substrate are in the passive film and the width of the cracks increases obviously (Fig. 8D).The illustration in Fig. 8 may explain all of the experimental results in this study well.
5. Conclusion The effects of elastic and plastic stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution have been studied. The experimental results proved that it is possible to use current oscillations to investigate the effects of tensile stresses on the formation and the dissolution of passive films. The effects of the tensile stresses on the active dissolution rate of X70 carbon steel were very small, while the effects on the oscillatory electrodissolution are significant. The tensile stresses degrade the stability and increase the defects of the passive film, leading to some changes in the frequency and the mode of the oscillations. Microplasticity occurs under the stress of 450 MPa, which is below the nominal yield strength. At last, a micro-crack model is presented to explain the damage of passive films by the tensile stresses. The model explains all of the
13
experimental results in this study well.
Acknowledgements This project has been supported by the Chinese National Science Fund (No. 21303077, No. 21110102049 and No. 21173180), the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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47 (2012) 155-159. [7] Y.Y. Zhu, L. Li, C. Wang, J.L. Luo, G.F. Gao, J.L. Zhang, Effects of elastic deformation
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15
[15] H. Chen, X.Z. Guo, W.Y. Chu, K.W. Gao, Y.B. Wang, Y.J. Su, L.J. Qiao, Martensite caused by passive film-induced stress during stress corrosion cracking in type 304 stainless steel, Mater. Sci. Eng. A358 (2003) 122-127. [16] B. Rush, J. Newman, Periodic behavior
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16
chloride-containing sulfuric acid solutions, J. Electroanal. Chem. 489 (2000) 1-16. [23] B.Y. Yuan, J.L. Zhang, G.F. Gao, L. Li, C. Wang, Dynamic observation of the diffusion layer in anodic processes of the Fe/H 2SO4 system with digital holography, Electrochem. Commun. 27 (2013) 116-119. [24] H. Gao, L. Zhang, W.D. Nix, C.V. Thompson, E. Arzt, Crack-like grain-boundary diffusion wedges in thin metal films, Acta Mater. 47 (1999) 2865-2878. [25] X.G. Feng, Y.M. Tang, Y. Zuo, Influence of stress on passive behaviour of steel bars in concrete pore solution, Corros. Sci. 53 (2011) 1304-1311. [26] X.G. Feng, Y. Zuo, Y.M. Tang, X.H. Zhao, X.Y. Lu, The degradation of passive
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Figure Captions Fig.1. The i-E polarization curve of X70 carbon steel in 0.50 mol/L H2SO4 solution at the scan rate of 0.001 V/s. Fig.2. Effects of the tensile stresses on the current oscillations of X70 carbon steel in 0.50 mol/L H2SO4 solution at E = 0.255 VSCE (A: 0 MPa; B: 200 MPa; C: 450 MPa; D: 580 MPa). Fig.3. Effects of the tensile stresses on the anodic dissolution of X70 carbon steel in 0.50 mol/L H2SO4 solution at E = 0.290 VSCE (A: 0 MPa; B: 200 MPa; C: 450 MPa; D: 580 MPa).
17
Fig.4. Effects of the tensile stress of 450 MPa on the active dissolution of X70 carbon steel at E = - 0.350 VSCE. Fig.5. Effects of the hydrogen charged into the electrode and the chloride ions in the electrolyte on the anodic dissolution of X70 carbon steel at E = 0.290 VSCE (A: The specimen had been cathodically pre-charged with hydrogen at 5 mA/cm2 in 0.50 mol/L H2SO4 solution for 20 min; B: The test solution was 0.50 mol/L H2SO4 + 0.010 mol/L NaCl). Fig.6. The relationship between the applied stress and the elastic extension of the specimen at E = 0.255 VSCE (20 min duration). Fig.7. The SEM images of the electrode surfaces after various time of anodic polarization at E = 0.275 VSCE with the stresses of 300 MPa (A and B: 20 min; C and D: 1 h) and 450 MPa (E and F: 20 min). Fig.8. Illustration for the development of micro-cracks in the passive films on the surface of the electrode (a: the X70 carbon steel substrate; b: the passive film; A: the unstressed specimen; B: under the elastic stress of 200 MPa; C: under the elastic stress of 450 MPa; D: under the plastic stress of 580 MPa).
