Electrochemical corrosion metal electrolyte systems

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material, corrosion products and possibly other deposits, a surface layer of water (often more or less polluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, which diffuses through the surface layer of water and deposits. As shown in Section 6.2.5, the thickness of the water film may have a large effect, but it is more familiar to relate atmospheric corrosion to other parameters. The main factors usually determining the accumulated corrosion effect are time of wetness, composition of surface electrolyte, and temperature. Figure 8.1 shows the result of corrosion under conditions implying frequent condensation of moisture in a relatively clean environment (humid, warm air in contact with cold metal). 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

The most important mechanism involved in the corrosion of metal is electrochemical dissolution. This is the basis of general metal loss, pitting corrosion, microbiologically induced corrosion and some aspects of stress corrosion cracking. Corrosion in aqueous systems and other circumstances where an electrolyte is present is generally electrochemical in nature. Other mechanisms operate in the absence of electrolyte, and some are discussed in Section 53.1.4. 

A layer that forms on top of a metal upon contact with the solution can be of three different types. If it is dense and nonconducting, it can protect the metal from further corrosion, but the system cannot be used as a battery, since the metal is totally isolated from the solution. If it is electronically and ionically conducting, reduction of the solvent at the film-electrolyte interface and oxidation of the metal at the metal-film interface can proceed freely, leading to a fast rate of self discharge. It is only when the film is both an ionic conductor and an electronic insulator that the chemical pathway of spontaneous reduction of the solvent at the anode is jirevented, whereas the electrochemical pathway of oxidation of the metal at the anode and reduction of the solvent at the cathode can proceed at a sufficient rate to allow the... 

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

An electrochemical method for fouling prevention could be effectively applied as the method works at the metal-electrolyte interface where the prevention is needed. Thus, an excess use of a chemical which is always necessary in the chemical mode of fouling prevention could be avoided in the electrochemical method. The method could be ideal for situations where biofouling has to be avoided at the earliest stage, for example, in an OTEC power plant. In some systems, it would be possible to prevent fouling and electrochemical corrosion simultaneously by a proper manipulation of the cathodic potential. 

Corrosion of metals is defined as their spontaneous deterioration by chemical interaction with the surrounding environment. It is a two-component system involving the interaction of the metal or alloy with a medium or environment. In the absence of an environment (e.g., vacuum), corrosion will not occur. Most corrosion reactions are electrochemical in natme, and for electrochemical corrosion to occur, a cell consisting of an anode, a cathode, an electrolyte, and a pathway for electron flow between the anode and the cathode is needed. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

Fig. 6 presents the system of A1 corrosion in 0.5 M NaCl solution, its frequency impedance characteristic in the form of Nyquist plot and the equivalent electrical circuit. Individual parts of the electric circuit reflect the electrochemical and electrical characteristics of the corrosion systems. In this arrangement, the spectral characteristic of the impedance in the Nyquist plot has the shape of a semicircle, whose intersection with the real axis in the high-frequency range determines the electrolyte solution resistance Rs. Conversely, the intersection of the real axis in the low-frequency range corresponds to the sum of Rs + Rci/ where Ret indicates the charge transfer resistance of the boundary metal/electrolyte, and characterizes the rate of corrosion. On the other hand, Cdi component of the circuit represents capacity of the double layer at the interface metal/electrolyte. 

Electrochemical reaction kinetics is essential in determining the rate of corrosion of a metal M exposed to a corrosive medium (electrolyte). On the other hand, thermodynamics predicts the possibility of corrosion, but it does not provide information on how slow or fast corrosion occurs. The kinetics of a reaction on a electrode surface depends on the electrode potential. Thus, a reaction rate strongly depends on the rate of electron flow to or from a metal-electrolyte interface. If the electrochemical system (electrode and electrolyte) is at equilibrium, then the net rate of reaction is zero. In comparison, reaction rates are governed by chemical kinetics, while corrosion rates are primarily governed by electrochemical kinetics. 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

The corrosion of steel or other metals in a boiler plant system takes place when an electrochemical cell is established. This occurs when two different metals (anode and cathode) are coupled together in water, which acts as the electrolyte in any steam-water circuit. 

Figure 29.4 shows an example, the energy diagram of a cell where n-type cadmium sulfide CdS is used as a photoanode, a metal that is corrosion resistant and catalytically active is used as the (dark) cathode, and an alkaline solution with S and S2 ions between which the redox equilibrium S + 2e 2S exists is used as the electrolyte. In this system, equilibrium is practically established, not only at the metal-solution interface but also at the semiconductor-solution interface. Hence, in the dark, the electrochemical potentials of the electrons in all three phases are identical. 

The most efficient system devised by Monsanto uses electrodes fabricated from carbon steel plate, electro-coated on one face with cadmium. These are stacked in parallel so that the electrolyte can be pumped through the gap between successive plates. Overall tire system forms a series of electrochemical cells with a cadmium cathode and a carbon steel anode. Each plate of metal forms the cathode of one cell and the anode of the next in the stack. Electric current is passed across the stack. The electrolyte contains phosphate and borate salts as corrosion inhibitors, EDTA to chelate any cadmium and iron ions generated by corrosion together with hex-amethylenebis(ethyldibutylammonium) phosphate to provide the necessary telraal-kylammonium ions. This electrolyte circulates through the cell from a reservoir and there is provision for the introduction of acjylonitrile and water as feedstock. The overall cell reaction is ... 

Modification of semiconductor electrode response with adsorbed or attached dye molecules is an attractive alternative to other photoelectrochemical systems (7-13). Metal oxides which are stable or have very low corrosion rates but are transparent to visible wavelength light can be used in light-assisted electrochemical reactions when modified with monolayers and multilayers of a wide variety of chromophores interposed between the electrode and electrolyte. With one exception, the initial reports of energy conversion efficiencies of electrodes with adsorbed dyes was disappointingly low. Recently however,... 

Probably the most extensive coatings employed in electrochemical systems are passive films that are formed on metal and semiconductor surfaces (21,22). These anodic films are responsible for the corrosion resistance of reactive metals, such as Fe, Cr, Ni, Ti, Zr, Zn, Cu, Sn, and Al, among others, in aqueous environments as well as for the operation of various electrochemical devices (e.g., electrolytic capacitors). Decorative coatings on aluminum, titanium, and zirconium are also formed anodically, with those for aluminum being very highly developed. The principal limitation in the knowledge of the growth and... 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

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