Silica passivation layer on aluminium brazing sheets

Abstract The request for more efficient fuel economy to save raw materials and to reduce air pollution becomes more and more crucial in the automotive industry. To fulfil this demand by weight reduction, conventional materials such as steel and copper are replaced by light metals. Due to its beneficial material properties e. g. low density, high strength, good formability and high thermal conductivity, aluminium becomes a frequently used candidate. Particularly in the heat-exchanger industry this change has already taken place. Heat-exchangers made of aluminium brazing sheets are used in various applications e. g. climate control, engine and transmission cooling and charge air cooling. Brazing sheets consist of a multilayer alloy system. A typical material combination in this field is an Al-Mn core alloy (AA3xxx) clad on one or both sides with an Al-Si alloy (AA4xxx). The benefit for this combination is given by the difference in the melting points of the two alloys. During the brazing process (around 590 °C) the core alloy provides structural integrity while the low melting point clad alloy melts and flows to provide upon cooling metallic bonding between the various components of the heat-exchanger. Due to the various conditions that a heat-exchanger is exposed to during its service life such as heating and cooling cycles, salt water environment on the surface and mechanical loading, the corrosion performance is a crucial point since it might cause perforation of the material resulting in a complete failure of the system. In order to understand the corrosion behaviour of the brazing sheets, it is necessary to have a deeper look at the microstructure, which is mainly determined by the composition of the alloying elements and heat treatments. The addition of alloying elements such as copper and iron provides beneficial material properties e. g. strength, good formability and brazeability, but can also change the corrosion mechanism from pitting, which is the preferred attack in pure aluminium, to more aggressive forms of localized attack, like intergranular and exfoliation corrosion. Furthermore thermal treatments such as solution heat treatment, aging, homogenisation or brazing significantly change the microstructure and hence modify the corrosion behaviour. To fulfil the requested corrosion performance, which is usually performed by salt spray testing (SWAAT), it is often necessary to apply a pre-treatment on the heat-exchanger. Besides protecting the aluminium brazing sheet of corrosion attack, the pre-treatment has to contain environmental friendly and low costs components. After accelerated corrosion testing of AA4045/3003/4045 brazing sheets by SWAAT and electrochemical techniques, the corrosion propagation was attributed to potential differences within the brazing sheet causing a galvanically driven perforation of the core material by the diffusion zone. Furthermore a correlation between the SWAAT and potentiodynamic and potentiostatic polarization measurements is observed, offering a substitution of the SWAAT by these electrochemical techniques and hence reducing the testing time from several weeks (SWAAT) to less that one day. Corrosion prevention is achieved by immersing of the brazing sheets in an aqueous sodium silicate solution. The resulting silica passivation layer consists mainly of a dense SiO2 network and provides a reduction of around 80% of the corrosion propagation during SWAAT exposure. It is shown that the barrier properties are dependent on the respective pre-treatments conditions such as bath concentrations, curing times, curing temperatures and dipping times