Channel electrodes in the study of pitting corrosion

The resistance of many metals and alloys to corrosion depends critically upon the presence of a thin (10-1000 A [168]) passive surface film [169]. In aggressive environments, this film may become damaged locally via several processes, e.g. surface stress effects (either flow-induced [170,171] or as a result of anion adsorption [168]), the impingement of small particles on the surface [169], spontaneous depassivation [169]. Retention of the protective film by the metal only results if repassivation of the unprotected area is feasible compared with pit growth. 

It is now very well established that the pitting corrosion of a number of metals and alloys is accompanied by the formation of metal salt films [172-181]. In particular, Beck and Alkire [181] predicted that the current densities resulting from pit initiation could be large enough to produce a supersaturated metal salt solution in the vicinity of the metal surface with subsequent, rapid precipitation of a salt film. For some metals, the precursor to the re-establishment of passivity is the precipitation of a metal salt film [172— 

180] for others (e.g. certain metals in halide-containing mineral acids [169, 

It can be readily envisaged that the rate of transfer of fresh solution to a pit will be an important factor in whether the precipitation process occurs and, by extension, in the fate (growth or death) of the pit. It has recently been recognised that channel electrodes are a useful experimental system by which the effect of flow on pitting corrosion can be studied [111-113,169] for the following reasons. 

Firstly, the effect of both laminar and turbulent flow regimes is readily investigated, as illustrated in Sects. 6.1 and 6.2, and secondly, direct observation of the electrode area under study may be achieved by incorporating a window in the channel cell, permitting light microscopy. In this way, electrochemical measurements can be linked directly to observable events on the surface, such as metal salt film precipitation and pit growth and death. 

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