Surface passive layers

Iron and Stainless Steel. The purpose of XPS investigations on typical corrosion systems like iron or stainless steel, is the determination of the composition of the passive surface layer, if possible, as a function of depth. As a consequence of the technical and economic relevance of corrosion reactions, XPS investigations on corrosion systems are numerous. With respect to the application of XPS, there is no difference between corrosion systems and any other electrochemical surface reaction like oxide formation on noble metals. Therefore, in this paragraph only a few recent typical results of such studies, using XPS, will be mentioned. For a detailed collection of XPS corrosion studies the reader is referred to references [43,104], A review of aqueous corrosion studies, using XPS, was given by McIntyre for the elements O, Cr, Mn, Fe, Co, Ni, Cu and Mo [105], The book edited by M. Froment [111] gives an impression of the research achieved on passivity of metals up to 1983. 

The lag between the time that nitinol, was first produced and the time it was used commercially in medical devices was due in part to the fear that nickel would leach from the metal and not be tolerable as a human implant. As it turns out, with a correct understanding of the surface electrochemistry and subsequent processing, a passivating surface layer can be induced by an anodizing process to form on the nitinol surface. It is comprised of titanium oxide approximately 20 mn thick. This layer actually acts as a barrier to prevent the electrochemical corrosion of the nitinol itself. Without an appreciation for the electrochemistry at its surface, nitinol would not be an FDA-approved biocompatible metal and an entire generation of medical devices would not have evolved. This is really a tribute to the understanding of surface electrochemistry within the context of implanted medical devices. 

This boundary condition might apply for solute absorption with its rate moderated by some thin passive surface layer. Note that the surface concentration at x = 0 must be a function of time to maintain the constant-flux condition (see Fig. 5.7). 

The reactivity of solids is sometimes reduced by the existence of unreactive (passivating) surface layers that are easily removed by mechanical energy. In this way the reactivity of hydrogenation catalysts such as nickel or platinum has been greatly increased. The use of metals and nonmetals in synthetic reactions under mechanical surface cleaning has been reviewed [50]. 

XPS study by Buckley and Woods (1984b) showed that freshly fractured chalcopyrite surfaces exposed to air formed a ferric oxyhydroxide overlayer with an iron-deficient region composed of CuSi. Acid-treated surfaces of fractured chalcopyrite showed an increase in the thickness of the CuS2 layer and the presence of elemental sulfur. Hackl et al. (1995) suggested that dissolution of chalcopyrite is passivated by a thin (< 1 pm) copper-rich surface layer that forms as a result of solid-state changes. The passivating surface layer consists of copper polysulhde, CuS , where n > 2. Hackl et al. (1995) described the dissolution kinetics as a mixed diffusion and chemical reaction whose rate is controlled by the rate at which the copper polysulhde is leached. The oxidation of chalcopyrite in the presence of ferric ions under acidic conditions can be expressed as... 

The effect of chlorate on the corrosion rate of Fe was studied [34] and it was reported that iron and steel corrode in 50% NaOH at 80°C at concentrations above 1% or below 0.01%. The corrosion rate decreases rapidly at concentrations of >1% and <0.01% NaQOs, because of the formation of passive surface layers (Fig. 14.31). Iron and steel passivate in concentrated NaOH at noble potentials in the presence of dissolved oxygen. However, the passive film is pitted in the presence of chloride ions [35], as shown in Fig. 14.32. 

As a result of its passive surface layer, aluminum alloys have a good corrosion resistance and a rather constant surface appearance. There are however some reasons, ex-emphfied in Table 37.4, for surface treatment of aluminum and its alloys. 

Figure 30 shows what happens to Li surfaces during Li deposition. As demonstrated in the AFM image and explained in the cartoon, fresh Li deposits emerge and grow at locations of high ionic conductivity at the surface films, out of the passivating surface layer. Hence, fresh Li is exposed to the solutions, reacts with solution species, and both Li and solution species are irreversibly... 

The reaction between the isocyanate groups near the droplet surface layer and water molecules are thought to generate more hydrophobic urea groups that form a passivated surface layer to retard further reaction of the isocyanate groups with water inside the droplets. 

While in alkaline solutions, the problem of aluminum corrosion has to be overcome by protecting the metal by adding inhibitors like zinc oxide [5], zincate ions [6], a combination of zinc species with organic additives [7], or by replacing water in the electrolyte with methanol [8] the aluminum anode in seawater has to be activated by breaking the passive surface layer with suitable etchants [9-12]. 

Figure 6.33. Iron is a base metal that decomposes by electrochemical corrosion in a humid environment. If iron (or steel) is embedded in concrete which is strongly alkaline, however, it forms a passivating surface layer of dense iron oxides. This passivation is decisive of the corrosion resistance of reinforcing bars.

Electroetching (cleaning) The electrolytic removal of material from an anodic surface without the presence of a passivating surface layer. See also Electropolishing. 

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