Anode reaction with passivation

It is now appropriate to consider the kinetics of the anodic reaction with particular reference to the phenomenon of passivity, but since the mechanism is dealt with in detail in Section l.S this discussion will place the emphasis on the anodic E-i curves. 

After the initiation period of corrosion pits, filiform corrosion dominates the morphology as narrow semi-cylindrical corrosion filaments project from the pit. Lunder et al. (1990) observed that propagation of the filaments occurs with voluminous gas evolution at the head while the body immediately behind passivates. Electrochemical transport of chloride ions to the head of the filament appears to be an essential component as is precipitation of insoluble Mg(OH)2 by the anodic reaction with Mg ions elsewhere along the filament (Ghali et al, 2004). 

Chromium plating from hexavalent baths is carried out with insoluble lead-lead peroxide anodes, since chromium anodes would be insoluble (passive). There are three main anode reactions oxidation of water, reoxidation of Cr ions (or more probably complex polychromate compounds) produced at the cathode and gradual thickening of the PbOj film. The anode current density must balance the reduction and reoxidation of trivalent chromium so that the concentration reaches a steady state. From time to time the PbOj film is removed as it increases electrical resistance. 

Passivity the state of a metal in which a low corrosion rate is brought about by reaction with its environment under a high anodic driving force through formation of a surface barrier film, usually an oxide. 

With regard to rechargeable cells, a number of laboratory studies have assessed the applicability of the rocking-chair concept to PAN-EC/PC electrolytes with various anode/cathode electrode couples [121-123], Performance studies on cells of the type Li°l PAN-EC/PC-based electrolyte lLiMn20 and carbon I PAN-EC/PC-based electrolyte ILiNi02 show some capacity decline with cycling [121]. For cells with a lithium anode, the capacity decay can be attributed mainly to passivation and loss of lithium by its reaction with... 

A 1.0 m NiS04(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction (b) the anode reaction, (c) With no overpotential or passivity at the... 

The electrochemical impedance gradually decreases but does not vary very much with the DDTC concentration increasing. It indicates that DDTC takes part in the electrochemical reaction and the reaction rate increases as the DDTC concentration enhances. As contrasted with it, the passivation of the collector-salts of reaction production on the mineral electrode further inhibits the anodic reaction so that the electrochemical resistance is wholly much bigger than that of self-corrosive reaction in the absence of DDTC. 

In considering the selection of anodes for high energy density (HED) storage (or secondary) batteries (SB), we note that there are some 19 metals whose free-energy density (TED) of reaction with oxidants such as O2, Cl2, and F2 are higher than those of Zn with the same oxidants. Most of these metals react violently with water. The remainder are passivated by water. Therefore other electrolytes must be considered for these metals, based on non-aqueous, molten salt, or solid-state ionic conductors. Much experimental work has been carried out during the last two decades on primary and secondary batteries based on anhydrous electrolytes, aimed at utilization of the active metals. 

A corrosion-inhibiting admixture is a chemical compound which, when added in small concentrations to concrete or mortar, effectively checks or retards corrosion. These admixtures can be grouped into three broad classes, anodic, cathodic and mixed, depending on whether they interfere with the corrosion reaction preferentially at the anodic or cathodic sites or whether both are involved [48]. Six types of mechanisms, viz. anodic (oxidizing passivators), anodic (non-oxidizing passivators), cathodic, precipitation... 

It is well known that the oxidation of phenolic compounds at solid electrodes produces phenoxy radicals, which couple to form a passivating polymeric film on the electrode surfaces [20,21]. The anodic reaction proceeds through an initial one-electron step to form phenoxy radicals, which subsequently can undergo either polymerization or further oxidation with the transfer of oxygen from hydroxyl radicals at the electrode... 

When a gold anode is rendered passive in hydrochloric acid solution the conditions appear to be somewhat different from those described above experiments show that the gold dissolves in the tervalent state to form AuCli" ions in solution. There is consequently a limiting c.d. determined by the maximum rate of diffusion of chloride ions to the anode (cf. p. 459) when this rate is exceeded the potential must rise so as to permit another process to occur. The gold then becomes covered with a layer of oxide, produced either by reaction of Au++" ions with water, or by the direct action of oxygen or hydroxyl radicals, and ceases to dissolve. ... 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

In deaerated 1 N H2SO4 (pH = 0.56), hydrogen-ion reduction is the cathodic reaction with the cathodic polarization curve intersecting the iron, nickel, and chromium curves in the active potential region. Hence, active corrosion occurs with hydrogen evolution, and the corrosion rates would be estimated by the intersections of the curves. The curves predict that the titanium will be passivated. However, the position ofthe cathodic hydrogen curve relative to the anodic curves for titanium and chromium indicates that if the exchange current density for the hydro-... 

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