High temperature-resistant paint and coatings are used in a variety of industries to prevent corrosion of steel subjected to extreme temperatures in service. This article lists the industries and markets that employ heat-resistant coatings, describes the generic types of products available.
High-temperature Service, which is considered an exposure environment where coatings are used on steel with surface temperatures typically ranging from 120°C (250°F) to over 760°C (1,400°F). High-temperature-resistant coatings are designed to withstand these conditions while providing corrosion protection.
High-temperature coatings are used in the aerospace, manufacturing, military, petrochemical and power industries for process piping, engines, offshore rigs, original equipment and various type of plants/facilities that use high-temperature processes.
One of the largest users of industrial high-temperature coatings is processing facilities such as power plants, petrochemical plants, and refineries. These facilities often have an extensive network of piping, vessels, and tanks that need protection from corrosion. Since high temperature steel is often insulated, corrosion under insulation (or CUI) is frequently a concern, as active corrosion cannot be seen unless the insulation is removed.
The requirements of high-temperature service coatings in process-based facilities vary depending on whether they are for new construction, maintenance, or hot-surface applications.
The primary requirements of a heat-resistant coating in new construction include performance over a wide temperature operating range, minimal application time, and transportation from the fabrication shop to the jobsite with minimal handling damage. The traditional use of a single coat of inorganic zinc (IOZ), or an inorganic zinc top coated with a thin film application of a high-heat silicone, or an epoxy (heat resistance typically is a maximum of 350°F) covers many new construction installations.
Maintenance applications of high-temperature liquid coatings is challenging. Abrasive blast cleaning is often not possible, and hand or power tool cleaning is the only possible method of surface preparation. However, many of the traditional coating systems for high temperature service require a minimum SSPC-SP 10 “Near-white Blast Cleaning” surface cleanliness; therefore, material selection is very limited.
Equipment/process outages for maintenance painting can be costly, so there is obvious interest in coating while the equipment remains in service. Epoxy phenolic coatings are often used for these applications; however, there are a considerable number of other generic types that can be applied to hot surfaces with an application surface temperature threshold well above the operating temperature of the equipment.
High Temperature Coating Types
Composed of either organic or inorganic materials, high-temperature coating types are commonly epoxy, epoxy phenolic, epoxy novolac, silicone, or a more specialized multi-polymeric matrix.
Epoxy coatings are commonly used in oilfield, off-shore, and petrochemical facility applications and are favored for their impact and abrasion resistance. Epoxy coatings are organic polymers created through chemical reactions between epoxy resins and co-reactants/hardeners/curatives. Epoxies are also thermosets, which means that once cured they cannot be melted and reformed like a vinyl or plastic can. Excessive heat will deteriorate chemical bonds within a thermoset and cause it to degrade, discolor, lose ductility, and/or become brittle. Epoxy resins are also vulnerable to solar radiation and will chalk when exposed to sun light.
Epoxy phenolic coatings are categorized as either ambient cure, in which the phenolic and epoxy resins chemically react at room temperature, or heat cure, where the coating is exposed to temperatures of 350-400°F to accelerate the cure or activate a catalyst or curing agent in the coating. Epoxy phenolics provide chemical, solvent and temperature resistance, and are commonly used for immersion service, tank linings, and high-temperature oil and brine immersion service. Other suitable applications are when severe chemical resistance is necessary, but a high degree of flexibility is not.
Advantages of epoxy phenolics include excellent adhesion properties, temperature resistance up to 400°F and resistance to solvents, chemicals, and abrasion. Limitations include decreased weatherability and flexibility, relatively slow air curing time and often the necessity of heat curing at relatively high-temperatures.
Epoxy novolac coatings exhibit improved heat resistance because of the presence of aromaticity in their molecular structure, coupled with more cross-linking compared to other epoxies. Novolac epoxies are typically heat resistant up to 350 -360°F. In general, novolac epoxies are known for having greater resistance to oxidizing and nonoxidizing acids, and aliphatic and aromatic solvents compared to other epoxies. These qualities make novolac epoxies an option for applications such as tank linings in contact with high-temperature acidic crude oil.
Silicone coatings contain resins that are either pure or hybrid polymers and consist of organic pendant groups attached to an inorganic backbone of alternating silicon and oxygen atoms. The polymer structure provides thermal stability and oxidation resistance. Silicones are essentially transparent to ultraviolet radiation from sunlight. High-temperature, 100% silicone coatings are single component and cure by heat-induced polymerization. These thin film paints dry by solvent evaporation to achieve sufficient mechanical strength for handling and transport. However, total cure is achieved only after exposure to temperatures in the 350-400o F range. Curing can be achieved as the equipment is returned to its operating temperature. Pure silicone coatings are used on exhaust stacks, boilers and other exterior steel surfaces at temperatures ranging from 400-1200o F.
Modified silicone coatings have lower resistance to elevated temperatures than 100% silicone coatings. Silicone acrylics are single package air-dry paints that have color and gloss retention to temperatures in the 350-400o F range. Similarly, silicone alkyds are single package air dry paints with similar color and gloss retention properties. However, the dry heat resistance of silicone alkyds is limited to about 225o F. Although most high-temperature silicones require ambient temperatures for application, special formulations are available that can be applied to steel up to 400o F.
Multi-polymeric matrix coatings are either single or multi-component inert, inorganic, and composed of resin combinations. Often, multi-polymeric coatings contain aluminum and micaceous iron oxide flake, or titanium. Results from manufacturer studies have revealed anti-corrosive performance with single coat applications (150-200 microns [6-8 mils]) between ambient and 400°C (752°F) in both atmospheric exposure and under insulation tests.
Contact Dampney Co. Inc to get more information about their high heat paint and coatings product offerings