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Hydrogen mapping across a crevice: Effect of applied potential
Scripta Materialia 53 (2005) 1219–1223 www.actamat-journals.com
Hydrogen mapping across a crevice: Effect of applied potential T. Sundararajan *, E. Akiyama, K. Tsuzaki Steel Research Center, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305 0047, Japan Received 1 July 2005; received in revised form 26 July 2005; accepted 9 August 2005 Available online 2 September 2005
Abstract Iron exposed to acetate buffer in the presence of a crevice assembly showed metal dissolution and hydrogen evolution. Permeated hydrogen was successfully mapped on the rear surface of the crevice using the silver decoration technique. Magnitude of silver deposit varied with the change in the applied potential, with inhomogeneous distribution. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Corrosion; Iron; Hydrogen diffusion; Silver decoration
1. Introduction The classic mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects, e.g., the acidification or the migration of aggressive ions into the crevice solution. These solution composition changes can cause breakdown of the passivating film and promote acceleration and auto catalyzation of crevice corrosion [1,2]. In the cases of constant pH and zero chloride concentration, crevice corrosion still occurred in iron and was shown to be caused by the IR drop which placed the local electrode potential existing on the crevice wall in the active peak region of the polarization curve [3,4]. Moreover, hydrogen evolution was witnessed inside the crevice, which may lead to the entry of hydrogen into the metal. In addition, this voltage drop mechanism has been shown to operate with other metals, including steel [5], nickel [6,7] and aluminum [8,9]. In our earlier investigation [10], we demonstrated that iron exposed to acetate buffer showed hydrogen evolution inside the crevice. A part of the hydrogen produced by the reduction reaction subsequently diffused into the metal and was mapped successfully for the first time using the
*
Corresponding author. Tel.: +81 29 8592132; fax: +81 29 8592101. E-mail address: [email protected] (T. Sundararajan).
silver decoration technique. The present study aimed to map the hydrogen distribution inside the crevice region with varying applied potential. 2. Materials and methods Pure iron samples of 0.5 mm thick (99.95%, vacuum annealed at 800 C, Nilaco, Japan) were cut into 35 · 12 mm rectangular pieces for crevice corrosion. After polishing to a mirror-like surface using 0.05 lm alumina and washing with acetone and ethanol, a specimen was fitted into the crevice assembly coupled with a silver decoration set up. The experimental set up is described elsewhere [10]. The iron sample was fixed on the Plexiglas with a window of 30 · 10 mm using silicone paste. Lacquer was used to eliminate the crevice between the iron sample and the neighboring silicone spacer. This resulted in a covering of around 1 mm along the edge of the iron samples. On one side of the sample surface, another piece of Plexiglas was fixed at a distance of about 600 lm, to create the crevice. Silicone spacers were used to create the crevice distance between the sample surface and the top Plexiglas. Except for the sample mouth all other portions were covered with the silicone paste to avoid the entry of solution inside the crevice. The rear surface of the sample was exposed to 4.3 mM potassium silver cyanide solution in order to detect the hydrogen using the silver decoration technique. The
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.08.016
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Fig. 1. Potentiodynamic polarization curve for iron in acetate buffer (0.5 M acetic acid + 0.5 M sodium acetate); the legends indicate the passive potentials chosen for crevice corrosion studies.
detailed mechanism by which the silver decoration technique is operated can be found elsewhere [11–14]. Acetate buffer (0.5 M acetic acid + 0.5 M sodium acetate) was used as the standard solution for crevice corrosion experiments. The solution used in this work was open to the air. A three electrode cell containing Pt wire as the counter electrode and a standard Ag/AgCl electrode as the reference were used for all electrochemical measurements. Before the crevice corrosion test, the potentiodynamic polarization curves were obtained for the flat iron specimen in the above solution with a scan rate of 10 mV/min. Fig. 1 shows the potentiodynamic polarization curves for iron exposed to the acetate buffer solution. The polarization curve exhibited an active loop around 550 mV to +200 mV indicating the active corrosion process at this potential segment. Above +200 mV, the system showed the least current density arising from the formation of the passive film on the surface, which retards the active dissolution of the metal. The passive potentials of +500, +700 and +900 mV applied for the crevice corrosion studies are marked in the figure. After 24 h of crevice experiments, the specimens were rinsed with deionized water and ethanol before being submitted to backscattered electron (BSE) image investigation. Potential inside the crevice was monitored in a separate electrochemical set up using Ag/AgCl microelectrodes. The microelectrodes were positioned in nine places vertically across the crevice and the potential variations were monitored for 24 h.