18
Table 1. Comparison of the parameters of the current oscillations under various stresses. stres
n
s
f
imax
(s-1)
(A/cm
(MP
2
)
ip
Q1
Q
(A/cm
(C/cm
(C/cm
2
)
2
)
2
0.000
2.176
28.29
90
1.825
29.20
0.001
1.318
30.16
1.017
31.53
)
a) 0 150 300
13.
0.03
0.370
5
38
0.360
17.
0.04
0.348
2
30
02 0.336
450 23.
0.05
0.001
5
88
09
31.
0.07
0.001
6
90
18
Note: n, the period number; f, the oscillation frequency; imax, the maximum current density; ip, the passive current density; Q1, the charge for one period; Q, the charge for the duration of 400 s.
19
Highlights The anodic dissolution of X70 steel/H2SO4 system is studied under various stresses. The elastic stress does not change the mode of the current oscillations. The plastic stress induces irregular current oscillations. Microplasticity occurs at the stress below the apparent yield strength. A micro-crack model is presented to explain the damage of the passive films.
20
S0010-938X(15)00098-0 http://dx.doi.org/10.1016/j.corsci.2015.02.023 CS 6229
To appear in:
Corrosion Science
Received Date: Accepted Date:
30 August 2014 19 February 2015
Please cite this article as: Y. Zhu, L. Li, C. Wang, Effects of tensile stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution, Corrosion Science (2015), doi: http://dx.doi.org/10.1016/j.corsci. 2015.02.023
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Effects of tensile stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution a, b
Yongyan Zhua, b, Liang Li
, Chao Wanga, b*
a. Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou 221116, China b. School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, China
Abstract: Effects of elastic and plastic stresses on the oscillatory electrodissolution of X70 carbon steel have been investigated in 0.50 mol/L H2SO4 solution. The elastic stress increases the frequency of the current oscillations, while the plastic stress induces irregular oscillations. This fact suggests that the elastic and plastic stresses degrade the stability and increase the defects of the passive film to different degrees. Microplasticity occurs at the stress below the apparent yield strength and it exerts obvious effects on the oscillations. Finally, a micro-crack model is presented to explain the damage of passive films by the tensile stresses. Keywords: A. elastic deformation; A. plastic deformation; B. microplasticity; C. oscillatory electrodissolution; C. current oscillations
*Corresponding author. Fax: +86 516 83536977. E-mail: [email protected] (C. Wang).
1
1. Introduction Tensile stresses, such as applied loads, residual stresses and thermal stresses exist widely in various metallic structures, and they would cause elastic and/or plastic deformations in the material. The interactions between the deformations and the electrochemical corrosion processes have been investigated by many people for more than 50 years
[1-15]
. However, the mechanisms of the interactions are far from being
fully understood due to their complexity and diversity. Many researchers have studied the active dissolution of iron-based material under various stresses. Tan and Nobe [1] observed that for mild steel in 1 mol/L FeSO4 solution the potential of the stressed electrode was more positive with respect to the unstressed electrode, and it returned to the original no-load value with the removal of the load. Despic et al.
[2]
suggested in the case of iron/acid solution system that the
shift of the corrosion potential was about 1 mV/1000 MPa and the over-all effect of the elastic stress was negligible. Lu et al.
[3]
found that the effect of the elastic
deformation, even the plastic deformation on the active dissolution rate of pipeline steel in near-neutral pH groundwater was very limited. Cheng et al. [4, 5] proposed that for X100 pipeline steel in a neutral pH bicarbonate solution, with an elastic deformation, there was no detectable effect on the electrochemical corrosion activity. In comparison, if the electrode surface is covered by a passive film, which constitutes a protective layer against metal dissolution, the electrochemical corrosion processes will be significantly affected by even elastic stresses. The stresses gave rise to the breakdown of the passive films, leading to an increase in the corrosion rate of
2
X70 carbon steel in concentrated phosphoric acid solution
[6]
. The elastic deformation
accelerated the anodic dissolution of X70 carbon steel in sulfuric acid solution by degrading the stability of the passive film, promoting the breakdown of the film, increasing the surfac
G) of the specimen and causing the effective potential
(Ev) to move negative [7]. Vignal et al. [8] proposed that for a type 316 L stainless steel (SS) in 0.5 mol/L NaCl solution at 50
, an elastic stress above 40% of the elastic
limit decreased the ability of the passive film to protect against localized corrosion. They also proposed that for a duplex SS in 0.5 mol/L NaCl solution at 50
, under an
elastic stress above 54% of the yield strength, the time to pit dropped [10]. Qiao et al. [15]
suggested that for a type 304 SS in 42% MgCl2 solution, the passive film on the
electrode surface generated an additive stress, which assisted the applied stress to enhance the corrosion processes. The additive stress had been also proved by experiments for X70 carbon steel in 0.50 mol/L H2SO4 solution [7]. Current oscillations are usually observed for metallic materials in acidic solutions, such as carbon steel in sulfuric acid solution or phosphate acid solution. They are typical non-linear phenomena resulting from the periodic formation and dissolution of the surface films at the electrode/electrolyte interfaces
[16]
. The
oscillatory electrodissolution processes are mainly determined by the physicochemical microenvironment at the interface, which is very sensitive to outside or inside perturbations. When the physicochemical environment at the interface was changed, artificial oscillations of Fe/H2SO4 system were induced, and the period of the artificial oscillations could be changed as designed
3
[17, 18]
. Since the oscillations are very
sensitive to any perturbation, it is possible to use current oscillations as indicators to investigate the effects of corrosion factors on the formation and the dissolution of passive films. Li et al.
