Enhanced corrosion protection by microstructural control of aluminium brazing sheet

Aluminium brazing sheet is a sandwich material made out of two aluminium alloys (AA4xxx/AA3xxx) and is widely used in automotive heat exchangers. One of the main performance criteria for heat exchanger units is the lifetime of the product. The lifetime of the heat exchanger units is determined by their corrosion resistance. The most common way to measure the corrosion resistance of heat exchanger materials is the so called Sea Water Acidified Accelerated Test (SWAAT). Unfortunately there is no agreement on how corrosion performance should be evaluated in SWAAT. The first aim of the current research work was to perform a statistical and in-depth analysis of the critical parameters of the SWAAT. By keeping the climatic parameters constant, the correlation between the variation of the test results and the actual fundamental differences in the material condition i.e. temper and brazing condition was elucidated. Parameters that potentially could influence the outcome of the SWAAT test were investigated. The results were used to narrow the operation window of the test and making it more reproducible and reliable. The possibility of the presence of some links between SWAAT and electrochemical characterization were also investigated. Corrosion mechanism and morphology of attack in SWAAT and potentiodynamic test were compared. The result confirmed that the SWAAT performance of the material can be reflected in potentiodynamic responses. Finally a new statistical reliability evaluation approach and a novel result interpretation method were developed. Nowadays cost reductions are key within automotive industry and the current corrosion resistant alloys become less competitive. There is a strong demand for low cost alloys but still with the excellent corrosion protection properties of the expensive alloys. In depth knowledge of how alloy composition, microstructure and thermo-mechanical processing alter the corrosion resistance of aluminium brazing sheet is crucial to allow production of cost effective alloys with sufficient properties in the near future. Therefore the second aim of this research was to develop a fundamental understanding over the correlation between microstructural features and the resulting electrochemical responses of aluminium brazing sheet materials. The detailed and fundamental mechanistic understanding of the effects of microstructural variations and the consequent electrochemical reactivities was aimed at providing solutions for enhanced corrosion protection by microstructural control of aluminium brazing sheet. A full microstructural characterization of the material was btained. A complete electrochemical characterization of the structure at macro, localized and micro scales was achieved and correlated to the microstructural features developed as consequences of brazing treatment. The results were used to explain the corrosion propagation mechanism through the structure of the brazed material and to investigate possibilities for improving the corrosion resistance of the brazed structure. Using the generated knowledge over the correlation between the microstructural features and the electrochemical responses, it was concluded that applying a proper post-braze heat treatment may improve the corrosion resistance of the aluminium brazing sheet. An innovative technique for improving corrosion resistance of a modified AA4xxx/AA3xxx aluminium brazing sheet has been developed. In this process the controlled atmosphere brazed (CAB) material is subjected to a thermal post-brazing treatment. The investigations of the corrosion resistance of the post-braze heat treated material confirmed the effectiveness of the developed thermal treatment for improving the corrosion resistance of the brazing sheet material. The relatively high tolerance to increased Si and Fe alloying element concentrations in this post-braze heat treated structure will provide the opportunity for further usage of recycled aluminium in its production process. Therefore this new material is referred to as a green corrosion resistant aluminium brazing sheet. This new generation of aluminium brazing sheet is able to tolerate a higher amount of previously considered detrimental alloying elements and thus a lower need for purification during primary and recycling processing saving energy, material and labor costs.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin