Porcelain enamel coatings

Porcelain enamel is used mainly in high-level performance appliances. The porcelain enamel coatings are more scratch resistant and heat resistant than the thinner liquid and powder coatings however, porcelain enamel is porous. Holidays are sometimes found in the porcelain (glass). Magnesium, zinc, and aluminum anodes are used in combination with porcelain enamel coatings in water heating systems to act as a sacrificial anode. 

C 743-87 (1993) Test Method for Continuity of Porcelain Enamel Coatings... 

Inorganic coatings include enamels, glass linings, and conversion coatings. Porcelain enamel coatings are inert in water and resistant to... 

Opacifiers are fine inorganic powders, usually white, that are used to reduce the transparency of ceramic gla2es and porcelain enamels. The coating becomes opaque because the particles of the opacifter scatter and reflect the incident light. When inorganic pigments are combined with white opacifiers, pastel colors are obtained. 

Finishes for aluminum products can be both decorative and useful. Processes in use include anodic oxidation, chemical conversion coating, electrochemical graining, electroplating (qv), thin film deposition, porcelain enameling, and painting. Some alloys respond better than others to such treatments. 

Porcelain enameling requires the use of frits and melting temperatures of 550 °C or below. Enamels are appHed over chemical conversion coatings that are compatible with the frit. AHoy selection is important to obtain good spall resistance. Alloys 1100, 3003, and 6061 are employed most extensively among wrought products and alloy 356 for castings. 

Another large appHcation is as an ingredient in the production of charcoal briquettes. The amount of sodium nitrate used in charcoal briquette manufacture depends on the type and amount of wood and coal used. Typically charcoal briquettes contain up to almost 3% sodium nitrate. Sodium nitrate is also used in the manufacture of enamels and porcelain as an oxidizing and fluxing agent. In porcelain—enamel frits used for metal coating, the amount of sodium nitrate in a batch varies with the various metal bases to be coated, typically from about 3.8 to 7.8 wt %. 

The uses of boric oxide relate to its behavior as a flux, an acid catalyst, or a chemical iatermediate. The fluxing action of B2O2 is important ia preparing many types of glass, gla2es, frits, ceramic coatings, and porcelain enamels (qv). 

For both economic and technical reasons, the most effective way to impart color to a ceramic product is to apply a ceramic coating that contains the colorant. The most common coatings, gla2es and porcelain enamels, are vitreous in nature. Hence, most appHcations for ceramic colorants involve the coloring of a vitreous material. 

Gold purple, often caHed Purple of Cassius, is a tin oxide gel colored by finely divided gold (7). It has good coverage and brilliance in low temperature coatings such as porcelain enamels. It is a very expensive pigment, because of its difficult preparation as well as the price of gold. 

Abrasion Resistance. Porcelain enamel is the most scratch resistant and hardest of commercial coatings (see Hardness). This property is used to distinguish between porcelain enamel and organic enamel or painted coatings. The rate of abrasive wear in surface abrasion increases with time, and the subsurface abrasion which follows exhibits a higher, but constant rate of wear. Abrasion resistance can be evaluated by loss of gloss or weight (35). 

Thermal shock resistance is a direct function of enamel thickness. The greater the residual compressive stress in the porcelain enamel, the greater is the resistance to thermal shock failure. Thin coatings, such as one-coat enamels or the two-coat enamels having alow expansion titania covet coat, provide excellent thermal, shock resistance. 

A.dherence to Porcelain Enamel Cover Coats Direct to Steel, PEI Bulletin T-29(72), Porcelaiu Enamel Institute, Washiagton, D.C., 1972. 

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