The Screening–Confirmation Gap in Corrosion Under Insulation Inspection: A Critical Review of NDT Evidence, Field Limitations and Asset Integrity Decision Support

Corrosion under insulation (CUI) remains a major asset integrity concern in process plants because degradation occurs beneath insulation, cladding, coatings, or protective wrapping, where direct visual access to the metal surface is restricted. A wide range of non-destructive testing (NDT) and monitoring methods have been used to manage CUI; however, these methods generate different levels of engineering evidence. Moisture detection, thermography, and sensor-based monitoring may indicate corrosion-favorable conditions or activity, whereas pulsed eddy current testing (PEC), guided wave testing (GWT), and radiography can support through-insulation screening of suspicious areas. In contrast, ultrasonic thickness measurement (UTM) and phased-array ultrasonic testing (PAUT)-based corrosion mapping are more appropriately positioned as local confirmation methods after access or inspection windows are created. This review examines the screening-confirmation gap in CUI inspection by classifying inspection and monitoring methods based on the evidence type and decision-support value. This review proposes an evidence-chain framework that links risk indication, screening indication, activity monitoring, local confirmation, and asset integrity decision support. This framework shows why screening and monitoring outputs are valuable for prioritization but should not be overclaimed as final evidence for repair planning, remaining life assessment, risk-based inspection (RBI) updates, or fitness-for-service (FFS) decisions. By distinguishing between indirect risk indicators, suspicious wall-loss responses, active-source evidence, and local confirmation data, this review provides a more defensible basis for integrating NDT results into CUI inspection planning and asset integrity management.