Corrosion of carbon and stainless steels under wet thermal insulation can be a serious problem and can be especially prevalent in the humid Gulf Coast area. This paper discusses an inspection program that has been in progress since late 1982 at a ten-year old chemical plant located at La Porte, TX. The program is intended to determine the extent of corrosion damage to major pieces of equipment that has occurred under inhibited calcium silicate insulation finished with aluminum jacketing and to recommend remedial action.

Key elements of a successful inspection program are discussed. A series of pictures is presented that correlate visual external telltale signs with corrosion beneath the insulation. At the time of writing, significant localized corrosion of carbon steel has been found, as well as evidence of incipient stress corrosion cracking of austenitic stainless steel.