Critical crevice width

If there is a gap between two components of unalloyed steel, then there is a possibility of crevice corrosion. For example, this can be the case for screwed connections. In principle, crevice corrosion is an aeration cell which results from a difference between the oxygen concentration in the crevice (low oxygen content) and that outside the crevice (higher oxygen content). This type of corrosion only occurs at critical crevice widths [16]. If the width of the crevice lies markedly above or below the critical value, there is no corrosive attack. This also explains the practical observation that corrosive attack does not occur in every crevice [13]. 

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. 

There is a close relationship between crevice corrosion and pitting corrosion. Crevice corrosion only occurs in waste waters that contain chloride ions, and is dependent on the conditions in the crevice. Crevice widths of more than 0.5 mm are generally not critical however, the crevice depth must also be taken into account [30]. Crevices between stainless steels and insulating materials, e.g. plastics, are particularly susceptible to crevice corrosion. Experience shows that the most important potential for crevice corrosion, the critical potential Uc, is clearly more negative than the pitting potential Up, as shown in Figure 4 [31]. Thus, for example, for a chloride concentration of 1 g/1, these potentials are Uc = -t 0.10 V and Up = -t 0.45 V. Crevice corrosion between stainless steel and stainless steel was also observed however the risk of corrosive attack is only to be expected at very high chloride concentrations [27]. For the use of the materials listed in Table 4, only a very low probability of corrosion is to be expected if there are crevices in the components [32]. 

The magnitude of crevice corrosion also depends on the depth of the crevice, width of the gap, number of crevices and ratio of exterior to interior crevice. It has been shown for types 316 and 304 stainless steel that smaller the gap, the less is the predicted time for initiation of crevice corrosion (Fig. 4.14). The reason is that when the ratio of crevice solution volume to creviced area is small, the acidity is increased and the critical value for initiation of crevice is achieved rapidly. The ratio of the bold area to the creviced area also affects crevice corrosion (Fig. 4.15). Generally, the larger is the bold area (cathodic) and smaller the creviced area (anodic), the larger is the probability of crevice corrosion. This has been shown by work on types 304 and 316 stainless steel and... 

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. 

The relationship between the equilibrium vapor pressures on a concave meniscus and that on a flat surface is given by the Kelvin equation Pr = Pf x exp (-A/r), where r is the radius of curvature of the concave meniscus. In the case of a crevice of constant width, under fixed environmental conditions, r is constant, and hence, there is a critical relative humidity of the atmosphere, at which the crevice is empty and above which the crevice is filled with water. In the case where, instead of a spherical particle on a flat surface, the radius of curvature of the meniscus at the sphere-plane point of contact tends to zero, and hence, the critical relative hnmidity also tends to zero, there does not exist a minimum value below, which, under the particle, there is no water. By increasing the relative humidity of the atmosphere, both the radius of curvature of the meniscus and the amount of water below the particle increase (Figure 12.26). 

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