Factors Affecting MoO₄^2– Inhibitor Release from Zn₂Al Based Layered Double Hydroxide and Their Implication in Protecting Hot Dip Galvanized Steel by Means of Organic Coatings

Zn₂Al/-layered double hydroxide (LDH) with intercalated MoO₄^2– was investigated as a potential source of soluble molybdate inhibitor in anticorrosion coatings for hot dip galvanized steel (HDG). The effect of solution pH, soluble chlorides, and carbonates on the release kinetics of the interleaved MoO₄^2– ions from the LDH powder immersed in solutions containing different anions was studied by X-ray diffraction, in situ attenuated total reflectance infrared (ATR-IR) spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The effect of the solution composition on the total release and the release kinetics was demonstrated. Less than 30% of the total amount of the intercalated MoO₄^2– was released after 24 h of the immersion in neutral 0.005–0.5 M NaCl and 0.1 M NaNO₃ solutions whereas the complete release of MoO₄^2– was observed after 1 h in 0.1 M NaHCO₃ or Na₂SO₄ and in alkaline solutions. The in situ ATR-IR experiments and quantification of the released soluble species by ICP-AES demonstrated the release by an anion exchange in neutral solutions and by the dissolution of Zn₂Al/-LDH in alkaline solutions. The anion exchange kinetics with monovalent anions was described by the reaction order <i>n</i> = 0.35 ± 0.05 suggesting the diffusion control; for divalent anions, <i>n</i> = 0.70 ± 0.06 suggested the control by a surface reaction. Dissolution of Zn from coated HDG with and without Zn₂Al/-MoO₄^2– fillers, leaching of MoO₄^2– from the coating, and the electrochemical impedance spectroscopy response of the coated systems were measured during the immersion in 0.5 M NaCl solutions with and without 0.1 M NaHCO₃. Without carbonates, the release of soluble MoO₄^2– was delayed for 24 h with no inhibiting effect whereas with 0.1 M NaHCO₃ the immediate release was accompanied by the immediate and strong inhibiting effect on Zn dissolution. The concept of controlling the inhibition performance of LDH hybrid coatings by means of the environment composition is discussed