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Use of the electrochemical microcell technique and the SVET for monitoring pitting corrosion at MnS inclusions
Electrochemistry Communications 6 (2004) 655–660 www.elsevier.com/locate/elecom
Use of the electrochemical microcell technique and the SVET for monitoring pitting corrosion at MnS inclusions H. Krawiec a, V. Vignal a
b,*
, R. Oltra
b
Department of Foundry, AGH University of Science and Technology, ul. Reymonta 23, 30-059 Cracow, Poland b LRRS, UMR 5613 CNRS, Universite de Bourgogne, BP 47870, 21078 Dijon Cedex, France Received 19 April 2004; received in revised form 4 May 2004; accepted 4 May 2004 Available online 25 May 2004
Abstract The purpose of this paper is to report on use of the electrochemical microcell technique and the scanning vibrating electrode technique for monitoring pitting corrosion on the same stainless steel microstructure. First, the electrochemical behaviour of sites containing a single inclusion was investigated in order to the determine both the successive steps occurring during the inclusions activation and some key-parameters such as the onset potential for MnS dissolution and the pitting potential. Then, the local current distribution around a pitting site was monitored at open circuit potential in order to locate anodic and cathodic regions and to obtain informations on the galvanic coupling between inclusions and the matrix. This was done using a part of a modified electrochemical microcell combined with the scanning vibrating electrode technique. 2004 Elsevier B.V. All rights reserved. Keywords: Manganese sulfide; Pitting corrosion; Stainless steel; SVET; Microcapillary
1. Introduction Materials have complex microstructure which could be composed of different metallic phases, non-metallic inclusions, intermetallic particles and second-phase precipitates. Such microstructures lead to heterogeneous distributions of oxido-reduction reactions at the solution/surface interface. It is hardly possible to assign electrochemical changes observed to corresponding phenomena and locations on the surface by using largescale electrochemical experiments that provide average data integrating over a large surface area. Microelectrochemical methods which have been developed over the past years appear as powerful techniques to study oxidoreduction reactions in the micro- and nano-range [1–4]. The scanning vibrating electrode technique (SVET) offers the possibility of mapping variations in current *
Corresponding author. Tel.: +33-380-396-160; fax: +33-380-396132. E-mail address: [email protected] (V. Vignal). 1388-2481/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.05.001
densities at the microscale over a corroding metals by measuring potential gradients developed in the solution due to the flow of ionic currents and to locate anodic and cathodic zones. Measurements at open circuit potential (OCP) have only been performed in specific cases such as for characterizing the current evolution during the initiation and growth of surface microcracks under straining conditions [5] and the deterioration of organic coatings [6,7] and for studying microbial influenced corrosion [8]. The main reason is that it is difficult to activate a single pit at OCP on sufficiently long time scales to perform local current measurements. The purpose of this paper is to report on use of the electrochemical microcell technique and the SVET for monitoring pitting corrosion on the same stainless steel microstructure. The electrochemical behaviour of sites containing a single inclusion was investigated in order to the determine both the successive steps occurring during the inclusions activation and some key-parameters such as the onset potential for MnS dissolution and the pitting potential. Then, the local current distribution
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H. Krawiec et al. / Electrochemistry Communications 6 (2004) 655–660
around a pitting site was monitored at OCP in order to obtain informations on the galvanic coupling between the inclusions and the matrix. This was done using a part of a modified electrochemical microcell combined with the SVET.
2. Experimental Measurements were performed on a resulfurized 304L type stainless steel (Ni: 8.75 wt%, Cr: 18.3, Mn: 1.7, S: 0.17, Si: 0.5, P < 0:035 and C: 0.05). The samples were machined from rolled plates (thickness: 20 mm, width: 4 m and length: 10 m) along the short transverse direction. Spheroidized inclusions were observed (depth <5 lm) and their size ranged between 5 and 40 lm, as shown in Fig. 1. These inclusions were heterogeneously distributed on the surface. The samples (diameter: 2.5 cm) were mechanically polished with silicon carbide emery papers down to 4000 grit, smoothed with diamond pastes down to 1 lm and ultrasonically rinsed in ethanol. After polishing, the depth of the inclusions emerging at the surface ranged from the nano to the microscale, depending on the quantity of MnS removed during polishing. The local electrochemical behaviour of specimens was studied at 25 C using the electrochemical microcell technique. This technique consists of a glass microcapillary which is filled with the electrolyte, as shown in Fig. 2(a). The microcapillary tip was sealed to the specimen surface with a layer of silicone rubber. The microcell was mounted on a microscope for precise positioning of the microcapillary on the surface and the entire setup was placed in a faradic cage. The diameter of the microcapillary tip was 50 lm and the counter electrode was a platinum wire. A modified high resolution potentiostat was used in order to have a current detection limit of 20 fA (Jaissle 1002T-NC-3). A potential of )500 mV corresponding to the cathodic zone (the OCP values ranged between )200 and )300 mV/ SCE in the different solutions) was applied for 3 min and
Fig. 1. Morphology of the shallow inclusions observed using optical microscopy.
