Behaviour of Crevice Corrosion in Iron

8/12/2019 Behaviour of Crevice Corrosion in Iron

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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 ArabiaReceived 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

(0.5M CH3COOH + 0.5M NaC2H3O2), pH = 4.66. The current and the potential gradient

within the crevice were measured at crevice depth (L) = 7.35, 8, 10, and 15mm, for a crevice

that was positioned facing the electrolyte in a downward position. A remarkable shift inpotential (>1.2V) in the active direction was measured inside the crevice cavity, when the

potential at the outer surface was held at 800mV(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 whichxpasslocation shifts shar-

ply towards the crevice bottom. Steeping of the potential gradient occurred with time indicat-

ing 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

0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2004.08.001

* Tel.: +966 5568 2242; fax: +966 2695 1754.

E-mail address: [email protected](M.I. Abdulsalam).

Corrosion Science 47 (2005) 13361351

www.elsevier.com/locate/corsci
mailto:[email protected]:[email protected]


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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. Well-known 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 in-

side 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 alu-minium[6]. Another proposed theory to explain the onset of crevice corrosion ad-

dresses 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 oc-

cluded 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 poten-

tial 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].

UnderIR drop mechanism controlled crevice corrosion, metal dissolves inside the

crevice and the anodic current flows through the crevice electrolyte to the outer sur-face where the oxidant is reduced. The resulting IR voltage translates into an elec-

trode 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 polariza-

tion of a crevice[14].Walton et al.[15]developed a reactive transport based theoret-

ical 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 [1618]. There-

fore, it is important to study the potential distribution inside the crevice and its rela-tion to the polarization curve in order to obtain a better understanding of the

mechanism by which crevice corrosion occurs.

M.I. Abdulsalam / Corrosion Science 47 (2005) 13361351 1337


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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 crev-

ice corrosion process[14,10,12,1921]. This transition boundary appeared at a cer-

tain distance into the crevice,xpass, which is located at the Epassvalue on the crevicewall. The appearance ofxpasson 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 theIRvoltage theory,Epassis 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 ofxpass on the crevice wall is predicted

by the relation[1,22]:

Du Ex0 Epass IRxpass 1

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 elec-

trolyte 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 sur-

face 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 re-

ported in an artificial crack [23].

Most available studies on the IRdrop mechanism of crevice-corrosion address the

effects of the oxidation power, gap-opening dimension, electrolyte composition andtemperature, while very few discuss the effects of the crevice depth. This paper de-

scribes 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 inelectromechanical relays, solenoids, magnetic pole and other flux-carrying

components.

1338 M.I. Abdulsalam / Corrosion Science 47 (2005) 13361351


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2. Experimental

The material used in this work was Carpenter Electrical iron; a low carbon com-

mercially 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 con-

dition was annealing at 843 C for 1h in dry hydrogen and cooled at 65.5 C per

hour. Rectangular specimens were cut to the size of 20 15 5mm. This size fits

with the size of the groove made on the Teflon block part of the electrode. The exper-

iments were performed in a strong-buffered acetate solution Fe/HAcNaAc (0.5M

CH3COOH + 0.5M NaC2H3O2). It was prepared with reagent grade sodium acetate

(NaC2H3O2 3H2O), acetic acid (CH3COOH) and double distilled water. The meas-

ured pH was 4.66, while the conductivity was: j= 0.03Scm1, at 24C. The exper-

iments were carried out at room temperature.

The crevice experimental set-up and procedure were similar as described previ-

ously [4,5,20]. The exposed Fe crevice wall (depth: L= 1cm; width: w= 0.5cm)

and outer (0.5cm 2cm) surface were polished to 0.05lm 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 hada gap (opening) dimension,a = 0.3mm. A schematic sketch of the electrode is shown

inFig. 1. The electrical connection was made with an insulated copper wire soldered

a

w

Crevice

wall Iron

L

x

Crevice

Cavity

Teflon

Plexiglas

Luggin

microprobe

SS Screw

Crevice mouth

L

Outer surface

w

w

Crevice

wall Iron

L

x

Teflon

Fig. 1. Schematic diagram of the electrode assembly used in crevice-corrosion experiments.



