Porcelain Enameling

Porcelain enamels are inorganic vitreous coatings that are matured by heat. The inherently good heat transfer, thermal stability, and rigidity of iron at firing temperatures, coupled with the excellent adherence of vitreous ceramic frits as they fuse onto the cast surface, make the combination of porcelain on iron an excellent product. General corrosion resistance or resistance to specific chemicals can be obtained by selecting the proper porcelain enamel. The scratch resistance and hardness of the enamel coating, which allow the surface to resist abrasion, are almost equal in importance to the corrosion resistance.

Four processes are used to apply enamels: dry, wet, thermal spray (or plasma spray), and electrostatic precipitation. The latter two methods are seldom used on iron castings. Preparations for enameling start by blasting with sand, steel shot, or iron grit.

Dry coating methods use formulations that are mainly silica; these formulations generate a surface with the hardness and abrasion resistance of glass. Fluxes and opacifiers are mixed into the silica, and the mixture is then melted, quenched, and ground to make frit. (Frit is the term applied to the basic coating materials.) After application of a bonding ground coat, relatively heavy, smooth coatings (such as those on sinks and bathtubs) can be obtained by multiple firing. Each firing is followed by hot dusting with additional powdered frit, until the desired finish has been achieved.

Wet methods produce thinner coatings. The powdered frit is suspended in a solution of electrolytes and water or in an organic solvent and is applied by spraying or dipping over the ground coat. The sprayed or dipped coating must be dried prior to firing.

Ground coats that wet iron readily, adhere well, and are compatible with the cover coat are essential to the enameling process. Good ground coats promote adhesion between the enamel layer and the substrate, seal and smooth the irregularities of the surface, and prevent oxidation of the iron casting at firing temperatures. Top coats (cover coats) must provide the desired appearance and must be compatible with the ground coat. Formulation of frits requires the judgment and experience of a frit manufacturer to ensure that the coating provides successful results.

Cast iron for enameling usually has a composition that falls within the following limits:

Constituent

Amount, %

Total carbon

3.20-3.60

Silicon

2.30-3.00

Manganese

0.30-0.60

Sulfur

0.50-0.12

Phosphorus

0.40-0.80

Total carbon and silicon should vary in opposite directions within the ranges shown. If both are low, the iron tends to be brittle and to blister during porcelain enameling. If both are high, the iron is soft and warps easily when reheated for porcelain enameling. Manganese and sulfur should range in the same direction, so that all of the sulfur is converted to manganese sulfide. Within the normal range, phosphorus has a negligible effect on the strength of the cast iron at porcelain enameling firing temperatures.

The primary applications for porcelain enameled cast irons are in the manufacture of bathtubs, kitchen sinks, and lavatories. See the article "Porcelain Enameling" in this Volume for more detailed information.

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