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A Primer for High-Heat Coatings

MONDAY, JULY 17, 2017

By Chris Ard, Tnemec Company, Inc.


High-temperature areas in power plants, pulp and paper mills, steel mills, refineries, or even water and wastewater plants, can often be the starting point of widespread corrosion. Whether hot water pipes and pumps or power plant stacks, these hot surfaces must be coated with the appropriate products to ensure that they operate properly and do not cause unwanted downtime for repairs.

Prior to beginning the high-temperature application process, owners, operators, engineers and applicators must consider several factors to ensure that the process goes as planned. When they do, the result is often a more efficient procedure and, in the end, a longer life cycle for the equipment or infrastructure. 

From power plants to water and wastewater facilities, high-temperature areas require specialized protective coatings to prevent accelerated corrosion. PHOTOS COURTESY TNEMEC COMPANY, INC.

BEFORE YOU PAINT

A common starting point when considering coating selection for high-temperature substrates is the current condition of the surface. Ideally, pipes, tanks or other hot surfaces will be prepared by abrasive blast. New equipment should be blasted — and possibly even coated — prior to installation, but in most other situations, applicators must prepare the surfaces on site.

In these cases, owners must consider the rest of the facility and the surrounding property. If a plant or facility is to remain in service during preparation, then a full blast may be difficult, and owners will need to determine if the substrate is a candidate for overcoating.

If an overcoat seems to be the best option, a generic analysis of the existing coating will need to be performed to ensure that the previously used products are compatible with the new and were applied at a proper thickness. Once the analysis shows that surfaces are appropriate for overcoating, they should be prepared, at the very least, in accordance with SSPC-SP2 Hand Tool Cleaning or SSPC-SP3 Power Tool Cleaning. If schedule allows, preparing a small section and applying one or more test patches can help identify the system with the best compatibility and performance.

Next, owners and applicators must consider the surface’s operating temperature. High-temperature processes are abundant in many types of facilities, and different processes are set up to peak at different temperatures. Knowing the typical operating temperature — as well as any high- or low-temperature spikes — will help to determine which coatings should be applied.

For continuous temperatures under 300 F (149 C), specifiers can use an organic-based coating, such as an epoxy, acrylic and urethane product. Most of these products can also handle intermittent temperatures up to 300 F (149 C), but above this level, organic matter begins to break down, compromising the coating’s abilities to protect the substrate.

For continuous temperatures above 300 F (149 C), inorganic or certain silicone-modified organic coatings should be used. A handful of silicone-based or multipolymeric coatings are able to protect structures in continuous service up to 1200 F (649 C) and can be hot-applied. Because the inorganic coatings contain little to no carbon, the film remains strong even when exposed to extreme temperatures.

When planning to coat any substrate, the engineer and/or owner must consider the facility’s climate along with the other exposures that might affect the surface. If the surface will be covered with insulating material or cladding, selected coatings should be able to withstand possible corrosion under insulation (CUI) conditions as well. Before applying material, applicators need to consider air temperature, surface temperature, relative humidity, dew point and the difference between surface temperature and dew point.

A handful of silicone-based or multipolymeric coatings are able to protect structures in continuous service up to 1200 F (649 C) and can be hot applied.

PREPARING TO PAINT

Owners need to think about what will happen once the coatings system is installed. This becomes especially important when planning to use a high-heat coating, as some of these coatings cure under ambient conditions and others are forced to cure by heat.

If a high-temperature coating is being applied while equipment is in service, the owner will need a product that can be hot-applied. These coatings may cure by heat or without heat, but they will need to be able to handle the surface temperature to which they are being applied.

If the product(s) will be applied while the equipment is off line or idle, either an ambient-curing coating or heat-cured coating can be used.

When using a heat-cure product, the equipment must return to service in a timely fashion to assist in the heat-curing process. If the equipment has an indefinite return-to-service time — and it may be months before the coated equipment will be in use — the manufacturer should be consulted to determine whether or not the coating will be affected by the surrounding conditions until it has completed curing.

Engineers, owners and applicators should match the surface temperature to the product data sheet of the specified product and ensure that the required curing temperature is met during and after the application process.

Another important issue to consider is the application process itself. Will the coatings be brush-, roller- or spray-applied? If spray-applied, what kind of equipment will be used?

When applying a high-temperature coating, it is typically recommended that only small sections be brush- and/or roller-applied. For most areas, spray application is used for efficiency and aesthetic reasons. Most high-heat product data sheets list air and airless spraying options at various pressures.

When spraying, the surrounding facility must be considered. Many facilities need to address concerns about contaminating or pacifying other processes, while also ensuring that the coating can be wiped clean from nearby surfaces, including company-owned and personal property. For these kinds of concerns, a “dry-fall” product, formulated to help prevent overspray, should be used.

For indoor spraying, applicators should ensure that proper ventilation is in place and use various containment structures, where needed, to avoid bothering personnel or other trades at the project site.

WHEN YOU PAINT

When completing a coating project for high-heat surfaces, it is important to have a crew that knows how to use the equipment and is familiar with the coatings themselves. Applicators and owners should always contact the coatings manufacturer with any questions or concerns about the products being utilized and ensure that all processes listed on the product data sheet are followed as closely as possible.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

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ABOUT THE AUTHOR
Chris Ard, Tnemec Company, Inc.

Chris Ard has more than 25 years of experience in the installation, project management and sales of high-performance protective coatings. He is a NACE III certified coating inspector and has held several positions with Tnemec Company, Inc., from technical services agent to his current position as industrial market manager.

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