Alkaline or acidic corrosion results when an alkali or acid and moisture, are present in certain fibrous or granular insulations. For hot service above 250o F, most of the water is driven off. This water vapor may condense at the edge of the insulation, and dissolve the alkaline or acidic chemicals there, resulting in corrosion of the aluminum or steel jacketing.

Some alkaline waters with aluminum produce etching and pitting. Pitting can be severe, especially when chloride ions are present. Insulating cement may also contain alkaline chemicals and water (while the cement is still drying). Below 250° F, an alkaline water may cause corrosion if the substrate or insulated surface is stainless steel, copper, brass or aluminum. Steel would normally not be affected in the time needed for the cement to dry. Fresh, potable water is recommended when mixing insulating cement.

Delahunt reported on leaching tests performed on polyurethane foam insulation containing fire retardant chemicals (i.e., brominated or chlorinated compounds). Distilled water was used, and aggressive acidic solutions were formed. The same was found true for phenolic foams. The pHs of the solutions were often two to three. Laboratory corrosion rates have been shown to be 15-20 mils/yr. Of the two foams, the phenolics are by far the more corrosive.