Gradient multilayer aluminium sheets used in automotive heat exchangers

The local electrochemical measurement and corrosion mechanism of the multi-layer aluminium sheets (AA4343/AA3xxx/AA4343 and AA7072/AA3xxx/AA4343) after brazing are determined by a combination of mechanical polishing and work function calculation based on density functional theory. The work functions of (100), (110), and (111) crystal planes of Si and alpha-Al(Fe,Mn)Si particles which are common in multi-layer aluminium sheets are calculated. The results demonstrate Si and alpha-Al(Fe,Mn)Si particles are considered to be cathodic with respect to the Al matrix, thus promoting cathodic reactions. The potential differences between the Si and alpha-Al(Fe,Mn)Si particles with respect to the Al matrix may lead to localized dissolution of the Al matrix. Besides, the microstructure and composition of multi-layered aluminium sheets are studied by optical microscopy, scanning electron microscopy, and glow discharge optical emission spectrometry and compared with those obtained for conventional multi-layer aluminium sheets (AA4343/AA3xxx/AA4343). The presence of the AA7072 layer reduces the outward diffusion of copper. In addition, the AA7072 sacrificial layer exhibits a large potential difference against the core material for a long time, thereby improving corrosion protection.