abstract
AA2024 aluminium alloy is widely used in the aerospace industry. However, it is known to be highly susceptible to localised corrosion, which is related to its complex microstructure, mainly the presence of numerous Cu-rich intermetallic particles. One of the effective solutions to increase their corrosion resistance relies on the formation of layered double hydroxide (LDH) based conversion coatings (CC). This investigation aims to understand how the conditions of in situ Li-Al LDH-CO 3 2- /OH - CC growth affect their further protective ability. For that purpose, concentration of reactants (0.05-0.15 M Li 2 CO 3 ), pH of the electrolyte (10-12), temperature (30-50 degrees C) and treatment time (15 min -30 h) were systematically varied. Additionally to that, NH 4 OH was applied as a chelating agent for the synthesis of Li-Al LDH-CO 3 2- /OH - as well as for the pH control. The obtained protective coatings were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES), Raman spectroscopy, salt spray test (SST) and electrochemical impedance spectroscopy (EIS). Among all varied parameters, involvement of chelating agent was found to be the most efficient for the formation of a coating with the highest level of corrosion protection. This is attributed to the formation of soluble complexes between NH 4 OH and Cu, resulting in dissolution of the intermetallics. It also prevents further redeposition of Cu species and consequently, the formation of new highly active Cu cathodes on the AA2024 alloy surface.
keywords
ALUMINUM-ALLOY; HEXAVALENT CHROMIUM; LITHIUM-CARBONATE; DOUBLE HYDROXIDES; COATINGS; FILMS; DISSOLUTION; INHIBITION; MECHANISM; AMMONIA
subject category
Chemistry; Materials Science; Physics
authors
Stephan, J; Kasneryk, V; Serdechnova, M; Scharnagl, N; Gazenbiller, E; Vaghefinazari, B; Volovitch, P; Starykevich, M; Blawert, C; Zheludkevich, ML
our authors
Maksim Starykevich
Junior Researcher
Mikhail Zheludkevich
Invited Principal Researcheracknowledgements
This investigation was carried out with the financial support of the COVER project in frame of LUFO V-3 Call 2018. The authors thank to Dr. Silke Gruenke and Dr. Stefan Kreling from Airbus (Germany) and Dr. Markus Becker from Fraunhofer IFAM (Germany). Dr. Valeryia Kasneryk thanks Alexander von Humboldt Foundation (Germany) and Christiane Nusslein-Volhard Foundation (Germany) for financial support. Dr. Maksim Starykevich acknowledges the support of the FCT-2020.00625. CEECIND (Portugal) grant. The authors thank to Daniel Strerath for spark analysis. The authors are also grateful to Deutsches Elektronen-Synchrotron PETRA III (Hamburg, Germany) for accepting and granting the proposal I-20211324 (P.08 end-station) and Dr. Florian Wieland and Dr. Vasyl M. Haramus for the experimental support during DESY beamline experiments. Finally, the authors thank to Mr. Volker Heitmann, Mr. Ulrich Burmester and Mr. Thymoty Naacke for technical support during the investigations.