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Behaviour of crevice corrosion in iron
Corrosion Science 47 (2005) 1336–1351 www.elsevier.com/locate/corsci
Behaviour of crevice corrosion in iron Mohammed Ismail Abdulsalam
*
Chemical and Materials Engineering Department, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia Received 23 May 2003; accepted 17 August 2004 Available online 27 October 2004
Abstract Crevice corrosion was investigated in iron exposed to a strong-buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2), pH = 4.66. The current and the potential gradient within the crevice were measured at crevice depth (L) = 7.35, 8, 10, and 15 mm, for a crevice that was positioned facing the electrolyte in a downward position. A remarkable shift in potential (>1.2 V) in the active direction was measured inside the crevice cavity, when the potential at the outer surface was held at 800 mV(SCE). Experimentation showed that there is a critical depth value, above which little changes occur on the transition boundary between passive and active regions on the crevice wall, xpass, and below which xpass location shifts sharply towards the crevice bottom. Steeping of the potential gradient occurred with time indicating enhancement of crevice corrosion, which was seen by the gradual increase in the current. These findings were in close agreement with the IR voltage theory and related mathematical model predictions. Morphological examination showed an intergranular attack around the active/passive boundary (xpass) on the crevice wall. 2004 Elsevier Ltd. All rights reserved. Keywords: IR voltage theory; Iron; Crevice corrosion
*
Tel.: +966 5568 2242; fax: +966 2695 1754. E-mail address: [email protected] (M.I. Abdulsalam).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.08.001
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1. Introduction Crevice corrosion is a dangerous form of localized corrosion, which occurs as a result of the occluded cell that forms under a crevice on the metal surface. Wellknown examples include flanges, gaskets, disbonded linings/coatings, fasteners, lap joints, weld zones and surface deposits. Systems relying on passive surface films for corrosion resistance can be particularly vulnerable to this form of corrosion. In these systems, which display active/passive transition in a corrosive environment, crevice corrosion can occur in the absence of pH change or chloride ion build-up inside crevices. Examples of these were reported in iron [1,2], and nickel [3,4]. In these cases Pickering and co-workers showed that crevice corrosion is caused by the IR voltage drop which placed the local electrode potential existing on the crevice wall in the active peak region of the polarization curve. In addition, IR voltage drop mechanism has been shown to operate with other metals including; steel [5], and aluminium [6]. Another proposed theory to explain the onset of crevice corrosion addresses the change in the chemical composition of the electrolyte and the formation of a critical crevice solution with concentrations of H+ and Cl that are large enough to breakdown the passive film [7]. Separation between the anodic and the cathodic reactions is necessary for the occurrence of crevice corrosion by the IR drop mechanism [8]. This condition prevails naturally for an open circuit experiment when an oxidant is added to the bulk solution where the potential at the outer surface (Esurf) is suddenly shifted from its open circuit condition in the active region into the passive region. Additionally, due to the occluded nature of the crevice geometry, the separation can still occur when the crevice solution becomes depleted of oxygen and other passivating oxidants originally present in the bulk solution. Alternatively, in laboratory controlled experiments this condition is achieved by a potentiostat. The potentiostatic control offers the advantage of a more quantitative analysis. Another practical significance of this experimental set up is in anodic protection industries. Under the same logic, it was reported that applied potential is unable to protect the entire structure due to the local electrode potential deep inside the crevice shifting to the active peak of the polarization curve [8,9]. Under IR drop mechanism controlled crevice corrosion, metal dissolves inside the crevice and the anodic current flows through the crevice electrolyte to the outer surface where the oxidant is reduced. The resulting IR voltage translates into an electrode potential on the crevice wall, E(x), that shifts in the less noble direction with increasing distance, x, into the crevice [10,11]. Recently, this concept was formalized [12,13], the results being in accordance with an earlier model for cathodic polarization of a crevice [14]. Walton et al. [15] developed a reactive transport based theoretical model and showed a good prediction to the measured potential distribution for crevice corrosion systems operating by the IR drop mechanism. It follows that the corrosion rate on the wall of the crevice is strongly position dependent as a result of the steep potential gradient in the depth direction of the crevice [16–18]. Therefore, it is important to study the potential distribution inside the crevice and its relation to the polarization curve in order to obtain a better understanding of the mechanism by which crevice corrosion occurs.
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Experimental studies on the IR drop mechanism of crevice corrosion showed a transition from passive to active dissolution on the crevice wall as results of the crevice corrosion process [1–4,10,12,19–21]. This transition boundary appeared at a certain distance into the crevice, xpass, which is located at the Epass value on the crevice wall. The appearance of xpass on the crevice wall indicates that the IR voltage drop inside the crevice is enough to shift the potential at the bottom of the crevice in the active region of the polarization curve, thereby creating active crevice corrosion. In accordance with the IR voltage theory, Epass is located in the active/passive transition region of the polarization curve. The transition boundary was seen to be a straight horizontal line when the resistance of the electrolyte inside the crevice is uniform throughout the crevice cavity. The location of xpass on the crevice wall is predicted by the relation [1,22]: Du ¼ Ex¼0 Epass ¼ IRxpass
ð1Þ
where Du* is the critical potential drop, Ex=0 is the passive applied potential at the crevice mouth, I is the current flowing out of the crevice, R = q/A, q is the electrolyte resistivity, and A is the cross-sectional area of the electrolyte column in the crevice. Analysis of the data is straightforward when the polarization curve for the crevice solution does not change during the experiment. The latter can be approached by using relatively open crevices with the ‘‘upside down’’ orientation with the outer surface facing downward in the solution (Fig. 1). It was shown that this crevice set-up keeps the pH value inside the crevice nearly the same as for the bulk solution, whereas it increased by a factor of four for the right side up orientation for which convective mixing did not occur [3]. Hence, the upside down orientation helps hold the pH constant due to the convective mixing of the crevice solution with the bulk solution [3,4]. The more dense corrosion products can easily move downward out of the crevice cavity in the direction of gravity, effectively maintaining a dilute ion concentration and the bulk solution pH. A similar finding of effective mixing was reported in an artificial crack [23]. Most available studies on the IR drop mechanism of crevice-corrosion address the effects of the oxidation power, gap-opening dimension, electrolyte composition and temperature, while very few discuss the effects of the crevice depth. This paper describes the characteristics of crevice corrosion of iron in an acetate buffered solution (constant pH) at room temperature, and addresses the role of the crevice depth. In order to keep the composition of the electrolyte from changing an artificial crevice with an ‘‘upside down’’ orientation was used instead of the ‘‘upside up’’ orientation reported previously [2,5,18,24]. The experiments were performed at different crevice depths, using an electrochemical microprobe technique to measure the potential distribution inside the crevice. Commercially pure iron known as Carpenter Electric Iron which has low-carbon content was used. Electric Iron is known for having good direct current soft magnetic properties after heat treatment. It has been used in electromechanical relays, solenoids, magnetic pole and other flux-carrying components.
M.I. Abdulsalam / Corrosion Science 47 (2005) 1336–1351
Luggin microprobe
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Teflon
w
Crevice Cavity
Plexiglas x
Crevice wall
on
Ir
L
SS Screw a w Crevice mouth L
Outer surface
Fig. 1. Schematic diagram of the electrode assembly used in crevice-corrosion experiments.
