Microstructural and in‐depth electrochemical characterization of Zn diffusion layers on aluminum 3xxx alloy

AA 3XXX alloys are widely used in heating, ventilation, and air conditioning (HVAC) field. Diffusion joining using a filler metal together with flux is employed in some applications as for heat exchangers. In this work, the effect of diffusion of a Zn‐based flux on both microstructure and electrochemical behavior has been investigated. In particular, an AA3xxx was coated with a Zn‐rich flux and subjected to controlled atmosphere brazing (CAB). Glow discharge optical emission spectroscopy (GDOES) composition profiles were acquired in order to determine the Zn distribution in the diffusion layer. The GDOES was also employed to produce a controlled erosion of the surface in order to obtain craters with defined depths in the Zn diffusion layer, in which electrochemical analyses could be performed. The Volta potential maps at different depths in the Zn diffusion layer were obtained by scanning Kelvin probe force microscope (SKPFM). The Zn diffusion layer was also investigated by means of Scanning Electron Microscope‐Energy Dispersive X‐ray Spectroscopy (SEM‐EDXS) and the chemical composition of the phases present in the regions was investigated by SKPFM. Finally, the electrochemical microcell was used in the produced craters in order to determine the electrochemical behavior along the Zn diffusion profile. SKPFM and microcell results showed a correlation between the Zn content and the electrochemical properties. In particular, a higher Zn content in the diffusion layer leads to an increase of the Volta potential difference between the intermetallic particles and the matrix. The electrochemical measurements also showed that the Zn diffusion layer provides galvanic protection to the underlaying aluminum alloy.