Fig. 2. (a) Experimental setup for local measurements using the electrochemical microcell technique and (b) use of the capillary part of the electrochemical microcell for activating a single inclusion (on the left side of the image) and the SVET for measuring the local current distribution above the reactive metal (on the right side of the image).
the potentiodynamic polarization curves were then measured at a scan rate 1 mV s1 . In order to monitor the electrochemical dissolution of MnS, measurements were performed in 1 M NaClO4 buffered with a mixture of 0.1 M Na2 C6 O7 H6 (39.9 ml) and 0.1 M HCl (60.1 ml) at pH 3. In order to study pitting corrosion at MnS, an unbuffered solution 1 M NaCl, pH 3 was used. This solution was altered to pH 3 with sulfuric acid. The microcapillary part of the electrochemical microcell was used to initiate locally a pit, as shown in Fig. 2(b). The microcell was connected to a microsyringe and this system was filled with a mixture of 1 M HCl (80 ml) and H2 O2 (0.3 ml). Hydrogen peroxide was used in order to increase locally the potential and to promote MnS dissolution whereas hydrochloric acid was used in order to initiate pits in the bare metal. Local current
H. Krawiec et al. / Electrochemistry Communications 6 (2004) 655–660
Current density / µA cm
-2
1000 100
P1 10 1 0.1 -600
(a)
-300 0 300 Potential / mV vs. SCE
8
600
P12
-2
imes-iBkg / µA cm
measurements were performed at OCP in a buffered solution 0.1 M C6 H8 O7 + 0.2 M Na2 HPO4 (pH 6.8 and j ¼ 2:3 ms cm1 ) using an Applicable Electronics SVET. Pt–Ir microelectrodes (MicroProbe Inc.) were black platinised and the sphere diameter after deposition was about 20 lm. The vibration amplitude was 20 lm and the vibration frequency was 600 and 200 Hz in the direction parallel and normal to the surface, respectively. The ASET Software (Science Wares Inc.) converted the potential drop measured by the microelectrode with Ohm’s law into a current density value after amplification. The displacement of the microelectrode was performed using a motorised and computer-controlled XYZ micromanipulator allowing 0.5 lm steps. Two video cameras (140 magnification) were used for imaging and controlling the distances between the microelectrode, the microcapillary and the specimen surface, as shown in Fig. 2(b). The distance between the microelectrode and the surface was fixed to 50 lm and the Y-component of the current was measured. A saturated calomel electrode (SCE), to which all the potentials in the text are referred, was used as reference.
657
6 4
P11
2 0 200
(b)
300 400 Potential / mV vs. SCE
500
3. Results The successive steps occurring during the activation of MnS inclusions in the absence of pitting corrosion was first studied. The electrochemical response of sites containing the pure matrix and the matrix with a single MnS inclusion in the buffered solution 1 M NaClO4 , pH 3 is shown in Fig. 3(a) and particular attention was paid to the electrochemical dissolution of the inclusion. This process was observed between 70 and 500 mV with a maximum of about 12 lA cm2 for a small and shallow inclusion, as shown in Fig. 3(a). This peak can be decomposed into two peaks, P11 and P12 , connected to different anodic reactions, as shown in Fig. 3(b). The background current related to the reactions occurring on the surrounding matrix was subtracted to the total current such that the current density reported in Fig. 3(b) could only be associated to the reactions occurring at the inclusion surface. Peak P11 observed at 320 mV was associated to the electrochemical dissolution of the shallow MnS inclusion. Three possible electrochemical reactions may be proposed for the dissolution of MnS [9–13]: þ 2MnS þ 3H2 O ! 2Mn2þ þ S2 O 3 þ 6H þ 8e
ð1aÞ
þ MnS þ 4H2 O ! Mn2þ þ SO2 4 þ 8H þ 8e
with EðmVÞ ¼ 18 60 pH MnS ! Mn
2þ
þ S þ 2e
S2 O 3 ions released during reaction (1a) are not stable at pH 3 within the potential range considered and therefore this reaction was not considered. Among the two remaining electrochemical reactions, reaction (1c) seems to be the most relevant since it occurs at lower pH values. At the top of peak P11 , the shallow inclusion was completely dissolved and the current increase associated to peak P12 observed at 380 mV corresponds to the active–passive transition of the bare metal which was exposed to the solution. No current peak associated to MnS dissolution was obtained when a second polarisation curve was performed on the same location, confirming that the inclusion dissolution was complete after the first curve. The total charge associated to the inclusion dissolution can be calculated from the area of peak P11 and the quantity of MnS dissolved was then deduced from the following equation: mMnS ðgÞ ¼
ð1bÞ
with EðmVÞ ¼ 363 þ log½Mn2þ
Fig. 3. (a) Local polarisation curves determined in a buffered solution 1 M NaClO4 , pH 3 on sites containing the pure matrix (gray curve) and the matrix with a shallow inclusion (black curve). (b) Shape of peak P1 at higher magnification which was decomposed into two peaks P11 and P12 . The electrical charge was calculated from this plot considering that 1 mV corresponds to 1 s.
