Hard anodizing of AA2099-T8 aluminum‑lithium‑copper alloy: Influence of electric cycle, electrolytic bath composition and temperature

AA2099-T8 samples have been hard anodized in a traditional sulphuric bath with the aim of studying the influence of i) electrolyte temperature, ii) Al3+ concentration and iii) different electric cycles in direct current, multi-step direct current and pulsed current (both completely anodic cycles and with cathodic "off" pulses). Anodic oxide thickness, volumetric expansion ratio (Vox/VAl), mean hardness (HV0.05), faradic efficiency and defective state have been analyzed. As temperature decreases, the oxide hardness increases but defectiveness steeply worsens; similarly a middle compromise of Al3+ concentration ensures the best performances. Regimes of high current, from one side, and of low current and current transient, from the other side, strongly affect oxide defective state introducing different types of flaws; an appropriate balance is crucial for a performing non-defected oxide. High current induces the occurrence of parasitic reactions which produce rough and defected Al/ox interface. Low current and current transient induce diffused cracking and detachments of coating fragments which decrease corrosion resistance as shown by potentiodynamic polarization tests. Li-based intermetallics play a crucial role in the last mentioned flaw-creation phenomena since they produce inhomogeneities in electric field distribution and contribute to create unstable oxide structures.