Atmospheric Corrosion Mechanisms of Painted Steel

Electron microscopy and microprobe analysis were used to study the spread of corrosion beneath an acrylic paint film on steel. Corrosion initiates beneath the paint film in crevices formed by scribing. Corrosion spreads as the build up of corrosion products beneath the paint film drives a crack along the paint/substrate interface. The buildup of corrosion products cause defects to form in the paint film. Phosphating reduces the corrosion rate by increasing the polymer/substrate adhesion and slowing down crack growth at the interface. The improved adhesion can be explained in terms of acid-base interactions between the polymer and substrate. Sulfur was identified in the corrosion products of the one sample which was studied. The sulfur is presumed to be from atmospheric pollutants.