ð1cÞ
nM ; ne qe N
ð2Þ
where n is the total charge associated to peak P11 (3.32 109 C). M is the molar mass of MnS (87 g mol1 ). qe is the charge of electrons (1.16 · 1019 C). ne is the number of
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electrons involved in reaction (1c) (ne ¼ 2). N is the Avogadro’s number (N ¼ 6:02 1023 mol1 ). The quantity of MnS was estimated to mMnS ¼ 1:5 1012 g. Considering that the density of pure MnS was d ¼ 4 g cm3 and the radius of the inclusion before dissolution was between 4 and 5 lm, the thickness dissolved was estimated between 5 and 10 nm from Eq. (3). This value is in agreement with the effect of polishing on the remaining volume of inclusions, as described in the experimental section. mMnS depth ðcmÞ ¼ ð3Þ p r2 d Once the successive processes involved in the activation of inclusions were identified, local experiments were performed in 1 M NaCl, pH 3 in order to determine precursor sites for pitting corrosion close to shallow inclusions and some key-parameters. It was found that MnS inclusions start to dissolve at about 70 mV, as shown in Fig. 4(a). From this potential the polarisation curve determined in the presence of an inclusion deviates from that obtained on the pure matrix. Stable pitting was systematically observed during this process, at around 250 mV. SEM observations revealed that pits were mainly located in the bare metal that was exposed
Fig. 4. Local polarisation curves determined in 1 M NaCl, pH 3 on sites containing the pure matrix (gray curve) and the matrix with a large and shallow inclusion (black curve). (b) Pits initiated during local measurements inside a large and shallow inclusion.
to the solution after complete dissolution of the inclusion, as shown in Fig. 4(b). The SVET was then used on the same microstructure in order to map the current distribution around the shallow inclusions at OCP. The main objective was to locate the cathodic and anodic regions and to estimate the current above pits. As explained previously, the sample was immersed into a citrate/citric buffered solution and a pitting site was activated by injecting a small quantity (of about 1.5 ll) of activating solution close to an inclusion. Macroscopic injections revealed that the presence of H2 O2 in the activating solution increased the potential of the overall system up to a value around 120 mV. As explained above, MnS inclusions underwent electrochemical dissolution within this potential range. Once the inclusion was dissolved, pits initiated in the bare metal due to the low value of the pH and the high density of chloride ions in the close vicinity of the selected site. As it was demonstrated [4], microinjection induced a current perturbation for about 10 s. This corresponded to the time necessary for removing the microcapillary far from the surface and for starting the SVET measurements and therefore, such a current perturbation had nearly no effects on these measurements. Distributions of the normal current density along scan lines across inclusions are shown in Fig. 5. The anodic current was always detected above the site where the bare metal was exposed to the solution after complete dissolution of the shallow inclusion. It can be seen that the anodic region extends over about 50 lm far from the inclusion. The highest anodic current density reached was 38 and 66 lA cm2 in Fig. 5(a) and (b), respectively. The cathodic current was observed on the passive metal surrounding the inclusion and roughly the same maxima was reached in both cases, of about )27 lA cm2 . These results are the first experimental evidence of the galvanic coupling between the bare metal exposed to the solution after MnS dissolution and the passive surface on stainless steels. The flow of current from a single site on the surface spreads rapidly in solution and the magnitude decreases precipitously with distance above the surface. As the SVET measurements were performed at a fixed distance above the surface and not at the metal surface, the current densities were lowered. To estimate the current density at the surface, the inclusion was divided into a number of square elements with a size of 0.2 0.2 lm2 . The probe height being much greater than the size of these elements, each element was replaced by point current source at its center. The potential, V, at a distance r from the point source i0 was given by [14]: V ðx; y; zÞ ¼
q i0 q i0 ¼ ; 2 p r 2 p ðx2 þ y 2 þ z2 Þ
ð4Þ
H. Krawiec et al. / Electrochemistry Communications 6 (2004) 655–660
659
Fig. 5. (a–b) Variations of the normal current density around and above two different shallow inclusions. Vectors that point upwards are representative of the anodic current and the length of vectors represents the magnitude of the current density (as shown in the schematic cross-section view). The background micrograph was recorded before the experiment. The time duration of a scan line was about 170 s.
