Corrosion process anodic dissolution

Typical examples of processes involving two or more adsorbed species are reactions of corrosion or anodic dissolution of metals, oxygen evolntion, etc. In the case of two adsorbed species B and C, the electrochemical... 

Fundamentals. The composition of liquids with respect to both identity and concentration of dissolved species can be determined with inductively coupled plasma atomic emission spectrometry (ICP-AES) [972]. The employed spectrometer can be coupled directly with an electrochemical cell wherein processes like corrosion or anodic dissolution occur. Continuous aspiration of very small liquid volumes transferred into the spectrometer allows determination of rates of dissolution as a function of various experimental parameters like electrode potential [973]. 

Since metals have very high conductivities, metal corrosion is usually electrochemical in nature. The tenn electrochemical is meant to imply the presence of an electrode process, i.e. a reaction in which free electrons participate. For metals, electrochemical corrosion can occur by loss of metal atoms tluough anodic dissolution, one of the fiindamental corrosion reactions. As an example, consider a piece of zinc, hereafter referred to as an electrode, inunersed in water. Zinc tends to dissolve in water, setting up a concentration of Zn ions very near the electrode... 

Ores are mined and are then refined in an energy intensive process to produce pure metals, which in turn are combined to make alloys (see Metallurgy Mineral RECOVERY and processing). Corrosion occurs because of the tendency of these refined materials to return to a more thermodynamically stable state (1—4). The key reaction in corrosion is the oxidation or anodic dissolution of the metal to produce metal ions and electrons... 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

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. ... 

Processes associated with two opposing electrode processes of a different nature, where the anodic process is the oxidation of a metal, are termed electrochemical corrosion processes. In the two above-mentioned cases, the surface of the metal phase is formed of a single metal, i.e. corrosion occurs on a chemically homogeneous surface. The fact that, for example, the surface of zinc is physically heterogeneous and that dissolution occurs according to the mechanism described in Section 5.8.3 is of secondary importance. 

The presence of water does not only create conditions for the existence of an electrolyte, but it acts as a solvent for the dissolution of contaminants [10], Oxygen plays an important role as oxidant element in the atmospheric corrosion process. The thickness of the water layer determines the oxygen diffusion toward the metallic surface and also the diffusion of the reaction products to the outside interface limited by the atmosphere. Another aspect of ISO definition is that a metallic surface is covered by adsorptive and/or liquid films of electrolyte . According to new results, the presence of adsorptive or liquid films of electrolyte perhaps could be not in the entire metallic surface, but in places where there is formed a central anodic drop due to the existence of hygroscopic particles or substances surrounded by microdrops where the cathodic process takes place. This phenomenon is particularly possible in indoor conditions [15-18],... 

Cathodic corrosion inhibitors reduce the corrosion rate indirectly by retarding the cathodic process which is related to anodic dissolution. In this process, access to the reducible species such as protons, to electroactive site on the steel, is restricted. Reaction products of cathodic inhibitors may not be bonded to the metal surface as strongly as those used as anodic inhibitors. The effectiveness of the cathodic inhibitor is related to its molecular structure. Increased overall electron density and spatial distribution of the branch groups determine the extent of chemisorption on the metal and hence its effectiveness. Commonly used cathodic inhibitor materials are bases, such as NaOH, Na2C03, or NH4OH, which increase the pH of the medium and thereby also decrease the... 

In most corrosion processes passivity is desirable because the rate of electrode dissolution is significantly reduced. The rate of aluminum corrosion in fresh water is relatively low because of the adherent oxide film that forms on the metal surface. A thicker film can be formed on the surface by subjecting it to an anodic current in a process known as anodizing. In most electrochemical conversion processes passive films reduce the reaction rate and are, therefore, undesirable. 

Consider a system consisting of a metal corroding in an electrolyte. The corrosion process involves a metal-dissolution deelectronation (anodic) reaction at electron-sink areas on the metal and an electronation (cathodic) reaction at electron-source areas. (This picture is applicable to a metal s corroding by a Wagner-Traud mechanism provided one imagines the sink and source areas shrunk to atomic-sized dimensions and considers the situation at one instant of time.)... 

The vast majority of engineering materials dissolve via electrochemical reactions. Chemical processes are often important, but the dissolution of metallic materials requires an oxidation of the metallic element in order to render it soluble in a liquid phase. In fact, there are four requirements for corrosion an anode (where oxidation of the metal occurs), a cathode (where reduction of a different species occurs), an electrolytic path for ionic conduction between the two reaction sites, and an electrical path for electron conduction between the reaction sites. These requirements are illustrated schematically in Fig. 1. 

Corrosion inhibitor - corrosion inhibitors are chemicals which are added to the electrolyte or a gas phase (gas phase inhibitors) which slow down the - kinetics of the corrosion process. Both partial reactions of the corrosion process may be inhibited, the anodic metal dissolution and/or the cathodic reduction of a redox-system [i]. In many cases organic chemicals or compounds after their reaction in solution are adsorbed at the metal surface and block the reactive centers. They may also form layers with metal cations, thus growing a protective film at the surface like anodic oxide films in case of passivity. Benzo-triazole is an example for the inhibition of copper cor-... 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

After a first sweep towards the positive which is not shown in the diagram and which is dominated by the dissolution of the airfoimed oxide layer, a sweep in the positive direction starts at the negative potential end of the cathodic part of the curve. In the first part, from A to the corrosion potential B where the curve becomes anodic, Hj evolution is the most important process. In this region both samples are very similar. The corrosion potential at B is nearly the same for unimplanted, with Cr implanted and with Ar bombarded iron. From B to C the anodic dissolution of the metal takes place and at C the active to passive transition starts. Here one observes the most significant difference between the two samples. The critical current density for passivation of implanted iron is more than one order of... 

From this physical model, an electrical model of the interface can be given. Free corrosion is the association of an anodic process (iron dissolution) and a cathodic process (electrolyte reduction). Ther ore, as discussed in Section 9.2.1, the total impedance of the system near the corrosion potential is equivalent to an anodic impedance Za in parallel with a cathodic impedance Zc with a solution resistance Re added in series as shoxvn in Figure 13.13(a). The anodic impedance Za is simply depicted by a double-layer capacitance in parallel with a charge-transfer resistance (Figure 13.13(b)). The cathodic branch is described, following the method of de Levie, by a distributed impedance in space as a transmission line in the conducting macropore (Figure 13.12). The interfacial impedance of the microporous... 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Dissolution of PS. The dissolution of PS during PS formation may be due to two proeesses a proeess in the dark and a proeess under illumination. Both are essentially eorrosion proeesses by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The corrosion process is responsible for the formation of micro PS of certain thickness (stain film) as well as the dissolution of the existing PS. The material in the PS which is at a certain distance from the pore tips is little affected by the extanal bias due to the high resistivity of PS and is essentially at an open-circuit condition (OCP). This dissolution process, which is often referred to as chemical dissolution, is an electrochemical process because it involves charge transfer across the interface. The anodic and cathodic reactions in the microscopic corrosion cells depend on factors such as surface potential and carrier concentration on the surface which can be affected by illumination and the presence of oxidants in the solution. 

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