Silicone-Based Coating Mitigates Corrosion Under Insulation (CUI) in Petrochemical Plants

Numerous coating systems have been used to attempt to curb CUI, with varying results based on environmental, startup and operating conditions. For example, standalone inorganic zinc-based coating systems without a topcoat have failed and are no longer recommended. Various epoxies have performed well, but not when asset temperatures operate at 450°F.

Fortunately, innovative silicone-based coatings, like Heat-Flex High-Temp 1200 from Sherwin- Williams, are proven not only to protect insulated assets from CUI but also to withstand a wide range of stressful conditions – including sudden temperature swings, thermal shocks and other upset conditions – that occur in petrochemical and refinery facilities (Figure 1). By mitigating these conditions, Heat-Flex High-Temp 1200 remains intact as a continuous protective layer under insulation for much longer periods compared to alternative coating systems.

THE INSULATION DILEMMA
Piping and other assets in petrochemical plants typically need insulation due to their operating temperatures. While some piping operates at ambient temperatures, other pipes are insulated because the materials flowing through them require heat to create operational efficiency flow. Some of these materials also require heat in storage so they don’t break down into byproducts. Without insulation, piping and storage tanks would lose too much heat, decreasing the efficiency of the process.

Insulation also protects workers who might accidentally come into contact with assets that have high operating temperatures and represent a risk for skin contact burns. Even some assets running at ambient temperatures require insulation to protect workers during steam cleaning operations. For example, piping can reach 350-450°F when it is cleaned or “steamed out,” which is an example of an upset condition.

Figure 1. Heat-Flex High-Temp 1200 is applied direct-to-metal to large pipes and valves before shipping to construction sites to protect insulated assets from CUI.

A NEW ERA IN CUI MITIGATION
Coatings can help prevent CUI, but many traditionally specified coating systems have failed in mitigating the issue. Epoxy resins, for example, are suitable for temperatures up to 400-450°F, if those temperatures remain constant. However, if thermal shock occurs, many epoxies tend to turn brittle and crack. That’s because most epoxies are rigid and inflexible and have a breaking point when heated or cooled rapidly.

For years, the primary coating used to mitigate CUI on pipes and assets in petrochemical plants was inorganic zinc, which is suitable in temperatures below 120°F and above 350°F. It is easy to apply and cheap when applied in a shop. Plus, only one coat was presumably needed for protection from CUI. However, field studies have determined that inorganic zinc coatings break down in the CUI sweet spot of 120-250°F when they don’t have a topcoat (Figure 2). New standards now discourage the use of inorganic zincs under insulation without a topcoat.

NACE International (now AAMP) has consistently recognized the challenges with CUI and in 2010 appointed a committee to devise solutions. In 2011 the committee published new CUI mitigation standards under the title, “Control of Corrosion Under Thermal Insulation and Fireproofing Materials – A Systems Approach,” which today is also known as SP0198-2017. The standards recommend which coatings to use, based on temperatures to which they’ll be exposed, on carbon steel and stainless steel assets.

The recommendations, last updated in 2018, include specifying innovative, silicone-based or silicone-based multipolymeric matrix coatings, such as Heat-Flex High-Temp 1200. It’s designed to expand and contract, along with the assets it’s protecting, during extreme temperature fluctuations. Therefore, unlike epoxies, it can perform well against the stresses of thermal shock.

Older types of silicone-based coatings, applied in a thin layer of 1 to 2 mils dry film thickness (DFT), are challenging to prepare and apply on carbon steel, and NACE’s CUI guidelines advise against using them. Heat-Flex High-Temp 1200, in contrast, includes micaceous iron oxide (MIO) and other pigments that add thickness to the coating, making it easier to apply in ambient temperatures. MIO also reinforces Heat-Flex High-Temp 1200, strengthening the product and making it resistant to impacts. MIO formulations, with silicone-oxygen bonding, are also stronger than epoxies, which have carbon-to-carbon bonding.

One important physical characteristic of MIO is that is creates a layered structure in the coating that acts like mirrors, deflecting ultraviolet light, which allows the coating to last longer and degrade slower when subjected to UV rays (Figure 3). This is vital because pipes and bulk valves are often shipped long distances from where they’re made to where they’re used. The journey can take six months to a year in some locations, and sunlight will pound the assets during that time. Epoxies, in comparison, don’t hold up well under UV exposure. Further, once the asset is installed, the layered structure of MIO pigments makes it more difficult for moisture to penetrate the coating and reach the steel substrate. Products with MIO are effective in temperatures ranging from 113°F to more than 1200°F in both open and closed areas. All of this has been verified in Sherwin-Williams labs, using rigorous standards and protocols by the American Society of Testing and Materials.

Heat-Flex High-Temp 1200 also contains a special additive that allows it to cure and harden at lower temperatures. Traditional silicone-based products dry at ambient temperatures but don’t fully cure until reaching 250-350°F. If these older coatings are applied at ambient temperatures without heat curing, the asset might sustain damage when moved or strapped down for transport or installed in the plant. Any spot of coating damage can weaken the coating’s protection and become a weak point for corrosion to attack. However, the curing additive in Heat-Flex High-Temp 1200 makes the coating harder than any other silicone coating in the petrochemical industry. When it comes to stopping CUI, Heat-Flex High-Temp 1200 from Sherwin-Williams offers the flexibility to handle thermal shock and temperature fluctuations, the chemical resistance to hold up in contact with contaminants, and the impact and damage resistance to retain a continuous film on coated assets during handling and installations. It can protect a company’s equipment, save money, prevent disastrous asset breakdowns and failures and keep workers safe.

Figure 2. Pipes coated with just an inorganic zinc-based primer  and no topcoat are prone to severe CUI when covered with insulation and operating in the CUI sweet spot of 120-250°F.

Figure 3. Micaceous iron oxide (MIO) pigments embedded in Heat-Flex High-Temp 1200 form a barrier that deflects ultraviolet rays. The layered pigment structure also mitigates the penetration of moisture into the coating.