Effect of Al3+ on the surface microstructure and film formation mechanism of Zr-based conversion coatings with improved corrosion resistance performance on cold rolled steel

Most Zr conversion coatings are prepared on Al alloys , with Al 3+ from the substrate contributing significantly to the film growth process. Thus, the effect of Al 3+ on the electrochemical characteristics, surface microstructure and film-formation mechanism of Zr-based conversion coatings on old- rolled steel (CRS) substrates was investigated by growing the coatings with and without Al 3+ as an additive. Electrochemical tests indicated that the anticorrosion performance of the Zr-Al coatings prepared in the presence of Al 3+ was greatly superior to that of Zr conversion coatings obtained in the absence of Al 3+ . Field-emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis indicated that adding of Al 3+ in the conversion bath changed the surface microstructure of the Zr coating, greatly promoted the deposition of Zr and F on the CRS substrate and increased the film thickness and density. Both coatings were embedded into the etched CRS substrates. The presence of Al 3+ additive induced the formation of different coating components . The Zr coating mainly consisted of ZrO 2 and some FeF x , whereas the Zr-Al coatings comprised of ZrO 2 , ZrO x F y , Al 2 O 3 , AlF x and some FeF x . The present study results confirm the key role of Al 3+ as an additive in the film formation and growth process of Zr coatings and its effect on enhancing the anticorrosion performance of CRS substrates in NaCl solutions.