Silane pre-treatments on copper and aluminium

Abstract

A large part of aluminium products are coated with an organic layer in order to improve the corrosion resistance. Copper surfaces are also sometimes protected with an organic coating to improve the durability or the aesthetic properties. Examples of industrial applications are household appliances and heat exchanger components. For these applications it is not rare to have the industrial need to treat at the same time components made of aluminium and copper. In order to extend the service life of the organic coated copper a specific surface pre-treatment is often required. Nevertheless, probably because of the limited market of this application, no specific pre-treatments for copper are industrially developed, with the exception of cleaning procedures, but simply extensions of existing pre-treatments optimised for other metals (aluminium, zinc) are used. The application of silane pre-treatments as adhesion promoters for organic coated metals is remarkably increasing in the last decade, because silanes offer very good performance together with high environmental compatibility. The idea is therefore to try to develop a specific silane based pre-treatment for copper. The starting point is the existing silane products for aluminium, optimising the composition and the application conditions (concentration, temperature, pH of the bath, etc.) in order to develop a high performance copper alloy pre-treatment increasing the protective properties and the adhesion of a successively applied organic coating. Moreover these pre-treatments could be used for aluminium alloys too and therefore could be suggested for multi-metals components. The deposits were analysed using FTIR spectroscopy and optical and electron microscopic observations. A careful electrochemical characterisation, mainly by electrochemical impedance spectroscopy measurements (EIS) was carried out to highlight the presence of silane and to evaluate the performance of the different deposits. In order to study an environmentally friendly protective system, an epoxy-polyester powder coating was deposited on the silane treated metal surfaces. The influence of the pre-treatments on the adhesion of an organic coating was evaluated by adhesion tests in dry and wet conditions and the delamination of the coating, starting from a defect under specific environmental conditions, was measured. Some preliminary results appear very interesting and the deposition conditions were optimised, in particular the thermal curing (200 °C) of the silane layer is necessary to have a good coverage in the case of a copper substrate. The good adhesion also in wet conditions of the silane pre-treatments was also proven. © 2006 Elsevier Ltd. All rights reserved.