Shape of corrosion pits

Figure 7.28 Different shapes of corrosion pits. (ASTM [7 21])...

Figure 7.46 Shape of corrosion pits (a) deep pit (b) occluded pit and (c) hemispherical pit.

Figure 6.11 Typical cross-sectional shapes of corrosion pits.

Studies on the effect of hydrodynamics on localized corrosion and electrochemical etching processes have been reviewed by West et al. Much of the work has been performed by Alkire and co-workers." They have used FIDAP, a commercial FEM code, to investigate the influence of fluid flow on geometries relevant to etching and to pitting corrosion. In most cases, Stokes flow was considered. The Stokes flow approximation is frequently valid inside the cavity because its characteristic dimension is small. However, the flow outside the cavity may not be in the Stokes flow regime. Since it is the external fluid motion that induces flow inside the cavity, under many (especially unsteady) situations, the use of the Stokes flow approximation may be problematic. Some of the work of Alkire and co-workers has been extended hy Shin and Economou, " who simulated the shape evolution of corrosion pits. Natural convection was also considered in their study. 

One of the recommended ways of recognizing the case of corrosion with MIC used to be by studying the shape of the pit. As Tatnall and Pope put it, Surface morphology relates to the chemistry at the metal surface, not to the presence or absence of miao-organisms. In fact, new studies have shown that the similarity between the dimensions of the bacterial cells that have adhered onto the surface and the size and dimensions of the pits may suggest that the pits have been initiated at the miaobial attachment sites. 

The shape of the pit is often responsible for its continued growth for the same reasons mentioned under crevice corrosion. A pit can be considered a type of self-forming crevice. Pitting can occur... 

At this point in the investigation, the relationship between the pits and the arrowhead-shaped regions of corrosion was uncertain. Several possible causes for the pitting were considered, such as siphonic gas exsolution, biological and/or microbiological activity, and debris (concrete chips, etc.) lodged in the tubes, but each was tentatively dismissed as improbable since none of the proposed mechanisms adequately accounted for all observations. 

Figure 11.19 Perforated pit downstream of an arrowhead-shaped area of corrosion. (Magnification 7.5x.)...

The shape of a vessel determines how well it drains (Figure 53.7). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive. This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils. 

Vetter, K. J. and Strehblow, H. H., Formation and Shape of Pitting Corrosion Pits in Iron and Theoretical Conclusions on Pitting Corrosion , Ber. Bunsenges Phys. Chem., 74, 1024 (1970) C.A., 74, 8865j... 

The study of corrosion properties of mild steel and stainless steel in aqueous solutions has received a great general attention. Mild steel is the most common structural material and is used in a wide range of environments. It is well known that when mild steel corrodes, anodic and cathodic areas develop over the corroded surface. Conventionally, these pits are known to change in shape and move across the surface, resulting in early corrosion that is approximately... 

Convective mass transport from small cavities is relevant to through-mask electrodeposition and to localized corrosion and has hence received much attention. In localized corrosion, mass transport is important for determination of the local environment inside an active pit. For through-mask deposition, an understanding of mass transport may be important for design of mixing methods or for analysis of measured deposit profiles. The electrodeposition in circular or rectangular cavities formed by a photoresist has been studied by Kondo etal. it was found that the shapes of deposited bumps can be explained by calculations of vortex evolution and penetration flow. 

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