The effect of conversion bath convection on the formation of Zr‐based thin‐film coatings on multi‐metal surfaces

Abstract

The present study investigates the effect of different conversion bath agitation and stirring conditions on the generation of Zr‐based thin film conversion coatings for steel, zinc galvanized steel, and aluminum substrates. Therefore, samples were immersed in a copper containing fluoric acid of zirconium. Film formation was monitored in situ via open circuit potential (OCP) measurements recorded during the pre‐treatment of the samples in solution. The resulting thickness and elemental distribution of the obtained Zr‐based coatings formed with and without stirring were analyzed ex situ by means of high‐resolution Auger electron spectroscopy (AES) depth profiles. Under the studied conditions, the conversion film thickness on AA6014 and cold rolled steel increased three and two times as a result of stirring. For hot dip galvanized steel, a thickness increase of 400% was achieved. The data point at mass transfer as the dominant factor for film formation. In this context, specific relations for each metal substrate type were detected.

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…7a. 108 For Al alloys in the 1xxx, 3xxx, 5xxx and 6xxx series, the large cathodic particles are predominantly Fe-containing. A Zr-based conversion coating was found not to uniformly cover the surface of AA1050, but the oxide deposition was concentrated on and around IMPs containing mainly Fe and Si.…”

“…33,34,36,37,47,51,52,55,59,61,62,[64][65][66]69,70,72,78,82,84,87,90,92,94,95,98,99,101,102,104,[106][107][108]110,[112][113][114] Elevated temperatures were also studied, i.e. between 30 and 55…”

“…This reduces the time required for oxide dissolution and increases the available time zirconium can deposit on the activated matrix. 108 Moreover, the content of copper in the coating, coming from Cu 2+ added to conversion bath to increase the number of cathodic sites at the surfaces, is enriched on the surface with stirring.…”

“…6,62,85-88, 108 When properly activated, even if the alloy itself does not contain copper (e.g. in AA1050), Cu-rich particles will deposit on the surface and promote the formation of Zr-based coating.…”

“…by a factor of 3 on CRS and HDG steel substrates, and by a factor of 0.5 on AA6014. 108 The content of Cu in the coating differed depending on the substrate. Areas with different concentrations of Cu were observed on AA6014, with islands of copper (0.2 to 2 μm) greater in size than those observed for CR and HDG steels.…”