Protection of Aluminum Alloy 3003 in Sodium Chloride and Simulated Acid Rain Solutions by Commercial Conversion Coatings Containing Zr and Cr

• Main Authors: Maja Mujdrica Kim, Barbara Kapun, Urša Tiringer, Gavrilo Šekularac, Ingrid Milošev
• Format: Article
• Language: English
• Published: MDPI AG 2019-09-01
• Series: Coatings
• Subjects: aluminum alloy 3003; trivalent chromium process (TCP); hexafluoro zirconate conversion coating; simulated acid rain; sodium chloride; X-ray photoelectron spectroscopy
• Online Access: https://www.mdpi.com/2079-6412/9/9/563

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.