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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 orienta-

tion 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 electrolytecompositional changes inside the crevice, such as acidification, due to the accumu-

lation 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 suscepti-

ble to acidification, a buffered solution will provide the assurance against

acidification.

In the experimental setup the solution level inside the crevice was maintained be-

low the top of the Teflon block but above the Fe sample. Experiments were con-ducted by keeping the outer surface (bottom area of the iron specimen shown in

Fig. 1) potential, Esurf, at 800mV(SCE) in the passive region of the polarization

curve for this system. This was done by using a potentiostat. The potential distribu-

tionE(x) on the crevice wall was measured using a fine glass microprobe of 0.03mm

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 micro-

probe. Thexpasslocation andEpassvalue 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 speci-

mens 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 0.5M CH3COOH + 0.5M 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 con-nection. Slow potentiodynamic DC polarization scans were run by using a potenti-

ostat 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 1000ml was used, and a saturated cal-

omel 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 0.15mV/s. One, was done in the passive-to-active direction, from 1.2 to0.7V(SCE). The other scan was done on another freshly prepared specimen in thereverse 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 ace-

tate solution (Fe/NaAcHAc 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) was630mV(SCE) for both scans. The Epassvalue:165mV(SCE), was measured inside the crevice by placing the tip of the microprobeat 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 polar-

ization curve (Fig. 2). The current at 165mV(SCE) inFig. 2is approximately onetenth of the peak current and noticeably larger than the passive current consistent

with easy visible observation of this boundary on the crevice wall. This agreementof 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/1N

H2SO

4 system, where a difference of 92mV in the value of E

pass was observed be-

tween the two scans (at a given current density) [4]. One possible interpretation

0.0001

0.001

0.01

0.1

1

10

100

-1000 -500 0 500 1000 1500

Potential, E, (mV,SCE)

Currentdensity

,i,(mAcm-2)

Reverse Scan

Forward Scan

Epass=-165mV(SCE

)

E*=-460mV(SCE)

ipeak(-283 mV, 18.43 mA cm-2

)

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 0.15mV/s. Area: 1.89cm2.



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for this is that the characteristic of the oxide film forming on the surface of the iron

in NaAcHAc solution is different from the one forming on nickel in 1N 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 roughsurface. In the works reported by Abdulsalam and Pickering on crevice corrosion in

nickel, it was shown that Epassdepends on the direction of the scan[4,20].The value

forEpassobtained 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 ofEpass.

3.2. Crevice corrosion characteristics

With the selected crevice parameters and conditions, crevice corrosion com-menced at the start of the experiment and continued throughout the test period.

The potential at the crevice bottom (EL) jumped from approximately

622mV(SCE), a value around Eoc, to more positive potentials as the crevice depthdecreased. 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 conditionL > Lc, whereLcis the critical crevice depth was met.Lcis the smallest crevice depth that results in crevice corrosion for the test conditions,

and is defined as the distance x at which the activepassive 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 neces-

sary and therefore crevice corrosion started immediately[1,17,26]. This implies that

theIR voltage drop was enough at the start of the experiment for the applied poten-

tial at the outer surface to shift part of the crevice wall atLinto 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. 3shows photographs of the on-going exper-

iment of crevice corrosion of iron in the buffered acetate solution (L= 7.35, 8 and

15mm), taken at dissimilar times throughout the experiment. The glass microprobe

appears inside the cavity. The morphology of the surface under the action of crevicecorrosion as viewed through the clear Plexiglas can be divided into three main re-

gions. The first (lowest) region is the passive region (un-attacked) that extends from

the crevice mouth to thexpassboundary. 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 esti-

mated from the potential profile, E(x), as the distance at which Ebecomes independ-

ent 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 crevicewall morphology to these three regions was reported for crevice corrosion in nickel

[4,20], and duplex stainless steel [21].



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Epass=165mV(SCE) was measured with the microprobe tip at xpass and wasfound 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 (bulksolution) 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

Fig. 3. In situ photographs of the corroding iron crevice wall (upside down orientation) in buffered acetate

solution (0.5M CH3COOH + 0.5M NaC2H3O2), showing the location ofxpassand the distinctive regions

that appeared during crevice corrosion. (a) 40min, (b) 15min, and (c) 7min. Magnification 5.5.