where q was the solution resistivity and (x, y, z) were the Cartesian coordinates with the origin at the point source. The field Fz normal to the surface could be obtained by partial differentiation of Eq. (4): Fz ðx; y; zÞ ¼
oV qi0 z ¼ : oz 2p ðx2 þ y 2 þ z2 Þ3=2
ð5Þ
Assuming uniformity of the solution resistivity, the current density normal to the surface was jðx; y; zÞ ¼
Fz i0 z ¼ : q 2p ðx2 þ y 2 þ z2 Þ3=2
ð6Þ
In the simulation, the current densities from each point source were summed as follows: jðx; y; zÞ ¼
a100 X b25 i0 X h 2p m¼a1 n¼b 1
z 2
2
ðx mÞ þ ðy nÞ þ z2
i3=2 ; ð7Þ
where (ai , bi ) were the position of the element i of the array. Fig. 6(a) and (b) show the variations of the calculated normal current density at probe heights of 1 and 50 lm, respectively. When the probe was close to the surface,
Fig. 6. Normal current density variations calculated for various heights of the probe: (a) 1 lm and (b) 50 lm. Simulations were performed on an inclusion (located between 0 and 20 lm in the x-direction and between 0 and 5 lm in the y-direction) that was divided into 100 25 square elements (point current sources).
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the anodic current was detected above the inclusion with a maxima of about 7.6 mA cm2 , as shown in Fig. 6(a). On the other hand, when the probe was located 50 lm above the inclusion (similar to the conditions used during the SVET measurements), the anodic current was detected at a distance of about 50 lm far from the source and the maxima above the inclusion was very close to that found experimentally, of about 60 lA cm2 , as shown in Fig. 6(b). Deconvoluting the current densities measured is then necessary to resolve contributions both from anodic and cathodic regions at the interface between the matrix and the inclusion.
4. Discussion and concluding remarks Understanding the mechanisms involved in the evolution of modified surfaces and predicting within reasonable accuracy the performance and durability of engineering materials remain open challenges. This requires to obtain in situ informations about real-time dynamics of these systems at the local scale and to determine the role of physical and chemical parameters in the surface reactivity. Regarding the corrosion and related degradation phenomena in high value metallurgical systems, such as aluminum alloys in aeronautics and stainless steels in power plants, the driving force is the galvanic coupling of which the dimensional aspect is fixed by a combination of scales which can be described at the electrolyte-metal interface taking into account the microstructure, the possible chemical changes, the electrolyte conductivity, etc. Therefore, combining the electrochemical microcell technique with the SVET opens a very large field of investigation in studying the role of passive films, metallurgical parameters (microstructure, alloying elements, etc) and surface preparation methods in the galvanic coupling and localized corrosion processes (and not only for bimetallic corrosion). As shown in this paper, metallurgical, mechanical and chemical criteria leading to
an enhancement of pitting corrosion could be then proposed. These methods will also provide quantitative data concerning the interactions of an aggressive environment with engineering materials which could be introduced in the numerical modelling of galvanic processes (for a more accurate prediction over long time scales of processes). It should be mentioned that the proposed strategy could be applied to various engineering systems by adjusting the composition of the activating solution used before the SVET measurements according to the electrochemical behaviour of each constituent phase determined with the electrochemical microcell technique.
Acknowledgements One of the authors (H.K.) thank NATO Science Fellowships National Administration for its financial support.
