Aqueous layer acid-dependent dissolution rate

The overall rate of ZnO aqueous leaching might be controlled by mass transfer in the boundary layer solid-liquid (16). At this layer, a constant low pH in the aqueous phase suggests a saturation of dissolved Zr. Kakovskii et al. (16) have reported, that at high acid concentrations the rate of ZnO dissolution is controlled by diffusion of Zn away from the siuface. This local saturation appears to influence eq. 4 and suggests a pH dependent dissolution-desorption step. This step was estimated with the Avrami-Erofeev rate law applying the results of the aqueous leaching data. 

One of the most important factors influencing the corrosion rate is the formation and protective ability of the corrosion products formed. The specific corrosion products formed are dependent on the peuticipating dissolved metal ions and the access to anions solved in the aqueous layer. Formation of the film of corrosion products take place in a sequence of consecutive steps— dissolution, coordination, reprecipitation. When the dissolution step is acid-dependent, coordination is based on the hard and soft acid base principle (i.e., hard acids are preferably coordinated with hard bases and soft acids are preferably coordinated with soft bases). Acids or bases with tightly held valence electrons that are not easily distorted are hard acids or bases. Acids or bases having valence electrons that are easily polarized or removed are considered to be soft acids or bases. 

As a consequence of exposure to the aqueous layer, the oxygen-containing film can dissolve— the corrosion rate being normally controlled by the rate of dissolution of the film formed [55], As mentioned in the preceding section, the dissolution rate of many oxides and other minerals is acid dependent and can be written in the form ... 

The atmospheric corrosion of zinc starts with the instant formation of a thin film of zinc hydroxide, which seems to occur in different crystal structures, and the subsequent formation of a protective layer of basic zinc carbonate, Zn5(C03)2(0H)g. The pH of the aqueous layer controls the stability of initial corrosion products and results in the dissolution of Zn +. From the HSAB principle one expects Zn, classified as an intermediate acid, to coordinate with a number of different bases. In accordance with this, the prolonged exposure of zinc can proceed along a variety of different paths of reaction sequences depending on the actual deposition rates of atmospheric constituents. Among these Cl and SO2 seem to be the most important. 

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