the increase in the applied potential. The specimen impressed at 500 mV showed 2 mm of polished surface inside the crevice. However, the applied potential of 700 and 900 mV exhibited distances of 3 and 4.5 mm, respectively. Beneath the passive region, severe corrosion attack was seen, which resulted in reduction in the specimen thickness. Although the crevice bottom did not show much metal loss, it evinced an etched look through loss of the mirror finish surface. During experiments, the bottom portion showed gas bubble formation, which may arise from the hydrogen reduction reaction. Fig. 2 shows the morphology of the specimen inside the crevice during potential impression of 900 mV. The specimen inside the crevice confirms the above features such as retention of polished surface near the crevice mouth, active metal dissolution in the intermediate portion and the presence of gas bubbles at the crevice bottom. Pickering et al. [4,5] showed that crevice corrosion is caused by the potential drop which placed the local electrode potential existing on the crevice wall in the active peak region of the polarization curve. A further potential drop will lead to the hydrogen reduction reaction. The potential drop occurs inside the crevice is defined as an IR drop and was measured using Luggin probes inserting inside the crevice [3–6]. In the present work, the potential drop characteristic for each applied potential was measured using static microelectrodes without disturbing the crevice electrolyte. Fig. 3 represents the change in potential across the crevice for different applied potentials. Measurements showed a gradual decrease in potential from the crevice mouth to the bottom. At 30 min duration, all applied potential conditions showed a steep fall in their potential across the
3. Results and discussion The morphologies of all tested specimens were visually examined and showed the following features: the specimens tested with plain acetate buffer showed three distinct regions inside the crevice. The region near the crevice mouth retained its mirror finish surface. The distance of this polished surface increased towards the crevice bottom with
Fig. 2. Morphology of the iron exposed to acetate buffer with crevice assembly at the applied potential of 900 mVAg/AgCl.
T. Sundararajan et al. / Scripta Materialia 53 (2005) 1219–1223
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Fig. 3. Potential measured across the crevice using Ag/AgCl microelectrodes under different applied potentials.
crevice except near the crevice mouth. The region adjacent to crevice mouth showed a very small potential drop compared to the rest of the crevice region. However, at longer durations, +500 mV condition showed the significant potential drop, whereas applied potentials of 700 and 900 mV showed a significant difference in the potential between the crevice mouth and bottom. At higher test durations, the intermediate portion showed the increased potential values for all test conditions. This is attributed to the bubbles produced in the crevice which might have disturbed the measurements in the microelectrodes. The dotted line indicates the reversible potential for hydrogen reaction in the acetate buffer. Applied potential of 500 mV showed the potential drop below the line (hydro-
gen reduction potential), whereas, the potential drop for higher applied potentials falls on the boundary. This indicated that a lower applied potential can facilitate the high magnitude of hydrogen reduction reaction. The silver decoration technique was applied to map the permeated hydrogen from the crevice. The BSE images were taken for all the test conditions on the rear surface of the crevice assembly. Fig. 4 compares the magnitude of silver deposits present in a similar location (17 mm from crevice mouth towards bottom) at different applied potentials. The specimen impressed at 500 mV showed a significant amount of silver deposits with the size range of 1–5 lm. In our earlier studies [10], the specimen tested under similar conditions (electrolyte and applied potential)
Fig. 4. BSE images show the distribution of silver deposit in the crevice specimen: (a) 500 (b) 700 and (c) 900 mVAg/AgCl. (Location: 17 mm from the crevice mouth towards bottom.)