2. Experimental The material used in this work was Carpenter Electrical iron; a low carbon commercially pure iron of a composition (wt%): C:0.012, Mn:0.10, Si:0.11, P:0.006, S:0.009, Cr:0.14, Ni:0.04, Mo:0.02, Cu:0.03, V:0.07, Fe:bal. The heat treatment condition was annealing at 843 C for 1 h in dry hydrogen and cooled at 65.5 C per hour. Rectangular specimens were cut to the size of 20 · 15 · 5 mm. This size fits with the size of the groove made on the Teflon block part of the electrode. The experiments were performed in a strong-buffered acetate solution Fe/HAc–NaAc (0.5 M CH3COOH + 0.5 M NaC2H3O2). It was prepared with reagent grade sodium acetate (NaC2H3O2 Æ 3H2O), acetic acid (CH3COOH) and double distilled water. The measured pH was 4.66, while the conductivity was: j = 0.03 S cm1, at 24 C. The experiments were carried out at room temperature. The crevice experimental set-up and procedure were similar as described previously [4,5,20]. The exposed Fe crevice wall (depth: L = 1 cm; width: w = 0.5 cm) and outer (0.5 cm · 2 cm) surface were polished to 0.05 lm A12O3. In the electrode preparation process, all other surfaces in contact with the Teflon mount and edges of the specimen were sealed with a resin to prevent crevice corrosion between these materials and the specimen. Plexiglas formed the other walls of the crevice which had a gap (opening) dimension, a = 0.3 mm. A schematic sketch of the electrode is shown in Fig. 1. The electrical connection was made with an insulated copper wire soldered
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to the backside of the sample. The sample was positioned so that the crevice mouth faced downward in the electrolyte (upside down orientation). This orientation allows corrosion products to come out of the crevice by the gravity effect [3,4]. The use of a strong buffered solution also helped to minimize electrolyte compositional changes inside the crevice, such as acidification, due to the accumulation of corrosion products. In the previous work on crevice corrosion for Ni in 1 N H2SO4 it was shown that gravity was sufficient to remove corrosion products out of the crevice [4]. In that very low pH system the chemical changes if they occurred, would have been deacidification rather than acidification, and this is a further step to retard acidification [3]. In the present system, which is susceptible to acidification, a buffered solution will provide the assurance against acidification. In the experimental setup the solution level inside the crevice was maintained below the top of the Teflon block but above the Fe sample. Experiments were conducted by keeping the outer surface (bottom area of the iron specimen shown in Fig. 1) potential, Esurf, at 800 mV(SCE) in the passive region of the polarization curve for this system. This was done by using a potentiostat. The potential distribution E(x) on the crevice wall was measured using a fine glass microprobe of 0.03 mm outside diameter and a saturated calomel electrode (SCE) via Luggin probe. The potential measurements were performed by using a voltmeter that had high input impedance in order to avoid creating potential drop effect in the very thin microprobe. The xpass location and Epass value were measured in situ using the microprobe connected to a three-directional micromanipulator with the aid of a macro lens viewer. The morphological characteristics of the crevice wall for some selected specimens were subsequently examined under both an optical microscopy system and a scanning electron microscopy (SEM). This was done after the experiment, where images of the characteristic features of the corroded-surface profile were recorded. The anodic polarization behaviour of the flat iron specimen without a crevice was investigated in a 0.5 M CH3COOH + 0.5 M NaC2H3O2 buffer solution. Prior to the polarization the specimen was mounted in epoxy resin, metallographically polished to mirror-like surface and soldered to a copper wire for the electrical connection. Slow potentiodynamic DC polarization scans were run by using a potentiostat and a three electrode flat specimen cell (Model K0235, EG&G, Princeton). The polished rectangular specimen surface (2 cm2) exposed to the solution was positioned side ways in the bulk solution. Two graphite electrodes were used as counter electrodes. An electrolyte volume of 1000 ml was used, and a saturated calomel electrode in conjunction with a Luggin probe was used as a reference electrode. For the purpose of investigating the effect of the scan direction on the polarization behaviour two scans of opposite direction were conducted in a deaerated solution at a scan rate of 0.15 mV/s. One, was done in the passive-to-active direction, from 1.2 to 0.7 V(SCE). The other scan was done on another freshly prepared specimen in the reverse active-to-passive direction.
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3. Results and discussion 3.1. Anodic polarization behaviour in a buffered acetate solution Fig. 2, shows the anodic polarization curves for iron in a deaerated buffered acetate solution (Fe/NaAc–HAc system) for two scans of opposite direction. Both polarization curves exhibit the active/passive transition with a large active peak. The open circuit potential (Eoc) was 630 mV(SCE) for both scans. The Epass value: 165 mV(SCE), was measured inside the crevice by placing the tip of the microprobe at the visible boundary between the passive and active regions on the crevice wall. This value is within the passive-to-active transition region of the bulk solution polarization curve (Fig. 2). The current at 165 mV(SCE) in Fig. 2 is approximately one tenth of the peak current and noticeably larger than the passive current consistent with easy visible observation of this boundary on the crevice wall. This agreement of the measured Epass and Epass of the bulk solution polarization curve (in Fig. 2) indicates that the composition of the crevice solution had not deviated significantly from that of the bulk solution. The measured value for Epass inside the crevice is in agreement to others reported in the literature for this system [2]. In the passive region, the current was lower for a scan in the passive to active direction (reverse scan); while the two scans almost coincide in the active region. A similar finding was reported for duplex stainless steel in aerated acidic/chloride solution [21]. However, this is a different finding than was seen in nickel/1 N H2SO4 system, where a difference of 92 mV in the value of Epass was observed between the two scans (at a given current density) [4]. One possible interpretation
100 -2
i peak (-283 mV, 18.43 mA cm )
-2
Current density, i , (mA cm )
10
0.01
E pass = -165 mV(SCE)
0.1
E * = -460 mV(SCE)
1
Forward Scan
0.001 Reverse Scan
0.0001 -1000
-500
0
500
1000
1500
Potential, E , (mV,SCE)
Fig. 2. Potentiodynamic polarization curves for iron in deaerated buffered acetate solution (0.5 M CH3COOH + 0.5M NaC2H3O2) for scans in the active-to-passive and passive-to-active direction. The scan rate was 0.15 mV/s. Area: 1.89 cm2.
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for this is that the characteristic of the oxide film forming on the surface of the iron in NaAc–HAc solution is different from the one forming on nickel in 1 N H2SO4. In nickel a relatively stable appearing (dense gold-coloured) film was observed, while the film on the iron was weak and easily dissolved from the surface, leaving a rough surface. In the works reported by Abdulsalam and Pickering on crevice corrosion in nickel, it was shown that Epass depends on the direction of the scan [4,20]. The value for Epass obtained through the reverse scan was found to be more in agreement with the experimentally measured value inside the crevice cavity [4,10]. However, for the crevice corrosion system in this paper there is little effect of the scan direction on the value of Epass. 3.2. Crevice corrosion characteristics With the selected crevice parameters and conditions, crevice corrosion commenced at the start of the experiment and continued throughout the test period. The potential at the crevice bottom (EL) jumped from approximately 622 mV(SCE), a value around Eoc, to more positive potentials as the crevice depth decreased. The change in surface appearance started to occur within a few seconds after the potentiostat was turned on, mA currents were measured immediately. This indicates that the condition L > Lc, where Lc is the critical crevice depth was met. Lc is the smallest crevice depth that results in crevice corrosion for the test conditions, and is defined as the distance x at which the active–passive transition coincides with the crevice depth, L, for the given metal/electrolyte system [12,25]. Also, since a large active peak exists on the anodic polarization curve, no induction period was necessary and therefore crevice corrosion started immediately [1,17,26]. This implies that the IR voltage drop was enough at the start of the experiment for the applied potential at the outer surface to shift part of the crevice wall at L into the active peak of the bulk solution polarization curve. At the onset of crevice corrosion, the upper mirror-like part of the crevice wall lost its lustre and became attacked. Fig. 3 shows photographs of the on-going experiment of crevice corrosion of iron in the buffered acetate solution (L = 7.35, 8 and 15 mm), taken at dissimilar times throughout the experiment. The glass microprobe appears inside the cavity. The morphology of the surface under the action of crevice corrosion as viewed through the clear Plexiglas can be divided into three main regions. The first (lowest) region is the passive region (un-attacked) that extends from the crevice mouth to the xpass boundary. In this passive region, a small band of light etching was observed at x 6 xpass that formed within the initial seconds of crevice corrosion. The second region starts at x > xpass is the severely attacked region that extends to xlim. The location of xlim cannot be determined visually, since it is not associated with an obvious change in the surface morphology, but it can be estimated from the potential profile, E(x), as the distance at which E becomes independent of x, (Fig. 4). The third region is the etched region extending from xlim to the crevice bottom (x = L), over which the potential is nearly constant. Similar crevice wall morphology to these three regions was reported for crevice corrosion in nickel [4,20], and duplex stainless steel [21].