References [1] C.H. Paik, H.S. White, R.C. Alkire, J. Electrochem. Soc. 147 (2000) 4120. [2] D.E. Williams, Y.Y. Zhu, J. Electrochem. Soc. 147 (2000) 1763. [3] J.O. Park, C.H. Paik, Y.H. Huang, R.C. Alkire, J. Electrochem. Soc. 146 (1999) 517. [4] B. Vuillemin, X. Philippe, R. Oltra, V. Vignal, L. Coudreuse, L.C. Dufour, E. Finot, Corr. Sci. 45 (2003) 1143. [5] H. Uchida, M. Yamashita, S. Inoue, K. Koterazawa, Mater. Sci. Eng. A 319–321 (2001) 496. [6] I. Sekine, M. Yuasa, N. Hirose, T. Tanaki, Prog. Org. Coat. 45 (2002) 1. [7] M. Khobaib, A. Rensi, T. Matakis, M.S. Donley, Prog. Org. Coat. 41 (2001) 266. [8] M.J. Franklin, D.C. White, H.S. Issacs, Corr. Sci. 32 (1991) 945. [9] G.S. Eklund, J. Electrochem. Soc. 121 (1974) 467. [10] G.S. Eklund, Scand. J. Metall. 1 (1972) 331. [11] J.E. Castle, R. Ke, Corr. Sci. 30 (1990) 409. [12] G. Wranglen, Corr. Sci. 14 (1974) 331. [13] S.E. Lott, R.C. Alkire, J. Electrochem. Soc. 136 (1989) 983. [14] H.S. Isaacs, Corrosion 46 (1990) 677.
Use of the electrochemical microcell technique and the SVET for monitoring pitting corrosion at MnS inclusions H. Krawiec a, V. Vignal a
b,*
, R. Oltra
b
Department of Foundry, AGH University of Science and Technology, ul. Reymonta 23, 30-059 Cracow, Poland b LRRS, UMR 5613 CNRS, Universite de Bourgogne, BP 47870, 21078 Dijon Cedex, France Received 19 April 2004; received in revised form 4 May 2004; accepted 4 May 2004 Available online 25 May 2004
Abstract The purpose of this paper is to report on use of the electrochemical microcell technique and the scanning vibrating electrode technique for monitoring pitting corrosion on the same stainless steel microstructure. First, the electrochemical behaviour of sites containing a single inclusion was investigated in order to the determine both the successive steps occurring during the inclusions activation and some key-parameters such as the onset potential for MnS dissolution and the pitting potential. Then, the local current distribution around a pitting site was monitored at open circuit potential in order to locate anodic and cathodic regions and to obtain informations on the galvanic coupling between inclusions and the matrix. This was done using a part of a modified electrochemical microcell combined with the scanning vibrating electrode technique. 2004 Elsevier B.V. All rights reserved. Keywords: Manganese sulfide; Pitting corrosion; Stainless steel; SVET; Microcapillary
1. Introduction Materials have complex microstructure which could be composed of different metallic phases, non-metallic inclusions, intermetallic particles and second-phase precipitates. Such microstructures lead to heterogeneous distributions of oxido-reduction reactions at the solution/surface interface. It is hardly possible to assign electrochemical changes observed to corresponding phenomena and locations on the surface by using largescale electrochemical experiments that provide average data integrating over a large surface area. Microelectrochemical methods which have been developed over the past years appear as powerful techniques to study oxidoreduction reactions in the micro- and nano-range [1–4]. The scanning vibrating electrode technique (SVET) offers the possibility of mapping variations in current *
Corresponding author. Tel.: +33-380-396-160; fax: +33-380-396132. E-mail address: [email protected] (V. Vignal). 1388-2481/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.05.001
densities at the microscale over a corroding metals by measuring potential gradients developed in the solution due to the flow of ionic currents and to locate anodic and cathodic zones. Measurements at open circuit potential (OCP) have only been performed in specific cases such as for characterizing the current evolution during the initiation and growth of surface microcracks under straining conditions [5] and the deterioration of organic coatings [6,7] and for studying microbial influenced corrosion [8]. The main reason is that it is difficult to activate a single pit at OCP on sufficiently long time scales to perform local current measurements. The purpose of this paper is to report on use of the electrochemical microcell technique and the SVET for monitoring pitting corrosion on the same stainless steel microstructure. The electrochemical behaviour of sites containing a single inclusion was investigated in order to the determine both the successive steps occurring during the inclusions activation and some key-parameters such as the onset potential for MnS dissolution and the pitting potential. Then, the local current distribution
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H. Krawiec et al. / Electrochemistry Communications 6 (2004) 655–660
around a pitting site was monitored at OCP in order to obtain informations on the galvanic coupling between the inclusions and the matrix. This was done using a part of a modified electrochemical microcell combined with the SVET.
