No abstract is provided for this article.
The presence of a heavily deformed and corrosion susceptible surface/near-surface region on rolled aluminium alloy has been amongst the most important problems which the aluminium manufacturers are dealing with. While primarily the composition of the alloying elements and the thermo-mechanical treatment play an important role in determining surface activation, the importance of several other secondary parameters cannot be ignored. This paper tries to identify the impact of several factors, associated with sheet metal production, on the corrosion behaviour e.g. (a) type of the cast bar fed into the hot rolling mill, whether scalped or not scalped, (b) importance of homogenisation and in-line heat treatment, in relation to pre-existing understanding of surface activation on AA3005, (c) effect of hot and cold rolling (d) effect of surface finish, etc. The alloy under investigation was recycled AA5050, containing high Fe and Si as impurities, and used commercially for architectural purpose, eg. window facades.
A full microstructural characterization of a modified AA4xxx/AA3xxx brazing sheet was obtained. The electrochemical nature and activity of the overall microstructure and microstructural heterogeneities were identified. Accumulation of alloying elements on the surface, localized corrosion susceptibility of the re-solidified clad material, and the increase in the Volta potential of the clad matrix which in turn reduces the cathodic protection power of the re-solidified clad towards the core, were found to be the major changes that occurred during the brazing process. The detailed understanding of the evolved electrochemical properties aims to develop strategies to enhance corrosion resistance by controlling the microstructure.
A better understanding of corrosion processes during the last years is leading to an increased demand for numerical corrosion models. Corrosion models become increasingly relevant toward simulations, lifetime predictions, and the optimization of corrosion prevention. Also, the improvements in computational power and capacity give rise to an increased number of research projects in this field. Totally different modeling approaches are applied to a wide range of different corrosion processes. This leads to a dispersion of information in the literature. The aim of this paper is to give an overview of the present approaches in atmospheric corrosion modeling. In the past, atmospheric corrosion has been modeled based on empirical, historical data. These models provide fitted functions of the corrosion rate or damage as a function of time for different environmental parameters. These methods give no or little information about the underlying physicochemical phenomena that determine the corrosion processes. During the last decade, more and more work has been done on causal approaches. Therefore, the focus will be on these recent advances in atmospheric corrosion modeling. The links will be made to the microscopic models of isolated corrosion phenomena. The examples are crevice corrosion, corrosion under porous layers, and the modeling of local electrochemical methods. Also, some more general, macroscopic approaches will be discussed. These macroscopic approaches will be compared to each other and positioned into the scope of multiscale modeling. The first goal of atmospheric corrosion modeling is to provide tools in the understanding and quantification of the processes under the influence of external atmospheric conditions. These models could reduce the current corrosion assessment methods that essentially rely on empirical models. On a longer term, modeling would aid in optimizing the material selection, structural design, and maintenance management.
Macro- and micro-electrochemical properties of clad and core surfaces of a modified AA4xxx/AA3xxx brazing sheet material, before and after brazing, have been evaluated and compared. By scanning Kelvin probe force microscopy (SKPFM), the Volta potential distribution over the brazed and non-brazed clad surfaces was measured. The changes in the Volta potential maps were correlated to the macro-electrochemical responses of the surfaces and the microstructural features that evolve as a result of brazing. By performing potentiodynamic polarization experiments and microscopic analysis of the corroded surfaces and cross sections, the suitability of SKPFM analysis for corrosion performance prediction of the aluminium brazing sheet material in a sea water acidified accelerated test (SWAAT) environment was confirmed. Considering the purity of Si phase in the structures of both brazed and non-brazed material, it is suggested that Si can be applied as a reliable local reference in both structures to compare the changes in Volta potential differences as the result of different heat treatments of aluminium brazing sheet. Increasing the copper content of the re-solidified clad material as a result of brazing treatment was found to increase the Volta potential of the matrix which in turn reduces the cathodic protection power of the re-solidified clad material towards the core material.