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Fig. 3. In situ photographs of the corroding iron crevice wall (upside down orientation) in buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2), showing the location of xpass and the distinctive regions that appeared during crevice corrosion. (a) 40 min, (b) 15 min, and (c) 7 min. Magnification 5.5·.
800
E applied = 800 mV (SCE) L = 8.0 mm
400
200 15 min 3.5 hr
0
12 hr
E pass = -165 mV(SCE)
-200
Direction of motion of x pass with time
x pass
-400
x pass x pass
Potential, E, (mV,SCE)
600
-600 -1
0
1
2
3
4
5
6
7
8
9
10
Distance into crevice, x, (mm)
Fig. 4. Variation of the potential distribution E(x) with time inside the crevice for iron in buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2), when L = 8.0 mm.
Epass = 165 mV(SCE) was measured with the microprobe tip at xpass and was found to be a constant value independent of time or L. This finding indicates that the convective mixing of the bulk and crevice solution is maintaining the initial (bulk solution) anodic polarization curve, and that the electrolyte composition inside the crevice is not changing [3,4,20]. This is in agreement with the literature where it
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was shown that Epass changes by approximately 150 mV for the Ni/1 N H2SO4, as the bulk solution pH changes from 0.3 to 2 when the bulk solution was saturated in Ni2+ ion [3]. In agreement with the observation reported in earlier works, xpass moved with time towards the crevice mouth [4,10,21]. Fig. 4 shows this behaviour through the E(x) distribution measured using the glass microprobe at different times for L = 8 mm. The location of xpass is indicated by the arrow at Epass, and listed in Table 1. These values are in close agreement with the measured ones physically at the xpass boundary. The advancement of xpass towards the crevice mouth with time is consistent with an increasing current. The measured current increased from the onset of applying Esurf from 1.26 mA initially to 2.23 mA at t = 12 h. Similar behaviour for other systems was reported in the literature [2–4]. It is worth mentioning here that while the active region on the crevice wall is located in the region between Epass and E* the most contributing part to the current is located only at a distance that is slightly greater than xpass. At this location, the peak current density will exist in agreement with the shape of the active peak of the polarization curve (Fig. 2). This is in agreement with the potential gradient showing a steep gradient in the region where the peak current is expected to exist. With time, the gradient at this region becomes steeper (Fig. 4), thereby, the current also increases with time. This was discussed in more detail and verified both experimentally and theoretically in earlier works [20]. This change on E(x) with time was less pronounced when L = 15 mm. In addition, with time Elim became more negative (Fig. 4). It is interesting to note that while in this work for the crevice with L = 10 mm the current was measured as 1.2 mA after 40 min, in other work this corresponds to 1.34 mA for a crevice in the right-side up orientation [2]. This is expected since in the right side up orientation, acidification is possibly occurring which increases the size of the active peak in the polarization curve, thereby, increasing the crevice corrosion current [4,24]. In addition EL was about 70 mV more negative with the rightside up orientation. This is explained by the increase of the electrolyte resistance that is allowed with this crevice orientation. The effect of L on the initial xpass value (at t P 0), measured directly by physically placing the tip of the microprobe in situ at the xpass boundary seen through the Plexiglas with the help of a macro lens viewer, is shown in Table 2, for L = 7.35, 8, 10 and 15 mm, being 4.5, 3.3, 3.5 and 3.7 mm, respectively. These measurements are in close agreements with the values obtained using the initially measured E(x) distributions
Table 1 Time dependency of xpass and Elim inside the crevice for iron in the buffered solution, L = 8.0 mm Time, h
xpassa (±0.1), mm
xpassb (±0.1), mm
Elima (±1), mV(SCE)
0.25 3.5 12.0
3.3 2.6 2.4
3.4 2.7 2.4
471 505 514
a b
Measured inside the crevice by using microprobe. Estimated from the potential profile at Epass (Fig. 4).
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Table 2 Depth dependency of the initially measured (within first 15 min): xpass, Elim, and I for a crevice in iron exposed to acetate buffered solution Depth (L), mm L<8 7.35 L>8 8.0 10.0 15.0 a
xpassa(±0.1), mm
Elima(±1), mV(SCE)
I, mA
4.5
418
1.4
3.3 3.5 3.7
471 528 600
1.26 1.2 1.05
Measured inside the crevice by using microprobe.
shown in Fig. 5, at the intersection of E(x) curve and Epass line. When L P 8 mm (more specifically in the range from L = 8 to 15 mm) the location of xpass at t P 0 was further into the crevice as the crevice depth, L, was increased. However, the amount of change of xpass was very moderate (0.4 mm for DL = 7 mm), in accordance with the moderately decreasing initial current, I, with L, Table 2. These results are consistent with the predication of the IR voltage drop theory, shown in Fig. 6 by the good agreement between the experimental value for xpass and the calculated one according to Eq. (1). On the other hand, when L < 8 mm (7.35 mm), xpass shifted in the opposite direction towards the crevice bottom for decreasing L, as shown in Fig. 5 and Table 2. The amount of change in xpass here is much more pronounced (1.2 mm for DL = 0.65 mm). Reported data for Lc is included in Fig. 6 and it is shown to follow
950
750
E applied = 800 mV (SCE)
Potential, E, (mV,SCE)
550 L = 8.0 mm
350
L = 15.0 mm
150 L = 7.35 mm
-50 E pass E @ i peak, (Fig. 2)
-250 x pass @ L = 10 mm
*
E , (Fig. 2)
-450
E L @ L = 10 mm
-650 -1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Distance into crevice, x, (mm)
Fig. 5. Variation of the initial (within 15 min) measured potential distributions with depth inside the crevice for iron in buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2).
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E applied = 800 mV (SCE)
10
L c = 6.7 (reference 12)
x pass, (mm)
8 No crevice corrosion
detected
6
4
2
Experimental data Calculated using Eq. (1)
0 2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Crevice depth, L , (mm) Fig. 6. The effect of the crevice depth (L) on the location of the transition boundary (xpass). Shown also are the expected xpass values according to theoretical prediction based on IR voltage theory.
