Impact of microstructural characteristics on the corrosion morphology of high strength brazing sheets

Conventional brazing sheets consist of a 3xxx (AlMn) core alloy, cladded with a near eutectic 4xxx (AlSi) layer. The substitution of the 3xxx alloy by an age hardenable alloy of the 7xxx (AlZnMg) series allows an increase of the mechanical properties of the automotive heat exchanger. Due to interdiffusion phenomena from the cladding layer into the 7xxx core alloy, a barrier interlayer has to be introduced.
Differences in the corrosion morphology of samples with changed chemical composition of both, core material and interlayer, are described. The corrosion potential difference between the AlZnMg core material and the AlZn interlayer were adjusted by variation of the Mn- and Si-contents of both materials. The samples were subjected to a modified SWAAT (Sea Water Acetic Acid Test) test. The corrosion morphology and the penetration depth were evaluated. SEM/EDX elemental map-pings helped to explain the exhibited difference of the corrosion attack mode.