Mechanisms of Corrosion Processes

Other factors are fretting and cavitation in a liqnid (impact of the liqnid). Corrosion can also occnr dne to electric cnrrents (stray cnrrents in soils). 

Almost all metals are snbject to corrosion, an exception being the noble metals (the platinnm metals, gold, silver) which nnder ordinary conditions do not corrode. Corrosion of iron is the most prominent problem since parts and strnctnres consisting of iron and steels are nsed so widely. 

Different parameters are nsed to characterize the corrosion rate the loss of mass by the metal sample within a certain length of time (per nnit area), the decrease in sample thickness, the eqnivalent electric cnrrent density, and so on. For most metals nndergoing nniform general corrosion, these parameters in order of magnitude can be interrelated (while allowing for atomic masses and densities) as 1 g/m -yr 10 mm/yr 10 A/m.  

FIGURE 22.2 Schematic polarization curves for spontaneous dissolution (a) of active metals (h) of passivated metals. (1,2) Anodic curves for active metals (3) cathodic curve for hydrogen evolution (4) cathodic curve for air-oxygen reduction (5) anodic curve of the passivated metal. 

Passivation of the metal and the associated sharp decline of its anodic dissolntion rate have a strong effect on corrosion rates (curve 5 and the point of intersection C in Fig. 22.2b). Passivation is encountered more often under the effect of oxidizing agents (e.g., in the presence of oxygen). 

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This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

Since kinetics and mechanism of corrosion is controlled by electrochemical principles, the technique based on electrochemical methods is used to determine the corrosion rate and understand the mechanism of corrosion process. The testing methods are based on principle of accelerating the corrosion process without changing the environment and the corrosion rates can be measured without removing the test specimens. 

Identify opportunities and advance scientific and engineering understanding of the mechanisms of corrosion processes, environmental materials degradation, and their mitigation. 

In most SECM experiments, the tip is held at a constant potential in an amperometric mode or scanned in the cyclic voltammetry (CV) mode. The substrate can also be subjected to various potential treatments. Studies involving transients or time-dependent signals are especially useful in obtaining information about adsorbed intermediates or products, as discussed in Chapter 16. Another time-dependent technique involves an AC signal applied to the tip, a form of electrochemical spectroscopy impedance (EIS). Examples of this approach have been discussed in Chapter 14 as applied to studies of the mechanism of corrosion processes. For example, in the... 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

The essential features of the electrochemical mechanism of corrosion were outlined at the beginning of the section, and it is now necessary to consider the factors that control the rate of corrosion of a single metal in more detail. However, before doing so it is helpful to examine the charge transfer processes that occur at the two separable electrodes of a well-defined electrochemical cell in order to show that since the two half reactions constituting the overall reaction are interdependent, their rates and extents will be equal. 

THE MECHANISM OF CORROSION PREVENTION BY INHIBITORS Effects of Inhibitors on Corrosion Processes... 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Many important processes such as electrochemical reactions, biological processes and corrosion take place at solid/liquid interfaces. To understand precisely the mechanism of these processes at solid/liquid interfaces, information on the structures of molecules at the electrode/electrolyte interface, including short-lived intermediates and solvent, is essential. Determination of the interfacial structures of the intermediate and solvent is, however, difficult by conventional surface vibrational techniques because the number of molecules at the interfaces is far less than the number of bulk molecules. 

Corrosion is the deterioration of a material by reaction with its enviromnent. Although the term is used primarily in conjunction with the deterioration of metals, the broader definition allows it to be used in conjunction with all types of materials. We will limit the description to corrosion of metals and alloys for the moment and will save the degradation of other types of materials, such as polymers, for a later section. In this section, we will see how corrosion is perhaps the clearest example of the battle between thermodynamics and kinetics for determining the likelihood of a given reaction occurring within a specified time period. We will also see how important this process is from an industrial standpoint. For example, a 1995 study showed that metallic corrosion costs the U.S. economy about 300 billion each year and that 30% of this cost could be prevented by using modem corrosion control techniques [9], It is important to understand the mechanisms of corrosion before we can attempt to control it. 

Electrodes of many metals can undergo corrosion or passivation— formation of a salt film on the surface—and other reactions, depending on the medium and experimental conditions. Electrochemical techniques can be used to investigate the mechanisms of these processes. 

Owing to the tremendous economic damage it can cause, corrosion has and continues to be the subject of extensive study especially with a view to its minimization at acceptable expense—economic and environmental. We attempt to give an idea of the forms of corrosion, how to investigate it by electrochemistry, and how it can be minimized, or at least reduced and controlled. As will be seen, given the complexity of corrosion processes, the mechanism of which can alter significantly depending on the local environment, the more specialized literature should be consulted for details on specific cases, for example Refs. 1-6. 

The fundamental causative factor for the dissolution of metals and the mechanism of the process are identical to those pertinent to the establishment of an electrode potential. We may begin consideration of die dissolution process with a discussion of the thermodynamically reversible electrode potential, Eeq, of a metal, M, and proceed to show that the dissolution reaction is a departure from the equilibrium conditions. With this approach we can appreciate the role played by the electrode potential in corrosion and gain an insight into the true nature of the process. 

In chemistry, labelled compounds are used to elucidate reaction mechanisms and to investigate diffusion and transport processes. Other applications are the study of transport processes in the geosphere, the biosphere and in special ecological systems, and the investigation of corrosion processes and of transport processes in industrial plants, in pipes or in motors. 

The corrosion of metals is one of the most significant problems faced by advanced industrial societies (Fig. 17.13). It has been estimated that in the United States alone, the annual cost of corrosion amonnts to tens of billions of dollars. Effects of corrosion are both visible (the formation of rnst on exposed iron snrfaces) and invisible (the cracking and resulting loss of strength of metal beneath the surface). The mechanism of corrosion must be understood before processes can be developed for its prevention. 

Ordinary corrosion is the redox process by which metals are oxidized by oxygen, O2, in the presence of moisture. There are other kinds, but this is the most common. The problem of corrosion and its prevention are of both theoretical and practical interest. Corrosion is responsible for the loss of billions of dollars annually in metal products. The mechanism of corrosion has been studied extensively. It is now known that the oxidation of metals occurs most readily at points of strain (where the metals are most active ). Thus, a steel nail, which is mostly iron (Section 22-7), first corrodes at the tip and head (Figure 21-11). A bent nail corrodes most readily at the bend. 

Particularly under the broad definition of corrosion as the deterioration of materials by reaction with the environment, the number of mechanisms whereby deterioration occurs is large. In general, a mechanism of corrosion is the actual atomic, molecular, or ionic transport process that takes place at the interface of a material. These processes usually involve more than one definable step, and the major interest is directed toward the slowest step that essentially controls the rate of the overall... 

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