the same trend [12]. In this range it is clear that xpass deviates from the linear relationship predicated by Eq. (1) as shown in Fig. 6. This scenario observed experimentally is in agreement with theoretical analysis of the mechanism for a similar localized corrosion problem, formulated by using the electric field within the film on the surface for different L values [25]. The model computation results showed that for a given crevice system there is a certain depth value, above which xpass varies little, and below which xpass increases substantially to Lc. Fig. 6 is the experimental plot that agrees with the model result in reference [25]. The initial measured current (within the first 15 min) at different L are listed in Table 2. Little changes occurred on the current until L was decreased beyond 8 mm. These current values can be shown to be consistent with the initial potential distributions shown in Fig. 5, where the amount of the current is proportional to the range of the peak current region of the polarization curve that operates in the severely attacked region of the crevice wall. Therefore, the spread of this region can possibly favour the likelihood of the increase of the crevice-wall area that is exposed to potentials of the ipeak region, thereby, increasing the total dissolution (crevice corrosion) current, I. Similar current behaviour was reported for the effect of a on the measured current in nickel/sulphuric acid crevice system [20]. The potential behaviour at x = L, EL, is also shown in Fig. 5, and listed in Table 2. The larger L is the more active EL, which is in agreement with theoretical model predictions [25]. Similar findings were also reported for the change of EL with the applied potential at the surface [4], and the crevice opening dimension [20]. Another observation shown in Fig. 5 is that for the potential profiles for L = 8 and 15 there is a clear existence of xlim, but at L = 7.35 this is not evident. In principle, the most
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negative potential that can be measured at EL is the open circuit potential (Eoc) which is 630 mV(SCE) for the current system [1,4]. However it is usually observed that a Elim is more positive than Eoc [3,10,20]. It is interesting to note that based on the current findings (Fig. 5), although Elim appears to be constant with distance, it is actually still decreasing with increasing crevice depth, L. This is shown in Fig. 5 where at L = 7.35 mm, EL = Elim = 418 mV(SCE) which gradually decreased to 600 mV(SCE) when L increased to 15 mm. Thus Elim is expected to approach Eoc for increasing L. Recently, Vankeerberghen et al. [12] developed a mathematical model for the potential drop into the crevice based on a Poisson-type field problem with non-linear boundary conditions that was described as a one-dimensional finite difference framework. The Poisson-type, second order differential equation used in the analysis is given by d2 UðxÞ P iðxÞ ¼ 2 dx rS
ð2Þ
where r is the conductivity of the solution, S is the cross-sectional area, P is the electrochemically active part of the perimeter, and U(x) is the potential in solution. The application of this model to a crevice corrosion system similar to the one used in this work showed for L = 10 mm, xpass = 3.2 mm. This value for xpass is in reasonable agreement with the experimentally determined one here, Table 2. 3.3. Morphology of xpass boundary on the crevice wall After and during the experiment, examination of the crevice wall revealed more penetration and attack in the region just further than xpass inside the crevice. Similar observation was reported on the crevice wall for other crevice corrosion systems operating by the IR mechanism [1,2,4,10,12,20–22,24]. Changes in the appearance of the crevice wall right at xpass were further examined under conventional optical microscopy and were photographed. Fig. 7 shows a photograph at 100· of the morphology of the xpass boundary after the experiment, which lasted 40 min. The general feature is that an intergranular attack becomes more severe when moving in a direction away from the crevice mouth. The intergranular attack is usually caused by a composition difference at the grain boundary and prior heat treatment condition. Thus, for another iron or heat treatment no intergranular attack may be seen. Fig. 8 shows scanning electron microscope (SEM) micrographs of the main features observed on the crevice-corrosion wall that included; xpass, severely attacked, and the etched (bottom) regions. At xpass, the attack is of intergranular nature; similar to that shown in Fig. 7 but at higher magnification with some scattered micropits are observed on the attacked grains. In this region and at xpass boundary the measured electrochemical potential is 162 mV(SCE). From the polarization curve shown in Fig. 2, this potential is in the transition (active/passive region), in agreements with the physical observations in this region. Conventionally this area is associated with
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Fig. 7. Post-experiment greyscale photograph of the crevice wall at xpass boundary showing the corrosion attack for a crevice in iron exposed to acetate buffered solution. Esurf = 800 mV(SCE), L = 0.85 cm and t = 40 min. Magnification: 100·.
stress corrosion cracking, which generally coincides with intergranular attack at grain boundaries. Therefore, the IR voltage drop can bring the crevice wall in a potential region where the intergranular attack associated with stress corrosion cracking can take place. In the severely attacked region Fig. 8(b), it appears as a dense mountainous region with threadlike appearance. This changes to a porous etched surface showing a few scattered simple crystallographic pits in the etched region located at the bottom of the crevice. Little penetration occurred in the etched region, Fig. 8(c), in agreement with the potential prevailing in this region being around Elim where the current is expected to be much lower than in the active severely attacked region.
4. Conclusions • Crevice corrosion occurred immediately without an induction time for a crevice in iron immersed in a strong-buffered acetate solution, under a condition where the crevice corrosion products were allowed to leave the crevice cavity by choosing a crevice in the upside down orientation. However, among other reported factors, the crevice geometry is important in determining the onset of crevice corrosion, where for the current crevice corrosion system, it is expected that no crevice corrosion will occur when L < 6.7 mm. • For a given crevice system there is a critical depth value, above which xpass varies little, and below which xpass increases substantially towards the crevice bottom. This continues until the depth L < Lc, where crevice corrosion will not occur immediately. Then, even if an induction period was allowed, crevice corrosion will be in question for the current crevice geometry that does not allow corrosion products to accumulate inside the crevice.
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Fig. 8. Post-experiment SEM micrographs of different regions on the crevice wall, inside a crevice in iron exposed to acetate buffered solution. Micrographs were taken after a crevice corrosion experiment with parameters: Esurf = 800 mV(SCE), L = 0.8 cm and t = 12 h.
• The active/passive boundary, xpass, on the crevice wall moved towards the crevice mouth (x = 0) for increasing time and increasing current flowing out of the crevice, in accordance with IR voltage theory and in agreement with other reported findings in the literature. The potential at xpass was found to be constant irrespective of time or crevice depth.
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• Potential distribution measurements inside the crevice, using a fine glass microprobe, showed a large potential drop inside the cavity. A large potential drop of about 1.4 V was measured at L = 15 mm. • The morphology of xpass boundary showed an intergranular attack that gets more severe when moving away from the crevice mouth. However, this is not conclusive for the current system as the intergranular attack is usually caused by a composition difference at the grain boundary and prior heat treatment condition.
Acknowledgments Acknowledgment is made to the Institute of Research & Consultation of King Abdulaziz University and to Saudi Basic Industries Corp. (SABIC) for their support of this research. Professor Howard W. Pickering, The Pennsylvania State University, provided helpful comments.
References [1] Howard W. Pickering, Corros. Sci. 29 (1989) 325; A. Valdes, H.W. Pickering, in Advances in Localized Corrosion, NACE-9, in: H. Isaacs, U. Bertacci, J. Kruger, Z. Smialowska (Eds.), Houston, TX, 1990, pp. 393–401. [2] K. Cho, M.I. Abdulsalam, H.W. Pickering, J. Electrochem. Soc. 145 (1998) 1862. [3] M. Wang, H.W. Pickering, Y. Xu, J. Electrochem. Soc. 142 (1995) 2986. [4] M.I. Abdulsalam, H.W. Pickering, Corros. Sci. 41 (1999) 351. [5] E.A. Nystrom, J.B. Lee, A.A. Sagu¨e`s, H.W. Pickering, J. Electrochem. Soc. 141 (1994) 358. [6] A.M. Abdullah, B.A. Shaw, H.W. Pickering, Crevice corrosion of aluminum 6XXX alloy, in: B.A. Shaw, R.G. Buchheit, J.P. Moran (Eds.), Proc. of Corrosion and Corrosion Prevention of Low Density Metals and Alloys, PV 2000-23, (ECS Phoenix Meeting), The Electrochemical Society, Penington, NJ, 2001, pp. 401–410. [7] J.W. Oldfield, W.H. Sutton, Br. Corros. J. 13 (1978) 13. [8] H.W. Pickering, J. Electrochem. Soc. 150 (2003) K1. [9] W.D. France, N.D. Greene, Corrosion (Houston) 24 (1968) 247. [10] M.I. Abdulsalam, Corrosion 58 (2002) 364. [11] M.Z. Yang, M. Wilmott, J.L. Luo, Thin Solid Films 326 (1998) 180. [12] M. Vankeerberghen, M.I. Abdulsalam, H.W. Pickering, J. Deconinck, J. Electrochem. Soc. 150 (9) (2003) B445. [13] Y. Xu, H.W. Pickering, J. Electrochem. Soc. 140 (1993) 658. [14] B.G. Ateya, H.W. Pickering, J. Electrochem. Soc. 122 (1975) 1018. [15] J.C. Walton, G. Cragnolino, S.K. Kalandros, Corrosion Sci. 38 (1996) 1. [16] M.K. Sawford, B.G. Ateya, A.M. Abdullah, H.W. Pickering, J. Electrochem. Soc. 149 (2002) B198. [17] B.A. Shaw, P.J. Moran, P.O. Gartland, Corros. Sci. 32 (1991) 707. [18] A. Valdes, H.W. Pickering, in Advances in Localized Corrosion, NACE-9, in: H. Isaacs, U. Bertacci, J. Kruger, Z. Smialowska (Eds.), Houston, TX, 1990, pp. 393–401. [19] H.W. Pickering, Corrosion 42 (1986) 125. [20] M.I. Abdulsalam, H.W. Pickering, J. Electrochem. Soc. 145 (1998) 2276. [21] J. Al-Khamis, H.W. Pickering, J. Electrochem. Soc. 148 (2001) B314. [22] B. DeForce, H. Pickering, J. Met., ‘‘JOM’’ 47 (1995) 22. [23] P.D. Peterson, A.P. Reynolds, M.A. Sutton, Corrosion 57 (2001) 693.