[19]
observed the oscillatory electrodissolution of X70 carbon
steel to study the effects of hydrogen on the electrochemical corrosion processes. By analyzing the variation of the current oscillations, Zhu et al. [7] and Lu et al. [6] studied the effects of elastic stresses on the anodic dissolution of X70 carbon steel in sulfuric acid solution and phosphate acid solution respectively. In this paper, current oscillations are utilized as an indicator to demonstrate the impacts of various tensile stresses on the stability of the passive films. The elastic and plastic stresses are applied during the anodic polarization processes, and then the effects of the deformations on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution are discussed.
2. Experimental The test material was X70 carbon steel. It has the nominal yield strength of 483 MPa and the chemical composition (in wt. %) of 0.22 C, 1.65 Mn, 0.24 Si, 0.015 S, 0.025 P and the balance Fe. The tensile specimens were used as the working electrodes, which were 25.6 mm in gauge length and 2.5 mm in diameter. The details of the specimens can be found in our previous publication
[7]
. In the electrochemical
measurement, only 5 mm of the working segment was exposed to the solution and the rest was carefully sealed with a thin layer of epoxy resin. The specimen was abraded by using a series of emery papers (up to 2000#), with the direction parallel to the applied stress, and then cleaned by alcohol and triply distilled water in an ultrasonic
4
bath before each experiment. The counter electrode was a platinum plate with a size of 10 × 5 mm2 and the reference electrode was a saturated calomel electrode (SCE) with a Luggin capillary tip set at 2 mm from the working electrode. All the potentials reported here are referred to the SCE. To study the effects of hydrogen on the anodic dissolution of X70 steel/0.50 mol/L H2 SO4 system, the working electrode had been cathodically polarized at 5 mA/cm2 for 20 min before the potentiostatic polarization measurement. All the solutions were prepared from reagents of analytical grade and triply distilled water. The electrochemical experiments were carried out by CHI 660 electrochemical analyzer. The scan rate of the i-E curve was 0.001 V/s. The stresses were applied by a Stress Corrosion Testing Machine (Letry, produced by Lichuang Machine Company, The tensile stresses of 200 MPa, 450 Mpa and 580 MPa (the tensile strength was about 640 MPa) used in the electrochemical experiments were about 40%, 90% and 120% of the nominal yield strength of X70 carbon steel. The surface morphologies of X70 carbon steel specimens after exposure to 0.50 mol/L H2SO4 solution in the presence of various potentials and stresses were examined by Hitachi S-3400N scanning electron microscope (SEM, Japan) instrument. All the experiments were carried out at room temperature.
