51316-7707-Transport Considerations in the Crevice Corrosion of Alloy 625: An Experimental and Modeling Approach

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Product Number: 51316-7707-SG

ISBN: 7707 2016 CP

Author: Robert Lillard

Publication Date: 2016

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We have recently shown that the crevice corrosion of alloy 625 begins with the formation of a critical crevice solution (CCS) deep within the crevice. With time the volume of this CCS expands and moves forward towards the mouth of the crevice. However the damage associated with this initial phase is relatively small penetration depths on the order of 1-2x10^-6 m. It is not until this CSS reaches a critical point close to the mouth presumably the location of IR* that the large damages associated with crevice corrosion propagation are observed. In this paper we will present experimental evidence as well as a quasi-steady state three stage model to describe the species dependent potential ionic species and damage in alloy 625 crevice corrosion. In stage I of the model restricted convection and continued anodic dissolution lead to deoxygenation of the crevice and separation of the anodic and cathodic sites. Here every electron that is produced is immediately consumed an electro-neutrality is immediately satisfied. Stage II then accounts for the induction time of crevice corrosion. Here anodic dissolution begins deep in the crevice and propagates toward the mouth leading to acidification and the formation of a critical crevice solution. Crevice corrosion initiation occurs when the CCS reaches the location where the IR drop exceeds the active/passive transition of the polarization curve (IR*). In stage III metal salt precipitation reactions allow for the breakdown of the passive layer and continued aggressive anodic dissolution. Here the localized damage propagates from the crevice mouth toward the tip and is consistent with a nearly well-mixed migration controlled corrosion system.