Localized corrosion concept

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

The phenomenology of localized corrosion helps to define certain requirements for localized corrosion that can be expressed in terms of the concepts already discussed in Chapter 2. In order for localized corrosion to occur, there must be a spatial variation in the electrochemical or metallurgical conditions. The occurrence of discrete sites of attack demonstrates that passivity must be able to coexist on the same surface with active regions. In fact, this is one of the scientifically interesting aspects of localized corrosion. Under normal circumstances, one would expect that a surface would either be completely passive or completely active, not a mixture of the two. Finally, there is a physical separation of the anodic and cathodic reaction sites during localized corrosion. In order to understand localized corrosion and thus how to test for resistance to localized corrosion, we must understand each of these aspects and their interrelations. 

In modem technology an increasing number of nonmetallic materials, such as semiconductors, oxides, ionic crystals, and polymers, is employed, which corrode or degrade via chemical rather than electrochemical mechanisms. Corrosion protection of these materials by inhibitors is currently only marginally studied and will be an important future challenge for inhibitor science. For the important case of oxides, similar concepts as employed for the stabilization of passive films in the inhibition of localized corrosion should be applicable. 

Stress also plays a role in localized corrosion owing to its connection with elastic-plastic incompatibilities between particles and matrices. For example, it has been proposed that stress or strain creates a crevice between a hard particle and a soft matrix where a special chemistry may subsequently develop. Stress effects are more complicated in many observed cases of SCC emanating from pits in complex alloys where the oxide film has already been ruptured. Because these driving forces are not well understood, the concept of accumulation of damage under stressed conditions cannot be used as a basis for quantitative life prediction. 

Statistical assessment of time to failure is a basic topic in reliability engineering for which many mathematical tools have been developed. Evans, who also pioneered the mixed potential theory to explain basic corrosion kinetics, launched the concept of corrosion probability in relation to localized corrosion. According to Evans, an exact knowledge of corrosion rate was less important than the ascertainment of the statistical risk of its initiation [12]. The following examples illustrate the application of empirical modeling in two areas of high criticality. 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

It follows that the corrosion potential on a heterogeneous metal corroding by local-cell action is virtually equal to the mixed potential at an electrode on which electronation and deelectronation reactions are occurring on spatially separated sinks and sources and is identical to a mixed potential when the metal is corroding homogeneously by a Wagner-Traud mechanism. The concept of the corrosion current /corr and the corrosion potential 40corr will now be treated quantitatively. 

Concepts of local equilibrium and charged particle motion under - electrochemical potential gradients, and the description of high-temperature -> corrosion processes, - ambipolar conductivity, and diffusion-controlled reactions (see also -> chemical potential, -> Wagner equation, -> Wagner factor, and - Wagner enhancement factor). 

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