3. Results 3.1. The polarization behavior of X70 carbon steel in sulfuric acid solution The i-E curve of X70 carbon steel in 0.50 mol/L H2SO4 solution, shown in Fig. 1, can be roughly divided into four regions: the active region (
5
), the limiting current
region (
), the oscillatory region (
) and the passive region ( ). In the active region,
the current density increased monotonously with the increase of the applied potential. The limiting current plateau was observed in region
due to the equal rates for the
formation and the dissolution of the surface film on the electrode. In the oscillatory region, a relaxation type of oscillations was observed if the potential was constantly controlled at E < EF (the Flade potential, which is approximately equal to the reactivation potential [20]). The current density decreased to almost zero for a uniform and compact oxide film (the film is composed of Fe3O4 and -Fe2O3
[21]
) was formed
on the electrode in the passive region in which the potential was higher than the passive potential, Ep. The potential regions of interest in this study include the oscillatory region and the initial stage of the passive region in which oscillatory electrodissolution occurs under various stresses. 3.2. Effects of tensile stresses on the i-t curves in the oscillatory region Regular current oscillations were observed for the unstressed specimen when the potential was controlled at E = 0.255 VSCE in the oscillatory region (Fig. 2A), indicating that a balance between the formation and the dissolution of the passive film had been formed. When an elastic stress was applied, the oscillation frequency and the accumulated charge (the integral of current timed the time for the experiments duration of 400 s) increased obviously, the amplitude decreased somewhat, while the oscillation mode was essentially unchanged, as shown in Fig. 2B (200 MPa) or Fig. 2C (450 MPa). Table 1 is the comparison of the parameters of the current oscillations
6
under various stresses. New balance with high frequency had been established under the applied stress meant that the electrodissolution of X70 carbon steel had been accelerated by the elastic stress. However, the application of a tensile stress exceeding the nominal yield strength changed the mode of the oscillations. When the plastic stress of 580 MPa was applied, the oscillations were irregular, as shown in Fig. 2D. 3.3. Effects of tensile stresses on the i-t curves at the initial stage of the passive region When the potential was controlled at E = 0.290 VSCE, the initial stage of the passive region, a small and stable current was detected for the unstressed specimen (Fig. 3A), indicating that the specimen was in a passive state. However, current oscillations appeared when the electrode was applied with an elastic stress, as shown in Fig. 3B (200 MPa) or Fig. 3C (450 MPa). The current oscillations under the small elastic stress of 200 MPa were almost regular, but when the stress was increased to 450 MPa, the current oscillations exhibited a regular pattern after a brief period of irregular behavior within the first 300 s. Similar to Fig. 2D, the plastic stress of 580 MPa induced irregular oscillations, as shown in Fig. 3D.
4. Discussion It has long been accepted that the accelerated dissolution caused by tensile stresses is due to the strain energy stored in metals during the deformations. Through thermodynamic analysis, Lu et al. [3] suggested that for pipeline steel in a near-neutral pH groundwater, the increment of the free energy due to the elastic stress or the plastic stress was not insufficient to alter the active dissolution kinetics substantially.
7
It had been also confirmed by our experimental results. At the potential of E = - 0.350 VSCE (in the initial of the active region), the current response to the tensile stress of 450 MPa was not remarkable (Fig. 4), which indicated that the impacts of the tensile stress on the active dissolution kinetics were very limited. According to Yuan et al, when the applied potential was in the initial of the active region, there was no passive film on the surface of the specimen in 0.50 mol/L H2SO4 solution
[23]
. So the impacts
of the tensile stresses on the oscillatory electrodissolution should be related to the changes in the passive films, as have been reported in the literatures [6, 7]. Vignal et al.
[8]
used the Mott-Schottky analysis to study the effects of
mechanical stresses on the conductivity of the passive films. The obtained results indicated that the semiconducting properties of the passive film on the surface of 316 L SS reflected a higher vacancies concentration than those formed in a stress-free state in the same sodium chloride solution. However, the semi-conductive properties of the passive films were not markedly modified by the stresses, and the kinetics of the charge transfer remained unchanged. So it could be concluded that the applied stresses increased the defect density in the passive film and decreased its ability to protect the substrate against localized corrosion. Generally, current oscillations have been attributed to the periodic formation and dissolution of the passive film formed on the surface of the electrode oscillations
will
be
modified
by
the
changes
in
the
[16]
. The
physicochemical
microenvironment at the electrode/electrolyte interface, such as charging hydrogen into the electrode and adding chloride ions in the electrolyte. The hydrogen charged
8
into the electrode caused some changes in the current oscillations (Fig. 5A) by leading to highly defective passive films with low stability on the electrode
[19]
. The chloride
ions in the sulfuric acid solution led to significant changes in the frequency and the mode of the oscillations (Fig. 5B) by changing both the electronic and ionic properties of the passive film on the iron electrode
[20-22]
. However, at the potential of 0.290 V SCE