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without crevice assembly showed an absence of silver deposits. This confirmed the effect of the crevice in producing hydrogen. The applied potentials of 700 and 900 mV showed a lower magnitude of silver deposits compared to the 500 mV condition. This revealed that increasing the applied potential led to decreased hydrogen reduction inside the crevice. This may attributed to the following reason: the potential drop inside the crevice for higher applied potentials did not reach the limit to facilitate the hydrogen reduction reaction. Fig. 3 confirms the above postulation. Fig. 5 shows the area coverage of silver particles deposited on the iron specimen at various applied potentials. The magnitude of silver deposits showed the variation with respect to the applied potential of the specimen. The specimen impressed at 500 mV showed the higher area coverage with the peak intensity of 1.1% compared to 0.5% and 0.3% for 700 and 900 mV, respectively. Also, the increased applied potentials shifted the peak intensity towards the bottom of the crevice. This may arise from the potential drop characteristics of respective specimens. The potential required to facilitate the hydrogen reduction shifted towards the bottom with the increasing applied potential. Each testing condition showed an inhomogeneous distribution of silver deposits from the crevice mouth towards the bottom. Regions adjacent to the crevice mouth showed very little silver deposits, irrespective of the applied potential. The intermediate portion yielded more silver deposits. From the crevice morphology, the peak intensity regions corresponded to the cathodic region. However, the crevice bottom showed a lower intensity though the segment represents the same cathodic region. This indicated that the hydrogen adsorption and diffusion is more active at the intermediate portion than at the crevice mouth and the bottom. The potential inside the crevice drops with depth. This led to the increased hydrogen generation with the depth. From Fig. 2, it can be clearly seen that the hydrogen bub-
bles were produced at the bottom of the crevice. It might be expected that more silver deposits would accumulate at the crevice bottom. Although the hydrogen reduction reaction would be higher at the bottom portion, the absence of silver deposits in the corresponding area is attributed to the following reason. The higher magnitude of the hydrogen reduction reaction led to the re-combination of H and resulted in the formation of H2 bubbles. The bubbles produced inside the crevice could not escape from the metal surface due to its crevice geometry. The cluster of bubbles at the region restricted the solution contact with the metal surface for further hydrogen reduction reaction. During crevice corrosion, the specimen thickness varied across the crevice. The active region showed remarkable metal losses, which resulted in the significant reduction in their thickness in this region. For the 500 mV condition, this lowest thickness region is identified at 2–6 mm from the crevice mouth. However, the magnitude of silver deposits at this region is very low compared to the intermediate position, where moderate metal loss was observed. This indicated that metal thinning would not be a major cause for the higher magnitude of silver deposits. Saitoh et al. [13] reported that hydrogen charged on platinum, palladium and 304 stainless steel showed a clear boundary between the charged and uncharged regions upon silver deposition. This had been witnessed until 24 h of charging duration. However, the present study employed body-centered cubic phase iron, which has faster hydrogen diffusion than Pd, Pt (102 times) and 304 stainless steel (104 times) [15]. It is expected that the permeated hydrogen could widen its length across the crevice with the increase in test duration. The calibration test showed that the permeated hydrogen expanded its length up to 2 mm until 24 h, which resulted in the widening of the silver deposits at this region. Considering 2 mm of linear spatial resolution and the low magnitude of silver deposits near the crevice mouth, permeation of hydrogen would be very low at passive and adjacent active regions. It was reported that the pH of the active region is significantly decreased during the test [16,17]. The present results indicated that H+ produced on this region did not much influence the permeated hydrogen. Additionally, the permeated hydrogen identified in this study could be attributed mostly to the hydrogen reduction reaction. 4. Conclusion
Fig. 5. Distribution of silver deposits in the rear surface of the crevice at different applied potentials (BSE image; 100· magnification).
Crevice corrosion operated with the potential drop mechanism for iron exposed to acetate buffer resulted in hydrogen entry along with metal dissolution. The diffused hydrogen showed inhomogeneous distribution into the metal across the crevice region. Variation in the applied potential without altering the electrolyte and the crevice set up could modify the distribution profile remarkably. Increasing the applied potential from +500 mV to +900 mV resulted in the lower magnitude of hydrogen entry.
