Pits, corrosion modeling

J.C. Velazquez, A. Valor, F. Caleyo, V. Venegas, J.H. Espina-Hernandez, J.M. HaUen, M.R. Lopez 2009. Pitting corrosion models improve integrity management, reliability. OU and Gas Journal 107 (28), 56-62. 

The discussion of concentrated solution models has indicated that, while the transport flux equations in their rigorous form (5) may be intractable, the use of the binary electrolyte approximation allows the convenient implementation of concentrated solution theory in pitting corrosion models. Engelhard et al. have shown that this approximation is valid over a surprisingly wide range of... 

Fig. 9. Corrosion model of silver development. As the haUde ion, X, is removed into solution at the etch pit, the silver ion,, travels interstitiaHy, Ag/ to the site of the latent image where it is converted to silver metal by reaction with the color developer, Dev. Dev represents oxidized developer.

Regions characterized by large anodic overpotentials. Under such conditions, complete passivation and severe oxidation of most metal surfaces occurs. A breakdown of passive oxide layers and pitting corrosion is observed for transition-metal model systems. In this section are considered also the surfaces of electropositive metals such as aluminum. 

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

If the data collected do not fit the simplest equivalent-circuit model (Fig. 6.18), more complex models are analyzed. A number of equivalent circuits have been developed to model corrosion processes involving diffusion control, porous films or coatings, pseudoinductive mechanisms, simultaneous electrochemical and chemical reactions, and pitting corrosion (Ref 14-18). 

This section provides a brief review of both the available theoretical models and the experiments relating to pitting corrosion at channel electrodes. A thorough coverage of pitting corrosion per se is beyond the scope of this chapter and the reader is referred to ref. 182 for further detail. 

In this sub-section, we present the available theoretical descriptions for the effect of convection on pitting corrosion and metal salt deposition. Three models described in the literature are relevant these are discussed in the next three sections. 

D.E. Williams, C. Westcott, M. Fleischman, Stochastic modek of pitting corrosion of stainless steek I. Modeling of the initiation and growth of pits at constant potential, J. Electrochem. Soc. 132 (1985) 1796-1804. 

Step of initiation of SCC [36], It has been reported that bound water, which occupies active sites for corrosion, initiates pitting corrosion [37]. Zhang et al. [38,39] suggested that the same model could be apphed to explain the inhibition and acceleration of anions [38] and cations [39] for the initiation of SCC. 

Figure 2.4 The classical corrosion model of pitting attack after Wranglen G. Corrosion Sci, 1974, 14 331. Reprinted with permission from Elsevier.

Figure 10.19 Model of pitting corrosion. A salt film stabilizes the potential difference between active iron surface in the pit and the passive surface.

The model can be run using a range of the rates. An example might be to vary one or more of the parameters, e.g., using a maximum SS pitting corrosion value, and a SOO year Furfurol(F) lifetime, as opposed to the lASAP recommended lifetime of 100 years. To illustrate this, the model was run using three sets of corrosion rates and barrier lifetimes. The rates or lifetimes were chosen at the upper and lower ends of the published or advised values (see Table XVII), and compared to the lASAP recommended values. 

Figure 2.4. The classical corrosion model of pitting attack.

Williams, D.E., Westcott, C. and Fleischmann, M. (1984) Stochastic models of the initiation of pitting corrosion on stainless steels. Journal of Electroanalytical Chemistry, 180, 549. 

---