What is corrosion under insulation (CUI) and how is it prevented in heat transfer equipment?

CUI occurs when moisture penetrates insulation and contacts metal surfaces, causing corrosion at temperatures between -4C and 175C (most severe 60-120C). Risk factors: insulation damage, poor weather barriers, cyclic temperature, and chloride contamination. Prevention: proper insulation system design (vapor barriers, weatherproofing, sealants at penetrations), coating systems under insulation (epoxy, TSA - thermal spray aluminum), regular inspection programs (thermography, profile radiography, pulsed eddy current), and maintenance of weather barriers.