Vitreous enamel, corrosion resistance

A vitreous enamel coating is, as the name implies, a coating of a glassy substance which has been fused onto the basis metal to give a tightly adherent hard finish resistant to many abrasive and corrosive materials. The purpose of modern vitreous enamels is twofold, i.e. to confer corrosion protection to the metal substrate and at the same time to provide permanent colour, gloss and other aesthetic values. 

In this section, chemical resistance will be divided into three parts, viz. acid, alkali (including detergents) and water (including atmosphere). Normally an enamel is formulated to withstand one of the corrosive agents more specifically than another, although vitreous enamel as a general finish has good all round resistance, with a few exceptions such as hydrofluoric acid and fused or hot concentrated solutions of caustic soda or potash. 

Organic acid Some glasses are more prone to attack by organic acids than by other acids with a lower pH value. These acids form complex ions in solution which increases the glass solubility. Many vitreous enamels contain lead oxide and are susceptible to attack by organic acids. Consequently their corrosion resistance is assessed by exposure to acetic or citric acidsThe test temperature is always stipulated because a rise of 10°C can double the rate of reaction between glass and acid. 

The sublimate can be reduced in a blast furnace similar to that used for smelting lead. The metal is alloyed with lead for battery plates and with tin in pewter, in both cases conferring greater hardness. It is also used with lead in corrosion-resistant piping, with lead and tin in type metals and with tin, copper and lead in bearing metals. The oxide, Sb Og, is employed in vitreous enamels and as a pigment. 

Combinations of ceramic and metal systems - sometimes referred to as "cermets" have excellent resistance to mechanical damage (one of the limitations associated with the use of vitreous enamel for process plant). The combination of ceramic and metal in surface coatings also provides oxidation and corrosion resistance to the underlying metal substrate. The technique is particularly applicable to regions of low temperature in air preheaters in combustion plant, i.e. near or below the acid dew point. Page [1980] reports some extensive tests. Over a five year period some 40 corrosion resisting steels, alloy steels and organic and... 

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