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Tham khảo tài liệu 'materials handbook 15th ed - g. brady_ h. clauser_ j. vaccari (mcgraw-hill_ 2011) ww part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 350 ENAMEL aluminum usually have a high proportion of lead oxide to lower the melting point and enamels for magnesium may be based on lithium oxide. Some enamels for low-melting-point metals have the ceramic frit bonded to the metal with monoaluminum phosphate at temperatures as low as 400 F 204 C . The mineral oxide coatings fused to metals are often called porcelain enamels but they are not porcelain and the term vitreous enamel is preferred in the industry although ceramic-lined tanks and pipe are very often referred to as glass-lined steel. The composition varies greatly one company having more than 3 000 formulas. Vitreous enameled metals are used for cooking utensils signs chemical tanks and piping clock and instrument dials and siding and roofing. Ground coats are usually no more than 0.004 in 0.010 cm thick and cover coats may be 0.003 to 0.008 in 0.006 to 0.020 cm thick. The hardness ranges from Knoop 150 to 500. Thick coatings on thin metals are fragile but thin coatings on heavy metals are flexible enough to be bent. Standard porcelain-type enamel has a smooth glossy surface with a light reflectance of at least 65 in the white color but pebbly surfaces that break up the reflected image may be used for architectural applications. High-temperature coatings may contain a very high percentage of zirconium and will withstand temperatures to 1650 F 899 C . Refractory enamels for coating superalloys to protect against the corrosion of hot gases to 2500 F 1371 C may be made with standard ceramic frits to which is added boron nitride with a lithium chromate or fluoride flux. Blue undercoats containing cobalt are generally used to obtain high adhesion on iron and steel but some of the enameling steels do not require an undercoat especially when a specially compounded frit or special flux is used. When sodium aluminum silicate Na2O Al2O3 6SiO xH2O is used instead of borax a white finish is produced without a ground coat. Mirac is a white enamel which gives good .
