Electrochemical Behavior and Self-Sealing Ability of Zirconium Conversion Coating Applied on Aluminum Alloy 3005 in 0.5 M NaCl Solution

Zirconium conversion coating (ZrCC) was prepared on aluminum-manganese alloy AA3005 by immersion in 200 ppm of H₂ZrF₆bath for 10 min at room temperature. Potentiodynamic polarization curves and electrochemical impedance spectra were measured up to 10 and 40 days, respectively, in 0.5 M NaCl solution. Microstructural characterization of samples was carried out using scanning electron microscopy equipped with energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, focus ion beam microscopy and transmission electron microscopy. The coating had a tri-layer structure with a thickness of 200 nm in the proximity of intermetallic particles and 30 nm far from intermetallic particles at the coating matrix. Electrochemical measurements showed that the corrosion resistance of ZrCC on AA3005 was improving during immersion in NaCl solution, i.e., impedance value at low frequency increased with immersion time reaching the average value 8.5 ∙ 10⁶Ω∙cm²at 3 mHz after 10 days immersion. Microstructural and compositional characterization showed that this behavior is related to the change in composition and structure of conversion coating including a transformation of ZrF₄/ZrO_xF_yto ZrO₂· 2H₂O_(s)and formation of Al(OH)₃in the top layer of ZrCC, respectively, accompanied by the change in thickness of individual layers within the coating.