T. Sundararajan et al. / Scripta Materialia 53 (2005) 1219–1223
References [1] Uhlig HH, Revie RW. Corrosion and corrosion control. 3rd ed. New York, NY: John Wiley and Sons; 1985. [2] Fontana MG. Corrosion engineering. 3rd ed. New York, NY: McGraw–Hill Book Co; 1986. [3] Cho K, Pickering HW. J Electrochem Soc 1990;137:3313. [4] Cho K, Pickering HW. J Electrochem Soc 1991;38:L56. [5] Nystrom EA, Lee JB, Sagu¨e`s AA, Pickering HW. J Electrochem Soc 1994;141:358. [6] Abdulsalam MI, Pickering HW. Corros Sci 1998;41:351. [7] Lee JS, Reed ML, Kelly RG. J Electrochem Soc 2004;151:B423. [8] Abdullah AM, Shaw BA, Pickering HW. Crevice corrosion of aluminum 6XXX alloy. In: Shaw BA, Buchheit RG, Moran JP, editors. Proceedings of corrosion and corrosion prevention of low
[9] [10] [11] [12] [13] [14] [15] [16] [17]
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density metals and alloys (ECS Phoenix Meeting), PV 2000-23, Penington, NJ: The Electrochemical Society; 2001. p. 401. Akiyama E, Frankel GS. J Electrochem Soc 1999;146:4095. Sundararajan T, Akiyama E, Tsuzaki K. Electrochem Solid State Lett 2005;8:B30. Schober T, Dieker C. Metall Trans A 1983;14:2440. Yao J, Cahoon JR. Metall Trans A 1990;21:603. Saitoh H, Hishi T, Misawa T. Mater Trans JIM 1996;37:373. Koyama K, Itoh G, Kanno M. J Jpn Inst Met 1998;62:742. Metals Data Book Ed. Maruzen, 3rd ed., Tokyo, Japan Institute for Metals; 1993. p. 21. Wolfe RC, Weil KG, Shaw BA, Pickering HW. J Electrochem Soc 2005;152:B82. Wolfe RC, Weil KG, Pickering HW. J Phys Chem B 2004;108: 14298.
Hydrogen mapping across a crevice: Effect of applied potential T. Sundararajan *, E. Akiyama, K. Tsuzaki Steel Research Center, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305 0047, Japan Received 1 July 2005; received in revised form 26 July 2005; accepted 9 August 2005 Available online 2 September 2005
Abstract Iron exposed to acetate buffer in the presence of a crevice assembly showed metal dissolution and hydrogen evolution. Permeated hydrogen was successfully mapped on the rear surface of the crevice using the silver decoration technique. Magnitude of silver deposit varied with the change in the applied potential, with inhomogeneous distribution. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Corrosion; Iron; Hydrogen diffusion; Silver decoration
1. Introduction The classic mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects, e.g., the acidification or the migration of aggressive ions into the crevice solution. These solution composition changes can cause breakdown of the passivating film and promote acceleration and auto catalyzation of crevice corrosion [1,2]. In the cases of constant pH and zero chloride concentration, crevice corrosion still occurred in iron and was shown to be caused by the IR drop which placed the local electrode potential existing on the crevice wall in the active peak region of the polarization curve [3,4]. Moreover, hydrogen evolution was witnessed inside the crevice, which may lead to the entry of hydrogen into the metal. In addition, this voltage drop mechanism has been shown to operate with other metals, including steel [5], nickel [6,7] and aluminum [8,9]. In our earlier investigation [10], we demonstrated that iron exposed to acetate buffer showed hydrogen evolution inside the crevice. A part of the hydrogen produced by the reduction reaction subsequently diffused into the metal and was mapped successfully for the first time using the
*
Corresponding author. Tel.: +81 29 8592132; fax: +81 29 8592101. E-mail address: [email protected] (T. Sundararajan).