without hydrogen, chloride ions or stress, as shown in Fig. 3A, the electrode is in a passive state. The effects of the elastic stress and the plastic stress on the current oscillations (Fig. 3B and D) resemble the impacts caused by the hydrogen charged into the electrode and the chloride ions in the electrolyte respectively (Fig. 5 A and B). So the tensile stresses may affect the oscillatory electrodissolution of X70 carbon steel by degrading the stability and increasing the defects of the passive film. At the potential of E = 0.255 VSCE, the frequency increased obviously (Fig. 2 and Table 1) when the elastic stresses were applied. The increase in the frequency of the current oscillations suggests that the elastic deformations accelerate the corrosion rate and reduce the stability of the passive film on the electrode surface, leading to the establishment of a new balance between the dissolution and the formation of the film. With the plastic stress of 580 MPa, the slip due to the plastic deformation gave rise to severe damage of the passive film, leading to irregular oscillations (Fig. 2D). The current density was small and stable in the absence of tensile stress at the potential of E = 0.290 VSCE, as shown in Fig. 3A, for a passive film had been formed on the surface of the electrode. However, current oscillations were induced by the stresses, as shown in Fig. 3B, C and D, which indicates that the deformations degrade
9
the stability of the passive films or lead to the formation of the films with many defects. The elastic stress makes it more difficult to keep the passive state all the time, so the electrode will transit from the passive region to the oscillatory region at the potential of E = 0.290 VSCE. Thus regular and periodic current oscillations were observed with the elastic stresses (Fig. 3B and C). Similar to the results at E = 0.255 VSCE, the plastic stress of 580 MPa induced irregular oscillations for the tremendous destruction in the passive film (Fig. 3D). From Fig. 2C, we found that the current oscillations with the stress of 450 MPa were different from those with the stress of 200 MPa (Fig. 2B). Although the mode of the oscillations remained essentially the same, irregular phenomena were observed in Fig. 2C at 170 s, 220 s and 380 s, indicating that more severe defects appeared in the passive films under the stress of 450 MPa. Irregular phenomena were also observed when the potential was controlled at E = 0.290 VSCE (the first 200 s in Fig. 3C). Sazou et al. [21] studied the effects of halides on the passive film of iron/sulfuric acid system and suggested that mono-periodic (regular) oscillations were associated with a uniform dissolution (general corrosion), while complex (irregular) oscillations were associated with a localized attack of the film. So it is deduced that the irregular phenomena in Fig. 2C and Fig. 3C are probably produced by the localized severe attack in the films. In this study, microplasticity is utilized to explain the irregular phenomena in the regular oscillations. Several studies have shown that microplasticity occurs in metallic alloys under the stress below the apparent yield strength
[8-10]
. The
term microplasticity is employed when applying a mechanical stress below the
10
apparent yield strength some grains of a polycrystalline material are plastically deformed
[9]
. Microplasticity is often neglected and ignored since it could not affect
the metallic structures obviously at the early stage after its appearance, but its accumulation does lead to irreversible structural changes even the final failure Vignal et al.
[8]
[8]
.
studied the electronic properties of the passive film formed on 316 L
SS in the acidic NaCl solution in the presence of the elastic stress and found that above 70% of the nominal yield strength, microplasticity was observed. Between 52% and 100% of the yield strength of a type 316 L SS in sodium chloride media, microplasticity was detected corresponding to the emergence of slip steps in some grains of the substrate
[9]
. They also studied the effects of an elastic stress on the time
to pit of a duplex SS immersed under potentiostatic control in 0.5 mol/L NaCl, pH 6.5 at 50
and proposed that above 54% of the nominal yield strength, irreversible
damage (microplascity) occurred [10]. Other experiments have been also performed in this study to verify the existence of the microplasticity under the stress of 450 MPa. Fig. 6 shows the elastic extensions of the specimens after applying each tensile stress for 20 min at the potential of E = 0.255 VSCE. It can be seen from the plot that the amount of the elastic extensions corresponding to the elastic stresses were very small and appeared linear. However, the extension corresponding to the stress of 450 MPa deviated from the linear relationship slightly (point a in Fig. 6). The SEM images with the stresses of 300 MPa and 450 MPa have been taken after anodic polarization at the potential of E = 0.275 VSCE (Fig. 7). No obvious
11
micro-fracture on the specimen was found at the stress of 300 MPa even after 3600 s, as shown in Fig. 7C and D. However, micro-fractures were observed at the stress of 450 MPa (Fig. 7E and F). Therefore, it can be concluded that microplasticity does occur at the stress of 450 MPa, which is about 90% of the nominal yield strength of X70 carbon steel. In the presence of the elastic stress of 450 MPa, where the microplasticity is observed, some defects in the passive film, even penetrate through the film to the substrate, could not be completely repaired after unloading, resulting in irreversible damage in the film. The new balance under the stress of 450 MPa was significantly affected by the damage, thus the rapid increase of the oscillation frequencies and the accumulated charge (Table 1), and the deviation from the linear relationship in the curve of the elastic extension vs. the elastic stresses (point a in Fig. 6), the irregular phenomena in the regular oscillations (Fig. 2C and Fig. 3C) and the micro-fractures in the SEM images (Fig. 7E and F) were observed. Gao et al. and Zuo et al.
