Corrosion under insulation – prevention, control and detection

Corrosion under insulation (CUI) is a real threat to the reliability of many plants, including sugar mills. CUI can cause failures in areas that are not normally of a primary concern to an inspection program. The failures are often the result of localised corrosion and not general wasting over a large area, making it very difficult to develop a cost-effective inspection program. CUI failures can be catastrophic or at least have an adverse safety and economic effect on the plant due to lengthy downtime and repairs following an unexpected failure. Whether the insulation material is hydrophilic or hydrophobic influences the likelihood of CUI, along with other key influences such as insulation system design and insulation material in- service breakdown products. A desktop comparison between hydrophilic Rockwool and hydrophobic Pyrogel insulation material was made to codes such as: BS 5422 Method for specifying thermal insulating materials for pipes, tanks, vessels, ductwork and equipment operating within the temperature range -40°C to +700°C; and 2BS5970 Thermal insulation of pipework, ductwork, associated equipment and other industrial installations in the temperature range of -100°C to +870°C. CUI is difficult to detect because it is hidden by the insulation masking the corrosion until it is too late. Removal of the insulation is expensive and time consuming, so an inspection solution is required. A desktop review of CUI detection techniques was also made as it is important to use a best-practice screening strategy, rather than that current status quo of reactive maintenance. The key findings were that hydrophobic insulation material influences the likelihood of CUI, more than if elevated temperature plant is protected by a paint coating, prior to insulation application. A pulsed eddy current (PEC) method is a suitable NDT screening tool that provides reliable and accurate results. Key words CUI, Insulation, pulsed eddy current