Passive alloys, crevice corrosion propagation

The rate of crevice corrosion propagation of stainless steel alloys, such as types 304 and 316, is decreased in natural seawater, when the temperature is increased from ambient to 70°C [8,9]. The tendency for initiation of crevice corrosion is not affected by increase in temperature. The above trend may be attributed to the decreased solubility of oxygen with increased temperature and changes in the nature of the passive film which is formed. 

On passivated alloys, and particularly on stainless alloys (Fe-Ni-Cr-Mo-... alloys), crevice corrosion occurs when the local conditions in the crevice cause passivity breakdown and the onset of active dissolution. As a consequence, crevice corrosion exhibits two different stages an incubation period during which the local environment in the crevice evolves toward critical conditions and a propagation period that starts at the initiation time corresponding to the passivity breakdown. 

The increase in cathodic kinetics due to the action of biofilms on passive alloy surfaces can also increase the propagation rate of galvanic corrosion. Potentiodynamic polarization studies show that cathodic kinetics are increased during biofilm formation on passive alloy surfaces. Tests on crevice corrosion samples of passive alloys S30400 and S31600 revealed that crevice initiation times were reduced when natural marine biofilms were allowed to form on the exposed external cathode surface. (Dexter)5... 

In this chapter, the basic mechanisms of crevice corrosion are briefly presented but most of the text is devoted to the crevice corrosion of passive alloys, particularly Fe-Ni-Cr-. .. alloys in aerated chloride environments. Phenomenological aspects, the mechanisms of initiation, the conditions of propagation, the modeling, the experimental techniques, and the possibility of prevention are successively described. 

Conversely, there is some agreement that a critical value (usually referred to as protection potential) exists that is the maximum possible value of below which no crevice corrosion may propagate significantly. This is consistent with the fact that crevice corrosion of passive alloys in chloride environments does not occur in deaerated environments. 

For a given crevice geometry, the critical potentials for crevice initiation and repassivation decrease with increasing chloride content (Fig. 10) and increasing temperature (Fig. 11) of the bulk solution. This means that the susceptibility to crevice corrosion of passivated alloys increases with the chloride content and the temperature. For example, titanium alloys become sensitive to crevice corrosion only in hot concentrated chloride solutions around 100/150°C [9,10]. Propagation rates also increase with temperature. 

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. 

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