[24-26]
used wedge models to discuss the diffusion of
micro-cracks in metal films. A similar model, shown in Fig. 8, has been proposed in this paper to explain the damage of the passive films by the elastic stresses and the plastic stress. The passive film is almost compact and complete for the unstressed electrode (Fig. 8A). Under the elastic stress of 200 MPa, there is only a small elastic deformation in the X70 carbon steel electrode. The small deformation may cause some perpendicular micro-cracks in the passive film (Fig. 8B). However, these cracks do not penetrate to the substrate and will be almost completely re-passivated after
12
unloading. As the stress magnitude increases to 450 MPa, more micro-cracks are formed in the passive film, and some of these cracks may penetrate through the film to the substrate (Fig. 8C). In such a case, it is difficult to reestablish the passive state after the stress has been removed, resulting in non-ignorable damage in the film, even in the substrate (microplasticity). An even more serious situation occurs when the plastic deformation takes place in the substrate as the passive film is subjected to the higher stress of 580 MPa. More micro-cracks, even macro-cracks penetrating through the film to the substrate are in the passive film and the width of the cracks increases obviously (Fig. 8D).The illustration in Fig. 8 may explain all of the experimental results in this study well.
5. Conclusion The effects of elastic and plastic stresses on the oscillatory electrodissolution of X70 carbon steel in sulfuric acid solution have been studied. The experimental results proved that it is possible to use current oscillations to investigate the effects of tensile stresses on the formation and the dissolution of passive films. The effects of the tensile stresses on the active dissolution rate of X70 carbon steel were very small, while the effects on the oscillatory electrodissolution are significant. The tensile stresses degrade the stability and increase the defects of the passive film, leading to some changes in the frequency and the mode of the oscillations. Microplasticity occurs under the stress of 450 MPa, which is below the nominal yield strength. At last, a micro-crack model is presented to explain the damage of passive films by the tensile stresses. The model explains all of the
13
experimental results in this study well.
Acknowledgements This project has been supported by the Chinese National Science Fund (No. 21303077, No. 21110102049 and No. 21173180), the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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47 (2012) 155-159. [7] Y.Y. Zhu, L. Li, C. Wang, J.L. Luo, G.F. Gao, J.L. Zhang, Effects of elastic deformation
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the
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carbon
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15
[15] H. Chen, X.Z. Guo, W.Y. Chu, K.W. Gao, Y.B. Wang, Y.J. Su, L.J. Qiao, Martensite caused by passive film-induced stress during stress corrosion cracking in type 304 stainless steel, Mater. Sci. Eng. A358 (2003) 122-127. [16] B. Rush, J. Newman, Periodic behavior
in the
iron/sulfuric
acid
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J. Electrochem. Soc. 142 (1995) 3770-3779. [17] H.M. An, S.H. Chen, H.T. Cui, X.G. Yang. Investigation into designed current oscillations during electrodissolution in sulfuric acid solution, J. Electrochem. Soc. 149 (2002) B174-B178. [18] M. Zeng, C. Wang, L. Li, Designed oscillations of the Fe/H2SO4 system with the flow injection in a partially-closed environment, Electrochem. Commun. 11 (2009) 1888-1891. [19] L. Li, J.L. Luo, J.G. Yu, Y.M. Zeng, B.T. Lu, S.H. Chen, Effects of hydrogen on current oscillations during electro-oxidation of X70 carbon steel in phosphoric acid, Electrochem. Commun. 