In-situ AFM and EIS study of waterborne acrylic latex coatings for corrosion protection of carbon steel

Corrosion protection for carbon steel by three waterborne styrene-acrylic latex coatings with different glass transition temperature (Tg) and levels of hydrophobicity has been studied by in-situ atomic force microscope (AFM) and electrochemical impedance spectroscopy (EIS) measurements in 3.0 wt% NaCl solution. The AFM images reveal the micro- and nano-structure of and pinholes in the coatings as well as their changes during exposure in the solution, whereas the EIS spectra vs. time of exposure show the barrier property, stability and degradation of the coatings. The coating with crosslinking and high level of hydrophobicity exhibited a high barrier property and long-term corrosion protection in the solution despite of some shallow pinholes. The coating without crosslinking but having high Tg and high level of hydrophobicity also displayed a high barrier property and certain stability although initial changes occur in the solution. However, for the coating without crosslinking and with low Tg and relative low level of hydrophobicity, some dissolution and open-up of defects occurred on the surface shortly after exposure and the coating lost its barrier property rapidly in the solution. The reasons for the different behaviors between the coatings are discussed with respect to the crosslinking, Tg and the hydrophobicity level of the coating.