Impact of the in-situ phosphatization on the corrosion resistance of steel coated with fluorinated waterborne binders assessed by SKP and EIS

A thin stand-alone waterborne coating that combines hydrophobicity and a built-in ability to in-situ phosphatize low carbon steel substrates, avoiding flash rust in the interface is presented. Despite the hydrophobicity of the coating is enhanced by the presence of a fluorinated comonomer, the barrier protection is substantially decreased as compared with a pure acrylate coating. The bad performance is independent of the thickness and it is mostly attributed to the lack of coalescence of the copolymer particles containing fluorinated comonomer (detected by Scanning Electron Microscopy (SEM)). The best way to hinder this harmful effect is to trigger the formation of the in-situ phosphatization layer on the metallic surface; namely slowly drying the coating at 60 % of relative humidity (R.H.). The combination of Electrochemical Impedance Spectroscopy (EIS) and Scanning Kelvin Probe (SKP) has shown the beneficial effect of having a protected interface when it is exposed either to bulk electrolyte or to a droplet of 3.5 wt.% NaCl. A methodology using both techniques to evaluate coatings with an artificial defect has proved that the phosphatization is governing the corrosion protection for these coatings.