Passive films duplex structure

The analysis of several pure metals and binary alloys yields generally at least a duplex and in some cases a multilayer structure of the passive film, as depicted schematically in Fig. 19. These systems have been examined with surface analytical methods, mainly XPS, but also ISS in some cases. The systematic variation of the electrochemical preparation parameters gives insight to the related changes of layer composition and layer development, and support a reliable interpretation of the results. Usually the lower valent species are found in the inner part and the higher valent species in the outer part of the passive layer. It is a consequence of the applied potential which of the species is dominating. Higher valent species are formed at sufficiently positive potentials only and may suppress the contribution of the lower... 

Duplex Films Various passive films are not homogeneous, but show a duplex structure or a continuous gradient... 

Fig. 45. Band structure model for passive layers on Cu for increasing potential explaining quantitatively the formation of the duplex film and the transpassive behavior [86],...

Silver is chemically close to copper. Oxide layers can be easily reduced on this seminoble metal and the polarization curve is qualitatively similar. However, there are also many differences for their passive behavior. Figure 5.21 shows the polarization curve in IM NaOH with two pronounced anodic and cathodic peaks [72]. Like in the case of Cu, Ag20 is formed at Al and it is further oxidized to AgO at A2. A duplex film structure is suggested for oxidation at E > 0.73 V (A2) in IM NaOH. 

The chemical structure of the passive layers formed in 0.1 M KOFI has been evaluated with XPS and ISS for Cu-4Sn and Cu-19Sn alloys as a function of potential and by potentio-static transients [123]. At E = -0.3 V, Cu-4Sn forms a thin film of SnO and Sn(OH)2 without Cu. When oxidation of Cu becomes possible at E = 0.2-0.8V, the passive layer consists of Cu oxides with only minor contributions of a few percents of Sn oxides decreasing with increasing potential. One observes a duplex film of CU2O/CUO, Cu(OH)2 like on pure copper. Only a small Sn content of less than 10% was observed in the center of the passive layer. At E = 0.5 V, this result was obtained for fp = 0.2 s to 90 min. 

Time-resolved studies are also important for a better understanding. It is a rule that the lower valent species are formed first and will be oxidized later. The layer structure develops within milliseconds or hours. This time scale depends on potential and thus on the driving force for the electrochemical reactions but also on the system. The duplex film on iron forms within a few seconds and it takes ca. 100s to get the final structure, which gets faster with increasing potentials. On the contrary, the accumulation Cr within passive layers on FeCr alloys and its change from a hydroxide to an oxide will get to its final stage after hours or days. 

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