M.I. Abdulsalam / Corrosion Science 47 (2005) 1336–1351 [24] K. Cho, H.W. Pickering, J. Electrochem. Soc. 138 (1991) L56. [25] Y. Xu, M. Wang, H.W. Pickering, J. Electrochem. Soc. 140 (1993) 3448. [26] R.S. Lillard, M.P. Jurinski, J.R. Scully, Corrosion 50 (1994) 251; R.S. Lillard, J.R. Scully, J. Electrochem. Soc. 141 (1994) 3006.
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Behaviour of crevice corrosion in iron Mohammed Ismail Abdulsalam
*
Chemical and Materials Engineering Department, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia Received 23 May 2003; accepted 17 August 2004 Available online 27 October 2004
Abstract Crevice corrosion was investigated in iron exposed to a strong-buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2), pH = 4.66. The current and the potential gradient within the crevice were measured at crevice depth (L) = 7.35, 8, 10, and 15 mm, for a crevice that was positioned facing the electrolyte in a downward position. A remarkable shift in potential (>1.2 V) in the active direction was measured inside the crevice cavity, when the potential at the outer surface was held at 800 mV(SCE). Experimentation showed that there is a critical depth value, above which little changes occur on the transition boundary between passive and active regions on the crevice wall, xpass, and below which xpass location shifts sharply towards the crevice bottom. Steeping of the potential gradient occurred with time indicating enhancement of crevice corrosion, which was seen by the gradual increase in the current. These findings were in close agreement with the IR voltage theory and related mathematical model predictions. Morphological examination showed an intergranular attack around the active/passive boundary (xpass) on the crevice wall. 2004 Elsevier Ltd. All rights reserved. Keywords: IR voltage theory; Iron; Crevice corrosion
*
Tel.: +966 5568 2242; fax: +966 2695 1754. E-mail address: [email protected] (M.I. Abdulsalam).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.08.001
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1. Introduction Crevice corrosion is a dangerous form of localized corrosion, which occurs as a result of the occluded cell that forms under a crevice on the metal surface. Wellknown examples include flanges, gaskets, disbonded linings/coatings, fasteners, lap joints, weld zones and surface deposits. Systems relying on passive surface films for corrosion resistance can be particularly vulnerable to this form of corrosion. In these systems, which display active/passive transition in a corrosive environment, crevice corrosion can occur in the absence of pH change or chloride ion build-up inside crevices. Examples of these were reported in iron [1,2], and nickel [3,4]. In these cases Pickering and co-workers showed that crevice corrosion is caused by the IR voltage drop which placed the local electrode potential existing on the crevice wall in the active peak region of the polarization curve. In addition, IR voltage drop mechanism has been shown to operate with other metals including; steel [5], and aluminium [6]. Another proposed theory to explain the onset of crevice corrosion addresses the change in the chemical composition of the electrolyte and the formation of a critical crevice solution with concentrations of H+ and Cl that are large enough to breakdown the passive film [7]. Separation between the anodic and the cathodic reactions is necessary for the occurrence of crevice corrosion by the IR drop mechanism [8]. This condition prevails naturally for an open circuit experiment when an oxidant is added to the bulk solution where the potential at the outer surface (Esurf) is suddenly shifted from its open circuit condition in the active region into the passive region. Additionally, due to the occluded nature of the crevice geometry, the separation can still occur when the crevice solution becomes depleted of oxygen and other passivating oxidants originally present in the bulk solution. Alternatively, in laboratory controlled experiments this condition is achieved by a potentiostat. The potentiostatic control offers the advantage of a more quantitative analysis. Another practical significance of this experimental set up is in anodic protection industries. Under the same logic, it was reported that applied potential is unable to protect the entire structure due to the local electrode potential deep inside the crevice shifting to the active peak of the polarization curve [8,9]. Under IR drop mechanism controlled crevice corrosion, metal dissolves inside the crevice and the anodic current flows through the crevice electrolyte to the outer surface where the oxidant is reduced. The resulting IR voltage translates into an electrode potential on the crevice wall, E(x), that shifts in the less noble direction with increasing distance, x, into the crevice [10,11]. Recently, this concept was formalized [12,13], the results being in accordance with an earlier model for cathodic polarization of a crevice [14]. Walton et al. [15] developed a reactive transport based theoretical model and showed a good prediction to the measured potential distribution for crevice corrosion systems operating by the IR drop mechanism. It follows that the corrosion rate on the wall of the crevice is strongly position dependent as a result of the steep potential gradient in the depth direction of the crevice [16–18]. Therefore, it is important to study the potential distribution inside the crevice and its relation to the polarization curve in order to obtain a better understanding of the mechanism by which crevice corrosion occurs.
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Experimental studies on the IR drop mechanism of crevice corrosion showed a transition from passive to active dissolution on the crevice wall as results of the crevice corrosion process [1–4,10,12,19–21]. This transition boundary appeared at a certain distance into the crevice, xpass, which is located at the Epass value on the crevice wall. The appearance of xpass on the crevice wall indicates that the IR voltage drop inside the crevice is enough to shift the potential at the bottom of the crevice in the active region of the polarization curve, thereby creating active crevice corrosion. In accordance with the IR voltage theory, Epass is located in the active/passive transition region of the polarization curve. The transition boundary was seen to be a straight horizontal line when the resistance of the electrolyte inside the crevice is uniform throughout the crevice cavity. The location of xpass on the crevice wall is predicted by the relation [1,22]: Du ¼ Ex¼0 Epass ¼ IRxpass
ð1Þ
where Du* is the critical potential drop, Ex=0 is the passive applied potential at the crevice mouth, I is the current flowing out of the crevice, R = q/A, q is the electrolyte resistivity, and A is the cross-sectional area of the electrolyte column in the crevice. Analysis of the data is straightforward when the polarization curve for the crevice solution does not change during the experiment. The latter can be approached by using relatively open crevices with the ‘‘upside down’’ orientation with the outer surface facing downward in the solution (Fig. 1). It was shown that this crevice set-up keeps the pH value inside the crevice nearly the same as for the bulk solution, whereas it increased by a factor of four for the right side up orientation for which convective mixing did not occur [3]. Hence, the upside down orientation helps hold the pH constant due to the convective mixing of the crevice solution with the bulk solution [3,4]. The more dense corrosion products can easily move downward out of the crevice cavity in the direction of gravity, effectively maintaining a dilute ion concentration and the bulk solution pH. A similar finding of effective mixing was reported in an artificial crack [23]. Most available studies on the IR drop mechanism of crevice-corrosion address the effects of the oxidation power, gap-opening dimension, electrolyte composition and temperature, while very few discuss the effects of the crevice depth. This paper describes the characteristics of crevice corrosion of iron in an acetate buffered solution (constant pH) at room temperature, and addresses the role of the crevice depth. In order to keep the composition of the electrolyte from changing an artificial crevice with an ‘‘upside down’’ orientation was used instead of the ‘‘upside up’’ orientation reported previously [2,5,18,24]. The experiments were performed at different crevice depths, using an electrochemical microprobe technique to measure the potential distribution inside the crevice. Commercially pure iron known as Carpenter Electric Iron which has low-carbon content was used. Electric Iron is known for having good direct current soft magnetic properties after heat treatment. It has been used in electromechanical relays, solenoids, magnetic pole and other flux-carrying components.
