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 problems as disbondment, discoloration and early failure.
BASICS: For high temperature applications, the coating system is expected to retain its appearance and integrity while protecting metal substrates at temperatures above (300F) (150C). The coating may be subjected to corrosion. In general, coatings are made up of a resin or vehicle, pigments and solvents. Conventional coatings, such as alkyds, use organic vehicles as pigments binders. However, these vehicles may decompose under heat, and this can cause premature failure.
To overcome this problem, high temperature coatings use heat resistant resins. These resins compounds have excellent thermal stability and resistance to oxidation. They are also essentially transparent to, and resistant to degradation by ultraviolet radiation.
The combination of heat resistant properties and weathering characteristics make these resins and coatings ideal for formulation into heat resistant maintenance coatings. Other coatings can be formulated with substitute resins which will reduce cost per gallon while improving properties such as adhesion, abrasion resistance and curing time.
The pigments used must be compatible with the resin and should not decompose at high temperatures. Pigments must also be color stable over the entire working temperature range of the coating. Thermally stable pigments keep their color over time, unlike other pigments on the market and so are used in high temperature coatings. Traditionally, only black and aluminum colored heat stable pigments were available. Now, there is a wide range of colors, including pigments that will support numerous color matching options.
OPERATING CONDITIONS: In specifying a high temperature coating system, the factors affecting performance must first be assessed. In addition to temperature, these include the nature of the substrate, its structure, stress due to thermal cycling, weathering, surface preparation and application limitations, corrosives and life expectancy of the coating .Two common pitfalls are made in specifying : 1. Assuming that a single high temperature coating will be right for all applications. 2. “Overspecifying” the coating. Too often, the substrate skin temperature 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 that will be encountered. Also in overspecifying, the coating may not dry/cure 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 polymerize. For this reaction, a coating rated at (1000F/540C) will not perform satisfactorily at a temperature below (450F-230C). Curing will never take place and then is a matter of time and temperature.
MEASURING TEMPERATURE: Correct application and substrate conditions are critical to writing a specification. Both the temperature range and the maximum temperature need to be identified. Surface thermometers and heat guns are now much more advanced today and are the most accurate to take temperature measurements. Temperature readings taken at the most accessible locations can be misleading. For example, at ground level, a stack may be heavily line with refractories. It will have skin temperature much lower than its upper reaches where the lining might be thinner. When contact measurements cannot be made, other methods must be used. One is infared emissivity measurement. An infared scan provides accurate temperature profiles of such equipment as smelters, blast furnaces, pipelines and kilns. Stack gas inlet temperature 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 height 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 temperature of the substrate to maintain this broad range of temperature, number of coats needed and rapid rise in temperature based on what is being painted. In cases where this is an extremely rapid temperature 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. Usually design changes can be made only on new construction, 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 poorly treated weldments. Sharp protrusions should be ground off, and welds abraded. 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 external 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 of welding might result in destructive alloying of the steel. Here, it is necessary to specify a coating that is free from chlorides , metallic zinc. Rusted or weathering steel may 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 will result in overheating of the equipment surface, destruction of the coating. Lesser refractory failures such as thinning or cracking may cause hot spot failures of the coating. Discolorations result, an 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 – will perform properly if it is not applied properly. The surface must be correctly 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 profile should not normally exceed 1-1.5 mil, 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 non chlorinated 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 previously applied coatings must be considered. Topcoats These topcoats should be applied only over either 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 surface. During priming, the dry film thickness of the primer should not exceed 1.5 mils for temperatures to (300F-150C). and less for higher temperature surfaces. Primers should usually be allowed at least 24 hour wait before top-coating to ensure complete drying and flash off of entrapped solvents.
FIELD APPLICATION METHODS- Equipment should be allowed to cool to ambient temperature 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 excessive 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 adhere properly, and will be extremely porous due to bubbling that results from rapid solvent evaporation. Contamination is often a problem. 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 other corrosive conditions 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 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.
APPLYING COATINGS TO HOT SURFACES – Most high temperature coatings are made to be applied to surfaces at ambient temperatures. What about equipment that is either rarely shut down or cannot be scheduled for painting due to short turnaround times? For such problems, special coatings can be used. These are made for in service painting of equipment as hot as 400-500F. Uses include coating of process vessels, piping, stacks and heat exchangers. Coatings that are hot applied are usually self – priming and can also be used for Corrosion Under Insulation (CUI).
PERFORMANCE LIMITATIONS- Although, typical high temperature coatings work well, they do have some limitations. They are not intended for immersion service. Splash zones, mists, gases and fumes can be a problem as well. When high temperature coatings are modified, film properties such as flexibility, chemical resistance, toughness and curing are improved. By also modifying high temperature coatings for maximum heat resistance, they can operate successfully to 1400F (760C). Keep in mind that high temperature coatings require maintenance. They must be inspected and repaired if damaged.
For more information on high temperature coatings contact Dampney Company Inc.
www.dampney.com