Benzotriazole modified Zn-Al layered double hydroxide conversion coating on galvanized steel for improved corrosion resistance

In-situ grown layered double hydroxide (LDH) conversion coatings could be modified with corrosion inhibitors to improve corrosion resistance of the films. In this paper, a Zn-Al layered double hydroxide conversion coating on galvanized steel was modified with benzotriazole, as an organic corrosion inhibitor, for the first time. To achieve the modification, the as-synthesized Zn-Al layered double hydroxide was firstly immersed in a methanol solution containing ammonium nitrate, followed by dipping in benzotriazole solution. Microstructural and elemental analysis as well as electrochemical experiments were performed to study the changes in the modified coatings. Results verified partly intercalation of benzotriazole (BTA) in the layered double hydroxide (LDH) structure. The electrochemical impedance spectroscopy (EIS) assessment indicated that modification with benzotriazole increased the impedance of the as-prepared films by 4 times. Ultraviolet–visible spectroscopy showed the BTA release in the sodium chloride solutions containing BTA loaded LDH plates. In order to analyze the active protection feature of the modified conversion coatings, a polymeric topcoat, based on epoxy-polyamide, was applied. The EIS results showed a 3.6-time increase in the impedance values of the samples with benzotriazole loaded conversion coating compared to those with as-synthesized LDH films. The data, gathered from scanning electron microscopy and energy-dispersive X-ray spectroscopy mapping, verified inhibitor release in the artificial scribe zones of the modified specimen.