Pitting corrosion from chlorides

Protection that concrete offers to steel against stray current ceases when corrosion of the reinforcement has initiated, e. g. due to carbonation, chloride contamination, or the stray current itself In this case, any current flowing through the steel will increase the corrosion rate at the anodic site, similarly as in buried steel structures. Figure 9.8 shows that even small driving voltages can lead to an increase in the corrosion rate on the anodic area (from to Furthermore, it has been observed that if steel is subjected to pitting corrosion in chloride-contaminated concrete, the anodic current increases the size of the attacked area [5]. 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

In the transportation industries, external corrosion of tanker trucks and railcar-mounted tanks is a common problem. Both general and pitting corrosion from the atmosphere and splash water from the roadway or rail bed can affect the tank s structured integrity and tightness. This problem is particularly severe in areas of the country with chloride sources such as road salt or airborne marine atmosphere and severe airborne industrial pollution. 

The propagation of crevice corrosion can also be arrested by decreasing the potential of the outside surfaces below a critical value (see earlier). The existence of a repassivation or protection potential was recognized very early, in particular by Pourbaix et al. [81] for pitting corrosion. From a practical point of view, the existence of a protection potential below which no crevice corrosion is possible is of major importance because it guarantees the immunity of passivated alloys in near-neutral chloride solutions in the absence of oxidizing species and because it makes possible the cathodic protection of stmctuies. 

Sulfate ions have reactions similar to those of chloride. They are corrosion-causative agents (similar to oxygen and hydrogen) of the various types of concentration cell corrosion. In addition, they also are depassivation agents and may greatly accelerate the risk of stress corrosion mechanisms. Saline corrosion pits resulting from high concentrations of chloride and sulfate salts also may be associated with low pH corrosion because hydrochloric acid and sulfuric acid can form within the pit, under deposits. 

Chlorides are responsible for the pitting corrosion of steel parts. Normal carbon steel can stand 1000 ppm of chlorides (=1000 g M 3), but stainless steel starts to corrode severely from 100 ppm on Attention for ladders, illumination sets etc. 

Pitting corrosion is a general term that can be considered a visible sign of the results of concentration cell corrosion and of further induced-corrosion processes such as when chloride attack occurs. Although pits can also occur with acid corrosion, etc., under-deposit corrosion, of course, can also involve direct metal surface attack, from, say, biologically induced corrosion (but that is discussed separately). 

Sodium nitrite is incorporated into formulations for both open and closed cooling systems and acts as an anodic inhibitor. It is a good passivator but requires a relatively high dose rate to ensure that all anodic areas within a system are protected from the risk of pitting corrosion. The dose rate has to be increased when high chlorides or sulfates are present. 

Fig. 3 Crevice and pitting corrosion of a stainless steel autoclave head. Note the crevice corrosion underneath the bolts (now removed) and in the gap between the two parts that are still assembled, and the pitting corrosion on the free surface. This corrosion was probably caused by chloride derived from thermal insulation. (View this art in color at www. dekker.com.)...

Localized corrosion occurs at a particular site, typically because of a breakdown of the passivation protection layer. This can often occur because of anionic attack from chlorides and similarly aggressive components in the vessel. Such localized corrosion is manifested as pitting at various sites across the surface or as crevice corrosion within the material extending beneath the surface. 

A passive metal, which is subject to pitting corrosion beyond the pitting potential, Epit, in the presence of chloride ions, will be inhibited from pitting corrosion if an n-type oxide makes the electrode potential of the metal less positive than Epit. Furthermore, for a corroding metal in the active... 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

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