Corrosion Under Insulation (CUI)

Corrosion under insulation is one of the costliest problems attacking industry today. According to one specifier from a major oil company, CUI problems account for more unexpected downtime than all other possible causes combined.

Corrosion under insulation is a unique problem. A CUI protective coating in this environment needs to be able to withstand multiple abuses and also have certain application characteristics in order to make it a viable solution. This article will address the issues relevant to the CUI market today.

Properties a CUI Coating Must Have In Order To Be Successful

1) Resistance to Boiling Water

People in the industry often question why boiling water resistance in a coating is necessary to prevent CUI. An illustration is necessary to answer these questions. Picture a smoke stack which has a surface temperature of 350°F. When rain comes in contact with the stack, the drops will evaporate on contact.

An insulated piece of equipment can have trapped moisture by two means: moisture can become trapped due to cyclic equipment and condensation forming and being trapped under insulation. The second and more common reason is rain. Rainwater will eventually enter weak points in the system and pour onto the surface under insulation. The difference when compared to a stack is that the water gets trapped because of the insulation and is not allowed to escape as readily as a coated stack. In the case of an insulated hot pipe or vessel, the trapped water is up against a 300°F or 400°F surface. What occurs when water surrounds and lays on this hot surface? It boils. It will continue to boil until it stops raining and all the water is boiled off.

Therefore, a coating being used to prevent CUI must be resistant to immersion in boiling water. Almost all the products being used today are NOT resistant to boiling water. When a boiling water situation occurs they will inevitably fail. At this point your corrosion cell will begin and continue to worsen over time. It is especially significant to understand that since these surfaces are out of sight, the damage can progress undetected and lead to a catastrophe.

Hi-Temp 1027 CUI is resistant to continuous immersion in boiling water. It was designed to resist the real world severe conditions of a continually changing harsh environment. For example, Hi-Temp 1027 CUI can withstand the repeated cycle of heat, then thermal shock, then immersion in boiling water, and then back to heat, etc.

2) Heat and Thermal Shock Resistance

CUI materials must be able to withstand thermal cycling and continuous exposure to elevated temperatures ranging from 200°F to 500°F along with peaks of up to 800°F (e.g. regeneration units). An excellent CUI coating must be able to withstand these varying temperatures and be resistant to boiling water, if it should it occur.

Thermal shock is a significant cause of CUI. Thermal shock is categorized by a dramatic rise or fall in the temperature of the equipment. Thermal shock may occur when a unit is turned on or off, during the normal cyclic conditions the unit may exhibit, or when the steel is exposed to water that has penetrated the jacketing.

Almost all of the generic CUI products available today use older technology and are epoxy based. These include phenolic systems, novolac systems, phenolic/novolacs, flake filled products, etc. It is an inherent characteristic of epoxies to break down when exposed to prolonged high temperatures and/or thermal shock. They will inevitably become brittle and crack. Once cracking begins, water is able to penetrate and corrosion will begin. This is pertinent with CUI because the insulated piping remains unseen for extended periods of time. It is common that the steel will be subjected to these temperatures until the equipment gets replaced, the insulation gets replaced, or they shut the plant down. Therefore a CUI coating must be able to withstand continuous exposure to elevated temperatures as well as thermal shock.

Hi-Temp 1027 CUI can withstand continuous exposure and severe thermal cycling from ambient to 1200°F with no effect to the coating.

3) Direct Application to Hot Steel (up to 500°F)

The characteristic of direct application to hot steel is one of the most important in respect to existing CUI issues. It is common that chemical plants and refineries have problems with CUI. These problems are often compounded by the fact that companies cannot or will not shut down their equipment. It is enormously expensive to shut down equipment. Often times it is not possible to address the issue during scheduled down time. Therefore, a coating used during maintenance of existing CUI must have the ability to apply directly to hot steel.

Most generic CUI materials CANNOT be applied to hot steel. When materials are applied to hot steel there are many different modes of failure which can occur; bubbling, pin holing, dry spray, poor adhesion, cracking, delamination, etc.