2. Experimental Measurements were performed on a resulfurized 304L type stainless steel (Ni: 8.75 wt%, Cr: 18.3, Mn: 1.7, S: 0.17, Si: 0.5, P < 0:035 and C: 0.05). The samples were machined from rolled plates (thickness: 20 mm, width: 4 m and length: 10 m) along the short transverse direction. Spheroidized inclusions were observed (depth <5 lm) and their size ranged between 5 and 40 lm, as shown in Fig. 1. These inclusions were heterogeneously distributed on the surface. The samples (diameter: 2.5 cm) were mechanically polished with silicon carbide emery papers down to 4000 grit, smoothed with diamond pastes down to 1 lm and ultrasonically rinsed in ethanol. After polishing, the depth of the inclusions emerging at the surface ranged from the nano to the microscale, depending on the quantity of MnS removed during polishing. The local electrochemical behaviour of specimens was studied at 25 C using the electrochemical microcell technique. This technique consists of a glass microcapillary which is filled with the electrolyte, as shown in Fig. 2(a). The microcapillary tip was sealed to the specimen surface with a layer of silicone rubber. The microcell was mounted on a microscope for precise positioning of the microcapillary on the surface and the entire setup was placed in a faradic cage. The diameter of the microcapillary tip was 50 lm and the counter electrode was a platinum wire. A modified high resolution potentiostat was used in order to have a current detection limit of 20 fA (Jaissle 1002T-NC-3). A potential of )500 mV corresponding to the cathodic zone (the OCP values ranged between )200 and )300 mV/ SCE in the different solutions) was applied for 3 min and
Fig. 1. Morphology of the shallow inclusions observed using optical microscopy.
Fig. 2. (a) Experimental setup for local measurements using the electrochemical microcell technique and (b) use of the capillary part of the electrochemical microcell for activating a single inclusion (on the left side of the image) and the SVET for measuring the local current distribution above the reactive metal (on the right side of the image).
the potentiodynamic polarization curves were then measured at a scan rate 1 mV s1 . In order to monitor the electrochemical dissolution of MnS, measurements were performed in 1 M NaClO4 buffered with a mixture of 0.1 M Na2 C6 O7 H6 (39.9 ml) and 0.1 M HCl (60.1 ml) at pH 3. In order to study pitting corrosion at MnS, an unbuffered solution 1 M NaCl, pH 3 was used. This solution was altered to pH 3 with sulfuric acid. The microcapillary part of the electrochemical microcell was used to initiate locally a pit, as shown in Fig. 2(b). The microcell was connected to a microsyringe and this system was filled with a mixture of 1 M HCl (80 ml) and H2 O2 (0.3 ml). Hydrogen peroxide was used in order to increase locally the potential and to promote MnS dissolution whereas hydrochloric acid was used in order to initiate pits in the bare metal. Local current
H. Krawiec et al. / Electrochemistry Communications 6 (2004) 655–660
Current density / µA cm
-2
1000 100
P1 10 1 0.1 -600
(a)
-300 0 300 Potential / mV vs. SCE
8
600
P12
-2
imes-iBkg / µA cm
measurements were performed at OCP in a buffered solution 0.1 M C6 H8 O7 + 0.2 M Na2 HPO4 (pH 6.8 and j ¼ 2:3 ms cm1 ) using an Applicable Electronics SVET. Pt–Ir microelectrodes (MicroProbe Inc.) were black platinised and the sphere diameter after deposition was about 20 lm. The vibration amplitude was 20 lm and the vibration frequency was 600 and 200 Hz in the direction parallel and normal to the surface, respectively. The ASET Software (Science Wares Inc.) converted the potential drop measured by the microelectrode with Ohm’s law into a current density value after amplification. The displacement of the microelectrode was performed using a motorised and computer-controlled XYZ micromanipulator allowing 0.5 lm steps. Two video cameras (140 magnification) were used for imaging and controlling the distances between the microelectrode, the microcapillary and the specimen surface, as shown in Fig. 2(b). The distance between the microelectrode and the surface was fixed to 50 lm and the Y-component of the current was measured. A saturated calomel electrode (SCE), to which all the potentials in the text are referred, was used as reference.
