Interface electrochemical/corrosive reactions

For the gaseous-layer effects, such as entrainment and detrainment of species across the liquid interface, chemical transformations in the gas phase, the effects of solar radiation on photosensitive atmospheric reactions, and temperature effects on the gas phase, reaction kinetics are important. In the interface regime, the transfer of molecules into the liquid layer prior to their chemical interaction in the liquid layer is studied. Not only does the liquid regime receive species from the gas phase, but species from the liquid are also volatilized into the gas phase. Important variables in the liquid regime include the aqueous film thickness and its effect on the concentration of species, chemical transformations in the liquid, and reactions involving metal ions originating from the electrochemical corrosion reactions. 

All of these factors will influence the corrosion performance. Careful studies by Tallman and co-workers have given support to the concept proposed by Elsenbaumer that the conducting polymer promotes the formation of a passive oxide at the polymer metal interface. It is believed that the dense nature of this oxide impedes the electrochemical corrosion reactions. Studies of the interface region have shown that oxide layers are present. Fiowever, many other factors may also be involved in the overall... 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

Section 8 deals with reactions which occur at gas—solid and solid—solid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reaction at the liquid—solid interface (and corrosion), involving electrochemical processes outside the coverage of this series, are not considered. With respect to chemical processes at gas-solid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. 

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

Figure 3 Electrical equivalent circuit model commonly used to represent an electrochemical interface undergoing corrosion. Rp is the polarization resistance, Cd] is the double layer capacitance, Rct is the charge transfer resistance in the absence of mass transport and reaction intermediates, RD is the diffusional resistance, and Rs is the solution resistance, (a) Rp = Rct when there are no mass transport limitations and electrochemical reactions involve no absorbed intermediates and nearly instantaneous charge transfer control prevails, (b) Rp = Rd + Rct in the case of mass transport limitations.

Chapter 4 describes how the electrical nature of corrosion reactions allows the interface to be modeled as an electrical circuit, as well as how this electrical circuit can be used to obtain information on corrosion rates. Chapter 5 focuses on how to characterize flow and how to include its effects in the test procedure. Chapter 6 describes the origins of the observed distributions in space and time of the reaction rate. Chapter 7 describes the applications of electrochemical measurements to predictive corrosion models, emphasizing their use in the long-term prediction of corrosion behavior of metallic packages for high-level nuclear waste. Chapter 8 outlines the electrochemical methods that have been applied to develop and test the effectiveness of surface treatments for metals and alloys. The final chapter gives experimental procedures that can be used to illustrate the principles described. 

Another essential requirement for the photocatalyst is its resistance to reactions at the solid/liquid interface that may result in a degradation of its properties. These reactions include electrochemical corrosion, photocorrosion, and dissolution (Morrison, 1980). A large group of photocatalysts with suitable semiconducting properties for solar energy conversion (CdS, GaP, etc.) are not stable in the water-oxidation reaction because the anions of these materials are more susceptible to oxidation than water, causing their degradation by oxidation of the material (Ellis et al., 1977 Williams, 1960). 

Depending on the environment of an adhesive joint, moisture or electrochemi-cal/corrosive reactions at the interface which lead to a chemical degradation of bonds could lead to a failure of the joint In a corrosive environment, electrochemical reactions dominate, in particular on reactive metals at room temperature, whereas the electrode potential characterizes the reactivity of the interface. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

The corrosion potential, defined by the rate of the electrochemical reactions, is a relevant property of corrosion reactions at the metal/adhesive interface as it reflects the kinetics of the electron-transfer and ion-transfer reactions. Depending on the system being observed, correlations exist between the measurable Volta potential difference and the corrosion potential. 

The pressurized blister test is an excellent method to combine electrochemical reactions at polymer/metal interfaces with a mechanical load. It allows the application of a mechanical stress from a homogeneously pressurized electrolyte on the adhesive/metal interface in a sample geometry that is accessible for the HR-SKP [28]. Depending on the adjusted conditions, information on the synergy of mechanical stresses, elastic or inelastic deformations of the adhesive, transport processes, and corrosive reactions could be obtained with this method. 

The thermodynamics of corrosion processes provides a tool to determine the theoretical tendency of metals to corrode. Thus, the role of corrosion thermodynamics is to determine the conditions under which the corrosion occurs and how to prevent corrosion at the metal/environment interface. Thermodynamics, however, cannot be used to predict the rate at which the corrosion reaction will proceed [1—6]. The corrosion rate must be estimated by Faraday s law and is controlled by the kinetics of the electrochemical reaction. 

The corrosion cell or any electrochemical discharge reaction at the electrode interface is represented in EIS by an electronic circuit consisting of capacitors and resistors. Because the frequency is not zero in the case of ac excitation. Ohm s law is represented by ... 

Electrochemical inhibitors retard or prevent the anodic and/or cathodic partial reactions (i.e they influence the reaction at the metal/corrosive medium interface). Chemical inhibitors can react both with the material and form protective coatings and with the medium itself or its constituents and thus diminish its aggressiveness. Physical inhibitors form adsorption layers on the metal surface, which block the corrosion reaction. Inhibitors that influence the electrochemical electrode reactions are subdivided according to their mode of action and site of action in the area of the metal/ medium phase boundary, with the subdivision being between interface inhibitors, electrolyte film inhibitors, membrane inhibitors, and passivators. 

Any chemical transformation that implies the transfer of charge across the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) is referred to as an electrochemical reaction. An electrochemical reaction can include one or several electrode reactions. For example the reaction (1.3) is an electrochemical reaction each atom of iron that passes into solution implies the exchange of two electrons between the metal and the protons. Two electrode reactions are involved the oxidation of the iron and the reduction of the proton. According to the definition given above, all corrosion reactions that involve metal oxidation are electrochemical reactions. In order to understand and control corrosion phenomena it is essential to study the thermodynamics and kinetics of electrochemical reactions. 

Oxidation and reduction processes are accompanied by the flow of electric charge through the interface metal-corrosive environment. In metals the charge carriers are electrons while in the corrosive environment charge flow is due to ions. Thus an active assessment of electrochemical corrosion processes can be achieved by assessing the electrical charge transfer process. In the reactions of corrosion that are controlled by the rate of charge transfer, the current - potential relationship can be described by the Butler-Vokner equation ... 

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