Summary
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. Defected TSA under mineral wool showed an initially high rate that decreased to <0.1 mpy (0.003 mm/y) within 30 days. For defected TSA under calcium silicate insulation, the corrosion rate initially increased with time over this period and had a prolonged period at corrosion rates between 1 and 10 mpy (0.03 and 0.25 mm/y) over the duration of the experiment.
