Crevice growth rate

The inward crevice growth rate (dy/dt) may be predicted using Faraday s law. Letting dy/dt = r in eq. (3.13b) 5delds 

Previous work by MacDonald et al. [29-30] and Liu et al. [31] suggest that pit growth data can be fitted to an empirical relationship of the form 

Despite the several mathematical models being developed for describing corrosion problems and their solutions, each model has its own degree of accuracy, which depends on the chosen variables and conditions, and numerical method for determining relevant parameters that describe a particular corrosion phenomenon. Comprehensive reviews of models for describing a particular problem, such as crevice cathodic protection, pitting growth rate, current and potential attenuation are available in the literature [2,3,18,25-32]. 

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Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

Because the film growth rate depends so strongly on the electric field across it (equation 1.115), separation of the anodic and cathodic sites for metals in open circuit is of little consequence, provided film growth is the exclusive reaction. Thus if one site is anodic, and an adjacent site cathodic, film thickening on the anodic site itself causes the two sites to swap roles so that the film on the former cathodic site also thickens correspondingly. Thus the anodic and cathodic sites of the stably passive metal dance over the surface. If however, permanent separation of sites can occur, as for example, where the anodic site has restricted access to the cathodic component in the electrolyte (as in crevice), then breakdown of passivity and associated corrosion can follow. 

Electrochemical deposition of copper at very slow rates leads to a columnar structure with deep crevices [83]. This structure is explained theoretically to be a result of competition between local and nonlocal growth effects. 

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. 

The factors accounting for rates of corrosion at crevices follow the same principles as those described for pit growth. The higher the electrolyte conductivity and the larger the cathode area outside the crevice, the higher the rate of attack at the anode. The initiation of crevice corrosion, however, does not depend... 

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