Kinetics of corrosion reactions

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. 

Thermogravimetry is the most commonly used method for studying the kinetics of corrosion reactions at high temperatures. The sample is placed in an oven with a controlled atmosphere, where one monitors the variation of its mass by means of a microbalance (Figure 9.3). The precision of the technique is on the order of micrograms. If all corrosion products remain on the surface of the sample, the measured mass increase as a function of time permits to identify the rate law that governs the reaction kinetics. 

Temperature and time also affects the corrosion reaction. It is generally known that the rate of a chemical reaction increases with increasing temperature. As a rule of thumb the reaction rate doubles for every 10° increase in temperature. The kinetics of corrosion reactions can develop differently. Fig. 1-8. A constant, linear rate of reaction is observed for acidic corrosion where no protective layer is formed (curve a). Decreasing rates of corrosion are often ob-... 

The kinetics of corrosion reactions 499 Several obvious points may be made ... 

The order of nobiKty observed in actual practice may differ from that predicted thermodynamically. The reasons are that some metals become covered with a passivatingfEm of reaction products which protects the metal from further attack. The dissolution reaction may be strongly irreversible so that a potential barrier must be overcome. In this case, corrosion may be inhibited even though it remains energetically favorable. The kinetics of corrosion reactions are not determined by the thermodynamics alone. 

Electrochemical techniques measure electrochemical potentials and currents that are fundamentally related to the thermodynamics and kinetics of corrosion reactions. They are often used to measure the rates of uniform corrosion, to determine the tendency of localized corrosion, to study a wide range of corrosion-related phenomena such as passivation, galvanic corrosion and sensitization effects. Electrochemical corrosion testing and monitoring can be performed in a diverse range of environments in the laboratory or in the field, in a pipeline or in an autoclave. For instance, they have been successfully employed to monitor corrosion in multiphase oil/water conditions with as little as 1-2% water " ... 

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