Crevice corrosion critical factors

Tsujikawa, S., Critical Depth for Initiation of Growing Crevice Corrosion, Critical Factors in Localized Corrosion, G. S. Frankel and R. C. Newmans, Eds., The Electrochemical Society, Pennington, NJ, 1992, pp. 378-388. 

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

Crevice corrosion is one of the forms of localized metal corrosion, in which the anodic metal dissolution inside a crevice is coupled with a cathodic reaction outside the crevice [58], This form of localized corrosion occurs only if the structural crevice is thinner than a certain width, for example, 30-40 p,m for stainless steels [59], and thus restricted mass transport through the crevice is responsible. For crevice corrosion to occur, a certain induction period of time is required, during which a local cell has formed between the inside and the outside of the crevice. It was also shown that scaling factors in crevice corrosion may be characterized by the aspect ratio, L/a, where L is the depth of crevices and a is the crevice opening. If the aspect ratio is greater than its critical ratio, crevice corrosion will occur, whereas no crevice corrosion will occur if the aspect ratio does not exceed its critical ratio [60]. 

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. 

Crevice corrosion is affected by several factors, of a metallurgical, environmental, electrochemical, surface physical, and last but not least, a geometrical nature. One of the most important factors is the crevice gap. This is indicated in Figure 7.18. We can see that the critical gap size, below which crevice corrosion is possible under the... 

Below the CPP, CT cannot displace adsorbed oxygen so long as the passive film remains intact hence, pitting is predicted not to occur. Should passivity break down because of factors other than those described [e.g., reduced oxygen or depolarizer concentration at a crevice (crevice corrosion), or cathodic polarization of local shielded areas], pitting could then initiate independent of whether the overall prevailing potential is above or below the critical value. But under conditions of uniform passivity for the entire metal surface, application of cathodic protection to avoid pitting corrosion need only shift the potential of the metal below the critical value. This is in contrast to the usual procedure of cathodic protection, which requires polarization of a metal to the much more active open-circuit anode potential. 

Ulanovskii, I. B., Conditions for Cathodic Protection of Stainless Steels in Crevices , Zashchita Metal, 1, 643 (1965) C.A., 64, 10751 Holmes, D. R. and Mann, G. M. W., A Critical Survey of Possible Factors Contributing to Internal Boiler Corrosion , Corrosion, 21, 370 (1965)... 

---