Corrosion process reduction-oxidation reaction

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-... 

Chemical and electrochemical processes that cause materials corrosion usually involve both reduction-oxidations and acid-base reactions. The reduction-oxidation reaction is dependent on the electron energy level of the particles involved in the reaction, and hence managing the electrode potential of corroding materials may control the corrosion reaction. The acid-base reaction, on the other side, is determined by the HSAB characteristics (hard and soft acids and bases) of the particles involved in the reaction. It is mainly through the acid-base property that the environmental substances such as aggressive salts affect the corrosion of solid materials. 

Our comprehensive understanding of materials corrosion fundamentals has advanced considerably over the decades. Modern corrosion science has made it clear that the corrosion process on metals and semiconductors consists of an anodic oxidation and a cathodic reduction both occurring across the material-aqua-solution interface. These reduction-oxidation reactions depend on the interfacial potential and hence on the electrode potential of materials. 

As the molten salt is electrolytic. Hot Corrosion processes involve electrochemical reactions like oxidation of the metal and reduction of melt components and dissolved gases. Hence, many of investigations of Hot Corrosion have been done by electrochemical techniques, mostly combined with conventional corrosion... 

The second chapter is by Aogaki and includes a review of nonequilibrium fluctuations in corrosion processes. Aogaki begins by stating that metal corrosion is not a single electrode reaction, but a complex reaction composed of the oxidation of metal atoms and the reduction of oxidants. He provides an example in the dissolution of iron in an acidic solution. He follows this with a discussion of electrochemical theories on corrosion and the different techniques involved in these theories. He proceeds to discuss nonequilibrium fluctuations and concludes that we can again point out that the reactivity in corrosion is determined, not by its distance from the reaction equilibrium but by the growth processes of the nonequilibrium fluctuations. ... 

Whether a particular corrosion process is possible is determined by whether the line for hydrogen evolution or for oxygen reduction lies above (at a more positive potential than) the boundary for the oxidation half-reaction. This corresponds to a total negative free energy change. Nevertheless passivation often occurs, blocking further corrosion. In Fig. 16.1 a an example would be the zone where Fe203 is formed. 

In a general sense, oxidation is a reaction in which a substance (molecule, atom or ion) loses electrons. These are transferred to another substance called - oxidant. The oxidation number of the substance being oxidized increases. Oxidation and reduction always occur simultaneously. In nature, oxidation reactions play an important role, e.g., in - respiration, metabolic processes, photooxidation, - corrosion and combustion, and, most importantly in electrochemistry, oxidation processes proceed at - anodes. 

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. 

Most metals occur naturally in their oxide or sulfide forms. The process of metal refining converts these ores into pure metals. Thermodynamically, a metal will return spontaneously to its original oxide form. Metal oxidation can occur at high temperatures, by direct reaction with O2, or at a moderate temperature by reaction with water, O2, and/or H+. The latter oxidation, commonly referred to as wet corrosion, has as its basis the combination of electrochemical cathodic reduction and anodic metal oxidation reactions into a corrosion cell. Thus, many corrosion processes are... 

During the corrosion process, it is important to realize that both the anodic reaction (oxidation, for example, M — Mm+ + me) and the cathodic reaction (reduction, for example, 2H+ + 2e — H2) occur on the same metal in this case, therefore, the electron-conducting phase for both the LHE and the RHE would be the metal, M. 

A fundamental understanding of oxidation-reduction reactions is vital to the inorganic chemist in contexts ranging from energy transduction - chemical to electrical and the converse, in technical matters in corrosion processes and metallurgy, redox processes in environmental chemistry and metalloenzymes and metallo-proteins involved in electron transfer. Electron-transfer reactions of transition metal complexes are accompanied by a change in the oxidation state of the metal... 

The chemical reactions that take place in corrosion processes are reduction-oxidation (redox) reactions. Such reactions require a species of material that is oxidized (the metal), and another that is reduced (the oxidizing agent). Thus the complete reaction can be divided into two partial reactions one, oxidation the other, reduction. In oxidation, the metal loses electrons. The zone in which this happens is known as the anode. In the rednction reaction, the oxidizing agent gains the electrons that have been shed by the metal, and the zone in which this happens is the cathode. 

The principles that govern electrochemistry at semiconductor electrodes can also be applied to redox processes in particle systems. In this case, one considers the rates of the oxidation and reduction half-reactions that occur on the particle, usually in terms of the current, as a function of particle potential. One can use current-potential curves to estimate the nature and rates of heterogeneous reactions on surfaces. This approach applies not only to semiconductor particles, but also to metal particles that behave as catalysts and to surfaces undergoing corrosion. 

Mossbauer spectroscopy has been established already as a very useful tool for studying reaction mechanisms in a number of solid systems (50,51). The geochemical and environmental applications include monitoring of alterations, weathering, and oxidation processes in minerals and monitoring of corrosion processes in metals. The chemistry of the environmental processes, such as oxidation and reduction in sediments, can also be followed by characterizing the chemical states of iron contained as in situ probes in such systems. 

You ve heard electrochemistry of corrosion as a lecture I shouldn t spend much time on it but I d like to describe some electrochemical effects for film formers. First the general principles. If you put a good electronic conductor (a metal) in an aqueous solution, you will typically find that an electrical potential is developed between the piece of conductor and the solution. When ions of the metal enter the solution and leave extra electrons behind a negative potential is developed. All oxidation reactions occurring on the surface are expected to produce this result. Similarly, reduction reactions that use electrons from the metal are expected to produce a more positive potential in the metal. The solution potential of the metal influences the rate of an electrochemical half-cell reaction in accordance with Le Chatelier s Principle, so it is possible to predict through the use of the Nernst Equation the potential that will exist when the only significantly rapid reactions are the oxidation and reduction parts of a reversible reaction. When more than one potentially reversible process occurs, the rate of oxidation will be expected to exceed the rate of reduction for at least one and the converse for at least one. At... 

Equilibrium is disturbed when a net forward or backward reaction occurs, producing current in the external circuit. The current induces a potential change and causes polarization. Charge conservation requires the total rate of oxidation be equal to the total rate of reduction for any corrosion process. To avoid accumulation charge in the electrode, the sum of anodic currents must equal the sum of cathodic currents. 

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