Role of Intermetallics and Copper in the Deposition of ZrO₂Conversion Coatings on AA6014

Abstract

Zirconium oxide based conversion coatings are nickel-free alternatives to trication phosphate coatings. Here, the role of intermetallic inclusions in aluminum alloy AA6014 on the formation of Cu-rich particles inside ZrO 2 -based conversion layers with a film thickness of 25-30 nm was investigated. The role of the intermetallic inclusions on the deposition mechanism was characterized by varying the immersion time of the substrate and analyzing the coatings by electron microscopy and atomic force microscopy. Statistical analysis on the growth of the copper rich particles showed instantaneous nucleation of these particles, with a subsequent homogeneous inplane growth at constant height. A strong preferential nucleation of the Cu-rich particles on the intermetallic particles was observed, and was attributed to the cathodic nature of the intermetallics. Raman spectroscopy indicated the presence of crystalline CuO and amorphous ZrO 2 in the film. Enhanced layer formation in the vicinity of intermetallic particles was observed. Conversion coatings are widely used to protect metal surfaces from corrosion and to promote the adhesion of subsequently applied organic paints.1 While phosphate-based conversion coatings still have a large range of applications and some aspects of their formation are subject to more recent work, 2,3 current efforts focus on the replacement of conventional, μm thick phosphate and chromate containing systems by thin coatings that results in layers with an average thickness of just few tens of nm, without toxic ingredients. [4][5][6][7] One class of candidates are rare-earth based coatings. 4,6,[8][9][10] Other candidates are nanoceramic conversion coatings, typically containing zirconium or titanium oxides. Such coatings are either deposited on the surface by sol-gel chemistry, or by immersion into a fluoric acidic of Zr/Ti. Nanoceramic coatings have been shown to provide good multi-metal corrosion protection for various substrates including steel, zinc, and Al alloys, [11][12][13][14][15][16][17] and combine good corrosion performance with excellent adhesion properties. 5,[18][19][20][21][22] A lot of work has been specifically focused on investigation of zirconium or titanium based pretreatment on Al alloys. Their basic film formation mechanism is (i) fluoride etching of the oxide layer (which can be replaced by alkaline treatment), 23 (ii) onset of hydrogen evolution reaction, i.e. water reduction with a consequent increase in pH near the interface, and (iii) subsequent precipitation of oxyhydroxides as oxide precursors.24-27 Inclusion of Cu II salts (or alternatively, Fe salts) in the bath has been shown to lead to higher overall growth rate of the film on cold rolled steel compared to the solutions without Cu II . 25,28 Coatings prepared by including copper in the solution were also shown to be thicker and showed enrichment of copper within the conversion film. 25,28 Painted cold-rolled steel pretreated with a Cu-containing ZrO 2 -based coating shows low frequency impedance moduli comparabl...

Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

“…22 In Figures 6b and 6c, the major coating components, in the form of zirconium and chromium oxides, were detected in both coated specimens by Raman shifts at 470 and 536 cm −1 respectively. 13,20 In addition, Raman shifts at 998 cm…”

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…4. 88 In some studies it was noticed that, although the coatings were thicker at longer im- 93 and low carbon steel. 63 The roughness increased with immersion time up to 60 s as a result of the nucleation, clustering and growth of zirconia nodules.…”

“…81 The majority of Zr/Ti based conversion coatings are in the category of thin coatings, between 10 nm and 50-80 nm. 6,[32][33][34]44,49,52,76,77,88,90,91,97,102,105,106,108,112 Coatings with thickness between 100 nm and 600 nm were reported as well, 48,81,94,127 and even in the micrometer thickness range from 1.0 μm up to 6 μm. 30,37,43,59,61,80,119,127 Thickness is related to other parameters such as substrate, pH, time of immersion and stirring, as discussed in Conversion bath parameters and Effect of substrate on coating formation sections.…”

“…• C. 88 terms of morphology, composition and corrosion resistance. 32,35,43,58,96 The optimal conditions were different for different coatings.…”

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

“…Layer growth kinetics and mechanisms can be influenced by the presence of species with high precipitation tendency in the conversion bath . For example, copper ions quickly deposit on the substrates, get reduced, and form copper and copper oxide agglomerates, which act as additional sites for the local alkalization required to support the generation of the conversion film .…”