Localized corrosion area effects

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

Pits occur as small areas of localized corrosion and vary in size, frequency of occurrence, and depth. Rapid penetration of the metal may occur, leading to metal perforation. Pits are often initiated because of inhomogeneity of the metal surface, deposits on the surface, or breaks in a passive film. The intensity of attack is related to the ratio of cathode area to anode ai ea (pit site), as well as the effect of the environment. Halide ions such as chlorides often stimulate pitting corrosion. Once a pit starts, a concentration-cell is developed since the base of the pit is less accessible to oxygen. 

Nonmetallic conductors and corrosion products. Carbon brick in vessels is strongly cathodic to the common structural alloys. Impervious graphite, especially in heat-exchangers, is cathodic to structural steel. Carbon-filled polymers can act as active cathodes. Some oxides or sulfates are conductors, such as mill scale (magnetite Fe304), iron sulfides on steel, lead sulfate on lead can act as effective cathodes with an important area to that of the anodes. Very frequently, the pores of the conductive film are the preferable anodic sites that leads to localized corrosion (pitting).5... 

One approach to using the AFM to study localized corrosion is to press hard vnth the tip or scratch the surface to stimulate passive film breakdown. Scratching with large tips has been used with success to study the repassivation process [120, 121]. By scratching in a controlled fashion over a small area with an AFM tip, it is possible to study the conditions under which the freshly bared surface will repassivate or propagate into localized corrosion [120, 121]. The effects of potential and environment, including inhibitors can be probed. 

Although there is little evidence for auto-catalysis in dechlorination by Fe , it is still possible that localized corrosion contributes to the remediation of contaminants in environmental applications. Various investigators have postulated that localized corrosion contributes through increased surface area (44) and creation of corrosion cell domains (49-51). The corrosion cell model works on the same principle as the electrochemical model described above (Figure 3), but invokes additional effects such as the reduction of protons as the major cathodic reaction, and the creation of an electrical double layer between the anode and cathode that permits transport due to electrical migration as well as diffusion. Although many aspects of these models are plausible, there are not yet any data that specifically support them, and a study that systematically addresses the role of localized corrosion in remediation applications of Fe remains to be done. 

The effects of flow rate (flow intensity, turbulence, shear stress) on corrosion have been known for a long time and have been variously called erosion corrosion or flow induced localized corrosion (FILC), which latter term is preferred because it denotes a purely aqueous (liquid) phenomenon, while erosion generally includes the presence of a solid phase. An attempt was made in 1990 to summarize the state of the art of FILC in a symposium [54]. The following is a brief synopsis of the developments in this area, with emphasis on corrosion inhibitor testing. 

CIPS (close interval potential survey) Enables detection of defects of coatings, evaluation of cathodic protection effectiveness, CP station current is used. Localization of area with insufficient protection, localization of coating defects, indication of defect size. Requires inspection of whole pipeline, may not show coating disbonding, does not determine corrosion. 

The role of sulfide inclusions in corrosion has been recognized in early woiks. The fact that sulfur-containing species are detrimental to the lesistanee of metals and alloys to localized corrosion has been established for a long time but the meehanisms have remained unclear until recently. In the area of passivity breakdown, where substantial research effort has been expended for several years, the effects of chloride ions have been investigated much more than the effects of sulfur. The aim of this chapter is to review the fundamental aspects of the mechanisms of S-induced corrosion, with special emphasis on the role played by adsorbed (or chemisorbed) sulfur. 

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