How to prevent CUI

Ways to prevent corrosion under insulation

 

Corrosion under insulation (CUI) is one of the costliest forms of corrosion in industry. CUI is a particularly severe form of localized corrosion that has been plaguing the chemical process industry since the mid 1970's. The reason for this was that operators did not insulate, but due to rising energy costs and health and safety we now insulate surface that operate above 60°c. The ASTM 1055 safe touch test says that you will receive 3rd degree burns on insulated surfaces above 60°c.

So what is the root cause of CUI, in very easy terms the culprit here is the insulation is there was no insulation there would be no CUI, in the diagram on the right we have a simple illustration on the CUI cycle. (Use your mouse to over the image to see a larger picture) In Figure 1 we can see the layout of a standard insulated pipe. Figure 2 shows water ingress this can be from may sources. Figure 3 show what happens inside the jacket the water boils and turns to steam condensing on the inside of the jacket turning back to water and the cycle starts again.

 

What are the Mechanism's of CUI

The mechanism of corrosion under insulation involves four requirements the availability of 

Steel ( carbon steel or 300 series stainless steels)

oxygen.

High temperature.

Solution

Normally as the temperature increases, the amount of oxygen dissolved in the aqueous solution decreases when the boiling point is reached. This results in reduced corrosion rates.

When the surface is insulated, a wet dry effect is created because the insulation holds in the water which makes it similar to a closed system, the measured corrosion rates associated with corrosion under insulation follow trends to higher corrosion rates commonly found in pressurized systems.

The water gets in much quicker than it can get out. The fact all systems are cyclic just at some point, due to breakdowns and maintenance this cyclic service just exacerbates the possibility of increased corrosion rates.

How do I prevent CUI

The coating system needs to be carefully selected. it must have resistance to boiling water and be able to stand thermal cycling, most of the coating systems available today are based on older technology these include epoxy phenolic and epoxy novalac systems. Zinc silicates have often been used to protect carbon steel but these are sacrificial these system work well but only for a time.  One of the recommended systems in NACE SPO198 is to use an Inert Poly Meric Matric Coating but even then care needs to be taken, as some of these systems need heat curing before they are put in to service which is risky if this get immersed in water before its sees service.

The Solution

1/ Protective Polymers have developed a true ambient cured Poly Meric Matrix Coating that does not require heat to cure this would be the preferred coating system to meet the requirements of NACE SPO 198 under traditionally insulated steels please see PolyMeric CUI 650

Polymeric CUI 650 is resistant to boiling water and cyclic service and can operate in temperature from -185°c to 650°c. It can be applied to hot surfaces up to 230°c. This allows operators and contractors to remove the insulation prepare the substrate and re-insulate without the need for costly shutdowns so has no impact on production

2/ The second option is to remove the existing insulation and replace this with a liquid applied insulation this system is monolithically bonded to the substrate allowing no moisture through to the steel, these coatings have a proven track record are very durable. These coatings show outstanding performance and are equivalent to most conventional insulation products but with the added benefit that it does not take up moisture and in so prevents corrosion under Insulation. It provides personal protection to the ASTM 1057 skin touch test. Please see AeroShield 180T15