Development of zinc ferrocyanide ion exchange compounds for corrosion-inhibiting and sensing pigments

The fabrication and application of zinc ferrocyanide ion exchange compounds for use as inhibiting and sensing pigments in protective paints and primers for metals and alloys are described in this paper. A distinctive property of these ferrocyanide pigments is an ability to store corrosion inhibitors and then release them when needed by an ion exchange process. The exchange process is triggered when moisture containing Na+ and K+ ions contacts inhibitor particles embedded in an organic coating. The exchange kinetics were studied by measuring the concentration of released zinc ions in the solution with inductively coupled plasma. The inhibitor storage capacity can be as high as 200 meq/100 g, and it is much higher than that found in other reported ion exchange compounds. Furthermore, structure changes from non-crystalline to crystalline, upon exchange with alkaline ions, can be detected by x-ray diffraction, providing an important indication of activation of the corrosion protection. This is a form of corrosion sensing. The inhibition performance of zinc ferrocyanide in the coating was examined by salt spray tests and electrochemical impedance spectroscopy, and both tests showed that zinc ferrocyanide could provide good protection on aluminum alloy 2024.