In situ grown gluconate-containing CaAl-LDH/organic coating for steel corrosion protection

This study reports the development of anticorrosive bilayer coatings consisting of in situ grown layered double hydroxides (LDH) covered with a polyurethane layer on a steel substrate. This design aims at providing corrosion protection via the controlled release of gluconate from LDH near the substrate, while at the same time contributing to the improvement of the polyurethane layer coating adhesion. The CaAl-LDH thin film was initially grown directly on AISI 1080/1010 carbon steel and modified with environmentally friendly gluconate molecules through an ion-exchange reaction. The effect of polyurethane treatments on the LDHs thin film was systematically explored: gluconate is either intercalated in LDHs or dispersed in polyurethane coatings, and the two systems are studied to understand the role of inhibitors in bilayer coating systems at defined conditions. The structural characteristics of the developed coatings were evaluated by scanning electrochemical microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (ATR-FTIR), and glow discharge optical microscopy (GDOES). The findings of electrochemical impedance spectroscopy (EIS) measurements on coated carbon steel substrates in NaCl solution demonstrated the significance of the bilayer film design for long-term corrosion protection, by combining active corrosion protection provided by the LDH conversion film with the passive barrier effect against electrolyte species rendered by the organic polyurethane layer. Additionally, improving the polyurethane coating's wet adhesion to the substrate when applied onto CaAl-LDH opens new directions toward the co-development of surface treatments with organic coatings.