Zn and Zn-Sn alloy coatings with and without chromate layers. Part I: Corrosion resistance and structural analysis

The aqueous corrosion resistances of Zn and Zn-Sn (similar to 20 wt% Sn) el ectrodeposits, passivated by immersion in chromating solution with differen t ratios of Cr(VI) to activating ions, are compared. The electrochemical be haviour of various chromated and nonchromated coatings were investigated in deaerated 0.5 mol dm(-3) Na2SO4/pH 5 solution using a.c. impedance and d.c . polarization techniques. The polarization curves revealed that the chroma te layers influence both the cathodic and anodic reactions. The corrosion r ate of each specimen decreases with time due to the accumulation of corrosi on products. The dark yellow (DY) chromate film on the Zn-Sn alloy and the iridiscent yellow (IY) on Zn yields the best protective ability in agreemen t with the assessment of prolonged salt spray chamber tests. These chromate layers resembling cracked mud become permeable to the electrolyte after im mersion and, as a consequence of the transformation and the leaching of cer tain Cr compounds, a very porous agglomerate of corrosion products forms. T he morphology and structure of dark yellow chromated Zn-Sn alloy was also i nvestigated by transmission electromicroscopy (TEM) and scanning electronmi croscopy with microprobe (SEM/EDS) analyses before and after corrosion. The depth profile of the corroded surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS).