High temperature coatings are specialized materials. These coatings are designed for temperatures of 300-1400F. Selection depends upon the temperature profile and type of substrate that is to be painted. Understanding how they work and how to specify and apply them will help to ensure proper service and eliminate such issues as disbandment, discoloration, and early failure.
Basics:
The coating system is expected to retain its appearance and integrity for high-temperature applications while protecting metal substrates at temperatures above (300F) (150C). The coating may be subjected to corrosion. In general, coatings are made up of resin or automobile, pigments, and solvents. Regular coatings, such as alkyds, use organic vehicles as pigment binders. However, these vehicles might decompose under temperature, and this can cause premature failure.
To overcome this problem, high-temperature coatings use temperature-resistant resins. These resin compounds have excellent thermal stability and resistance to oxidation. Also, they are essentially transparent to and resistant to degradation by ultraviolet radiation.
The combination of temperature-resistant properties and weathering characteristics helps make these resins and coatings ideal for formulation into temperature-resistant maintenance coatings. Additional coatings can be formulated with substitute resins, reducing cost per gallon while improving properties such as adhesion, abrasion resistance, and curing time.
The pigments must be compatible with the resin and should not decompose at high temperatures. Pigments must also be color-stable over the entire working heat range of the coating. Thermally stable pigments keep their color over time, unlike other pigments that are used in high-temperature coatings. Traditionally, only black and aluminum-colored heat-stable pigments were available. Now, a wide range of colors, including pigments, will support numerous color-matching options.
Operating Conditions:
The factors affecting performance must be assessed when specifying a high-temperature coating system. In addition to heat, these include the nature of the substrate, its structure, stress due to thermal cycling, weathering, surface preparation and application limitations, corrosives, and the life span of the coating. Two common pitfalls are made in specifying: 1. Assuming that a single high-temperature coating shall be suitable for all applications. 2. “Overspecifying” the coating. Too often, the substrate skin heat is guessed at, and the guess is made on the high side for safety. Thus, the coating system specified may be suitable for operating temperatures much higher than those encountered. Also, if overspecified, the coating may need to be dry/cured properly. High-temperature coatings usually require curing at elevated temperatures to achieve optimum film properties. A threshold temperature must be achieved before the coating fully cures/crosslinks or polymerizes. For this reaction, a coating rated at (1000F/540C) will not perform satisfactorily at a heat below (450F-230C). Curing will never take place and is a matter of time and temperature.
Measuring Heat:
Correct program and substrate conditions are critical to writing a specification. Both the temperature range and the maximum temperature need to be identified. Surface thermometers and temperature guns are now much more advanced and are the most accurate way to take temperature measurements. Heat readings taken at the most accessible locations can be misleading. For example, at ground level, a stack may be heavily ranged with refractories. It will have a skin temperature much lower than its upper reaches, where the lining may be thinner. When contact measurements cannot be made, other methods must be used. One is infrared emissivity measurement. An infrared scan provides accurate temperature profiles of such equipment as smelters, blast furnaces, pipelines, and kilns. Stack gas inlet heat can be determined from the process control temperature recorder. Once this temperature is known, the exit gas temperature can be found for an unlined stack of known elevation and diameter.
Range Of Applications:
There are two broad categories of high-temperature coatings: those for service below (500F-260C) and those for service above (500F-to 1200F-650C). Formulations of these coating systems change when the temperature requirement exceeds these temperatures. Coatings must be formulated specifically for the application and operating heat of the substrate to maintain this broad range of temperatures, the number of coats needed, and the quick temperature rise that is predicted by what is being painted. In cases where there is an extremely rapid heat rise, it is unlikely that any coating will work. This is because of the thermal stress caused by the difference in coefficients of expansion between the substrate and the coating.
Design And Maintenance Factors:
In writing a specification for a high-temperature coating, the equipment design and its condition must be considered. Design changes can be made only on new construction usually and only when a coating specialist is consulted before fabrication begins. If proper measurements are not taken, premature coating failure can be caused by items such as bolts, rivets, corners, edges, inverted channels, and badly treated weldments. Sharp protrusions should surface off and be welded. Such areas should be spot-primed with a high-temperature zinc dust primer. The makeup of the substrate must be considered since not all equipment is made of carbon steel. Stainless steel that is to be insulated should be coated to prevent externally induced chloride stress corrosion cracking.
This coating system must be chloride-free. Any type of zinc-containing coating should be kept away from stainless steel because welding might result in destructive alloying of the steel. Here, it is necessary to specify a coating that is free from chlorides and metallic zinc. Rusted or weathering steel might need painting. All products of oxidation must be removed from it before coatings are applied. Mil scale must also be removed from any metal surface. Upon heating, the scale eventually shatters, disbands, and separates from the parent metal. When refractories are used, their condition must be considered. A failure of a refractory lining shall result in overheating of the equipment surface and destruction of the coating. Lesser refractory failures such as cracking or thinning may cause hot spot failures of the coating. Discolorations result in a dare followed by disbanding, peeling, and flaking.
Surface Preparation:
Once the conditions of application are known, the coating can be specified. However, no coating- no matter how well specified – can perform if it is not applied properly. The surface must be prepared. Contaminates must be removed. The SSPC should be followed for each type of substrate, along with the coating manufacturer’s suggestion/recommendation. For carbon steel, abrasive blasting is the preferred method. It removes contaminants and creates a mechanical anchor pattern to hold the coating. The account should not normally exceed 1-1.5 mils since the high-temperature coatings are applied in thin films to reduce internal thermal stresses. For stainless steel, the removal of oil and grease can be done with light brush blasting or solvent cleaning with specialized nonchlorinated solvents.
PRIMING To avoid recontamination, priming should be done as soon as possible after surface preparation is finished. For carbon steel, a high-temperature zinc dust primer should be used. For indoor exposure in nonaggressive environments, a two-coat topcoat system offers a viable option. When high-temperature equipment is to be painted, the nature of the applied coatings must be considered previously. Topcoats These topcoats should be applied only over clean, dry surfaces or over primers that are compatible with the topcoat. If the composition of the existing coatings cannot be determined, remove all coatings from the surface. During priming, the dry film thickness of the primer ought not to exceed 1.5 mils for temperatures (300F-150C). And less for higher-temperature surfaces. Primers should usually be allowed at least a 24-hour wait before top coating to ensure full drying and flash off of entrapped solvents.
Field Application Methods-
Equipment should be allowed to cool to ambient heat before it is painted. The only exception is coatings that are formulated to be applied to hot surfaces. If equipment is hot, in some cases, brush and rollers could produce excessively thick films and could fail due to cracking and flaking caused by thermal stress in the film. Spray applications on hot surfaces can result in a condition similar to dry spray. The film will not properly adhere and will be extremely porous due to bubbling that results from quick solvent evaporation. Contamination is a problem often.
Apply the topcoat over a primer as soon as possible. If too much time passes after the primer is applied, remove any contaminates from its surface to promote adhesion. Avoid prolonged exposure to wet weather, salt fog, or additional corrosive circumstances before a high-temperature coating is cured. Work should be scheduled so that equipment exposed to such environments can be put back into service as quickly as possible. Poor control of film thickness can be a nagging problem. If the film is too thick, it can crack and lift. The total system dry film thickness should be considered as per the technical data sheet of both the primer and topcoat.
