Crevice depth

FIGURE 22.30 Schematic polarization curves of a cyhndrical crevice in an anode of stainless steel in neutral solutions of three different chloride concentrations [59] h = crevice depth, Icrev — anodic crevice dissolution current, ccr = chloride concentration in the solution bulk, crcv = crevice protection potential, and /Crev— minimum crevice dissolution current at the critical crevice (protection) potential crcv. 

It is in fact the acidification of the occluded crevice solution that triggers the crevice corrosion. The critical acid concentration, < , , for crevice corrosion to occur corresponds to what we call the passivation-depassivation pH, beyond which the metal spontaneously passivates. This critical acidity determines the crevice passivation-depassivation potential, and hence the crevice protection potential Ecrev. The electrode potential actually measured consists of the crevice passivation-depassivation potential, E -ev, and the IR drop, A/iIR, due to the ion migration through the crevice. Assuming the diffusion current from the crevice bottom to the solution outside, we obtain AEm = icmv x h constant, where crcv is the diffusion-controlled metal dissolution current density at the crevice bottom and h is the crevice depth [62], Since anodic metal dissolution at the crevice bottom follows a Tafel relation, we obtain Eciev as a logarithmic function of the crevice depth ... 

In fact, a linear relationship was found to hold between the crevice protection potential and the logarithm of the crevice depth with b = 0.06 V decade 1 for a cylindrical crevice in austenitic stainless steel in chloride solution [62]. 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

Pit or crevice depth and pit density may be determined according to ASTM G 46, Standard Guide for Examination and Evaluation of Pitting Corrosion, but this can be labor-intensive. Results for highly resistant and very susceptible materials can be reproducAle, but data for replicates with marginal resistance can be quite variable. Because of this variation these tests are generally difficult to use for quality control purposes. 

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]. 

Penetration rate calculated from maximum pit or crevice depth. 

The severity of the crevice regarding the initiation stage strongly depends on its geometry, i.e., the crevice gap (h) and the crevice depth (L). 

The crevice depth L (Fig. 2) controls the transport processes the deeper the crevice, the slower the transport kinetics between the crevice tip and the bulk environment. A minimum crevice depth is often required for corrosion to occur [2]. 

All models confirm that the crevice geometry is of major importance. As previously mentioned, Bernhardsson et al. [4] introduced an U/h geometric factor (in fact, a severity factor S = iJJ-/ i, Figure 10.34) that appears to control the environment evolution. The trends shown in Figure 10.34 are one of the main reasons to study in more detail the effect of crevice depth/width ratio on the crevice repassivation (see Section 10.4.4). 

Curve Crevice Depth Curve Crevice Depth... 

Evaluation of Results After the specimens have been reweighed, they should be examined carefully. LocaHzed attack such as pits, crevice corrosion, stress-acceleratedcorrosion, crackiug, or intergranular corrosion should be measured for depth and area affected. 

Evaluation of attack if other than general, such as crevice corrosion under suppoi t rod, pit depth and distribution, and results of microscopic examination or bend tests... 

If the depth of crevices, vias, or similar details on a cathode is small (about 10 tm to 1 cm), the distribution of current and thus that of the deposit should be uniform. In most cases, however, one observes that deposits are thicker over micro peaks (bumps)... 

If the depth of the crevices, vias, or similar on the cathode is small (about 10 /zm to 1 cm), the distribution of current and thus that of the deposit should be uniform. In most cases, however, one observes that deposit is thicker over micro peaks (bumps) than, say, micro valleys. Such state of affairs is referred to as bad micro throw. When the opposite is true, one talks about true leveling. From simple geometric considerations it follows that given a V-shaped recess, even in the case of uniform metal distribution, still at the bottom the deposit would be expected to be thicker than at the top. However, pronounced leveling is obtainable by using suitable additives. These kinds of additives are known as levelers. ... 

The majority of experimental studies in this field have used pig carcasses as models for human decomposition. However, one study has been reported that used human cadavers in an experimental capacity (Rodriguez and Bass 1985). The study conducted in Knoxville, Tennessee, involved the burial of six unembalmed human cadavers at varying depths and subsequent exhumation at varying intervals. Carrion insect activity was only observed on the bodies buried at a depth of approximately 30 cm (1 ft.). The insects were identified as larvae, pupae, and adults of the family Calliphoridae and Sarcophagidae. It was speculated that the adult flies laid their eggs in the small crevices in the soils above the remains and that the larvae then burrowed to the cadaver where further development ensued. The study was able to demonstrate that the depth at which the cadaver was buried directly affected access by carrion insects and subsequently the rate of decomposition. 

In general, metals or alloys that are used are covered with oxide or hydroxide films. Formation of cracks and fissures can destroy the passivation. The depth of crevices increases rapidly because it is only there that the metal is not covered with a protective layer of oxide/hydroxide (see Fig. 16.8). The result is an increase in surface roughness and possible problems due to reduction in mechanical strength. 

Many models exist to predict the conditions within these sites (e.g., 34,35). However, if the primary need is to determine the extent of corrosion damage (e.g., the depth of corrosion penetration), these models are not sufficient. Generally, electrochemical techniques contain no spatial information, since the current measured is the sum of currents from all individual corrosion sites. In the case of pitting, this limitation is being slowly erased as scanning techniques capable of spatial resolution are being developed. However, the ability to resolve local corrosion sites within fixed occluded areas such as cracks and crevices remains minimal. 

Figure 31 Maximum penetration depths measured on Ti-2 specimens crevice corroded in 0.27 mol dm 3 NaCl at 100°C ( ) and 150°C(1) for various amounts of oxygen consumed.

Figure 34 The steps involved in determining the depth of container wall penetration under Canadian nuclear waste disposal conditions using data obtained in an electrochemical galvanic coupling experiment. (A) Crevice propagation rate (R cc Ic) as a function of temperature (T) (B) RCc as a function of 02 concentration [02] (C) calculated evolution of container surface temperatures and vault 02 concentrations with time in the vault (D) flux of 02 (Jo2) to the container surface as a function of time (E) predicted evolution of Rcc up to the time of repassivation (i.e., at [02]p) (F) total extent of crevice corrosion damage expressed as the total amount of 02 consumed (Q) up to the time of repassivation (G) experimentally determined maximum depth of wall penetration (Pw) as a function of 02 consumed (Q) (H) predicted maximum value of Pw up to the time of repassivation (fP)-...

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