Metal corrosion kinetics

Pourbaix, M., Recent Applications of Electrode Potential Measurements in the Thermodynamics and Kinetics of Corrosion of Metals , Corrosion, 25, 267 (1%9)... 

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. 

SECM is a powerful tool for studying structures and heterogeneous processes on the micrometer and nanometer scale [8], It can probe electron, ion, and molecule transfers, and other reactions at solid-liquid, liquid-liquid, and liquid-air interfaces [9]. This versatility allows for the investigation of a wide variety of processes, from metal corrosion to adsorption to membrane transport, as discussed below. Other physicochemical applications of this method include measurements of fast homogeneous kinetics in solution and electrocatalytic processes, and characterization of redox processes in biological cells. 

Electrochemical protection can be achieved by forming an electrolytic cell in which the anode material is more easily corroded than the metal it is desired to protect. This is the case of zinc in contact with iron (Fig. 16.11) in this example there is a sort of cathodic protection. Protection of ship hulls, of subterranean pipeline tubings, of oil rigs, etc. is often done using sacrificial anodes that are substituted as necessary. The requisites for a good sacrificial anode are, besides its preferential corrosion, slow corrosion kinetics and non-passivation. Sacrificial anodes in use are, for this reason, normally of zinc, magnesium, or aluminium... 

Metallic corrosion occurs because of the coupling of two different electrochemical reactions on the material surface. If, as assumed in the discussion of iron dissolution kinetics above, only iron oxidation and reduction were possible, the conservation of charge would require that in the absence of external polarization, the iron be in thermodynamic equilibrium. Under those conditions, no net dissolution would occur. In real systems, that assumption is invalid, and metallic dissolution occurs with regularity, keeping corrosionists employed and off the street. 

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Here we wish to exemplify how metal corrosion can be interpreted from both a thermodynamic and electrochemical kinetic point of view. This simple introduction may serve to direct readers to some of the more detailed literature on the chemistry of corrosion. 

Figure 1. Linear rates (in millimeters per year) of chemical kinetic and macroscopic transport processes in surficial aquatic and sedimentary environments. Individual processes are coupled to the driving forces, identified as three main groups of chemical (C), hydrological (H), and physical (P) driving forces. Data sources mineral dissolution rates, Tables 4 and 5, and Berthelin (1988) mineral dehydration, compilation in Bodek and Lerman (1988) metal corrosion, Coburn (1968), Costa (1982), and Haynie and Upham(1970) uplift, Lajoie(1986), Stallard (1988) physical erosion, Table 3 chemical weathering, soil formation and chemical denudation, Table 6.

Almost aU metals corrode, but many metals corrode very slowly under normal environmental conditions, due in part to kinetic limitations of the metal dissolution reaction. Thus, the rate of metal corrosion can be anticipated and controlled by developing kinetic rate expressions for metal oxidation reactions. There is a major difference, however, between classical electrochemical metal dissolution kinetics and metal dissolution in a corrosion system, that difference being the occurrence of one or more oxidation and reduction reactions on the same metal. 

The use of Tafel plots for the analysis of metal corrosion systems indicates how dissolved O2 in solution and the subsequent O2 reduction (which is under kinetic control) accelerates metal corrosion. Due to the high value of E for O2 reduction (+1.23 V vs. SHE), the intersection of the oxygen reduction and metal dissolution Tafel lines occurs at high values of E and When the reduction of both H+ and O2 drives metal corrosion (with the reduction reactions under kinetic control), one simply adds together the current-voltage Tafel lines for the two reduction reactions. A new line is then drawn for the sum of the cathodic currents on the corroding metal. The intersection of this new line with the metal oxidation Tafel line gives E and... 

These materials are highly efficient as a means of corrosion inhibition due to their ability to realize almost all inhibition mechanisms of metal corrosion, namely the barrier mechanism connected with the impenetrability of polymers for most corrosion media the inhibition mechanism induced by a specific action of the inhibitors on the corrosion process kinetics the protecting mechanism related to the effect of the polarizing charge formed in the plastic upon distribution of electrode potentials within the corrosive system,... 

Proceeding from the above, it can be concluded that polymers and compositions on a polymer base can become, under the action of operating conditions, the sources of products changing corrosion kinetics in the polymer-metal systems. 

Cl are chemical compounds or their blends able to retard metal corrosion even in small additions in aggressive media. Cl change the kinetics of electrochemical reactions that bring about corrosion so that the corrosion process rate retards significantly. 

Corrosion may be defined as the spontaneous deterioration of a structure or part of a structure due to the action of the total environment or individual environmental agents. For the purposes of this chapter, the structure is assumed to be metallic and the environment is assumed to be aqueous. Using this definition and the constraints noted, this chapter will outline the electrochemical techniques used to develop criteria of corrosion and those used in the study of corrosion kinetics. 

If in fact the iron sulfide building up on the surface of the metal represents a resistance to the corrosion processes,then one would expect that the corrosion rate decreases as the iron sulfide film increases. In other words the corrosion kinetics would... 

Corrosion of310 SS was studied in the presence ofNaClandNaS04 salt mixtures at 750 C [28]. Five different mixtures ofNaCl/Na2S04 with the following wt% were used 100/0,75/ 25, 50/50,25/75, and 0/100. The salts were apphedto specimens of 310 SS and placed in air at 750 C. Figure 11.19 shows parabolic metal oxidation kinetics for the control specimen with no applied salt (labeled oxidation in the legend) after about 3 h [28]. The specimens that underwent hot corrosion exhibited various types of rate laws for their oxidation. 

The tendency of metals to change with loss of energy into the disordered, more stable thermodynamic state represented by cathode local corrosion processes is particularly critical. Compounds are the reason for corrosion. However, the kinetic of metals to change with loss of energy into the disordered, more stable thermodynamic state represented by compounds is the reason for corrosion. Kinetic barriers, mainly provided by the formation of protective layers, often make metals resistant to corrosion and allow them to be used in technological applications. 

SECM has been applied to the investigation of various technologically important materials and interfaces, for example, metallic corrosion [91-96], fuel cell electrocatalysts [97], semiconductor photocatalysts [12, 60-63, 98], conducting polymers [49, 50, 85, 86, 99-103], liquid-liquid and liquid-gas interfaces [29, 30, 68]. The SECM may be used to image the substrate topography and/or reactivity, or with the tip at a fixed location, to study the local kinetics of the interfacial reactions of interest. 

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