Metal-electrolyte interface anodic process

The model for atmospheric corrosion tmder high chloride concentration su ested by Kamimura et al. [32] is based on the separation of cathode and anode sites under the rust and the thin electrolyte. The pH at the anode compartment is affected by the chloride ion concentration and decreased to 1.5 by the hydrolysis of ferric ions and the formation of P-FeOOH. Chloride ions accumrrlate at the anode site and initiate the oxidation of ferrous ions to ferric ions. Accumulated chloride ions increase ferric ion solubility in the electrolyte and accelerate the hydrolysis of ferric ions, causing the pH at the anode to decrease. Low pH at the metal-electrolyte interface accelerated the formation of P-FeOOH. The atmospheric corrosion process is summarized as follows ... 

Since corrosion is an electrochanical process, its progress may be studied by measuring the changes that occur in metal potential with time or with applied electrical currents. Conversely, the rate of corrosion reactions may be controlled by passing anodic or cathodic currents into the metal. If, for example, electrons are passed into the metal and reach the metal/electrolyte interface (a cathodic current), the anodic reaction will be stifled while the cathodic reaction rate increases. This process is called cathodic protection and can be applied only if there is a suitable conducting medium such as earth or water through which a current can flow to the metal to be protected. 

Ehiring corrosion (oxidation) process, both anodic and cathodic reaction rates are coupled together on the electrode surface at a specific current density known ds icorv This is an electrochemical phenomenon which dictates that both reactions must occur on different sites on the metal/electrolyte interface. For a uniform process under steady state conditions, the current densities at equilibrium are related as o = — c = ieorr Ecorr- Assume that corrosion is uniform and there is no oxide film deposited on the metal electrode surface otherwise, complications would arise making matters very complex. The objective at this point is to determine both Ecorr and icorr either using the Tafel Extrapolation or Linear Polarization techniques. It is important to point out that icorr cannot be measured at Ecorr since ia = —ic and current wfll not flow through an external current-measuring device [3]. 

Most corrosion processes, e.g., metal dissolution, hydrogen or oxygen evolution, and passive film formation, involve at least one adsorption step as a part of the overall reaction. This step can be significantly affected by the presence on the metal surface of a monolayer of nonmetal species. As evidenced by studies described in this chapter, adsorbed species may act by loosening the metal-metal bond or changing the electric field at the metal-electrolyte interface. They can also favor or inhibit the adsorption or the recombination of adsorbed atoms normally involved in the anodic or cathodic reactions. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

This draft represents a simplified model when an anodic film of low-solubility intermediates is typical. The film conductivity is regarded to be both ionic and metallic (see Sect. 4.1). The Faradaic process at the inner junction (that is, metal/ film interface) is bound up with the ionic current component giving rise to the film growth. The metallic part of the conductivity causes oxidation of low-valence intermediates at the outer junction (film/electrolyte), both transported from the bulk by the flux /a and the film s constituents. 

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