Corrosion process cathodic dissolution

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). 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

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. ... 

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],... 

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.)... 

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-... 

Babu and coworkers [26,27] investigated the copper dissolution in the presence of Fe using a copper rotating disk electrode (RDE). The cathodic reaction was separately studied using a platinum rotating disk electrode, while the overall corrosion process was measured on rotating disks. It was... 

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. 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

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. 

The tip and substrate current spikes in Figure 46 are generally well correlated (particularly at times greater than 8 s), suggesting that the breakdown of the passive layer (substrate current) involves the release of Fe2+ from the iron surface, which was detected by reduction to Fe(0) at the tip UME. Evidence for the presence of Fe(0) at the tip came from the visual observation of a reddish-brown film at the electrode surface after such measurements and cyclic voltammograms (CVs) recorded with the tip positioned close to the iron surface, before and after a corrosion experiment. Prior to corrosion measurements, the tip CV displayed features consistent only with the reduction of TCA, while after corrosion the CV also showed a cathodic wave, possibly due to the reduction of Fe2+ to Fe and a corresponding anodic stripping peak. The latter occurred at the same potential as the anodic dissolution of iron, and was thus attributed to the reoxidation of Fe(0). Denuault and Tan (68,69) used a similar approach to identify the dissolution products for mild steel subjected to an acidic corrosive environment. In contrast to the work of Wipf and Still, the tip electrode was used only as a detector and not as an initiator of the corrosion process. CVs recorded with the tip placed close to the substrate detected the presence of Fe2+ and H2. 

The rate of a corrosion reaction is affected by pH (via H reduction and hydroxide formation), the partial pressnre of O, (the solubility/concentration of oxygen in solution), fluid agitation, and electrolyte condnctivity. Corrosion processes are analyzed using the thermodynamics of electrode reactions, mass transfer of the cathode reactants O2 and/or H, and the kinetics of metal dissolution reactions [157, 158]. 

Tafel plots of E vs. log /, such as those shown in Figure 26.31, are often used to determine the rate of a corrosion process. For a corroding metal (anode) that is driven by a single kinetically controlled reduction reaction (such as hydrogen evolution from an acid-containing solution), one can write the following Tafel equations for cathodic proton reduction and anodic metal dissolution ... 

It is generally assumed that ions which can accelerate either or both partial reactions in a corrosion process are capable of being adsorbed on the iron surface. Thus it is known that hydrogen sulfide ions which accelerate both partial reactions of acid corrosion (although predominantly the anodic one), and formic acid molecules which catalyze the cathodic partial reaction but inhibit the anodic one, as well as commercial inhibitors which reduce both partial reactions, are in fact adsorbed on the iron surface. As a consequence the mere fact that adsorption takes place cannot be used to predict an expected change in corrosion rate as it is also known that halide ions cat-alize the anodic dissolution of indium, while hydroxyl adsorption catalyzes the anodic dissolution of iron. Furthermore, it is also known that certain ions can act either as a catalyst or an inhibitor when adsorbed on the metal surface depending on the type of metal considered. Kolotyrkin (18) observed that the adsorp-... 

The passive film that forms on zinc not only reduces the rate of the anodic process (zinc dissolution), but even hinders cathodic reactions of oxygen reduction and hydrogen development. In conditions of passivity, the corrosion potential of galvanized steel is therefore much lower than that of carbon steel. Values typically measured are between -600 and -500 mV SCE compared to values above -200 mV usually found for passive carbon-steel reinforcement. 

Whether the total corrosion process is determined by the kinetics of anodic metal dissolution or the cathodic process depends on the size of the cathode and the kinetics of the partial electrode processes. The slowest reaction is the rate-determining step, as is usual in kinetics. In the case of a well-passivated valve metal, this is most probably the cathodic reaction, whereas for metals with semiconducting oxides, the rate-determining step win he anodic metal dissolution. In order to study the partial reactions of pitting corrosion separately, potentiostatic experiments are preferred. The cathodic process is replaced hy the electronic circuit of the potentiostat to investigate the anodic metal... 

When more and less noble materials are placed in contact, the more noble material offers an extra area for the cathodic reaction. Therefore flie total rate of the cathodie reaction is increased, and this is balanced with an increased anodic reaction, i.e. increased dissolution of the less noble material (galvanic corrosion. Section 7.3). If the more noble material (the cathodic material) has a large surface area and the less noble metal (the anodic metal) has a relatively small area, a large cathodic reaction must be balanced by a correspondingly large anodic reaction concentrated in a small area. The intensity of the anodic reaction, i.e. the corrosion rate (material loss per area unit and time unit) becomes high. Thus, the area ratio between the cathodic and the anodic materials is very important and should be kept as low as possible. It should be mentioned that in a galvanic corrosion process, the more noble material is more or less protected. This is an example of cathodic protection, by which the less noble material acts as a sacrificial anode (see next section). 

A mechanism describing corrosion protection must specify the partial reaction of the corrosion process, which will be inhibited. This can either be the anodic metal dissolution or the cathodic process, which in most cases is oxygen reduction. A long-discussed mechanism dating back to Wessling is based on the assumed passivation of the metal surface by the positive redox potential of the conducting polymer, which in this case is polyaniline. 

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