Applications of Enamel

Enameled articles are used both in the home and in industry. Typical household applications are  

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Sheet steel is normally prepared for application of enamel by a sequence of operations including thorough degreasing, acid pickling and neutralisation. A nickel dip stage is often included to deposit a thin, porous layer of nickel applied at about 1 g/m especially when conventional groundcoat is not used (see Section 13.7). 

After being removed from the neutralizing solution, the ware is transferred to a dryer maintained at about 110—120°C that has good air circulation. This ensures quick and complete drying without rusting of the metal. After being dried, the sheet ware is ready for application of enamel. 

Aluminum. For aluminum alloys such as 1100 and 3003 only a cleaning operation is necessary before application of enamel. For alloy 6061, deoxidation and alkaline-chromate pretreatment processes may also be used to improve both the cleanliness of the surface and the development of enamel adherence. The spent chromium-containing solutions can present disposal problems and should be handled in accordance with existing regulations (19). 

Vitreous enamels have been used for many years as liners for reactors and other chemical processing equipment but it is only relatively in recent times that the technology has been used in conjunction with heat transfer equipment. Conventional acid resisting vitreous enamels soften at around 450°C but developments have enabled enamels to be used at much higher temperatures. Page [1980] has reviewed the opportunities for the application of enamels to heat exchangers, particularly for use in acidic conditions at high temperatures. 

Copperhead. A fault (reddish-brown spots) liable to appear in the ground-coat during vitreous enamelling the spots are exposed areas of oxidized base metal. Causes include boiling from the base-metal, inadequate metal preparation, very thin application of enamel, and the presence of acid salts in the enamel slip. 

A similar defect liable to occur in vitreous enamelling when one coating of enamel is fired over another coating that has already been fired. Causes include a too-heavy application of enamel, poorly controlled drying, and the use of enamel that has been too finely ground. 

These paints are resin based and the paints required for sheet-metal surfaces are genertilly alkyd-based resins. For general industrial applications, tiny of the following types of enamel paints may be used,... 

In the application of magnesium anodes for enamelled boilers, the consumption rate of the anodes is determined less by current supply than by self-corrosion. The calculation of life from data on protection current requirement, /, and anode mass, m, is difficult because the a value is so low. 

The main application of poly (vinyl formal) is as a wire enamel in conjunction with a phenolic resin. For this purpose, polymers with low hydroxyl (5-6%) and acetate (9.5-13%) content are used. Similar grades are used in structural adhesive (e.g. Redux) which are also used in conjunction with phenolic resin. Poly(vinyl formal) finds some use as a can coating and with wash primers. Injection mouldings have no commercial significance since they have no features justifying their use at current commercial prices. 

The first commercial applications of polypyromellitimides were as wire enamels, as insulating varnishes and for coating glass-cloth (Pyre.ML, Du Pont). In film form (Kapton) many of the outstanding properties of the polymer may be more fully utilised. These include excellent electrical properties, solvent resistance, flame resistance, outstanding abrasion resistance and exceptional heat resistance. After 1000 hours exposure to air at 300°C the polymer retained 90% of its tensile strength. 

Tubing used to convey fluids at elevated temperatures and/or pressures. Suitable for head applications, it is fabricated to exact o.d. and decimal wall tliickness in sizes ranging from V2-6 in. o.d. inclusive and to ASTM specifications. Coating that is applied to a surface before application of an adhesive, enamel, etc. The purpose is to improve bonding. 

Primer Coating that is applied to a surface before application of an adhesive, enamel, etc. The purpose is to improve bonding. 

Coal tar enamel This is derived from the coking of coal and is further distilled to produce coal tar pitches. It is used for hot application on-site. It will crack and craze if exposed to sunlight but has been employed successfully for over 50 years for the protection of underground or immersed structures. The main use is now for the exteriors of buried or immersed pipelines. Different types of enamel are available to give various degrees of heat resistance. It is now generally used for pipelines below 155 mm diameter. 