657
6 4
P11
2 0 200
(b)
300 400 Potential / mV vs. SCE
500
3. Results The successive steps occurring during the activation of MnS inclusions in the absence of pitting corrosion was first studied. The electrochemical response of sites containing the pure matrix and the matrix with a single MnS inclusion in the buffered solution 1 M NaClO4 , pH 3 is shown in Fig. 3(a) and particular attention was paid to the electrochemical dissolution of the inclusion. This process was observed between 70 and 500 mV with a maximum of about 12 lA cm2 for a small and shallow inclusion, as shown in Fig. 3(a). This peak can be decomposed into two peaks, P11 and P12 , connected to different anodic reactions, as shown in Fig. 3(b). The background current related to the reactions occurring on the surrounding matrix was subtracted to the total current such that the current density reported in Fig. 3(b) could only be associated to the reactions occurring at the inclusion surface. Peak P11 observed at 320 mV was associated to the electrochemical dissolution of the shallow MnS inclusion. Three possible electrochemical reactions may be proposed for the dissolution of MnS [9–13]: þ 2MnS þ 3H2 O ! 2Mn2þ þ S2 O 3 þ 6H þ 8e
ð1aÞ
þ MnS þ 4H2 O ! Mn2þ þ SO2 4 þ 8H þ 8e
with EðmVÞ ¼ 18 60 pH MnS ! Mn
2þ
þ S þ 2e
S2 O 3 ions released during reaction (1a) are not stable at pH 3 within the potential range considered and therefore this reaction was not considered. Among the two remaining electrochemical reactions, reaction (1c) seems to be the most relevant since it occurs at lower pH values. At the top of peak P11 , the shallow inclusion was completely dissolved and the current increase associated to peak P12 observed at 380 mV corresponds to the active–passive transition of the bare metal which was exposed to the solution. No current peak associated to MnS dissolution was obtained when a second polarisation curve was performed on the same location, confirming that the inclusion dissolution was complete after the first curve. The total charge associated to the inclusion dissolution can be calculated from the area of peak P11 and the quantity of MnS dissolved was then deduced from the following equation: mMnS ðgÞ ¼
ð1bÞ
with EðmVÞ ¼ 363 þ log½Mn2þ
Fig. 3. (a) Local polarisation curves determined in a buffered solution 1 M NaClO4 , pH 3 on sites containing the pure matrix (gray curve) and the matrix with a shallow inclusion (black curve). (b) Shape of peak P1 at higher magnification which was decomposed into two peaks P11 and P12 . The electrical charge was calculated from this plot considering that 1 mV corresponds to 1 s.
ð1cÞ
nM ; ne qe N
ð2Þ
where n is the total charge associated to peak P11 (3.32 109 C). M is the molar mass of MnS (87 g mol1 ). qe is the charge of electrons (1.16 · 1019 C). ne is the number of
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electrons involved in reaction (1c) (ne ¼ 2). N is the Avogadro’s number (N ¼ 6:02 1023 mol1 ). The quantity of MnS was estimated to mMnS ¼ 1:5 1012 g. Considering that the density of pure MnS was d ¼ 4 g cm3 and the radius of the inclusion before dissolution was between 4 and 5 lm, the thickness dissolved was estimated between 5 and 10 nm from Eq. (3). This value is in agreement with the effect of polishing on the remaining volume of inclusions, as described in the experimental section. mMnS depth ðcmÞ ¼ ð3Þ p r2 d Once the successive processes involved in the activation of inclusions were identified, local experiments were performed in 1 M NaCl, pH 3 in order to determine precursor sites for pitting corrosion close to shallow inclusions and some key-parameters. It was found that MnS inclusions start to dissolve at about 70 mV, as shown in Fig. 4(a). From this potential the polarisation curve determined in the presence of an inclusion deviates from that obtained on the pure matrix. Stable pitting was systematically observed during this process, at around 250 mV. SEM observations revealed that pits were mainly located in the bare metal that was exposed
Fig. 4. Local polarisation curves determined in 1 M NaCl, pH 3 on sites containing the pure matrix (gray curve) and the matrix with a large and shallow inclusion (black curve). (b) Pits initiated during local measurements inside a large and shallow inclusion.
to the solution after complete dissolution of the inclusion, as shown in Fig. 4(b). The SVET was then used on the same microstructure in order to map the current distribution around the shallow inclusions at OCP. The main objective was to locate the cathodic and anodic regions and to estimate the current above pits. As explained previously, the sample was immersed into a citrate/citric buffered solution and a pitting site was activated by injecting a small quantity (of about 1.5 ll) of activating solution close to an inclusion. Macroscopic injections revealed that the presence of H2 O2 in the activating solution increased the potential of the overall system up to a value around 120 mV. As explained above, MnS inclusions underwent electrochemical dissolution within this potential range. Once the inclusion was dissolved, pits initiated in the bare metal due to the low value of the pH and the high density of chloride ions in the close vicinity of the selected site. As it was demonstrated [4], microinjection induced a current perturbation for about 10 s. This corresponded to the time necessary for removing the microcapillary far from the surface and for starting the SVET measurements and therefore, such a current perturbation had nearly no effects on these measurements. Distributions of the normal current density along scan lines across inclusions are shown in Fig. 5. The anodic current was always detected above the site where the bare metal was exposed to the solution after complete dissolution of the shallow inclusion. It can be seen that the anodic region extends over about 50 lm far from the inclusion. The highest anodic current density reached was 38 and 66 lA cm2 in Fig. 5(a) and (b), respectively. The cathodic current was observed on the passive metal surrounding the inclusion and roughly the same maxima was reached in both cases, of about )27 lA cm2 . These results are the first experimental evidence of the galvanic coupling between the bare metal exposed to the solution after MnS dissolution and the passive surface on stainless steels. The flow of current from a single site on the surface spreads rapidly in solution and the magnitude decreases precipitously with distance above the surface. As the SVET measurements were performed at a fixed distance above the surface and not at the metal surface, the current densities were lowered. To estimate the current density at the surface, the inclusion was divided into a number of square elements with a size of 0.2 0.2 lm2 . The probe height being much greater than the size of these elements, each element was replaced by point current source at its center. The potential, V, at a distance r from the point source i0 was given by [14]: V ðx; y; zÞ ¼
q i0 q i0 ¼ ; 2 p r 2 p ðx2 þ y 2 þ z2 Þ
ð4Þ
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659
Fig. 5. (a–b) Variations of the normal current density around and above two different shallow inclusions. Vectors that point upwards are representative of the anodic current and the length of vectors represents the magnitude of the current density (as shown in the schematic cross-section view). The background micrograph was recorded before the experiment. The time duration of a scan line was about 170 s.
