Understanding corrosion mechanism of Sn-Zn alloys in NaCl solution via corrosion products characterization

The corrosion products formed on Sn–Zn solder alloys were systematically studied to elucidate the corrosion mechanism of Sn–Zn alloys in chloride‐containing solution. Sn–Zn alloys were subjected to immersion and polarization treatment in a 0.5 M NaCl solution. Ex situ analysis, including surface and cross‐section, of the corrosion products was carried out by scanning electron microscopy (FE‐SEM) and energy dispersive X‐ray spectroscopy (EDX). The corrosion products presented porous microstructure formed by a fine interlinked network. An increase of zinc content resulted in more pores and cracks of the corrosion products, indicating degradation of corrosion resistance. Intriguing is the finding that a gradient of chlorine concentration from the outer layer inwards the alloys through the Zn‐rich precipitates is observed, which demonstrates the migration of chloride ions inwards the alloys. Based on the results, a corrosion mechanism was proposed and discussed that explains the observations. Particular attention was paid to the anodic dissolution of Zn‐rich precipitates.