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...

Phosphating is a crucial process in the corrosion protection of metals. Here, activation and fluoride-assisted tricationic phosphating is investigated on aluminum-silicon (AS) coated steel surfaces. Dynamic light scattering results from the activation bath show a bimodal size distribution, with hydrodynamic radii of ~400 nm and ~10 μm. For the smaller particle fraction, static light scattering results are consistent with the interpretation of disklike particles as scattering objects. Particles of the larger fraction sediment with time. In the presence of electrolyte, the scattering intensity from the larger particle fraction increases. Coagulation with time is suggested to be related to the decrease in activity of the activation bath. Scanning Auger microscopy (SAM) shows a higher phosphorus concentration after titanium phosphate activation in the Al-rich areas compared to the Si-rich areas of the AS coatings. There is no correlation between the size of the species in the activation bath, and the size of the phosphate-containing regions on the activated surface. Phosphating was performed in the presence of hexafluorosilicic acid, H2SiF6, ammonium hydrogen difluoride, NH4HF2, and both, at an initial pH of 2.5. The absence of crystals after phosphating with H2SiF6 is an indication that SiF6(2-) is the final product of the oxide dissolution in the presence of fluoride. In the presence of NH4HF2, the Si-rich regions of the surface are phosphated before the Si-poor (Al-rich) regions. Hence, the phosphate distribution after activation and after phosphating are opposite. These results show that a high surface concentration of phosphate after activation is not sufficient for a high coverage with phosphate crystals after phosphating.

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