Corrosion behaviour of Zn and Zn alloy coatings in alkaline media

The corrosion behaviour of electrodeposited Zn, Zn-Co, Zn-Fe and Zn-Ni coatings without and with chromate films was studied in alkaline solutions and mortar probes. DC polarization measurements were used for electrochemical characterization of the investigated samples, while AES/XPS techniques were applied for the chromate film analysis. The stability of the anodically formed oxide films on unchromated coatings appeared to be dependent on their composition. The oxide layer stability in Cl-free NaOH solution was higher for Zn-Ni and Zn-Co alloys, while introduction of Cl-ions caused lowering of stability especially for Zn-Ni coating. The detected corrosion current values implied that unchromated Zn alloys did not possess higher corrosion resistance in alkaline media containing Cl-ions. Meanwhile, chromated Zn-Co coatings exhibited the beneficial effect of alloying for corrosion in alkaline solutions and concrete. A higher concentration of Cr(VI) compounds in the passive chromate layer of the alloyed sample may be the reason for its superior corrosion resistance.