This study involved a laboratory investigation of corrosion under insulation (CUI) using methods given
in ASTM G189 for cyclic temperature, wet/dry conditions. Two long term exposure tests were
conducted including three material conditions (bare steel, thermal sprayed aluminum (TSA) coated steel
and a defected TSA coating) and two insulative materials (mineral wool and calcium silicate insulations).
All specimens and insulation materials were heat and/or solution aged prior to exposure in the CUI test
cell to reduce initial transients associated with new materials. The highest corrosion rates in this program
were observed during CUI exposures for bare steel under mineral wool insulation. Maximum corrosion
rates were up to 40 mpy (1.0 mm/yr). TSA coated specimens (non-defected) showed the lowest
corrosion rates for all cases with corrosion rate of <0.1 mpy (.003 mm/y) under mineral wool insulation
and up to 1 mpy (0.03 mm/y) for calcium silicate insulation. Exposed steel in defected TSA coated
specimens showed corrosion rates up to 10 mpy (0.25 mm/y) under mineral wool and calcium silicate
insulations with the periods of maximum corrosion activity being different in each case.