Electrochemical aspects of protection afforded by finish coat on zinc‐rich primers

A barrier type of finish coat on zinc‐rich primers can effectively prevent corrosion on mild steel structures in marine environments. Chlorinated rubber is well known for its impermeability of water vapours and corrosive ions. Micaceous iron oxide (MIO) and titanium dioxide (Ti0₂) are the best barrier pigments available in the world. The chlorinated rubber based top coatings have been prepared by incorporating these two pigments separately and applied over three types of zinc‐rich primers (ZRP) (butyl titanate ZRP, cashew nut shell liquid ZRP and epoxy polyamide ZRP). The electro chemical aspects of protection afforded by these coating systems have been evaluated on mild steel substrate in 3 percent NaCI solution by potential‐time, polarisation and impedance techniques. This study revealed an interesting correlation between the polarisation and impedance observations. The inorganic ZRP with top coated systems behave differently to organic ZRP with top coated systems. The MIO pigmented finish coat affords equally higher duration of protection of steel substrate from corrosive saline environment, even though the PVC value is considerably lower than the TiO₂ pigmented top coat systems.