An investigation was performed of the changes that occur in a typical epoxy-coated aluminium system due to exposure to water. The adhesion of the epoxy coating upon exposure to water was evaluated for different exposure temperatures and periods. The adhesion test results showed an initial loss of adhesion of the coating but after this the adhesion improved again and even significantly exceeded adhesion prior to exposure. The amount of adhesion improvement and the speed with which adhesion improvement occurred was found to be larger for higher exposure temperatures. The changes that occur in the epoxy-coated aluminium system due to exposure to water were investigated using a number of different analytical techniques. Based on this, a model was proposed for the processes that take place during exposure and which can explain the adhesion test results. First, the adhesion of the epoxy coating is lost upon exposure due to the accumulation of a significant amount of water at the interface. The water at the interface causes formation and growth of a thin oxyhydroxide layer underneath the epoxy coating. After some time, this oxyhydroxide layer re-establishes contact with the epoxy coating and forms a new, water-stable bond, hence explaining the improvement of the adhesion after its initial loss. The temperature-dependence observed in the adhesion test results is explained by the fact that at a higher exposure temperature, more water accumulates at the interface, the oxyhydroxide layer grows faster and also attains a larger limiting thickness.
The macroporous structure of poly(styrene-co-divinylbenzene) monolithic capillary columns has been optimized for the gradient separation of peptides. To exploit monolithic supports with porosity exceeding 70%, the thermodynamic properties of the polymerization mixture were carefully tailored to yield homogeneous monolithic materials featuring macropore and polymer microglobule sizes in the range of 50–200nm. The effects of (i) initiator content, (ii) composition of porogenic mixture, comprising tetrahydrofuran and 1-decanol, (iii) percentage of divinylbenzene crosslinker, and (iv) monomers to porogen ratio on the morphology was investigated. The resulting column structures were investigated using scanning electron microscopy and the prepared monolithic columns were tested for the separation of a tryptic digest of cytochrome c while applying a fixed flow rate and gradient time. To obtain a better understanding of the effects of macropore and microglobule size, and structure homogeneity on the separation performance in gradient elution, both in terms of peak capacity and gradient plate height, separations were also carried out at different flow rates while maintaining a constant gradient steepness. Furthermore, performance limits were determined applying ultra-high pressure conditions up to the maximum system pressure of 80MPa. The potential of monolithic nanostructured columns is demonstrated for the separation of tryptic digests of cytochrome c and bovine serum albumin.
Susceptibility to localised corrosion is strongly affected by heat treatments performed on Al–Zn–Cu–Mg alloys. In order to study how galvanic coupling between intermetallics and matrix is affected by solution heat treatment, AA7075-T6 and solution heat treated AA7075 have been characterised by means of scanning electron microscopy and scanning Kelvin probe force microscopy. Solution heat treatment strongly increased the Volta potential difference between the intermetallics and the surrounding matrix showing a strong increase in galvanic coupling. This is explained by Zn and Mg enrichment of the matrix caused by dissolution of strengthening particles during solution heat treatment.
Using a temperature controlling electrode holder anodizing of aluminium is studied under conditions of applied electrode temperature. At different current densities the influence of the electrode temperature on the anode potential and oxide morphology is evaluated. Additionally, the results are compared to those of experiments performed under conditions of uncontrolled electrode temperatures at various electrolyte temperatures. By increasing the electrode temperature the anode potential decreases at all considered current densities, this even at a constant electrolyte temperature. The stationary, as well as the initial maximum potential decline; though, at high electrode temperatures the latter is more sensitive to temperature variations. Due to the observed large decline of the maximum potential at high electrode temperature, the possibility to grow oxide layers at very high current densities without encountering burning is evaluated at low electrolyte temperatures. At high electrode temperatures of 65°C the initial maximum potential is reduced to such an extent that at an electrolyte temperature of 25°C anodizing can be performed up to 25A/dm2 without observing burning. By increasing the electrolyte temperature up to 45°C, even at 30A/dm2 burning does not occur during anodizing at applied electrode temperature of 65°C. This is not possible under conditions of uncontrolled electrode temperature, which require higher electrolyte temperatures to grow oxide layers at very high current densities without encountering burning.
No abstract is provided for this article.
No abstract is provided for this article.
In this work the AC-graining in hydrochloric acid of the aluminium alloy 1050 was studied. The influence of the electric charge and of the current density on the smut layer formation and on the graining morphology was investigated.
No abstract is provided for this article.
Atmospheric corrosion predictions for design and engineering require time-consuming exposure experiments and provide only a rough estimation of the corrosion rate. At the other hand, the multi-scale problem of atmospheric corrosion involves thin film electrolytes, evaporating -highly concentrated- solutions where local variations are controlling the overall process. Conventional (bulk) electrochemistry is therefore inadequate for the quantification or even the understanding of atmospheric corrosion mechanisms. The Multi-Ion Transport and Reaction Model (MITReM) is the link between the underlying electrochemical physics and corrosion rate predictions. With a numerical finite element method we simulate the effect of electrolyte thickness, electrode geometry and NaCl concentrations during evaporation in ultra-thin (sub 50 µm) electrolytes. The zinc corrosion rate in these thin films is function of the calculated local, time-dependent variations in the electrolyte. The two-dimensional simulations demonstrate the geometrical effect on the local current density and its deviation from simple limiting current equations. The time-dependency also describes the evolution of the electrolyte concentrations and the current transients. The simulated formation of corrosion products is compared with experimental measurements of the initial oxidation of zinc in humidified air. The evaporation/condensation dynamics of ultra-thin films in combination with complex electrolyte compositions introduces high computational costs. The application of these models is therefore limited to small or highly simplified geometries. It also implies limits on the time scales that can be simulated. To reduce the computational cost a decoupling method is developed to decrease the number of finite elements and hence the number of unknowns in the system. The transport in the direction of the electrode plane and the perpendicular direction are decoupled. The decrease in computational cost enables numerical simulations of complex electrochemical systems on extended complex geometrical shapes and for longer time scales. The decoupling method has an additional benefit; the complexity of moving triangular meshes is avoided since the electrolyte height geometry is a combination of one-dimensional meshes. The reduced decoupling model is compared with the full-scale simulations of a galvanic couple (zinc-steel). The results show that the decoupled model approaches the full-scale model for thinner electrolyte layers. This observation is supported by the evolution of the edge-effect, illustrated with two-dimensional simulations. In thin layers, the minimal accuracy loss is acceptable in comparison to the gain in computational time. The use of the dimension reduction for the simulation of atmospheric corrosion conditions is therefore justified. The presented work improves the developments of quantification and prediction tools for atmospheric corrosion and electrochemistry in non-conventional (micro-)setups. A better description and prediction of the corrosion process is obtained in ‘dynamic’ thin electrolyte layers.
