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 use them will help to ensure proper program and eliminate such complications as disbondment, discoloration and early failure.
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.
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To overcome this problem, high temperature coatings use heat resistant resins. These resins compounds have great thermal stability and level of 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 that may reduce cost per gallon while improving properties such as adhesion, abrasion level of 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 heat 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 coloured heat stable pigments were available. Right now, there is a wide range of colours, including pigments that may support numerous color matching options.
In specifying a high temperature coating system, the factors affecting performance have to first be assessed. In addition to heat range, these include the nature of the substrate, its framework, stress due to thermal cycling, weathering, surface preparation and application limitations, corrosives and life expectancy of the coating.Two normal 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 pores and skin temperature can be guessed at, and the guess is made on the high part for safety. Therefore, 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 healing at elevated temperatures to accomplish optimum film properties. A threshold temperature must be achieved before the coating fully treatments/crosslinks or polymerize. For this reaction, a coating rated at (1000F/540C) will not perform satisfactorily at a heat range below (450F-230C). Curing will never take place and is a matter of time and temperature.
MEASURING Heat range:
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. Heat range 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 in fact the lining may 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 devices as smelters, blast furnaces, pipelines and kilns. Stack gas inlet heat range 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 program below (500F-260C) and those for service above (500F-to 1200F-650C). Formulations of these coating systems change when the temperature requirement exceeds these temps. Coatings must be formulated specifically for the application and operating heat range of the substrate to keep up 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. It is because of the thermal stress caused by the difference in coefficients of expansion between the substrate and the coating.
DESIGN AND MAINTENANCE Aspects:
In writing a specification for a high temperature coating, the equipment design and its condition must be considered. Usually design adjustments can be made only on new construction, and only when a coating specialist can be 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 aside 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.
Once the conditions of software are known, the coating can be specified. However, no coating- no matter how properly specified - will perform properly if it is not applied properly. The surface must be correctly prepared. Contaminates must be eliminated. The SSPC should be adopted for each type of substrate combined with the coating manufacturer's suggestion/recommendation. For carbon steel, abrasive blasting is the preferred method. It removes contaminants and produces 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 carried out with light brush blasting or solvent clean-up 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 coating topcoat system offers a viable option. When high temperature equipment is to be painted, the nature of the earlier 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, eliminate 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-
Devices should be allowed to cool to ambient heat range before it is painted. The only exception is coatings that are formulated to be applied to hot surfaces. If equipment is popular, in some cases, brush and rollers could create excessive thick films and could fail due to cracking and flaking caused by thermal stress in the film. Spray applications on popular surfaces can result in a condition similar to dry spray. The film will not adhere properly, and will be porous due to bubbling that results from fast solvent evaporation extremely. Contamination is often a problem. The topcoat over a primer as soon as possible apply. If too much time passes after the primer is applied, eliminate 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 solid, 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.
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