M.I. Abdulsalam / Corrosion Science 47 (2005) 1336–1351
Luggin microprobe
1339
Teflon
w
Crevice Cavity
Plexiglas x
Crevice wall
on
Ir
L
SS Screw a w Crevice mouth L
Outer surface
Fig. 1. Schematic diagram of the electrode assembly used in crevice-corrosion experiments.
2. Experimental The material used in this work was Carpenter Electrical iron; a low carbon commercially pure iron of a composition (wt%): C:0.012, Mn:0.10, Si:0.11, P:0.006, S:0.009, Cr:0.14, Ni:0.04, Mo:0.02, Cu:0.03, V:0.07, Fe:bal. The heat treatment condition was annealing at 843 C for 1 h in dry hydrogen and cooled at 65.5 C per hour. Rectangular specimens were cut to the size of 20 · 15 · 5 mm. This size fits with the size of the groove made on the Teflon block part of the electrode. The experiments were performed in a strong-buffered acetate solution Fe/HAc–NaAc (0.5 M CH3COOH + 0.5 M NaC2H3O2). It was prepared with reagent grade sodium acetate (NaC2H3O2 Æ 3H2O), acetic acid (CH3COOH) and double distilled water. The measured pH was 4.66, while the conductivity was: j = 0.03 S cm1, at 24 C. The experiments were carried out at room temperature. The crevice experimental set-up and procedure were similar as described previously [4,5,20]. The exposed Fe crevice wall (depth: L = 1 cm; width: w = 0.5 cm) and outer (0.5 cm · 2 cm) surface were polished to 0.05 lm A12O3. In the electrode preparation process, all other surfaces in contact with the Teflon mount and edges of the specimen were sealed with a resin to prevent crevice corrosion between these materials and the specimen. Plexiglas formed the other walls of the crevice which had a gap (opening) dimension, a = 0.3 mm. A schematic sketch of the electrode is shown in Fig. 1. The electrical connection was made with an insulated copper wire soldered
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to the backside of the sample. The sample was positioned so that the crevice mouth faced downward in the electrolyte (upside down orientation). This orientation allows corrosion products to come out of the crevice by the gravity effect [3,4]. The use of a strong buffered solution also helped to minimize electrolyte compositional changes inside the crevice, such as acidification, due to the accumulation of corrosion products. In the previous work on crevice corrosion for Ni in 1 N H2SO4 it was shown that gravity was sufficient to remove corrosion products out of the crevice [4]. In that very low pH system the chemical changes if they occurred, would have been deacidification rather than acidification, and this is a further step to retard acidification [3]. In the present system, which is susceptible to acidification, a buffered solution will provide the assurance against acidification. In the experimental setup the solution level inside the crevice was maintained below the top of the Teflon block but above the Fe sample. Experiments were conducted by keeping the outer surface (bottom area of the iron specimen shown in Fig. 1) potential, Esurf, at 800 mV(SCE) in the passive region of the polarization curve for this system. This was done by using a potentiostat. The potential distribution E(x) on the crevice wall was measured using a fine glass microprobe of 0.03 mm outside diameter and a saturated calomel electrode (SCE) via Luggin probe. The potential measurements were performed by using a voltmeter that had high input impedance in order to avoid creating potential drop effect in the very thin microprobe. The xpass location and Epass value were measured in situ using the microprobe connected to a three-directional micromanipulator with the aid of a macro lens viewer. The morphological characteristics of the crevice wall for some selected specimens were subsequently examined under both an optical microscopy system and a scanning electron microscopy (SEM). This was done after the experiment, where images of the characteristic features of the corroded-surface profile were recorded. The anodic polarization behaviour of the flat iron specimen without a crevice was investigated in a 0.5 M CH3COOH + 0.5 M NaC2H3O2 buffer solution. Prior to the polarization the specimen was mounted in epoxy resin, metallographically polished to mirror-like surface and soldered to a copper wire for the electrical connection. Slow potentiodynamic DC polarization scans were run by using a potentiostat and a three electrode flat specimen cell (Model K0235, EG&G, Princeton). The polished rectangular specimen surface (2 cm2) exposed to the solution was positioned side ways in the bulk solution. Two graphite electrodes were used as counter electrodes. An electrolyte volume of 1000 ml was used, and a saturated calomel electrode in conjunction with a Luggin probe was used as a reference electrode. For the purpose of investigating the effect of the scan direction on the polarization behaviour two scans of opposite direction were conducted in a deaerated solution at a scan rate of 0.15 mV/s. One, was done in the passive-to-active direction, from 1.2 to 0.7 V(SCE). The other scan was done on another freshly prepared specimen in the reverse active-to-passive direction.
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3. Results and discussion 3.1. Anodic polarization behaviour in a buffered acetate solution Fig. 2, shows the anodic polarization curves for iron in a deaerated buffered acetate solution (Fe/NaAc–HAc system) for two scans of opposite direction. Both polarization curves exhibit the active/passive transition with a large active peak. The open circuit potential (Eoc) was 630 mV(SCE) for both scans. The Epass value: 165 mV(SCE), was measured inside the crevice by placing the tip of the microprobe at the visible boundary between the passive and active regions on the crevice wall. This value is within the passive-to-active transition region of the bulk solution polarization curve (Fig. 2). The current at 165 mV(SCE) in Fig. 2 is approximately one tenth of the peak current and noticeably larger than the passive current consistent with easy visible observation of this boundary on the crevice wall. This agreement of the measured Epass and Epass of the bulk solution polarization curve (in Fig. 2) indicates that the composition of the crevice solution had not deviated significantly from that of the bulk solution. The measured value for Epass inside the crevice is in agreement to others reported in the literature for this system [2]. In the passive region, the current was lower for a scan in the passive to active direction (reverse scan); while the two scans almost coincide in the active region. A similar finding was reported for duplex stainless steel in aerated acidic/chloride solution [21]. However, this is a different finding than was seen in nickel/1 N H2SO4 system, where a difference of 92 mV in the value of Epass was observed between the two scans (at a given current density) [4]. One possible interpretation
100 -2
i peak (-283 mV, 18.43 mA cm )
-2
Current density, i , (mA cm )
10
0.01
E pass = -165 mV(SCE)
0.1
E * = -460 mV(SCE)
1
Forward Scan
0.001 Reverse Scan
0.0001 -1000
-500
0
500
1000
1500
Potential, E , (mV,SCE)
Fig. 2. Potentiodynamic polarization curves for iron in deaerated buffered acetate solution (0.5 M CH3COOH + 0.5M NaC2H3O2) for scans in the active-to-passive and passive-to-active direction. The scan rate was 0.15 mV/s. Area: 1.89 cm2.
