GB674359A - Improvements relating to low emissivity coatings for metal surfaces - Google Patents

Abstract

674,359. Low emissivity coatings for metal surfaces. FULMER RESEARCH INSTITUTE, Ltd. Jan. 12, 1951 [Feb. 13, 1950], No. 3662/50. Drawings to Specification. Class 56. [Also in Group V] Metal surfaces are coated with a material comprising by weight 5-30 per cent of enamel frit in dried and ground condition mixed with 95-70 per cent of a low emissivity refractory substance in the form of particles of from 0.1 to 0.001 mm. thickness or diameter, the coating adhering to the metal and reducing the radiant heat transference to and from the surface. Suitable refractory substances are fused SiO 2 , stabilised ZrO 2 , fused or recrystallised MgO, CeO 2 , or Al 2 O 3 , or cordierite (approximately 2 MgO, 2 Al 2 O 3 , 5 SiO 2 ), or mixtures of these substances. The nature and composition of the first and refractory must be such that no appreciable change in the character of the refractory particles occurs when the mixture is fired, thus it is shown that with a coating consisting of 40 per cent Al 2 O 3 , 40 per cent fused SiO 2 , and 20 per cent enamel frit the refractory particles react at about 1100‹ C. and the emissivity rises. The emissivity of a coating decreases with increased thickness but the adherence also decreases, and a coating of 0.003-0.015 inch is desirable. The enamel may contain feldspar, borax and quartz, and soda ash, fluorspar, barium oxide, calcium oxide, vanadium pentoxide, or zirconia may also be added. The refractory substance is crushed and ground dry, and may be treated to remove iron, and is then melted with the frit with an aqueous or alcoholic medium which may contain an organic binder or wetting agent. The metal surface is first prepared by shot or vapour blasting, washing in acetone, and drying. The coatings are particularly suitable for application to heat resisting alloys containing chromium, but mild steel may first be treated to give a high surface content of chromium. The mixture applied as a slip or water suspension by dipping, spraying &c. is dried and fired for say 30 minutes at 900- 1000‹ C. in an atmosphere whose oxygen content depends on the nature of the metal. Examples of coatings are as follows: (1) 20 per cent by weight of a frit formed from 20.9 per cent by weight feldspar, 40.2 per 'cent borax, 22.9 per cent quartz, 5.1 per cent soda ash, 4.8 per cent sodium nitrate, and 6.1 per cent fluorspar, and 80 per cent by weight of crushed fused SiO 2 are milled with a medium containing 500 ccs. of a 10 per cent solution of carbowax 4000, 500 ccs. of distilled water, 10 gms. sodium chloride, and 2.5 ccs. octyl alcohol, in the proportion of 100 gms. solid to 100-150 ccs. of medium, and the mixture applied to sheets of Nimonic 75, dried, and fired at 900‹ C. for 15- 30 minutes. The coating' is hard and white. (2) A mixture of 20 per cent of a frit formed from 33.8 per cent feldspar, 8.9 per cent borax, 15:9 per cent sodium carbonate, 4.0 per cent sodium nitrate, 3.7 per cent alumina, 4.0 per cent sodium silico fluoride, 10.4 per cent barium carbonate, 8.9 per cent zinc oxide, 7.7 per cent calcium carbonate, and 2.7 per cent vanadium pentoxide, with 80 per cent of pure cerium oxide, is used as in (1). Further examples in which the silica and ceria of examples (1) and (2) were replaced by fused magnesia required to be used within two or three hours of their preparation. The coatings are fired in air when applied to nimonic 75 or chromised mild steel, but when example (1) is applied to a stainless steel containing 18 per cent chromium and 8 per cent nickel it is fired in nitrogen of commercial purity. The coatings are stable during prolonged heating, do not flake as the result of thermal shock, e.g. quenching, and are flexible.