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: miabdul@yahoo.com(M.I. Abdulsalam).

    Corrosion Science 47 (2005) 13361351

    www.elsevier.com/locate/corsci

    mailto:miabdul@yahoo.commailto:miabdul@yahoo.com
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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,