Abstract
The morphology, composition and corrosion properties of commercial hexafluoro-zirconate trivalent chromium coatings (SurTec® 650) deposited on chemically cleaned aluminum alloy 3003 were studied. The coatings were deposited at room temperature using different concentrations of SurTec® 650 (10, 25 and 50 vol.%) and different conversion times (90 s, 11 min and 18 min). Scanning electron microscopy with energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy and time-of-flight secondary ion spectrometry were employed to investigate the surface morphology, composition and thickness of uncoated and coated AA3003 samples. The morphology of the coating varied from uniform nodular to non-uniform and cracked; coatings were deposited at intermetallic particles and at the alloy matrix. The main constituents of conversion coatings were Zr(IV) and Cr(III) oxides; in addition to oxides, fluorides were also formed. The corrosion properties were investigated in two solutions: more aggressive sodium NaCl and less aggressive simulated acid rain. These commercial conversion coatings exhibited a good corrosion resistance but only after longer immersion in solution, i.e., 24 h. The results reveal an interesting behavior of zirconate-based coatings on aluminum-manganese alloy.
Highlights
Chemical conversion coatings are widely used on aluminum-based alloys and other technologically important materials with the aim to protect the metal against corrosion and to promote adhesion of the primer coating to the substrate
The results demonstrated that the chromate layer promoted the highest corrosion resistance among the tested surface treatments, and the highest corrosion resistance was found for the self-assembling molecules (SAM) treatment
For the samples prepared under different conditions, the potentiodynamic polarization curves in simulated acid rain were measured; electrochemical parameters—Rp, Ecorr and jcorr —served as decisive parameters for selecting the optimal condition
Summary
Chemical conversion coatings are widely used on aluminum-based alloys and other technologically important materials with the aim to protect the metal against corrosion and to promote adhesion of the primer coating to the substrate. Conversion coatings are formed by immersing a metallic substrate in a chemical bath. Due to high toxicity and health concerns related to hexavalent chromium, numerous studies are currently devoted to alternatives for the replacement of Cr(VI)-based surface treatments. These include chemical conversion coatings, organic sol-gel coatings, nanocomposites, Cr(VI)-free anodization, metal rich primers, etc. Non-chromate treatment should provide protection that is equivalent or Coatings 2019, 9, 563; doi:10.3390/coatings9090563 www.mdpi.com/journal/coatings
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