Aqueous layer anodic reaction rate

Under most atmospheric corrosion conditions, the anode reaction rather than the cathode reaction is observed to be the rate-limiting step [2]. Upon evaporation of the aqueous layer, a film of corrosion products—consisting of metal hydroxides or metal oxyhydroxides—may precipitate. With repeated condensation-evaporation cycles, this film usually hinders the transport of ions through the corrosion product or the transport of Me from the anodic site. Hence, the anodic reaction rate is lowered and, thereby, the atmospheric corrosion rate. 

When the aqueous layer is thin enough (less than 10 pm or so, see below) to permit ample access of oxygen to the metal surface, the anode reaction rather than the cathode reaction is rate limiting. This is the most common situation in atmospheric corrosion [5]. However, the surface and exposure conditions alter over a dry-wet-dry cycle or with extended exposure time and may eventually reach a situation in which the cathode reaction becomes the rate-limiting part (Sect. 3.1.2.3). 

Electrochemically generated nickei(lll) oxide, deposited onto a nickel plate, is generally useful for the oxidation of alcohols in aqueous alkali [49]. The immersion of nickel in aqueous alkali results in the formation of a surface layer of nickel(ll) oxide which undergoes reversible electrochemical oxidation to form nickel(lll) oxide with a current maximum in cyclic voltammetry at 1.13 V vj. see, observed before the evolution of oxygen occurs [50]. This electrochemical step is fast and oxidation at a prepared oxide film, of an alcohol in solution, is governed by the rate of the chemical reaction between nickel oxide and the substrate [51]. When the film thickness is increased to about 0.1 pm, the oxidation rate of organic species increases to a rate that is fairly indifferent to further increases in the film thickness. This is probably due to an initial increase in the surface area of the electrode [52], In laboratory scale experiments, the nickel oxide electrode layer is prepared by prior electrolysis of nickel sulphate at a nickel anode [53]. It is used in an undivided cell with a stainless steel cathode and an alkaline electrolyte. 

If the product layer is nearly free of pores, then the anodic dissolution of metal will practically cease. The metal is then said to be passivated . The thickness of the compact product layer will reach a stationary value. For oxide products which are essentially electronic conductors, this stationary thickness will be determined by the very low ionic conductivity in the oxide on the one hand, and by the rate of dissolution of the oxide in the electrolyte on the other. However, in many cases the oxide layers are porous, so that the electrolyte can continue to attack the metal, independently of the transport of ions and electrons in the oxide. From the above discussion it can be seen that corrosion reactions in aqueous ionic solutions in which a solid product layer is formed on a metal are among the most complicated of all heterogeneous solid state reactions. The reasons for this are the electrochemical nature of these reactions, the great number of possible elementary steps which can occur at the various phase boundaries, and electrical space charge phenomena which occur in the reaction product. 

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