Efficiency of layered double hydroxides in epoxy coating as corrosion inhibitor reservoirs for carbon steel

Layered double hydroxides (LDH) are considered as interesting containers for corrosion inhibitors, since they exhibit high ability to release inhibitor as well as to capture aggressive ions from aqueous solution. Epoxy containing organo-modified LDH filler intercalated with ethylenediamine-N,N′-disuccinate anions (EDDS), [Zn2Al(OH)6]+ [EDDS]4−0.25 . 2H2O (LDH-EDDS) with loading from 0% to 1.25% were coated onto carbon steel plates. The efficiency of these coatings against corrosion (barrier properties and release of inhibitors) was assessed by using electrochemical impedance spectroscopy. The evolution of the impedance modulus |Z| at low frequency range during time showed that the 0.75% organo-modified LDH-containing epoxy (thickness 80 μm) exhibited the highest corrosion resistance in the series and the associated value of |Z| at 3 mHz (1010 Ω cm2 after 21 days of immersion in 0.1 mol L−1 NaCl solution) placed it among the best solutions published so far using organo-modified LDH as cargo carrying a corrosion inhibitor. The behavior of this polymer composite was compared to the free-LDH coating by plotting the resistivity profiles along the coating thickness as a function of time. These results showed that (i) the presence of 0.75% LDH-EDDS in the epoxy coating delayed the penetration of water molecules, and (ii) limited corrosion of carbon steel. The efficiency of the organo-modified filler to inhibit the corrosion process was explained by an exchange between interleaved species occurring from released EDDS4− anions and aggressive Cl− ions being captured.