silver decoration technique. The present study aimed to map the hydrogen distribution inside the crevice region with varying applied potential. 2. Materials and methods Pure iron samples of 0.5 mm thick (99.95%, vacuum annealed at 800 C, Nilaco, Japan) were cut into 35 · 12 mm rectangular pieces for crevice corrosion. After polishing to a mirror-like surface using 0.05 lm alumina and washing with acetone and ethanol, a specimen was fitted into the crevice assembly coupled with a silver decoration set up. The experimental set up is described elsewhere [10]. The iron sample was fixed on the Plexiglas with a window of 30 · 10 mm using silicone paste. Lacquer was used to eliminate the crevice between the iron sample and the neighboring silicone spacer. This resulted in a covering of around 1 mm along the edge of the iron samples. On one side of the sample surface, another piece of Plexiglas was fixed at a distance of about 600 lm, to create the crevice. Silicone spacers were used to create the crevice distance between the sample surface and the top Plexiglas. Except for the sample mouth all other portions were covered with the silicone paste to avoid the entry of solution inside the crevice. The rear surface of the sample was exposed to 4.3 mM potassium silver cyanide solution in order to detect the hydrogen using the silver decoration technique. The
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.08.016
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T. Sundararajan et al. / Scripta Materialia 53 (2005) 1219–1223
Fig. 1. Potentiodynamic polarization curve for iron in acetate buffer (0.5 M acetic acid + 0.5 M sodium acetate); the legends indicate the passive potentials chosen for crevice corrosion studies.
detailed mechanism by which the silver decoration technique is operated can be found elsewhere [11–14]. Acetate buffer (0.5 M acetic acid + 0.5 M sodium acetate) was used as the standard solution for crevice corrosion experiments. The solution used in this work was open to the air. A three electrode cell containing Pt wire as the counter electrode and a standard Ag/AgCl electrode as the reference were used for all electrochemical measurements. Before the crevice corrosion test, the potentiodynamic polarization curves were obtained for the flat iron specimen in the above solution with a scan rate of 10 mV/min. Fig. 1 shows the potentiodynamic polarization curves for iron exposed to the acetate buffer solution. The polarization curve exhibited an active loop around 550 mV to +200 mV indicating the active corrosion process at this potential segment. Above +200 mV, the system showed the least current density arising from the formation of the passive film on the surface, which retards the active dissolution of the metal. The passive potentials of +500, +700 and +900 mV applied for the crevice corrosion studies are marked in the figure. After 24 h of crevice experiments, the specimens were rinsed with deionized water and ethanol before being submitted to backscattered electron (BSE) image investigation. Potential inside the crevice was monitored in a separate electrochemical set up using Ag/AgCl microelectrodes. The microelectrodes were positioned in nine places vertically across the crevice and the potential variations were monitored for 24 h.
the increase in the applied potential. The specimen impressed at 500 mV showed 2 mm of polished surface inside the crevice. However, the applied potential of 700 and 900 mV exhibited distances of 3 and 4.5 mm, respectively. Beneath the passive region, severe corrosion attack was seen, which resulted in reduction in the specimen thickness. Although the crevice bottom did not show much metal loss, it evinced an etched look through loss of the mirror finish surface. During experiments, the bottom portion showed gas bubble formation, which may arise from the hydrogen reduction reaction. Fig. 2 shows the morphology of the specimen inside the crevice during potential impression of 900 mV. The specimen inside the crevice confirms the above features such as retention of polished surface near the crevice mouth, active metal dissolution in the intermediate portion and the presence of gas bubbles at the crevice bottom. Pickering et al. [4,5] showed that crevice corrosion is caused by the potential drop which placed the local electrode potential existing on the crevice wall in the active peak region of the polarization curve. A further potential drop will lead to the hydrogen reduction reaction. The potential drop occurs inside the crevice is defined as an IR drop and was measured using Luggin probes inserting inside the crevice [3–6]. In the present work, the potential drop characteristic for each applied potential was measured using static microelectrodes without disturbing the crevice electrolyte. Fig. 3 represents the change in potential across the crevice for different applied potentials. Measurements showed a gradual decrease in potential from the crevice mouth to the bottom. At 30 min duration, all applied potential conditions showed a steep fall in their potential across the
3. Results and discussion The morphologies of all tested specimens were visually examined and showed the following features: the specimens tested with plain acetate buffer showed three distinct regions inside the crevice. The region near the crevice mouth retained its mirror finish surface. The distance of this polished surface increased towards the crevice bottom with
Fig. 2. Morphology of the iron exposed to acetate buffer with crevice assembly at the applied potential of 900 mVAg/AgCl.
