Vitreous Enamel: Part One
Vitreous enameling is the process of applying a glassy inorganic coating to a metal substrate through thermal diffusion of temperatures in excess of 800°C.
Vitreous enameling can be seen in all areas of our lives, from home appliances to industrial steel tanks and silos and this is mainly due to the excellent properties which can be gained from enameling, including, pH, chemical and temperature resistivity, improvements in strength properties and being completely physiologically and bacteriologically safe.
Vitreous enamel is defined as a substantially vitreous or glassy inorganic coating bonded to a metal substrate by thermal fusion at temperatures above 800°C. Vitreous enamels represent smooth durable coatings with many excellent properties including abrasion, wear and chemical resistance, high hardness, brilliant colors and incombustibility. A tendency to crack or shatter when the substrate is stressed or bent is the main disadvantage of such coatings.
Vitreous Enamel is simply a thin layer of glass fused at high temperature on to the surface of a metal. The word enamel comes from the high German word “smelzan” and later from old French term “esmail”. Vitreous esmail is more specifically used on a metal base and is thus defined as a vitreous, glass-like coating fused on to a metallic base. In American English it is referred to as Porcelain Enamel. Vitreous enamel is part of everyday life and found all around us.
- In the home - baths, washing machines, dishwashers, heaters, fire places, gas and electric cookers, BBQs, clock faces and cookware.
- In places you can't see - hot water services, storage tanks, car exhaust systems, printed circuits and heat exchanges;
- In places you can see - street signs, railway signs, nostalgia signs and murals;
- In the building industry: interior and exterior architectural panels, fascias, spandrels and partitions.
During the last decade, the use of vitreous enamel for industrial applications has grown a lot and this application area became the second largest enamel consuming market after household appliances.
Industrial applications of vitreous enamel include hot water tanks, silos/tanks, heat exchangers and chemical vessels. For these products, the vitreous enameled steel is chosen from an engineering point of view because of the combination of following excellent technical properties:
- Inorganic chemical resistance over a wide pH-range
- Excellent organic chemical resistance
- Temperature resistance at high temperatures
- Very smooth, non-reactive and non-sticky surface
- Scratch and abrasion resistance
- Physiologically safe
- Zero bacteriological growth
- Overall strength of the enameled steel.
Vitreous enameled coated bolted steel tanks/silos have been on the market for around 50 years. They are used for the storage of a high variety of liquids, slurries and dry bulk products in municipal and industrial areas. Thanks to the very good chemical resistance of the vitreous enamels and the flexible and quick construction, not obtained by other silo technologies, there is a growing interest for enameled tanks/silos. The complete production process of the panels is reviewed, including steel requirements, enamel properties and final product quality. The development of a new family of enamels for the fish scale free enameling of non enamelable construction steel is discussed.
Depending on the required properties, different enamel coatings are used, ranging from a direct on enamel coating with good chemical resistance towards a 3 coat system with a top coat that has a perfect chemical resistance in the range pH 1-14. The total layer thickness varies correspondingly with the number of coatings. It is important to avoid excessive layer thickness at the borders and around the bolt holes, in order to avoid cracks and chips during the construction and the bolting of the panels. The enamel is mostly applied by wet spraying or dry electrostatic powdering. The PEMCO enamel assortment contains all enamel qualities needed for silo/tank enameling, both for wet and powder application, fulfilling the properties asked for by the given standards and beyond.
Table 1: Requirements of the vitreous enamel coating on silos