Abstract
The present study intends to elucidate the effect of treatment solution composition on the formation and structure of Trivalent Chromium-based Conversion (TCC) coatings containing cobalt on zinc substrates. Model solutions with two different complexing agents, viz. fluoride and oxalate, with and without cobalt were applied to zinc plated steel. The scanning electron microscopy and atomic force microscopy images revealed a morphology with microstructural defects that can be improved to a more uniform and adherent structure by adding cobalt to the passivating bath. The elemental composition of the layer was investigated by auger electron spectroscopy (AES). Furthermore, the amounts of Cr and Co in the coatings were also measured with the aid of inductively coupled plasma optical emission spectroscopy (ICP-OES). In good agreement with AES, cobalt was also detected in the layers via ICP-OES measurement. The results of accelerated corrosion tests suggested that the formation of a densely packed layer is crucial for a good corrosion resistance of the coating.
Highlights
Hesamedini, Sanez; Ecke, Gernot; Bund, Andreas: Structure and formation of trivalent chromium conversion coatings containing cobalt on zinc plated steel
Morphology and physical characteristics of the coating.— Figs. 1a–1d depicts the FIB-assisted cross-sectional scanning electron microscope (SEM) micrographs corresponding to the layers formed by different treatment solutions with the immersion time of 60 s
Results obtained in the present study imply that the Trivalent Chromium-based Conversion (TCC) coating is deposited in a layered structure, which is thinned through heat-treatment
Summary
Hesamedini, Sanez; Ecke, Gernot; Bund, Andreas: Structure and formation of trivalent chromium conversion coatings containing cobalt on zinc plated steel. The Cr(III)-based conversion layer acts as a barrier,[6] without heattreatment, the corrosion protection of Cr(VI) coatings is generally better. Studies[14,15] showed that transition metal ions such as Co(II), Ni(II), and Fe(II) incorporated into Cr(III)-based treating solutions induce a better corrosion protection. Is the corrosion resistance of the coating improved in the presence of cobalt ions,[16,17,18,19] and, the layer formed in cobalt-containing treatment solutions has the advantage of desired surface appearance.[20] According to some studies,[17,21] cobalt was not detected in the layer formed in cobaltcontaining Cr(III)-based passivation solutions. To prepare Cr(III)-based passivation solutions, apart from a Cr(III) salt and additional transition metal ions to increase the corrosion resistance, another component to form a complex with Cr(III), which is Experimental
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