Corrosion Under Insulation (CUI) | Materials And Corrosion Control - PDFYAR

Corrosion Under Insulation (CUI) | Materials And Corrosion Control

Damage Mechanism	Corrosion Under Insulation (CUI)
Damage Description	Corrosion of piping, pressure vessels and structural components resulting from water trapped under insulation or fireproofing.
Affected Materials	Carbon steel, low alloy steels; 300 Series SS and duplex stainless steels.
Control Methodology	· Use appropriate paints/coatings and maintain the insulation/sealing/vapor barriers to prevent moisture ingress. · High quality coatings, properly applied, can provide long term protection. · Use low chloride insulation on 300 Series SS to minimize the potential for pitting and chloride SCC.
Monitoring Techniques	· Utilize multiple inspection techniques to produce the most cost effective approach, including: o Partial and/or full stripping of insulation for visual examination. o UT for thickness verification. o Real-time profile X-ray (for small bore piping). o Neutron backscatter techniques for identifying wet insulation. o Deep penetrating eddy-current inspection (can be automated with a robotic crawler). o IR thermography looking for wet insulation and/or damaged and missing insulation under the jacket. o Guided wave UT.
Inspection Frequency	· UT, coupons and visual inspection at T&I
KPIs	· Corrosion Rate < 5 mpy
Reference Resources (Standards/GIs/BPs)	API RP 571 (DM #46) NACE RP0198

Notes:

Design of insulation system, insulation type, temperature and environment (humidity, rainfall and chlorides from marine environment) are critical factors.
High quality coatings, properly applied, can provide long term protection.
Corrosion rates increase with increasing metal temperature up to the point where the water evaporates quickly.
Corrosion becomes more severe at metal temperatures between the boiling point 212°F (100°C) and 250°F (121°C), where water is less likely to vaporize and insulation stays wet longer.
For SSs, the temperature range of 60 – 150°C presents the greatest risk of CUI.
Equipment that operates below the water dew point tends to condense water on the metal surface thus providing a wet environment and increasing the risk of corrosion.
CUI can be found on equipment with damaged insulation, vapor barriers, weatherproofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges.