Corrosion under insulation or CUI has always been a difficult problem to prevent.  Sadly this prevention is hardly helped by industry practices which seem to focus solely on CAPEX and completely ignore OPEX.  In its simplest form, corrosion under insulation manifests as general or uniform corrosion which occurs on carbon steel due to moisture which has penetrated the cladding and lagging used.  In more specialized or complex scenarios, chemical salts such as chlorides, nitrates, and sulfates can leach from the insulation or enter through rainwater and concentrate as the substrate cyclically heats and cools.  On carbon steel, these chemical salts especially as they concentrate and at elevated temperatures can dramatically increase corrosion rates to as high as 250 mils per year (mpy hereafter) where normal corrosion rates are 3mpy.  Worse, on stainless steel substrates chlorides can cause stress corrosion cracking in which cracks propagate at inches per second leading to catastrophic failures.  There are three possible means of preventing CUI and because all three add slightly to capital expenditures, they tend to be ignored leading to dramatic operating expenses later.  For CUI to occur, water or another electrolyte needs to penetrate the cladding, move through the insulation, and contact the outside wall of the pipe or vessel to initiate a corrosion cell.  Properly selected and installed coatings can prevent the initial ingress into the cladding and/or prevent contact of the electrolyte with the outer skin on the pipe or vessel.  Hydrophobic closed-cell insulation such as expanded pearlite can prevent water from travelling through the insulation despite being slightly more expensive than mineral wool.

Metal cladding is impermeable to both liquid and vapor when sealed properly which can be virtually impossible in complex piping networks.  Moreover, this impermeability is often viewed as a strength but it can just as easily be a weakness.  In many piping networks there are often “bath tubs” or low spots which would naturally collect liquid between the cladding and the pipe and within the lagging.  A small leak above these bath tubs allows for them to gradually fill with liquid water.  Because the cladding is a vapor barrier, this liquid remains trapped rendering the insulation thermally ineffective and allowing corrosion to proceed undetected.  Metal cladding is also easily deformed through pressure or impact.  It is very common in tightly packed pipe racks to find cladding and lagging crushed because plant personnel have stood on pipework during maintenance procedures.  This not only decreases the thermal performance of the underlying insulation but it greatly increases the likelihood of compromising the caulking which allows water ingress and corrosion beneath this defect.

Belzona® 3211 (Lagseal) was created specifically to address these known issues.  Because Belzona® 3211 (Lagseal) is a liquid-applied coating; there are no seams to seal regardless of the complexity of the underlying lagging.  Because Belzona® 3211 (Lagseal) is a vapor retarder rather than a vapor barrier it is able to expel water vapor when the internal relative humidity exceeds the external relative humidity.  This eliminates the possibility of any bath tubs and allows damp insulation to dry over time thus increasing its thermal performance.  Finally, because Belzona® 3211 (Lagseal) is elastic, if pipework is walked on or tools are dropped on it, the Lagseal and the lagging will return to their original shape and thickness.  It is important to note that Belzona® 3211 (Lagseal) provides an additional benefit which conventional cladding cannot provide; ease of inspection.  Lagging and the underlying pipe/vessel exterior can be inspected and the Lagseal resealed in a matter of minutes.  This is because the Lagseal and insulation can be cut with a knife and removed for inspection.  Once inspection is complete, the plug is reinserted and additional Lagseal can be applied to reseal the incision.  This frees asset owners to conduct inspections of critical or at-risk areas without the associated labor expense or risk of compromising the cladding further.

In order to guard against CUI some refiners such as ExxonMobil have bet heavily on thermal sprayed aluminum, TSA hereafter, because metallurgists gave plant management 40 year inspection cycles on TSA coated assets.  However, after just a few years certain assets began showing signs of corrosion and now ExxonMobil is turning back to organic coatings for CUI prevention.  TSA is nothing more than a thick buildup of spattered droplets which do not flow together before solidifying.  Thus there are pore networks which can and do allow moisture access to the pipe skin.  In addition, TSA looks identical at 2 mils thickness as it does at 20 mils thickness and these both look identical to grit blasted steel ready to accept TSA.  Because it is electrically conductive, TSA cannot be inspected for holidays or misses so one must hope that sufficient thickness is present.  These characteristics create a false sense of security in which an asset owner has a coating which is very easy to apply under thickly, with breaks in continuity, which cannot be inspected.  Other refiners have used atmospheric coatings which cannot survive the rigors of immersion which invariably occur due to breaks in cladding and low points which collect fluid.  Belzona offers a number of CUI coatings which were designed to be applied either at ambient temperatures or as hot as 400°F.  Others are formulated to tolerate minimal surface preparation such as wire brushing only, and still others are designed to cope with damp or oily substrates which cannot be cleaned.  All of these coatings are designed and tested for full immersion resistance. 

How can we help you with your CUI concerns?

Chris Lucas
clucas@rumfordgroup.com
937-435-4650
RumfordGroup.com

Time sure does fly, 35 years have gone by

On January 11, 1980 Rumford Industrial Group was incorporated, becoming the Great Lakes Region distributor for Belzona^® polymeric repair compounds and industrial protective coatings.

We are honored to know that you rely on us to repair, protect and improve your machinery, equipment, buildings and structures when things stop working.  We trust that you’ll allow us to continue to be a trusted business partner for the next 35 years and beyond.

On behalf of our employees and associates, thank you so very much for the opportunity to be of service.

Jim Rumford

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