Corrosion inhibition by phenylphosphonate and Zn2+

A formulation consisting of 300ppm sodium salt of phenyl phosphonic acid (PPA) and 50ppm Zn2+ is 95 per cent efficient in inhibiting the corrosion of mild steel in a neutral aqueous environment containing 60ppm chloride. A higher concentration of Zn2+ lowered inhibition efficiency. This is due to the formation of an insoluble Zn2+-PPA complex in the solution. It is found that, in solution, both the phosphonate group and the phenyl group of PPA are involved in the formation of the complex with both Zn2+ and Fe2+, whereas in the solid state, only the phosphonate group is involved.