Passivation phenomena

Oxide, hydroxide and basic salts of aluminum are less soluble at pH values of about 7 than those of zinc [17], which explains the easy passivatability. Galvanic anodes of aluminum alloys are primarily employed in the area of offshore technology. The anodes work in relatively pure seawater flowing with a high velocity so that by using suitable alloys, passivation phenomena are rare. Their low weight is particularly favorable in view of a service time of 20 to 30 years. 

Palladium and gold Palladium electrodeposition is of special interest for catalysis and for nanotechnology. It has been reported [49] that it can be deposited from basic chloroaluminate liquids, while in the acidic regime the low solubility of PdCl2 and passivation phenomena complicate the deposition. In our experience, however, thick Pd layers are difficult to obtain from basic chloroaluminates. With different melt compositions and special electrochemical techniques at temperatures up to 100 °C we succeeded in depositing mirror-bright and thick nanocrystalline palladium coatings [10]. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

The character of the passivation phenomena that can be observed at a given metal strongly depends on the composition of the electrolyte solution, particularly on solution pH and the anions present. The passivating tendency as a rule increases with increasing pH, but sometimes it decreases again in concentrated alkali solutions. A number of anions, particularly the halide ions CU and Br, are strong activators. 

Passivation phenomena on the whole are highly multifarious and complex. One must distinguish between the primal onset of the passive state and the secondary phenomena that arise when passivation has already occurred (i.e., as a result of passivation). It has been demonstrated for many systems by now that passivation is caused by adsorbed layers, and that the phase layers are formed when passivation has already been initiated. In other cases, passivation may be produced by the formation of thin phase layers on the electrode surface. Relatively thick porous layers can form both before and after the start of passivation. Their effects, as a rule, amount to an increase in true current density and to higher concentration gradients in the solution layer next to the electrode. Therefore, they do not themselves passivate the electrode but are conducive to the onset of a passive state having different origins. 

From the above discussion, it is clear that the stabilization or failure of graphite electrodes depends on a delicate balance between passivation phenomena (due to the formation of highly cohesive and adhesive surface films) and a buildup of internal pressure due to the reduction of solution species inside crevices in the graphite particles. This delicate balance can be attenuated by both solution composition (EC-DMC vs. EC-PC or PC, etc.) and the morphology of the graphite particles (i.e. the structure of the edge planes and the presence of crevices). 

In alkaline (NaOH) solutions [153], the role of Pb(OH)2 and PbO is important for passivity phenomena, and the voltammetric curves represent active, passive, and transpassive regions [335]. 

Solvents were initially selected primarily on the basis of the conductivity of their salt solutions, the classical example being propylene carbonate (PC). However, solutions based on PC on its own were soon found to cause poor cyclability of the lithium electrode, due to uncontrolled passivation phenomena. Solvent mixtures or blends were therefore developed and selection currently focuses on a combination of high dielectric solvents (e.g. ethylene carbonate, EC) with an alkyl carbonate (e.g. dimethy(carbonate, DMC), to stabilize the protective passivation film on the lithium electrode, and/or with a low viscosity solvent [e.g. 1,2-dimethoxyethane (DME) or methyl formate (MF)], to ensure adequate conductivity. 

On most corroding metals, the above reactions occur at an oxidized surface and, depending on the peroperties of the surface layer, passivation may occur by which the kinetics of metal dissolution are substantially supressed either by ohmic, ionic, or electronic transport at a surface passivating film or by electrocatalytic hindrance. In passivation phenomena, a steady state with a balance between the formation and dissolution of the surface film takes place. As a result, the ionic flux of metal ions dissolving through the passivating film is highly reduced. 

In usual polar aprotic solvents, all active metals (e.g., Li, Mg, Ca, and Al) are covered with surface films due to the reduction of solution components by the active metal and the consequent precipitation of insoluble species, but in the above molten salts these corrosion/passivation phenomena may be much less pronounced. Thus, deposition of divalent metals such as magnesium and calcium, which is not possible in usual polar aprotic systems because of the surface film barrier, may be feasible in electrolyte systems based on these molten salts. 

The last electrolyte system to be mentioned in connection with lithium electrodes is the room temperature chloroaluminate molten salt. (AlCl3 LiCl l-/ -3/ "-imidazolium chloride. R and R" are alkyl groups, usually methyl and ethyl, respectively.) These ionic liquids were examined by Carlin et al. [227-229] as electrolyte systems for Li batteries. They studied the reversibility of Li deposition-dissolution processes. It appears that lithium electrodes may be stable in these systems, depending on their acidity [227], It is suggested that Li stability in these systems relates to passivation phenomena. However, the surface chemistry of lithium in these systems has not yet been studied. 

Surface Chemistry of Lithiated Carbons and Passivation Phenomena... 

The effect of oxidizing agents on metallic dissolution has been studied rather extensively in recent years, particularly with respect to passivity phenomena. Stretcher (41) has made a detailed study of such reactions on stainless steels. Evans (42) has shown that metals such as Mg and Zn yield onlv hydrogen in dilute nitric acid but also nitrogen oxides in concentrated acid. The noble metals yield chiefly the nitrogen oxides. 

R. M. Hurd and N. Hackerman, Passivity phenomena at the silicon/electrolyte interface, Electrochim. Acta 9, 1633, 1964. 

Davis, H.J. Kinnibrugh, D.R. Passivation phenomena and potentiostatic corrosion in molten alkali metal carbonates. J. Electrochem. Soc. 1970, 117 (3), 392-396. 

Gallium is only slightly more noble than Zn. However, it dissolves in mineral acids slowly due to surface passivity phenomena. Hot, concentrated nitric acid is the most effective, dissolving 5 g. of Ga in 10 hours. Sebba and Pugh report achieving rapid solution of Ga In concentrated nitric acid if the metal, which disperses in tiny spheres due to the action of hot acid, is alternately cooled to a powdery acid-metal mixture and then reheated. 

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