Localized corrosion stability

Figure 27 Schematic (a) Evans diagram and (b) corrosion potential vs. time behavior for localized corrosion stabilization. Line a on the Evans diagram represents the electrochemical behavior of the material before localized corrosion initiates, while line b represents the electrochemical behavior of the material in the localized corrosion site. Due to the low Tafel slope of the active site, the corrosion potential of the passive surface/local-ized corrosion site falls. If repassivation occurs, the anodic behavior reverts back to line a, and the corrosion potential increases again (line c). If repassivation does not occur, the corrosion potential will remain low (line d).

Inconel alloy 622 —Modified composition and special thermal mechanical processing give this alloy superior thermal stability and resistance to intergranular attack and localized corrosion. The alloy is particularly suited to acidified halide environments, especially those containing oxidizing acids. 

The propagation of localized corrosion thus becomes an issue of stability. For the localized corrosion site to exist, the CCS must be maintained locally. However, the solution inside the localized corrosion site is constantly diffusing away. The higher the concentrations required in the CCS, the larger the Fickian driving force. In order to maintain the CCS within the localized corrosion site, the anodic reactions inside the localized corrosion site must occur at a high rate and must be balanced by cathodic reactions either inside or outside the localized corrosion site. Any process that upsets this balance will cause the diffusion of... 

CCS to dominate and the localized corrosion site to repassivate. In studying the localized corrosion test techniques, these three aspects of localized corrosion, the CCS, the separation of anode and cathode, and the stability of these processes, should be kept in mind. 

N Austenitic stabilizer, economical substitute for nickel Enhances localized corrosion resistance, thermal stability and mechanical properties... 

Remarkably enough, statements of high chemical stability under harsh corrosive environments such as pH < 3 or pH > 9 are not substantiated in the literature by reliable measurements on membrane systems and much more work is needed here. Problems that occur with the bursting pressure of support tubes after long-term usage might indicate local corrosion at the contact points between the ceramic particles making up the microstructure. 

Oxide stabilized refers to materials, such as aluminum and the stainless steels, whose corrosion resistance depends on the formation and stability of a very thin surface oxide layer that is inert, easily healed if damaged, and tenacious. When the oxide layer has been disrupted and not healed, the material usually has little corrosion resistance. Both active and passive states sometimes exist adjacent to each other on the surface, resulting in rapid local corrosion. Crevice corrosion in stainless... 

V. Cihal, Metallurgical Aspects of Knife-Line Attack on Stabilized Stainless Steels, Localized Corrosion NACE 3, R.W. Staehle, B.F. 

One may therefore conclude that the rate-determining step for the repassivation is the transport of accumulated aggressive anions out of small pits. This result coincides well with the explanation that localized corrosion is stabilized at its initial stage by the accumulation of aggressive anions, which prevent the formation of a passive layer at the active pit surface. 

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