where q was the solution resistivity and (x, y, z) were the Cartesian coordinates with the origin at the point source. The field Fz normal to the surface could be obtained by partial differentiation of Eq. (4): Fz ðx; y; zÞ ¼
oV qi0 z ¼ : oz 2p ðx2 þ y 2 þ z2 Þ3=2
ð5Þ
Assuming uniformity of the solution resistivity, the current density normal to the surface was jðx; y; zÞ ¼
Fz i0 z ¼ : q 2p ðx2 þ y 2 þ z2 Þ3=2
ð6Þ
In the simulation, the current densities from each point source were summed as follows: jðx; y; zÞ ¼
a100 X b25 i0 X h 2p m¼a1 n¼b 1
z 2
2
ðx mÞ þ ðy nÞ þ z2
i3=2 ; ð7Þ
where (ai , bi ) were the position of the element i of the array. Fig. 6(a) and (b) show the variations of the calculated normal current density at probe heights of 1 and 50 lm, respectively. When the probe was close to the surface,
Fig. 6. Normal current density variations calculated for various heights of the probe: (a) 1 lm and (b) 50 lm. Simulations were performed on an inclusion (located between 0 and 20 lm in the x-direction and between 0 and 5 lm in the y-direction) that was divided into 100 25 square elements (point current sources).
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the anodic current was detected above the inclusion with a maxima of about 7.6 mA cm2 , as shown in Fig. 6(a). On the other hand, when the probe was located 50 lm above the inclusion (similar to the conditions used during the SVET measurements), the anodic current was detected at a distance of about 50 lm far from the source and the maxima above the inclusion was very close to that found experimentally, of about 60 lA cm2 , as shown in Fig. 6(b). Deconvoluting the current densities measured is then necessary to resolve contributions both from anodic and cathodic regions at the interface between the matrix and the inclusion.
4. Discussion and concluding remarks Understanding the mechanisms involved in the evolution of modified surfaces and predicting within reasonable accuracy the performance and durability of engineering materials remain open challenges. This requires to obtain in situ informations about real-time dynamics of these systems at the local scale and to determine the role of physical and chemical parameters in the surface reactivity. Regarding the corrosion and related degradation phenomena in high value metallurgical systems, such as aluminum alloys in aeronautics and stainless steels in power plants, the driving force is the galvanic coupling of which the dimensional aspect is fixed by a combination of scales which can be described at the electrolyte-metal interface taking into account the microstructure, the possible chemical changes, the electrolyte conductivity, etc. Therefore, combining the electrochemical microcell technique with the SVET opens a very large field of investigation in studying the role of passive films, metallurgical parameters (microstructure, alloying elements, etc) and surface preparation methods in the galvanic coupling and localized corrosion processes (and not only for bimetallic corrosion). As shown in this paper, metallurgical, mechanical and chemical criteria leading to
an enhancement of pitting corrosion could be then proposed. These methods will also provide quantitative data concerning the interactions of an aggressive environment with engineering materials which could be introduced in the numerical modelling of galvanic processes (for a more accurate prediction over long time scales of processes). It should be mentioned that the proposed strategy could be applied to various engineering systems by adjusting the composition of the activating solution used before the SVET measurements according to the electrochemical behaviour of each constituent phase determined with the electrochemical microcell technique.
Acknowledgements One of the authors (H.K.) thank NATO Science Fellowships National Administration for its financial support.
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