Corrosion copper, cathodic process

For example, in the case of copper coatings on iron, copper is usually cathodic compared to iron, and the corrosion products of copper may act as reagents for cathodic processes on the iron. Both the galvanic coupling and the corrosion products of the coating contribute to favor the anodic dissolution processes of the substrate. If the corrosion problem was the rate of penetration into the substrate, the coating would be inherently unreliable. 

The contaminants may be deposited on the surfaces of the materials in the form of anhydrous or hydrated species. Some pollutants, like CO2, SO, NO, and HCl, are typical of urban and industrial areas, give rise to acid rains, and might contribute to the cathodic processes, while others, such as chlorides, are typical but not exclusive of marine and coastal areas and give rise to hygroscopic salts that increase the duration of wetting of surfaces, increase the conductivity of solutions, and make less protective the corrosion products. Some others, such as the sulfides, which can result from microbiological activity, alter the composition of the corrosion products, their protective capability, and the nobility of the metal often they are semiconductors, depolarize the cathodic process of hydrogen evolution, and may be oxidized to sulfuric acid by bacteria. Ammonia alters the composition of corrosion products and the solubility of metal ions it has particularly drastic effects on copper alloys and their corrosion forms. In the transport of these contaminants toward the surfaces, an important role is exerted by the wind and by the orientation of the surfaces, which can promote or hinder the washout by the rains. 

If the cell is permitted to operate long enough, the anode will deteriorate, but the cathode will not. This is because anode ions are going into solution in the electrolyte, forming an oxide or similar compounds. This process releases hydrogen ions from the solution which migrate to the cathode and deposit electrons on it. If the anode were steel with a copper cathode, the steel would corrode and provide protection to the copper. This is the basic mechanism of a corrosion cell. 

The IBM group led by Brusic et al. [57,58] also studied the use of polyaniline derivatives for corrosion protection of copper as well as silver. The unsubstituted polyaniline, in neutral base form, provided good corrosion protection both at open-circuit potential and at high anodic potentials. The dissolution of metal (both Cu and Ag) was decreased by a factor of 100 when the metal surface was completely covered by the neutral polyaniline. However, polyaniline doped with dodecylbenzene-sulfonic acid (the conductive form of the polymer) increased the corrosion rate of Cu and Ag in water. The doped polymer in contact with the metal is spontaneously reduced at a rate faster than the oxygen reduction rate. The faster cathodic process in turn increases the overall rate of the anodic reaction, which is the dissolution of Cu and Ag, as opposed to the formation of a passive oxide layer. 

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 soluble copper ammonia ion passes through the condensate system and plates out as a cathode on steel surfaces in the deaerator, heaters, economizer, and the boiler itself. A secondary galvanic corrosion process is initiated that damages the surrounding steel by forming ferrous hydroxide and releasing copper and ammonia. The ammonia carries over into the steam, and the entire corrosion process repeats itself. 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

In certain chemical plants, the process solution being cooled is under pressure or is very corrosive. It is found expedient in some cases to put the low-pressure sea water on the shell side of the heat exchanger. Under these conditions, the steel shell will suffer more rapid attack because of galvanic coupling to the copper-base alloy tubing. However, only the outer tubes are seen by the shell in this couple. Nevertheless, this represents a large cathode. 

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... 

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. 

Figure 21.21 The effect of metal-metal contact on the corrosion of iron. A, When iron is in contact with a less active metal, such as copper, the iron loses electrons more readily (is more anodic), so it corrodes faster. B, When iron is in contact with a more active metal, such as zinc, the zinc acts as the anode and loses electrons. Therefore, the iron is cathodic, so it does not corrode. The process is known as cathodic protection.

Because of zinc s high electronegativity, this element must have participated in some manner with the corrosion processes. About the only possible indications from the electrochemical and x-ray evaluations made, are the small reduction peaks observed at about — 1.15 V for protein solutions on the 5 V/min cyclic voltamograms. These cathodic peaks for use as evidence in showing zinc corrosion may just as well be reduction of copper products, since cathodic peaks are shifted negatively with respect to their redox potentials at faster sweep rates. 

It is necessary to exceed the critical anodic potential (23) bd for the electrochemical breakdown of passivation by pitting and consists of these factors (i) presence of halides at the interface (ii) induction time for the initiation of the breakdown process, leading to localized conditions that may increase the localized corrosion current density (iii) development of favorable conditions inside the pits for propagation when the local sites become immobile and localized at certain sites. Electrochemical breakdown of some metal oxides is possible in the case of copper, lead, and tin cathodically to metal while ferric oxide is reduced to the ferrous ion in aqueous solutions. Zinc and aluminum oxides are not cathodically reducible and in these cases hydrogen is reduced. The vigorous evolution of hydrogen assisted by electron conducting zinc oxide can accelerate the breakdown of passivity. 

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