Polymerised forms in the zirconium conversion coatings on cold-rolled steel: proof of concept

Abstract

This study validates the proposed polymerised structure, including tetrameric polynuclear species, of solid amorphous oxyhydroxide zirconium conversion coatings on cold-rolled steel using ToF-SIMS. Tetramers are formed at pH near 4 (and possibly higher), with thickness increasing over extended conversion times. EIS in simulated acid rain further demonstrates that optimal coating formation requires a pH of at least 4 and a sufficient conversion time for adequate thickness, confirmed by the high-frequency EIS loop. Tetramer forms were not observed when the coatings were prepared at lower pH or shorter conversion time, proving that the polymerisation step is crucial for obtaining the coatings offering adequate corrosion protection.