Electrochimica Acta 54 (2009) 4696–4703 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Crevice corrosion cathodic reactions and crevice scaling laws Glyn F. Kennell, Richard W. Evitts ∗ Department of Chemical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 article info Article history: Received 12 December 2008 Received in revised form 27 March 2009 Accepted 27 March 2009 Available online 7 April 2009 Keywords: Stainless steel Polarization Modeling studies Crevice corrosion Scaling laws abstract A numerical model that predicts the rates of metal dissolution and electrolyte composition along the length of a metallic crevice was used to simulate the crevice corrosion of AISI 304 stainless steel. The model considers both the forward and reverse electrochemical reactions that might take place during the corrosion process. The environment exterior to the crevice, where a net cathodic current is produced, was simultaneously modeled. It was found that cathodic reactions are likely to occur towards the tip of the crevice. For the case when the hydrogen evolution reaction was considered as a possible cathodic reaction in the crevice, it is shown how the delayed instigation of this reaction may be the cause of an experimentally observed increase in pH at the crevice tip. Two critical crevice scaling laws were exam- ined using model predictions and one scaling law fit the model predictions very well. This scaling law differentiates between crevices that will undergo active corrosion and those that will remain indefinitely passive. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Crevice corrosion is a localized form of corrosion that can cause considerable damage. It can occur on passive metals under condi- tions that do not normally cause high levels of uniform corrosion. These facts, combined with the fact that crevice corrosion can be difficult to detect, may lead to situations where the structural integrity of equipment is challenged by crevice corrosion under conditions that corrosion was not expected. The damage presented by crevice corrosion creates the need for a detailed understand- ing of the complex phenomena causing this type of corrosion. This understanding would help enable the prediction and prevention of crevice corrosion, as well as provide insight into related forms of localized corrosion, such as stress corrosion cracking. Crevice corrosion occurs in occluded spaces, possibly caused by a flange or under sediment deposited on the inside of a pipe. In a crevice a microenvironment may form in which the concentrations of species may be considerably different from those in the bulk electrolyte. A concentration gradient between the crevice and bulk electrolyte can be caused by a very low rate of corrosion that man- ifests as a small leakage or passive current, which is always present with passive metals. A passive current density is often much less than 1 A/cm 2 . For metal surfaces exposed to the bulk electrolyte, the build up of species due to the passive current is overwhelmed by mass transfer of the species away from the surface. Therefore, the concentrations ∗ Corresponding author. Tel.: +1 306 966 4766; fax: +1 306 966 4777. E-mail address: Richard.Evitts@usask.ca (R.W. Evitts). of species in proximity to the metal are similar to bulk electrolyte concentrations. However, for a stagnant electrolyte within a crevice mass transfer is limited by the width and gap of the crevice mouth through which all species must traverse. This mass transfer lim- itation allows the build up and depletion of species in the crevice electrolyte due to the passive current. The oxygen in the crevice may become depleted if oxygen reduction occurs faster than the trans- port rate of oxygen through the crevice mouth. Depletion of oxygen in a crevice is the first stage of crevice corrosion and this may be followed by an incubation period during which the crevice acidifies due to metal ion hydrolysis. At the end of the incubation period the passive film on the crevice will be damaged. Conversely an infinite incubation corresponds to an absence of crevice corrosion. After the oxygen in the crevice becomes depleted the reduc- tion of oxygen continues on the bold surface and anodic reactions may still occur in the crevice if they are supported by bold surface cathodic reactions. These cathodic reactions can support crevice anodic reactions if ions and electrons can move between the cathodic and anodic areas. A complete electrical circuit is devel- oped with aqueous ionic charge transfer and electronic conduction in the solid phase. The conduction of charged ions through the elec- trolyte may cause a significant drop in potential, commonly referred to as the iR drop [1]. Chloride anions also migrate into the crevice, attracted to the anodic metal surface. The increased concentrations of hydrogen and chloride ions in the crevice attack the passive film protecting the metal and cause the passive current to increase. An increase of cor- rosion current through the crevice increases the potential drop into the crevice if the change in conductivity of the electrolyte is neg- ligible. With this decrease in potential some regions in the crevice 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.03.080