T. Sundararajan et al. / Scripta Materialia 53 (2005) 1219–1223
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Fig. 3. Potential measured across the crevice using Ag/AgCl microelectrodes under different applied potentials.
crevice except near the crevice mouth. The region adjacent to crevice mouth showed a very small potential drop compared to the rest of the crevice region. However, at longer durations, +500 mV condition showed the significant potential drop, whereas applied potentials of 700 and 900 mV showed a significant difference in the potential between the crevice mouth and bottom. At higher test durations, the intermediate portion showed the increased potential values for all test conditions. This is attributed to the bubbles produced in the crevice which might have disturbed the measurements in the microelectrodes. The dotted line indicates the reversible potential for hydrogen reaction in the acetate buffer. Applied potential of 500 mV showed the potential drop below the line (hydro-
gen reduction potential), whereas, the potential drop for higher applied potentials falls on the boundary. This indicated that a lower applied potential can facilitate the high magnitude of hydrogen reduction reaction. The silver decoration technique was applied to map the permeated hydrogen from the crevice. The BSE images were taken for all the test conditions on the rear surface of the crevice assembly. Fig. 4 compares the magnitude of silver deposits present in a similar location (17 mm from crevice mouth towards bottom) at different applied potentials. The specimen impressed at 500 mV showed a significant amount of silver deposits with the size range of 1–5 lm. In our earlier studies [10], the specimen tested under similar conditions (electrolyte and applied potential)
Fig. 4. BSE images show the distribution of silver deposit in the crevice specimen: (a) 500 (b) 700 and (c) 900 mVAg/AgCl. (Location: 17 mm from the crevice mouth towards bottom.)
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without crevice assembly showed an absence of silver deposits. This confirmed the effect of the crevice in producing hydrogen. The applied potentials of 700 and 900 mV showed a lower magnitude of silver deposits compared to the 500 mV condition. This revealed that increasing the applied potential led to decreased hydrogen reduction inside the crevice. This may attributed to the following reason: the potential drop inside the crevice for higher applied potentials did not reach the limit to facilitate the hydrogen reduction reaction. Fig. 3 confirms the above postulation. Fig. 5 shows the area coverage of silver particles deposited on the iron specimen at various applied potentials. The magnitude of silver deposits showed the variation with respect to the applied potential of the specimen. The specimen impressed at 500 mV showed the higher area coverage with the peak intensity of 1.1% compared to 0.5% and 0.3% for 700 and 900 mV, respectively. Also, the increased applied potentials shifted the peak intensity towards the bottom of the crevice. This may arise from the potential drop characteristics of respective specimens. The potential required to facilitate the hydrogen reduction shifted towards the bottom with the increasing applied potential. Each testing condition showed an inhomogeneous distribution of silver deposits from the crevice mouth towards the bottom. Regions adjacent to the crevice mouth showed very little silver deposits, irrespective of the applied potential. The intermediate portion yielded more silver deposits. From the crevice morphology, the peak intensity regions corresponded to the cathodic region. However, the crevice bottom showed a lower intensity though the segment represents the same cathodic region. This indicated that the hydrogen adsorption and diffusion is more active at the intermediate portion than at the crevice mouth and the bottom. The potential inside the crevice drops with depth. This led to the increased hydrogen generation with the depth. From Fig. 2, it can be clearly seen that the hydrogen bub-
