Metals processing crevice corrosion

It is appropriate to consider first the crevice corrosion of mild steel in oxygenated neutral sodium chloride, and then to consider systems in which the metal is readily passivated. Initially, the whole surface will be in contact with a solution containing oxygen so that attack, with oxygen reduction providing the cathodic process, occurs on both the freely exposed surface and the surface within the crevice (Fig. 1.50). However, whereas the freely exposed surface will be accessible to dissolved oxygen by convection and diffusion, access of oxygen to the solution within the crevice can occur only... 

Pitting may be defined as a limiting case of localised attack in which only small areas of the metal surface are attacked whilst the remainder is largely unaffected, and this definition is applicable irrespective of the mechanism involved dezincification, crevice corrosion and impingement attack can all result in pitting, although the mechanisms of these three processes are quite different. 

The rate of metal wastage of this indirect form of corrosion may be increased by the presence of other direct corrosion influences in the deposit or foulant. Also (and similar to crevice corrosion), there may be general oxygen corrosion occurring at the same time or perhaps acting as an initiator to the under-deposit corrosion process. 

Uniform attack on a metal results in uniform corrosion. This is exploited in the processing and finishing of metals (Chapter 15). However, metallic structures are rarely homogeneous and surfaces are rough corrosion occurs, preferentially within fissures in the surface (crevice corrosion), making corrosion faster. 

Crevice corrosion occurs as a result of a non-uniform concentration of eletrolyte solution. The local electrode potential varies with the concentration of electrolyte corrosion currents (electron and ion migration) that occur in the metal and solution. For instance a piece of copper immersed in copper sulphate solution of varying composition, tends to be more positive in the region of high copper sulphate concentration due to the enhanced rate of deposition of Cu ions. A flow of electrons through the metal towards this region occurs and the ion deposition is a continuous process. As a result corrosion occurs in the metal where the solution concentration is at its lowest, and Cu ions pass into solution. 

When metal parts rub in an electrolyte, it is possible to form short-circuit galvanic microelements (Fig. 1.7). Potentials 1E3 and 2E3 appear at the metal-electrolyte interface and contact potential difference 1E2 in the contact sites of the parts. The electromotive force of these elements promotes electrode processes on the friction surfaces. The processes appear even though lEs = 2E3. because of the galvanic elements resulting from crevice corrosion in the friction zone. 

Crevice corrosion starts because of hampered access of electrolyte and oxygen in the space of the clearance. This changes the pH of the electrolyte solution in the clearance, spurs the anodic and retards the cathodic processes. As a result, the metal electrode potential shifts to the negative with respect to the potential on the open surface. These processes bring about electrochemical microelements of the slot-open-surface t3Tje, in which the metal in the clearance serves as an anode [14]. 

Moreover, corrosion processes may run on metals covered with varnish -and paint materials in an atmosphere and lead to the formation of numerous tortuous filiform areas of damage. This type of failure, called filiform corrosion, is a specific form of crevice corrosion. The filiform type of corrosion does not, as a rule, lead to metal failure but only impairs its appearance. Filiform... 

Taken together, the processes described above tend to lead to a concentrated, acidic solution of metal salts within the crevice. Metallic materials tend to dissolve rapidly upon polarization in these solutions. Thus, maintenance of the occluded solution composition is critical to stability of crevice corrosion. As mentioned above, diffusion acts to destabilize crevices by dispersing the concentrated solution, whereas migration effects tend to concentrate aggressive anionic species into the crevice. 

Precipitates and suspended solids in process streams will deposit in stagnant areas (e.g., comers in vessels or bends in a pipe) and lead to corrosion of metals by the same mechanism as that involved in crevice corrosion. This is called deposit attack. Filtration of process fluids can prevent these problems. 

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