5 (2003) 396-402. [20] M. Pagitsas, D. Sazou, Current oscillations induced by chlorides during the passive-active transition of iron in a sulfuric acid solution, J. Electroanal. Chem. 471 (1999) 132-145. [21] M. Pagitsas, A. Diamantopoulou, D. Sazou, General and pitting corrosion deduced from current oscillations in the passive-active transition state of the Fe/H2SO4 electrochemical system, Electrochim. Acta 47 (2002) 4163-4179. [22] D. Sazou, A. Diamantopoulou, M. Pagitsas, Complex periodic and chaotic current oscillations related to different states of the localized corrosion of iron in
16
chloride-containing sulfuric acid solutions, J. Electroanal. Chem. 489 (2000) 1-16. [23] B.Y. Yuan, J.L. Zhang, G.F. Gao, L. Li, C. Wang, Dynamic observation of the diffusion layer in anodic processes of the Fe/H 2SO4 system with digital holography, Electrochem. Commun. 27 (2013) 116-119. [24] H. Gao, L. Zhang, W.D. Nix, C.V. Thompson, E. Arzt, Crack-like grain-boundary diffusion wedges in thin metal films, Acta Mater. 47 (1999) 2865-2878. [25] X.G. Feng, Y.M. Tang, Y. Zuo, Influence of stress on passive behaviour of steel bars in concrete pore solution, Corros. Sci. 53 (2011) 1304-1311. [26] X.G. Feng, Y. Zuo, Y.M. Tang, X.H. Zhao, X.Y. Lu, The degradation of passive
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Figure Captions Fig.1. The i-E polarization curve of X70 carbon steel in 0.50 mol/L H2SO4 solution at the scan rate of 0.001 V/s. Fig.2. Effects of the tensile stresses on the current oscillations of X70 carbon steel in 0.50 mol/L H2SO4 solution at E = 0.255 VSCE (A: 0 MPa; B: 200 MPa; C: 450 MPa; D: 580 MPa). Fig.3. Effects of the tensile stresses on the anodic dissolution of X70 carbon steel in 0.50 mol/L H2SO4 solution at E = 0.290 VSCE (A: 0 MPa; B: 200 MPa; C: 450 MPa; D: 580 MPa).
17
Fig.4. Effects of the tensile stress of 450 MPa on the active dissolution of X70 carbon steel at E = - 0.350 VSCE. Fig.5. Effects of the hydrogen charged into the electrode and the chloride ions in the electrolyte on the anodic dissolution of X70 carbon steel at E = 0.290 VSCE (A: The specimen had been cathodically pre-charged with hydrogen at 5 mA/cm2 in 0.50 mol/L H2SO4 solution for 20 min; B: The test solution was 0.50 mol/L H2SO4 + 0.010 mol/L NaCl). Fig.6. The relationship between the applied stress and the elastic extension of the specimen at E = 0.255 VSCE (20 min duration). Fig.7. The SEM images of the electrode surfaces after various time of anodic polarization at E = 0.275 VSCE with the stresses of 300 MPa (A and B: 20 min; C and D: 1 h) and 450 MPa (E and F: 20 min). Fig.8. Illustration for the development of micro-cracks in the passive films on the surface of the electrode (a: the X70 carbon steel substrate; b: the passive film; A: the unstressed specimen; B: under the elastic stress of 200 MPa; C: under the elastic stress of 450 MPa; D: under the plastic stress of 580 MPa).
18
Table 1. Comparison of the parameters of the current oscillations under various stresses. stres
n
s
f
imax
(s-1)
(A/cm
(MP
2
)
ip
Q1
Q
(A/cm
(C/cm
(C/cm
2
)
2
)
2
0.000
2.176
28.29
90
1.825
29.20
0.001
1.318
30.16
1.017
31.53
)
a) 0 150 300
13.
0.03
0.370
5
38
0.360
17.
0.04
0.348
2
30
02 0.336
450 23.
0.05
0.001
5
88
09
31.
0.07
0.001
6
90
18
Note: n, the period number; f, the oscillation frequency; imax, the maximum current density; ip, the passive current density; Q1, the charge for one period; Q, the charge for the duration of 400 s.
19
Highlights The anodic dissolution of X70 steel/H2SO4 system is studied under various stresses. The elastic stress does not change the mode of the current oscillations. The plastic stress induces irregular current oscillations. Microplasticity occurs at the stress below the apparent yield strength. A micro-crack model is presented to explain the damage of the passive films.
20