• EC • Season Cracking • SSC • LME • MIC • SCC • Fatigue • Electrolysis • Stray Current • AC Corrosion • CUI • CUF • DewPoint Corrosion  • Index

Different Types of Corrosion
- Recognition, Mechanisms, Prevention and Prediction

Corrosion Under Insulation (CUI)

Recognition of Corrosion Under Insulation (CUI)

What is CUI? CUI refers to corrosion of piping, pressure vessels and structural components resulting from water trapped under insulation.

How to identify CUI? CUI Damage may be highly localized. In some localized cases, the corrosion can appear to be carbuncle type pitting (usually found under a failed paint/coating system). For 300 Series SS, specifically in older calcium silicate insulation, localized pitting and external chloride stress corrosion cracking (ECSCC) can occur.

What structures are affected by CUI? CUI can be found on

(1) equipment with damaged insulation, vapor barriers, weatherproofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges;

(2) Equipment designed with insulation support rings welded directly to the vessel wall;

(3) piping or equipment with damaged/leaking steam tracing;

(4) localized damage at paint and/or coating systems;

(5) locations where moisture/water will naturally collect before evaporating and improperly terminated fireproofing .

Mechanisms of Corrosion Under Insulation (CUI)

What causes CUI? The Equipment that operates below the water dew point tends to condense water on the metal surface thus providing a wet environment sustaining the corrosion process. Plants located in areas with high annual rainfall or warmer, marine locations are more prone to CUI than plants located in cooler, drier, mid-continent locations.
Environments that provide airborne contaminants such as chlorides or SO2/SO3 are more prone to CUI. Corrosion rates increase with increasing metal temperature up to the point where the water evaporates quickly. Cyclic thermal operation or intermittent service is also conducive to CUI.
 

Prevention and Prediction of Corrosion Under Insulation (CUI)

How to prevent corrosion under insulation? CUI Mitigation is best achieved by using appropriate paints/ coatings and maintaining the insulation/sealing/vapor barriers. Low chloride insulation should be used on 300 Series SS to minimize the potential for pitting and chloride SCC. Multiple inspection techniques should be considered for cost-effective approach.

How to predict dew point corrosion? Software tools such as CUI-Compass and CRA-Compass can be used to predict the risk of corrosion under insulation. Click the link for details on the software.

For more details on Corrosion Under Insulation

Where can I learn more about corrosion under insulation? More details on corrosion under insulation are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses:

A Basic Course in Corrosion and Its Prevention (5-day module)
Corrosion, Metallurgy, Failure Analysis and Prevention (5 days)
Methods and Materials for Corrosion Control and Prevention (5 days)

API 583 Corrosion Under Insulation and Fireproofing -Detection, Mitigation and Prevention (5 days)

API 584 Integrity Operating Windows (5 days)

Corrosion Control by Materials Selection and Design (5 days)

If you require corrosion expert witness or corrosion consulting service on corrosion under insulation (CUI), our NACE certified Corrosion Specialist is able to help. Contact us for a quote.

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