Repassivation

Once pitting has initiated, circulation of current in the anodic region provokes and maintains an increase in acidity and chloride content, so that propagation may take place even if the potential of the steel is reduced, e. g. owing to an external cathodic polarization. 

Depending on the potential of steel and chloride content in the concrete, it is possible to define different domains where pitting corrosion can or cannot initiate and propagate, and other effects may take place. These matters are further explored in Chapter 20. 

F ie 7.9 Schematic representation of a cyclic anodic polarization curve of an active-passive material in a chloride-containing environment pitting potential ( pu) and protection potential ( p ) are identified [1] 

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In the potential range catliodic to one frequently observes so-called metastable pitting. A number of pit growtli events are initiated, but tire pits immediately repassivate (an oxide film is fonned in tire pit) because the conditions witliin tire pit are such that no stable pit growtli can be maintained. This results in a polarization curve witli strong current oscillations iU [Pg.2728]

Pitting and Crevice Corrosion The general literature for pre-dic ting pitting tendency with the slow scan reviews the use of the reverse scan if a hysteresis loop develops that comes back to the repassivation potential below the FCP (E ) the alloy will pit at... 

Film is locally removed by dissolution, surface shear stress or particle/bubble impact but it can repassivate. Erosion corrosion rate is a function of the frequency of film removal, bare metal dissolution rate and subsequent repassivation rate. 

Compared with ferritic carbon and low-alloy steels, relatively little information is available in the literature concerning stainless steels or nickel-base alloys. From the preceding section concerning low-alloy steels in high temperature aqueous environments, where environmental effects depend critically on water chemistry and dissolution and repassivation kinetics when protective oxide films are ruptured, it can be anticipated that this factor would be of even more importance for more highly alloyed corrosion-resistant materials. 

Where pH levels have dropped below 7.0 due to glycol degradation, there is usually little alternative but to drain, flush, repassivate, and refill with a new inhibited glycol solution. 

Whatever the source of this type of contamination, it must be quickly and effectively removed from the boiler to prevent the problem from recurring. The standard procedure is to blow down the boiler to the maximum extent possible until the oil is removed. In more serious cases, it may be necessary to drop the boiler and provide a boil-out program. A boiler boil-out uses a simple formulation (usually based on caustic or strongly alkaline phosphates) and is employed to re-expose and repassivate the metal surfaces. 

NOTE Where cross-contamination takes place, rapid pitting corrosion of depassivated surfaces may develop. This in turn requires a cleaning and repassivation program to be initiated. 

At the area between the breakdown potential Eb and the critical pitting potential pit local film breakdown occurs, which leads to the creation of pit nuclei. However, these nuclei are immediately repassivated. Consequently, in this potential region it is concluded that breakdown and repair are continuously repeated without creating pit growth. 

If we define the passive-state and active-state modes to be stable and unstable, respectively (as long as pits grow as mentioned in the following sections, nonequilibrium fluctuations are always unstable), in the case of repassivation, pits grow stably in the polishing-state mode, finally being... 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

In this case, if the electrode potential inside the pit remains in the passive potential region, repassivation occurs immediately. 

Figure II. Schematic anodic polarization curves at a fixed temperature. Determination of either transpassive potential ( ,) or pitting potential ( p and repassivation potential ( ,) at the critical current density (/ ). t rcvis the current density at which the scan is reversed. ...

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