Crevice corrosion geometry

The localized nature of pitting attack can be associated with component geometry, the mechanics of the corrosion process, compositional inhomogeneity, or imperfection within the material itself. The growth of pits, once initiated, is closely related to another corrosion mechanism, crevice corrosion. 

The major effects of geometry relate to vulnerability of process equipment to crevice corrosion and erosion-corrosion. 

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 the crevice corrosion occurs. Factors that influence both the potential distribution and the polarization curve include the metal environment reaction products and the crevice geometry as well as the well-known promoters of crevice corrosion acidification and chloride ion build-up within the crevice. When excess oxygen entered the crevice via flushing air-saturated solution through it, the corrosion rate decreased which was consistent with the proposed need to keep the cathodic reaction outside the crevice, separate from the anodic reaction, for a stable crevice corrosion to occur [151]. 

In view of this sequence, the crevice geometry parameters of gap width and depth become important. If the gap is sufficiently wide and shallow, oxygen depletion and chloride-ion influx will decrease and metal-ion buildup will be less due to increased diffusion of corrosion products from the crevice. The pH decrease due to hydrolysis of cations will be less, the passive film may be preserved, and if so, crevice corrosion will not occur. These factors are reversed for deep, narrow crevices, and at some critical geometry, crevice corrosion will occur. As with pitting, increased concentration of chloride ions in the environment will increase chloride-ion concentration in the crevice and increase the probability of initiating crevice corrosion. 

A generic geometry for crevice corrosion can be seen in Fig. 1 in cross section. The substrate is the metalKc material of interest. It is separated from the crevice former by a gap, g, which, for an ideal crevice, is constant over the length of the crevice, 1. The fully exposed surface is that area of the substrate outside the crevice former, which is fully immersed in the electrolyte. Its properties can strongly influence the rate of crevice corrosion [2, 3]. 

This section describes the balance of these opposing mass transport forces for the geometry of crevice corrosion. Mass transport of species in aqueous solution can occur by three processes migration, diffusion, or convection. In most cases of crevice corrosion, convection can be ignored owing to the restricted geometry involved. 

Pickering and coworkers [31, 34, 35] have demonstrated both experimentally and computationally that for systems that meet the criteria of the IR theory, lA is predicted. The amount of potential drop increases as one moves into the crevice because of the current leaving the crevice. If the geometry, solution conductivity, and passive current density of the material in the environment conspire to create sufficient ohmic drop, then the potential of some portion of the material within the crevice falls to the primary passive potential. Under these circumstances, the passive film is not stable and active dissolution occurs. The potential difference between the applied potential and the primary passivation potential is referred to as IR. Deeper still into the crevice the ohmic drop leads to decreased dissolution as the overpotential for the anodic reaction decreases. Thus, ohmic drop is responsible for the initiation and stabihzation of crevice corrosion according to this model. 

Whether crevice corrosion occurs largely depends on the crevice geometry. Crevice widths are particularly critical when the distance between the crevice-forming surface is less than 1 mm. Since the reactions taking place in the crevice cannot be influenced from the outside, damage due to crevice corrosion can only be avoided through the prevention of crevices. 

Testing for shorter periods may be appropriate when the test sample configuration intentionally provides more severe conditions than anticipated in service. Crevice corrosion tests using severe artificial crevice geometries may only require 30-90 days of exposure to establish relative crevice corrosion resistance. 

In the ensuing crevice corrosion process, chloride-rich corrosion products with high iron and chromium contents are produced. The pH level of the electrolyte solution in the crevice is reduced by hydrolysis of these salts, accelerating the corrosion process. The probability of corrosion for a surface with crevices is thus greater than for a surface with no crevices. The level of the critical potential is determined mainly by the crevice geometry and the material involved [20, 21]. 

Pitting and crevice corrosion can be avoided by means of cathodic corrosion protection (CCP). The effectiveness of cathodic protection for crevice corrosion is, however, limited by the crevice geometry. 

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