Chromium(III)-substituted hydroxyapatite/silica sol–gel coating: towards novel green coating for corrosion protection of AA2024

A novel Cr-substituted hydroxyapatite (Cr-HA) nanoparticles were synthesized via continuous hydrothermal technique. Moreover, the synthesized material was embedded into a silica sol–gel matrix and applied on an AA2024 substrate to evaluate the corrosion inhibition efficiency of the coating. TEM and SEM micrographs confirmed the development of Cr-HA nanorods of 20 nm width and 6 µm length. XRD diffractograms demonstrated the evolution of a new crystalline structure; the XRD pattern was analyzed by Material Studio software which confirms the replacement of Ca 2+ by Cr 3+ . The EDX mapping revealed a uniform distribution of Ca and Cr ions within the Cr-HA crystal structure. The atomic ratio of Ca 2+ : Cr 3+ was reported to be 4:1 respectively. The Cr-HA nanoparticles were uniformly distributed in a silica sol–gel matrix and applied on an AA2024 substrate. The corrosion performance of the Cr-HA sol–gel coating composite was evaluated using Electrochemical Impedance Spectroscopy (EIS) in an aerated 3.5% NaCl solution and the results compared to those of neat silica sol–gel coating. Whereas pitting corrosion was also observed in the case of a neat sol–gel coated sample within 5 days of immersion, Cr-HA sol–gel coated AA2024 exhibited prolonged pitting resistance over 110 days with no sign of corrosion or delamination. The EIS data fitting suggested the formation of a protective layer that is responsible for the extended corrosion resistance of the Cr-HA-coated sample. The scratch test indicated that the Cr-HA nanocomposite coating might offer short-term self-healing properties in the 3.5% NaCl corrosive media. Graphical Abstract