-600

-400

-200

0

200

400

600

800

-1 0 1 2 3 4 5 6 7 8 9 10

Distance into crevice,x, (mm)

Potential,E,

(mV,S

CE)

Epass= -165 mV(SCE)

Eapplied= 800 mV (SCE)

L = 8.0 mm

3.5 hr

15 min

12 hr

Direction of motion

ofxpasswith time xpass

xpass

xpass

Fig. 4. Variation of the potential distributionE(x) with time inside the crevice for iron in buffered acetate

solution (0.5M CH3COOH + 0.5M NaC2H3O2), whenL= 8.0mm.



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was shown thatEpasschanges by approximately 150mV for the Ni/1N 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 withtime 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= 8mm. The location ofxpassis indicated by the arrow atEpass, and listed inTable

1. These values are in close agreement with the measured ones physically at the xpassboundary. The advancement ofxpasstowards the crevice mouth with time is consist-

ent with an increasing current. The measured current increased from the onset of

applying Esurf from 1.26mA initially to 2.23mA at t= 12h. Similar behaviour for

other systems was reported in the literature [24]. It is worth mentioning here that

while the active region on the crevice wall is located in the region between Epassand E* the most contributing part to the current is located only at a distance thatis 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 be-

comes steeper (Fig. 4), thereby, the current also increases with time. This was dis-

cussed 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= 15mm.

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 = 10mm the

current was measured as 1.2mA after 40min, in other work this corresponds to

1.34mA 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 cor-

rosion current[4,24]. In addition ELwas about 70mV more negative with the right-

side up orientation. This is explained by the increase of the electrolyte resistance that

is allowed with this crevice orientation.

The effect ofL on the initialxpassvalue (attP 0), measured directly by physically

placing the tip of the microprobe in situ at the xpassboundary seen through the Plexi-

glas with the help of a macro lens viewer, is shown inTable 2,for L = 7.35, 8, 10 and15mm, being 4.5, 3.3, 3.5 and 3.7mm, respectively. These measurements are in close

agreements with the values obtained using the initially measured E(x) distributions

Table 1

Time dependency ofxpass and Elim inside the crevice for iron in the buffered solution, L= 8.0mm

Time, h xpassa (0.1), mm xpass

b (0.1), mm Elima (1), mV(SCE)

0.25 3.3 3.4 4713.5 2.6 2.7 505

12.0 2.4 2.4 514a Measured inside the crevice by using microprobe.b Estimated from the potential profile at Epass (Fig. 4).



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shown in Fig. 5, at the intersection of E(x) curve and Epass line. When LP 8 mm

(more specifically in the range from L= 8 to 15mm) the location ofxpass at tP 0was further into the crevice as the crevice depth, L, was increased. However, the

amount of change ofxpasswas very moderate (0.4mm for DL= 7mm), 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< 8mm (7.35mm), xpassshifted in the opposite direc-

tion towards the crevice bottom for decreasing L, as shown in Fig. 5andTable 2.

The amount of change in xpass here is much more pronounced (1.2mm for

DL= 0.65mm). Reported data for Lcis included inFig. 6and it is shown to follow

Table 2

Depth dependency of the initially measured (within first 15min): xpass, Elim, and I for a crevice in iron

exposed to acetate buffered solution

Depth (L), mm xpassa

(0.1), mm Elima

(1), mV(SCE) I, mAL< 8

7.35 4.5 418 1.4L> 8

8.0 3.3 471 1.2610.0 3.5 528 1.215.0 3.7 600 1.05

a Measured inside the crevice by using microprobe.

-650

-450

-250

-50

150

350

550

750

950

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Distance into crevice,x, (mm)

Potential,E,

(m

V,S

CE)

Epass

xpass @ L = 10 mm


L = 15.0 mm

L = 7.35 mm

L = 8.0 mm

E@ ipeak, (Fig. 2)

E*, (Fig. 2)

EL @ L = 10 mm

Fig. 5. Variation of the initial (within 15min) measured potential distributions with depth inside the

crevice for iron in buffered acetate solution (0.5M CH3COOH + 0.5M NaC2H3O2).



11/16

the same trend[12]. In this range it is clear that xpass deviates from the linear rela-

tionship predicated by Eq.(1)as shown inFig. 6. This scenario observed experimen-

tally is in agreement with theoretical analysis of the mechanism for a similarlocalized corrosion problem, formulated by using the electric field within the film

on the surface for differentL values[25]. The model computation results showed that

for a given crevice system there is a certain depth value, above which xpassvaries lit-

tle, and below whichxpassincreases substantially toLc.Fig. 6is the experimental plot

that agrees with the model result in reference [25].

The initial measured current (within the first 15min) at different L are listed in

Table 2. Little changes occurred on the current until L was decreased beyond

8mm. 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

ofa on the measured current in nickel/sulphuric acid crevice system [20].

The potential behaviour atx= L,EL, is also shown inFig. 5, and listed inTable 2.

The largerL is the more active EL, which is in agreement with theoretical model pre-

dictions[25].Similar findings were also reported for the change ofEL with the ap-

plied potential at the surface [4], and the crevice opening dimension [20]. Anotherobservation shown inFig. 5is that for the potential profiles for L= 8 and 15 there

is a clear existence ofxlim, but at L= 7.35 this is not evident. In principle, the most

0

2

4

6

8

10

12

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Crevice depth, L , (mm)

xpass,

(mm)

Experimental data

Calculated using Eq. (1)

Lc= 6.7 (reference 12)


No crevice corrosion

detected

Fig. 6. The effect of the crevice depth (L) on the location of the transition boundary (xpass). Shown also are

the expectedxpass values according to theoretical prediction based on IR voltage theory.



12/16

negative potential that can be measured at EL is the open circuit potential (Eoc)

which is630mV(SCE) for the current system[1,4]. However it is usually observedthat a Elimis more positive than Eoc[3,10,20]. It is interesting to note that based on

the current findings (Fig. 5), althoughElimappears to be constant with distance, it isactually still decreasing with increasing crevice depth, L. This is shown in Fig. 5

where at L= 7.35mm, EL= Elim=418mV(SCE) which gradually decreased to600mV(SCE) when L increased to 15mm. Thus Elim is expected to approach Eocfor 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 frame-

work. The Poisson-type, second order differential equation used in the analysis is

given by

d2Ux

dx2

P

r Six 2

wherer is the conductivity of the solution, Sis the cross-sectional area,Pis the elec-

trochemically 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= 10mm, xpass= 3.2mm. 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 xpassinside the crevice. Similar

observation was reported on the crevice wall for other crevice corrosion systems

operating by the IR mechanism [1,2,4,10,12,2022,24]. 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 40min. The

general feature is that an intergranular attack becomes more severe when movingin 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. 8shows scanning electron microscope (SEM) micrographs of the main fea-

tures observed on the crevice-corrosion wall that included; xpass, severely attacked,

and the etched (bottom) regions. At xpass, the attack is of intergranular nature; sim-

ilar to that shown inFig. 7but at higher magnification with some scattered micropits

are observed on the attacked grains. In this region and at xpassboundary the meas-

ured electrochemical potential is162mV(SCE). From the polarization curve showninFig. 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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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 re-

gion 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 se-

verely attacked region.

4. Conclusions

Crevice corrosion occurred immediately without an induction time for a crevice iniron 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 cor-

rosion will occur when L< 6.7mm.

For a given crevice system there is a critical depth value, above whichxpassvaries

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 willbe in question for the current crevice geometry that does not allow corrosion

products to accumulate inside the crevice.

Fig. 7. Post-experiment greyscale photograph of the crevice wall atxpassboundary showing the corrosion

attack for a crevice in iron exposed to acetate buffered solution. Esurf= 800mV(SCE), L= 0.85 cm and

t= 40min. Magnification: 100.



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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 crev-

ice, in accordance with IR voltage theory and in agreement with other reportedfindings in the literature. The potential at xpasswas found to be constant irrespec-

tive of time or crevice depth.

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= 800mV(SCE), L= 0.8cm andt = 12h.



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Potential distribution measurements inside the crevice, using a fine glass micro-

probe, showed a large potential drop inside the cavity. A large potential drop

of about 1.4V was measured at L= 15mm.

The morphology ofxpassboundary showed an intergranular attack that gets moresevere 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 compo-

sition 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 supportof this research. Professor Howard W. Pickering, The Pennsylvania State University,

provided helpful comments.

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Behaviour of Crevice Corrosion in Iron