Protective measures range from chemical conversion coatings and anodising to the application of more substantial protective layers, e.g. enamels. For a more detailed treatment of the subject, the reader is advised to consult References 19, 20, 21 and 22. 

An objection to this form of wrap is that its mode of application renders it extremely difficult to obtain a good uniform bond between the wrap and the enamel. In view of this, water could become trapped under the armour wrap, and because of the non-conducting nature of the wrap itself the effective application of cathodic protection would be difficult. 

The hardness of an enamel surface is an important property for such items as enamelled sink units, domestic appliances, washing machine tubs which have to withstand the abrasive action of buttons, etc. On Moh s scale most enamels have a hardness of up to 6 (orthoclase). There are two types of hardness of importance to users of enamel, viz. surface and subsurface. The former is more important for domestic uses when one considers the scratching action of cutlery, pans, etc. whereas subsurface hardness is the prime factor in prolonging the life of enamelled scoops, buckets, etc. in such applications as elevators or conveyors of coal and other minerals. 

Figure 5.20 The effect of a citric acid solution on tooth structure (a) enamel surface before application, (b) enamel surface after application showing etching, (c) dentine surface before application, (d) dentine surface after application showing the opening-up of the dental tubules (Powis et al, 1982).

The porcelain enameling industry consists of at least 116 plants enameling approximately 150 million square meters (150 km2) of steel, iron, aluminum, and copper each year (each coat of multiple coats is considered in this total). Porcelain enameling is the application of glass-like coatings to the metals mentioned above. The purpose of the coating is to improve resistance to chemicals, abrasion, and water, and to improve thermal stability, electrical resistance, and appearance. The coating... 

The introduction of enamel into the wastestream results in an increase in the concentration of metals, but these metals (antimony, titanium, zirconium, tin, cobalt, and manganese) are in solid form whereas the metals generated by surface preparation are normally in dissolved form. These solid metals increase the suspended solids concentration of the stream. Other metals that may be found in the enamel preparation and application wastestream in significant amounts include aluminum, copper, iron, lead, nickel, and zinc. Table 8.2 presents pollutant sampling data for the processes used in the porcelain enameling on steel industry. 

The investment cost, operating and maintenance costs, and energy costs for the application of control technologies to the wastewater of the porcelain enameling industry have been analyzed. These costs were developed to reflect the conventional use of technologies in this industry. The detailed presentation of the cost methodology and cost information is characterized as... 

Coating operations. This includes all of the operations associated with preparation and application of the vitreous coating. Usually this incorporates ball milling, slip transport, application of slip to the workpieces, cleaning and recovery of faulty parts, and firing (fusing) of the enamel coat. 

Plate-out is observed with various hardeners if P.V.23 is incorporated in most types of powder coatings, for instance in epoxy systems. This phenomenon, however, is of no consequence for the applicability of the pigment only minor amounts of P.V.23 are required in these media to lend a blue shade to white enamels. 

A further important application of these pigments is in paints. The natural red iron oxides are also used in primers for steel structures and cars, for marine coatings and for anti-fouling paints. In the USA, the metallic browns are used for these purposes. The level of soluble salts in the latter pigments is low and this reduces corrosion problems. The metallic browns are also used in heat resistant enamels. 

The pure compound is used in glazes and enamels as an opacifying agent. It also is used to make colored glass, imparting yellow color to the glass. The crude form is used in flaming arc lamps. Another application of this compound is in the preparation of several other cerium salts. 

Zinc oxide occurs in nature as mineral zincite. It is the most important zinc compound and has numerous industrial applications. Zinc oxide is the pigment in white paints. It is used to make enamels, white printing inks, white glue, opaque glasses, rubber products and floor tiles. It is used in cosmetics, soaps, pharmaceuticals, dental cements, storage batteries, electrical equipment, and piezoelectric devices. Other applications are as a flame retardant, as a UV absorber in plastics, and a reagent in analytical chemistry. A major application of zinc oxide is in the preparation of most zinc salts. In medicine, the compound is used as an antiseptic, an astringent and a topical protectant. 

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