Herein, pH-sensitive microcapsules containing NaNO2 corrosion inhibitors for protection of steel reinforced concrete were synthesized via water-in-oil-in-water (W/O/W) double emulsion using colophony as the wall material. The average microcapsule size was 79.07 μm in diameter and exhibited a high encapsulation efficiency of 83.2%. Study of the release of corrosion inhibitors from microcapsules in deionized water (DI water, pH 6.8), carbonate/bicarbonate buffer solution (CBS, pH 9.1), and simulated concrete pore solution (SCPS, pH 12.6) demonstrates that the microcapsules are sensitive to pH and display higher release in alkaline media. This is the first study of colophony as an encapsulating agent for corrosion inhibitors. Furthermore, the alkaline pH-triggered release shows the suitability of its use in reinforced concrete systems. A wide thermal stability range was also found for the colophony microcapsules up to 100 °C. These high pH environments (CBS and SCPS) present pH values above the pKa of colophony (7.2), thus triggering enhanced inhibitor release by the ionization and deprotonation of colophony shell. The higher release in CBS and SCPS is demonstrated by the increases of the corrosion inhibitor diffusion coefficient by an order of magnitude from 3.30 × 10-17 m2/s in DI water up to 1.66 × 10-16 m2/s for SCPS. The release performance indicates that the proposed approach can be used to encapsulate a variety of inhibitors for the protection of steel reinforcements. After immersion in different pH solutions, the corrosion potentials of a carbon steel substrate with microcapsules containing nitrite were more noble than when immersed without microcapsules and the corrosion current densities showed comparable values to free corrosion inhibitors. The formation of a passive ferric oxide layer was confirmed by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.
Keywords: W/O/W double emulsion; concrete pore solution; controlled release; corrosion inhibitors; diffusion; microcapsules.