Hi-Temp 1027 CUI can be applied at both ambient temperatures and directly to hot operating equipment having a metal temperature up to 500°F. This allows workers to strip the insulation, prepare the surface, apply the 1027, and reinsulate without having to shut down. There is no effect on production and no impact on profits.

4) Application to Ambient Steel (new construction)

The application characteristics of a product to ambient steel are also of tremendous importance. The coating should dry quickly, be able to receive a topcoat, and ship as soon as possible. The coating should not damage easily. It must also be resistant to UV rays. This is pertinent because a fabricated pipe or vessel may sit for up to a year before being installed at a site and be continuously exposed to harmful UV light. If a coating is not resistant to UV it will begin to degrade and its effectiveness will be reduced leading to premature failure.

As previously mentioned almost all the generic CUI coatings available today are epoxy based. Epoxies are not resistant to UV and will begin to break down before the coated equipment is even installed. The application of these generic epoxies is a two-coat system. The second coat can only be applied the following day thereby increasing the projectås cost. Epoxy based CUI coatings also have short recoat windows. Most CUI specifications call for at least two coats. Once the first coat is applied the second coat must be applied before the initial coat fully cures. Recoat windows for generic CUI systems vary from 1 to 3 days. If the second coat is applied after the primary coat has cured, intercoat delamination may occur leading to the failure of the system.

Therefore a coating being used to prevent CUI in a new construction application must have excellent constructability characteristics as well as be UV resistant.

Hi-Temp 1027 CUI has excellent constructability characteristics. It can also be applied to ambient steel. One can apply a coat in the early morning and apply a second coat by mid-afternoon. The material is also completely resistant to UV rays and will not break down prior to installation. Hi-Temp 1027 CUI also has no recoat window.

5) High Film Build Capability while Applying Directly to Hot Steel

This ability is key to the solution of todayås CUI problem. It is imperative that a coating be able to cover the peaks and valleys of a badly pitted area of piping. A thin film coating will not provide the coverage necessary to accomplish this task. High film build coatings allow for the proper coverage of bolts and flanges under insulation. A thin film coating would not allow any build up on these difficult to reach spots and will inevitably fail.

Epoxies, which are used in almost all generic CUI products, does allow for a high film build when applied to ambient steel. However, when the temperature is raised the epoxies cannot withstand the internal stresses and they crack. An increase in thickness combined with exposure to heat and thermal shock is a formula for failure.

Therefore a coating being used to prevent CUI must have high film build capability while applying to hot steel.

Hi-Temp 1027 CUI can be built up to 18 mils DFT while applying to either ambient steel or hot steel (500°F). At 18 mils DFT it can withstand severe thermal cycling to 1200°F. Hi-Temp 1027 CUI can also be built up to 45 mils DFT when applied directly to hot steel having a metal temperature of 400°F. At 45 mils DFT it can withstand severe thermal cycling to 400°F.

6) Simple to Use

Since most generic CUI systems are epoxy based and therefore contain at least two components, they are inherently more difficult to work with. Many of these systems are multi-ratio products. Human error must be taken into consideration when mixing these ratios. This will lead to an immediate failure in the CUI coating. Working with a two-component product is slower due to the mixing process. It will cost more time and money as compared to working with a single component system. Any unused portion of these generic epoxy systems must be thrown away. This results in a significant amount of wasted material which will drive up costs.

Epoxy based CUI coatings are more prone to human error, they accumulate more waste, and are more time consuming to prepare and apply. It would be ideal to work with a simpler product.

Hi-Temp 1027 CUI is a single component material.

7) VOC and Environmental Concerns

Most CUI coatings today are low in VOC content and are safe for the environment.

In conclusion, Hi-Temp 1027 CUI meets all of the criteria a CUI coating needs in order to perform exceptionally well and be a viable solution. These criteria are:

• 1) Withstand boiling water
• 2) Heat and thermal shock resistance
• 3) Direct application to hot steel (up to 500°F)
• 4) Excellent constructability when applying to ambient steel
• 5) High film capability while applying to directly to hot steel
• 6) Simple to use
• 7) Low VOC content and environmentally safe

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