Effects of SO₂ deposition on painted steel surfaces

The effects of 1 ppm SO₂ with high humidity on alkyd‐painted steel was investigated using x‐ray photoelectron spectroscopy (XPS) and adhesion tensile tests. The SO₂ caused a spotted discoloration on the specimens on which dew is allowed to dry and accelerated the loss of adhesion between the primer and the steel. The brown spots are characterized by a high S content and form in areas where the condensed dew becomes more concentrated in H₂SO₃ or H₂SO₄ as it evaporates. Specimens not exposed to dew also have high S concentration on the surface, but do not discolor. High humidity alone caused a loss of adhesion at the primer–metal interface, but SO₂ increased the rate of this de‐adhesion. Such a loss of adhesion decreases the protection provided by the paint system and the presence of Fe on both sides of some failures indicated that the substrate was beginning to corrode.