Whether coating structural steel or steel pipe, an elevated water tank or an aboveground storage tank, many considerations must be made to ensure a coating project goes according to plan. When coating high-temperature surfaces, these considerations become even greater and care must be taken before the application process begins.
Below are five major considerations anyone should make prior to beginning the high-temperature application process.
1. Maximum Operating Temperature
Many factors will come into play when coating high-temperature substrates, but a common starting point when considering coating selection is the surface’s operating temperature. Whether oil and gas facilities, power stations or even wastewater plants, high-temp processes are abundant in many facilities and they are set up to peak at every temperature imaginable. Knowing the typical operating temperature – as well as any spikes – will help you determine which coatings should be applied.
For continuous temperatures under 250°F - 300°F (121°C - 149°C), coating specifiers can utilize an organic-based coating, such as an epoxy, acrylic and urethane product. The majority of these products can handle intermittent temps up to 300°F (149°C), but above these levels, organic matter begins to break down, causing issues with the coating’s abilities to protect the substrate.
For continuous temperatures above 300°F (149°C), inorganic or some silicone-modified organic coatings should be utilized. 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.
2. Substrate Condition and Surface Preparation
Chances are, if you’re thinking of coating or recoating a surface, the current condition of the substrate is not perfect. The current condition of the surface can help determine which coatings to apply. Ideally, pipes, tanks or other hot surfaces will be prepared by abrasive blasting, but this isn’t always an option. (Discover if wet blasting is a favorable alternative in the article Understanding the Industry Shift to Wet Abrasive Blasting.) New infrastructure or equipment can be blasted – or even coated – prior to installation, but for most situations, applicators must prepare the surfaces onsite.
When performing surface preparation onsite, specifiers must consider the surrounding property. If a plant will remain in service, then a full blast may be difficult. If this is the case, specifiers will need to determine if the substrate is a candidate for overcoating. To do this, a generic analysis of the existing coating should be performed to ensure that the previous products are compatible and applied at an acceptable thickness.
If the performed analysis is favorable, surfaces 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 determine the system with the best compatibility and performance. (For further reading on this topic, see Substrate Surface Preparation for Corrosion Prevention.)
3. Environment and Climate
When coating any substrate, one should consider the surrounding environment. Before applying material, applicators need to consider air temperature, surface temperature, relative humidity, dew point and the difference between surface temp and dew point. But before application, the engineer and/or owner must consider the facility’s climate along with the other exposures that might affect the surface.
For example, if specifying a high-temperature coating at a coal facility in Louisiana, the engineer would need to consider the extreme humidity typical of the area while also thinking about possible exposure to dust and other nearby pollutants. The specifier should also consider the overall climate classification by referring to ISO environmental standards to determine if the conditions would be considered moderate or otherwise.
Another consideration should be whether or not the surface will be covered with insulating material and/or cladding. If this is the case, coatings should be able to withstand possible corrosion under insulation (CUI) conditions. (Learn more about CUI in An Introduction to the Root Causes of CUI with Monica Chauviere.)
4. Application Method
Another important issue to consider is the plan for the application process. Will the coatings be applied using brush, roller or spray equipment? If being spray-applied, what kind of equipment will be used?
When applying a high-temp coating, it is typically recommended that only small sections are brush- and/or roller-applied. These coatings are usually spray-applied for efficiency and aesthetic reasons. Most high-heat product data sheets list air and airless spraying options at various pressures.
But when spraying, you must consider the surrounding facility. Many facilities need to prevent overspray to limit concerns with 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 should be used.
If being applied indoors, applicators should ensure that proper ventilation is provided and use various containment structures where needed to prevent bothering personnel or other trades at the project site.
5. Coating Curing Schedule
Before application begins, one should consider what will happen after the coating process is over. This becomes especially important when using a high-heat coating, as some of these coatings cure under ambient conditions and others are forced to cure by heat.
If a high-temp coating is being applied while equipment is in service, the owner would likely need a coating that can be hot-applied. These coatings may be ambient- or heat-cured, 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 offline, or while the equipment remains idle, then either an ambient-curing coating or heat-cured coating can be used.
When using a heat-curing 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 would be affected by the surrounding conditions until it has completed curing by reaching the necessary temperature.
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.