An Investigation of CUI Behaviors of Carbon Steel via Experimental Testing and RBI Methodology

Abstract Thermally insulated systems are often subjected to temperature swing down to ambient temperature(s). Also, these systems are often decommissioned for various operational, maintenance, and economic reasons. Any moisture infiltration via intrusion and/or condensation results in the high time of wetness (TOW) for the metal surface under the insulation which leads to corrosion under insulation (CUI). This study investigates the low temperature CUI behavior of carbon steel under two typical commonly used insulations namely fibrous mineral wool and hydrophobic blanket using a conventional (i.e., non-egress) design. In terms of experimental testing, the carbon steel coupons were installed in test rigs and wrapped with respective insulations. The assemblies were exposed to fresh water for two days followed by outdoor exposures for one year. The assemblies were dismantled, and the characterizations were performed on the corroded coupons to measure CUI rates. This study also compares the CUI rates with those calculated via Risk-Based Inspection (RBI) methodology and to those that were reported in the previous studies. It then discusses the deviations and reasoning behind the CUI rates.