bles were produced at the bottom of the crevice. It might be expected that more silver deposits would accumulate at the crevice bottom. Although the hydrogen reduction reaction would be higher at the bottom portion, the absence of silver deposits in the corresponding area is attributed to the following reason. The higher magnitude of the hydrogen reduction reaction led to the re-combination of H and resulted in the formation of H2 bubbles. The bubbles produced inside the crevice could not escape from the metal surface due to its crevice geometry. The cluster of bubbles at the region restricted the solution contact with the metal surface for further hydrogen reduction reaction. During crevice corrosion, the specimen thickness varied across the crevice. The active region showed remarkable metal losses, which resulted in the significant reduction in their thickness in this region. For the 500 mV condition, this lowest thickness region is identified at 2–6 mm from the crevice mouth. However, the magnitude of silver deposits at this region is very low compared to the intermediate position, where moderate metal loss was observed. This indicated that metal thinning would not be a major cause for the higher magnitude of silver deposits. Saitoh et al. [13] reported that hydrogen charged on platinum, palladium and 304 stainless steel showed a clear boundary between the charged and uncharged regions upon silver deposition. This had been witnessed until 24 h of charging duration. However, the present study employed body-centered cubic phase iron, which has faster hydrogen diffusion than Pd, Pt (102 times) and 304 stainless steel (104 times) [15]. It is expected that the permeated hydrogen could widen its length across the crevice with the increase in test duration. The calibration test showed that the permeated hydrogen expanded its length up to 2 mm until 24 h, which resulted in the widening of the silver deposits at this region. Considering 2 mm of linear spatial resolution and the low magnitude of silver deposits near the crevice mouth, permeation of hydrogen would be very low at passive and adjacent active regions. It was reported that the pH of the active region is significantly decreased during the test [16,17]. The present results indicated that H+ produced on this region did not much influence the permeated hydrogen. Additionally, the permeated hydrogen identified in this study could be attributed mostly to the hydrogen reduction reaction. 4. Conclusion
Fig. 5. Distribution of silver deposits in the rear surface of the crevice at different applied potentials (BSE image; 100· magnification).
Crevice corrosion operated with the potential drop mechanism for iron exposed to acetate buffer resulted in hydrogen entry along with metal dissolution. The diffused hydrogen showed inhomogeneous distribution into the metal across the crevice region. Variation in the applied potential without altering the electrolyte and the crevice set up could modify the distribution profile remarkably. Increasing the applied potential from +500 mV to +900 mV resulted in the lower magnitude of hydrogen entry.
T. Sundararajan et al. / Scripta Materialia 53 (2005) 1219–1223
References [1] Uhlig HH, Revie RW. Corrosion and corrosion control. 3rd ed. New York, NY: John Wiley and Sons; 1985. [2] Fontana MG. Corrosion engineering. 3rd ed. New York, NY: McGraw–Hill Book Co; 1986. [3] Cho K, Pickering HW. J Electrochem Soc 1990;137:3313. [4] Cho K, Pickering HW. J Electrochem Soc 1991;38:L56. [5] Nystrom EA, Lee JB, Sagu¨e`s AA, Pickering HW. J Electrochem Soc 1994;141:358. [6] Abdulsalam MI, Pickering HW. Corros Sci 1998;41:351. [7] Lee JS, Reed ML, Kelly RG. J Electrochem Soc 2004;151:B423. [8] Abdullah AM, Shaw BA, Pickering HW. Crevice corrosion of aluminum 6XXX alloy. In: Shaw BA, Buchheit RG, Moran JP, editors. Proceedings of corrosion and corrosion prevention of low
[9] [10] [11] [12] [13] [14] [15] [16] [17]
1223
density metals and alloys (ECS Phoenix Meeting), PV 2000-23, Penington, NJ: The Electrochemical Society; 2001. p. 401. Akiyama E, Frankel GS. J Electrochem Soc 1999;146:4095. Sundararajan T, Akiyama E, Tsuzaki K. Electrochem Solid State Lett 2005;8:B30. Schober T, Dieker C. Metall Trans A 1983;14:2440. Yao J, Cahoon JR. Metall Trans A 1990;21:603. Saitoh H, Hishi T, Misawa T. Mater Trans JIM 1996;37:373. Koyama K, Itoh G, Kanno M. J Jpn Inst Met 1998;62:742. Metals Data Book Ed. Maruzen, 3rd ed., Tokyo, Japan Institute for Metals; 1993. p. 21. Wolfe RC, Weil KG, Shaw BA, Pickering HW. J Electrochem Soc 2005;152:B82. Wolfe RC, Weil KG, Pickering HW. J Phys Chem B 2004;108: 14298.