A combined electron probe micro analysis and scanning Kelvin probe force microscopy study of a modified AA4xxx/AA3xxx aluminium brazing sheet

The electrochemical and microstructural properties of the clad and core from a modified AA3xxx/AA4xxx brazing sheet were investigated before and after brazing. For this, scanning Kelvin probe force microscopy (SKPFM) and field emission electron probe micro analysis (FE-EPMA) were used. The Volta potential difference (VPD) was measured as a function of depth for the brazed and non-brazed sheets. This was correlated with the cross-sectional variation of chemistry and microstructure that result from brazing. Furthermore, potentiodynamic polarization experiments and subsequent microscopic analysis of the corroded samples were used to explore the corrosion mechanism of the modified brazed sheet. The investigation revealed that the major consequences of brazing for the microstructure are: an increase in the Si content of the matrix in the heat affected zone (HAZ); a non-uniform distribution of Cu in the HAZ; an accumulation of Cu in Al–Si eutectics and around the grain boundaries, including α-Al(Mn,Fe)Si and Al2Cu intermetallics; the presence of some continuous grain boundaries in both the clad and core of the sheet; and the non-uniform precipitation of intermetallics at Al–Si eutectic phases. The impact of these microstructural changes on the corrosion behaviour of the brazed sheet was: to increase the VPD of the re-solidified clad matrix; to introduce localized corrosion susceptibility on the brazed clad and core structures; and to reduce the cathodic protection power of the re-solidified clad material. As a result of the correlation between microstructure and electrochemical behaviour, it was concluded that corrosion resistance of the modified aluminium brazing sheet could be improved by applying a post-braze heat treatment.