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for this is that the characteristic of the oxide film forming on the surface of the iron in NaAc–HAc solution is different from the one forming on nickel in 1 N H2SO4. In nickel a relatively stable appearing (dense gold-coloured) film was observed, while the film on the iron was weak and easily dissolved from the surface, leaving a rough surface. In the works reported by Abdulsalam and Pickering on crevice corrosion in nickel, it was shown that Epass depends on the direction of the scan [4,20]. The value for Epass obtained through the reverse scan was found to be more in agreement with the experimentally measured value inside the crevice cavity [4,10]. However, for the crevice corrosion system in this paper there is little effect of the scan direction on the value of Epass. 3.2. Crevice corrosion characteristics With the selected crevice parameters and conditions, crevice corrosion commenced at the start of the experiment and continued throughout the test period. The potential at the crevice bottom (EL) jumped from approximately 622 mV(SCE), a value around Eoc, to more positive potentials as the crevice depth decreased. The change in surface appearance started to occur within a few seconds after the potentiostat was turned on, mA currents were measured immediately. This indicates that the condition L > Lc, where Lc is the critical crevice depth was met. Lc is the smallest crevice depth that results in crevice corrosion for the test conditions, and is defined as the distance x at which the active–passive transition coincides with the crevice depth, L, for the given metal/electrolyte system [12,25]. Also, since a large active peak exists on the anodic polarization curve, no induction period was necessary and therefore crevice corrosion started immediately [1,17,26]. This implies that the IR voltage drop was enough at the start of the experiment for the applied potential at the outer surface to shift part of the crevice wall at L into the active peak of the bulk solution polarization curve. At the onset of crevice corrosion, the upper mirror-like part of the crevice wall lost its lustre and became attacked. Fig. 3 shows photographs of the on-going experiment of crevice corrosion of iron in the buffered acetate solution (L = 7.35, 8 and 15 mm), taken at dissimilar times throughout the experiment. The glass microprobe appears inside the cavity. The morphology of the surface under the action of crevice corrosion as viewed through the clear Plexiglas can be divided into three main regions. The first (lowest) region is the passive region (un-attacked) that extends from the crevice mouth to the xpass boundary. In this passive region, a small band of light etching was observed at x 6 xpass that formed within the initial seconds of crevice corrosion. The second region starts at x > xpass is the severely attacked region that extends to xlim. The location of xlim cannot be determined visually, since it is not associated with an obvious change in the surface morphology, but it can be estimated from the potential profile, E(x), as the distance at which E becomes independent of x, (Fig. 4). The third region is the etched region extending from xlim to the crevice bottom (x = L), over which the potential is nearly constant. Similar crevice wall morphology to these three regions was reported for crevice corrosion in nickel [4,20], and duplex stainless steel [21].
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Fig. 3. In situ photographs of the corroding iron crevice wall (upside down orientation) in buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2), showing the location of xpass and the distinctive regions that appeared during crevice corrosion. (a) 40 min, (b) 15 min, and (c) 7 min. Magnification 5.5·.
800
E applied = 800 mV (SCE) L = 8.0 mm
400
200 15 min 3.5 hr
0
12 hr
E pass = -165 mV(SCE)
-200
Direction of motion of x pass with time
x pass
-400
x pass x pass
Potential, E, (mV,SCE)
600
-600 -1
0
1
2
3
4
5
6
7
8
9
10
Distance into crevice, x, (mm)
Fig. 4. Variation of the potential distribution E(x) with time inside the crevice for iron in buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2), when L = 8.0 mm.
Epass = 165 mV(SCE) was measured with the microprobe tip at xpass and was found to be a constant value independent of time or L. This finding indicates that the convective mixing of the bulk and crevice solution is maintaining the initial (bulk solution) anodic polarization curve, and that the electrolyte composition inside the crevice is not changing [3,4,20]. This is in agreement with the literature where it
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was shown that Epass changes by approximately 150 mV for the Ni/1 N H2SO4, as the bulk solution pH changes from 0.3 to 2 when the bulk solution was saturated in Ni2+ ion [3]. In agreement with the observation reported in earlier works, xpass moved with time towards the crevice mouth [4,10,21]. Fig. 4 shows this behaviour through the E(x) distribution measured using the glass microprobe at different times for L = 8 mm. The location of xpass is indicated by the arrow at Epass, and listed in Table 1. These values are in close agreement with the measured ones physically at the xpass boundary. The advancement of xpass towards the crevice mouth with time is consistent with an increasing current. The measured current increased from the onset of applying Esurf from 1.26 mA initially to 2.23 mA at t = 12 h. Similar behaviour for other systems was reported in the literature [2–4]. It is worth mentioning here that while the active region on the crevice wall is located in the region between Epass and E* the most contributing part to the current is located only at a distance that is slightly greater than xpass. At this location, the peak current density will exist in agreement with the shape of the active peak of the polarization curve (Fig. 2). This is in agreement with the potential gradient showing a steep gradient in the region where the peak current is expected to exist. With time, the gradient at this region becomes steeper (Fig. 4), thereby, the current also increases with time. This was discussed in more detail and verified both experimentally and theoretically in earlier works [20]. This change on E(x) with time was less pronounced when L = 15 mm. In addition, with time Elim became more negative (Fig. 4). It is interesting to note that while in this work for the crevice with L = 10 mm the current was measured as 1.2 mA after 40 min, in other work this corresponds to 1.34 mA for a crevice in the right-side up orientation [2]. This is expected since in the right side up orientation, acidification is possibly occurring which increases the size of the active peak in the polarization curve, thereby, increasing the crevice corrosion current [4,24]. In addition EL was about 70 mV more negative with the rightside up orientation. This is explained by the increase of the electrolyte resistance that is allowed with this crevice orientation. The effect of L on the initial xpass value (at t P 0), measured directly by physically placing the tip of the microprobe in situ at the xpass boundary seen through the Plexiglas with the help of a macro lens viewer, is shown in Table 2, for L = 7.35, 8, 10 and 15 mm, being 4.5, 3.3, 3.5 and 3.7 mm, respectively. These measurements are in close agreements with the values obtained using the initially measured E(x) distributions
Table 1 Time dependency of xpass and Elim inside the crevice for iron in the buffered solution, L = 8.0 mm Time, h
xpassa (±0.1), mm
xpassb (±0.1), mm
Elima (±1), mV(SCE)
0.25 3.5 12.0
3.3 2.6 2.4
3.4 2.7 2.4
471 505 514
a b
Measured inside the crevice by using microprobe. Estimated from the potential profile at Epass (Fig. 4).
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Table 2 Depth dependency of the initially measured (within first 15 min): xpass, Elim, and I for a crevice in iron exposed to acetate buffered solution Depth (L), mm L<8 7.35 L>8 8.0 10.0 15.0 a
xpassa(±0.1), mm
Elima(±1), mV(SCE)
I, mA
4.5
418
1.4
3.3 3.5 3.7
471 528 600
1.26 1.2 1.05
Measured inside the crevice by using microprobe.
shown in Fig. 5, at the intersection of E(x) curve and Epass line. When L P 8 mm (more specifically in the range from L = 8 to 15 mm) the location of xpass at t P 0 was further into the crevice as the crevice depth, L, was increased. However, the amount of change of xpass was very moderate (0.4 mm for DL = 7 mm), in accordance with the moderately decreasing initial current, I, with L, Table 2. These results are consistent with the predication of the IR voltage drop theory, shown in Fig. 6 by the good agreement between the experimental value for xpass and the calculated one according to Eq. (1). On the other hand, when L < 8 mm (7.35 mm), xpass shifted in the opposite direction towards the crevice bottom for decreasing L, as shown in Fig. 5 and Table 2. The amount of change in xpass here is much more pronounced (1.2 mm for DL = 0.65 mm). Reported data for Lc is included in Fig. 6 and it is shown to follow
950
750
E applied = 800 mV (SCE)
Potential, E, (mV,SCE)
550 L = 8.0 mm
350
L = 15.0 mm
150 L = 7.35 mm
-50 E pass E @ i peak, (Fig. 2)
-250 x pass @ L = 10 mm
*
E , (Fig. 2)
-450
E L @ L = 10 mm
-650 -1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Distance into crevice, x, (mm)
Fig. 5. Variation of the initial (within 15 min) measured potential distributions with depth inside the crevice for iron in buffered acetate solution (0.5 M CH3COOH + 0.5 M NaC2H3O2).
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E applied = 800 mV (SCE)
10
L c = 6.7 (reference 12)
x pass, (mm)
8 No crevice corrosion
detected
6
4
2
Experimental data Calculated using Eq. (1)
0 2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Crevice depth, L , (mm) Fig. 6. The effect of the crevice depth (L) on the location of the transition boundary (xpass). Shown also are the expected xpass values according to theoretical prediction based on IR voltage theory.
the same trend [12]. In this range it is clear that xpass deviates from the linear relationship predicated by Eq. (1) as shown in Fig. 6. This scenario observed experimentally is in agreement with theoretical analysis of the mechanism for a similar localized corrosion problem, formulated by using the electric field within the film on the surface for different L values [25]. The model computation results showed that for a given crevice system there is a certain depth value, above which xpass varies little, and below which xpass increases substantially to Lc. Fig. 6 is the experimental plot that agrees with the model result in reference [25]. The initial measured current (within the first 15 min) at different L are listed in Table 2. Little changes occurred on the current until L was decreased beyond 8 mm. These current values can be shown to be consistent with the initial potential distributions shown in Fig. 5, where the amount of the current is proportional to the range of the peak current region of the polarization curve that operates in the severely attacked region of the crevice wall. Therefore, the spread of this region can possibly favour the likelihood of the increase of the crevice-wall area that is exposed to potentials of the ipeak region, thereby, increasing the total dissolution (crevice corrosion) current, I. Similar current behaviour was reported for the effect of a on the measured current in nickel/sulphuric acid crevice system [20]. The potential behaviour at x = L, EL, is also shown in Fig. 5, and listed in Table 2. The larger L is the more active EL, which is in agreement with theoretical model predictions [25]. Similar findings were also reported for the change of EL with the applied potential at the surface [4], and the crevice opening dimension [20]. Another observation shown in Fig. 5 is that for the potential profiles for L = 8 and 15 there is a clear existence of xlim, but at L = 7.35 this is not evident. In principle, the most
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negative potential that can be measured at EL is the open circuit potential (Eoc) which is 630 mV(SCE) for the current system [1,4]. However it is usually observed that a Elim is more positive than Eoc [3,10,20]. It is interesting to note that based on the current findings (Fig. 5), although Elim appears to be constant with distance, it is actually still decreasing with increasing crevice depth, L. This is shown in Fig. 5 where at L = 7.35 mm, EL = Elim = 418 mV(SCE) which gradually decreased to 600 mV(SCE) when L increased to 15 mm. Thus Elim is expected to approach Eoc for increasing L. Recently, Vankeerberghen et al. [12] developed a mathematical model for the potential drop into the crevice based on a Poisson-type field problem with non-linear boundary conditions that was described as a one-dimensional finite difference framework. The Poisson-type, second order differential equation used in the analysis is given by d2 UðxÞ P iðxÞ ¼ 2 dx rS
ð2Þ
where r is the conductivity of the solution, S is the cross-sectional area, P is the electrochemically active part of the perimeter, and U(x) is the potential in solution. The application of this model to a crevice corrosion system similar to the one used in this work showed for L = 10 mm, xpass = 3.2 mm. This value for xpass is in reasonable agreement with the experimentally determined one here, Table 2. 3.3. Morphology of xpass boundary on the crevice wall After and during the experiment, examination of the crevice wall revealed more penetration and attack in the region just further than xpass inside the crevice. Similar observation was reported on the crevice wall for other crevice corrosion systems operating by the IR mechanism [1,2,4,10,12,20–22,24]. Changes in the appearance of the crevice wall right at xpass were further examined under conventional optical microscopy and were photographed. Fig. 7 shows a photograph at 100· of the morphology of the xpass boundary after the experiment, which lasted 40 min. The general feature is that an intergranular attack becomes more severe when moving in a direction away from the crevice mouth. The intergranular attack is usually caused by a composition difference at the grain boundary and prior heat treatment condition. Thus, for another iron or heat treatment no intergranular attack may be seen. Fig. 8 shows scanning electron microscope (SEM) micrographs of the main features observed on the crevice-corrosion wall that included; xpass, severely attacked, and the etched (bottom) regions. At xpass, the attack is of intergranular nature; similar to that shown in Fig. 7 but at higher magnification with some scattered micropits are observed on the attacked grains. In this region and at xpass boundary the measured electrochemical potential is 162 mV(SCE). From the polarization curve shown in Fig. 2, this potential is in the transition (active/passive region), in agreements with the physical observations in this region. Conventionally this area is associated with
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Fig. 7. Post-experiment greyscale photograph of the crevice wall at xpass boundary showing the corrosion attack for a crevice in iron exposed to acetate buffered solution. Esurf = 800 mV(SCE), L = 0.85 cm and t = 40 min. Magnification: 100·.
stress corrosion cracking, which generally coincides with intergranular attack at grain boundaries. Therefore, the IR voltage drop can bring the crevice wall in a potential region where the intergranular attack associated with stress corrosion cracking can take place. In the severely attacked region Fig. 8(b), it appears as a dense mountainous region with threadlike appearance. This changes to a porous etched surface showing a few scattered simple crystallographic pits in the etched region located at the bottom of the crevice. Little penetration occurred in the etched region, Fig. 8(c), in agreement with the potential prevailing in this region being around Elim where the current is expected to be much lower than in the active severely attacked region.
4. Conclusions • Crevice corrosion occurred immediately without an induction time for a crevice in iron immersed in a strong-buffered acetate solution, under a condition where the crevice corrosion products were allowed to leave the crevice cavity by choosing a crevice in the upside down orientation. However, among other reported factors, the crevice geometry is important in determining the onset of crevice corrosion, where for the current crevice corrosion system, it is expected that no crevice corrosion will occur when L < 6.7 mm. • For a given crevice system there is a critical depth value, above which xpass varies little, and below which xpass increases substantially towards the crevice bottom. This continues until the depth L < Lc, where crevice corrosion will not occur immediately. Then, even if an induction period was allowed, crevice corrosion will be in question for the current crevice geometry that does not allow corrosion products to accumulate inside the crevice.
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Fig. 8. Post-experiment SEM micrographs of different regions on the crevice wall, inside a crevice in iron exposed to acetate buffered solution. Micrographs were taken after a crevice corrosion experiment with parameters: Esurf = 800 mV(SCE), L = 0.8 cm and t = 12 h.
• The active/passive boundary, xpass, on the crevice wall moved towards the crevice mouth (x = 0) for increasing time and increasing current flowing out of the crevice, in accordance with IR voltage theory and in agreement with other reported findings in the literature. The potential at xpass was found to be constant irrespective of time or crevice depth.
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• Potential distribution measurements inside the crevice, using a fine glass microprobe, showed a large potential drop inside the cavity. A large potential drop of about 1.4 V was measured at L = 15 mm. • The morphology of xpass boundary showed an intergranular attack that gets more severe when moving away from the crevice mouth. However, this is not conclusive for the current system as the intergranular attack is usually caused by a composition difference at the grain boundary and prior heat treatment condition.
Acknowledgments Acknowledgment is made to the Institute of Research & Consultation of King Abdulaziz University and to Saudi Basic Industries Corp. (SABIC) for their support of this research. Professor Howard W. Pickering, The Pennsylvania State University, provided helpful comments.
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