Corrosion of mild steel under insulation–the effect of dissolved metal ions

Corrosion under insulation (CUI) is a form of corrosion that occurs when moisture ingress through insulation and comes into contact with underlying metallic equipment. The rate of CUI depends on the ionic conductivity of the insulation, which, in turn, is related to the moisture distribution and metal ion concentration within the moist insulation. Metal ions are introduced into the moist insulation by metal corrosion and also from leaching from the insulation itself. These metal ions cause an increase in ionic conductivity through the moist insulation, thus influencing CUI. It was also observed that contact of steel with moist insulation, intrinsically increases its corrosion due to the formation of differential aeration cells resulting from variable oxygen diffusion arising from the non-homogenous packing of the insulation. Herein, the effect of such dissolved ions on CUI was studied. It was observed that the rate of CUI increases with time due to the release of metallic ions into the moist insulation.