504 - High Temperature
Porcelain enamel is defined as a substantially vitreous or glassy inorganic coating bonded to metal by fusion at a temperature above 800°F.
INTRODUCTION Abrupt and severe temperature changes do not affect porcelain enamels applied at normal thicknesses. Porcelain enamel's thermal properties are excellent in the range of temperature from below freezing to high heats. Because of this thermal stability and thermal shock resistance, porcelain enamel is a suitable coating for many heat-related applications. The thermal properties of porcelain enamel provide a coating that readily withstands the expansion and contraction of the base metal under changing temperature conditions.
RESISTANCE TO OXIDATION AND CORROSION Porcelain enamels and ceramic coatings-glass coatings for metal used for applications above 1000 degree F (538 degree C)-are noted for their ability to appreciably reduce the oxidation of base metals. As a result, they lower costs and improve performance in one or more of the following ways: (1) permit the use of less costly metal alloys without loss of performance; (2) extend base metal life; (3) increase the metal's maximum operating temperature, thereby improving the efficiency of operation. The ability of porcelain enamels and ceramic coatings to protect against oxidation is largely due to the fact that the coatings themselves are fully oxidized and do not undergo further oxidation at elevated temperatures. They also form an effective barrier to diffusion of oxygen. The protective ability of a porcelain enamel depends on the temperature at which it starts to soften and become more fluid (ordinarily at about 400 degree F (204 degree C) below firing temperature). However, special ceramic coatings designed to be effective above the firing temperature can protect metals from oxidation up to 2000 degree F (1093 degree C). Specially formulated porcelain enamel coatings resist the corrosive effects of acid and alkali solutions at the elevated temperatures required for many industrial processes and reactions. These coatings provide both protection of the metal and ease of cleaning. The smooth, abrasion resistant surfaces also allow for more efficient flow of fluids over the surfaces.
THERMAL STABILITYPorcelain enamels have the ability to withstand intermittent or prolonged heat without changing physical, chemical or appearance properties. The maximum temperature which a coating will withstand for extended periods is related to its firing temperature and its formulation. In general, the coating remains inert to a temperature about 400 degree F (204 degree C) below its firing temperature. The conventional firing temperatures using a steel substrate are from 1450 degree F to 1550 degree F (788 degree C to 843 degree C). Thus, the thermal stability of these coatings vary from 1050 degree F to 1150 degree F (566 degree C to 621 degree C). Of course, special formulations designed to fire at higher temperatures up to 1700 degree F (927 degree C) on steel can withstand proportionately higher temperatures for long service periods. When applied to an aluminum metal base, the usual firing temperature is in the 900 degree F to 1000 degree F (482 degree C to 538 degree C) range. Thus the temperature to which the coating can be subjected for prolonged periods is between 500 degree F and 600 degree F (260 degree C and 316 degree C). Above these temperatures, there may be some surface damage, such as crazing to the coating, but ordinarily the substrate remains protected. Special high temperature porcelain enamels and ceramic coatings can be engineered to take sustained heat from 1200 degree F to 2000 degree F (649 degree C to 1093 degree C).
THERMAL SHOCK RESISTANCEThermal shock resistance of a porcelain enamel varies inversely with its thickness. Thermal shock failure is caused by the rapid chilling of the surface. It manifests itself in a crack and occurs when the thermal gradient perpendicular to the surface is large enough to cause excessive differential shrinkage and tensile stress. Important considerations are the porcelain enamel's thickness and its coefficient of thermal expansion. Thermal shock resistance is also affected by the design and thickness of the part. Flexing of the metal due to localized thermal gradients parallel to the surface can produce bending and tensile stresses in the coating. Thus, any increase in the strength or rigidity of a part helps increase the coatings resistance to thermal shock. Most porcelain enamels applied at conventional thickness can be expected to take abrupt temperature drops of 200 degree F to 300 degree F (93 degree C to 149 degree C) without damage. Thinner coatings have a proportionately higher thermal shock resistance. Other things being equal, the best thermal shock resistance is achieved with coatings of 5-mils or less. Special ceramic coatings will successfully undergo repeated tests of heating to 1700 degree F (927 degree C) and quenching in water.
EMISSIVITYEmissivity is defined as the power of a surface to release heat by radiation. Porcelain enamels have high total emittance at high temperatures. The color of a porcelain enamel has practically no effect on total emittance at room temperature, but will have an effect at high temperatures. White porcelain enamels, for example, generally have lower emittance than black at high temperatures. A typical white porcelain enamel has low spectural emittance at short wavelengths of about 0.2 to 0.5 u. Spectural emittance increases as wavelength increases to about 5 u and remains high at all longer wavelengths. A black porcelain enamel has appreciably higher spectural emittance than a white one at short wavelengths; they are about the same at wavelengths longer than about 5 u.