Vitreous enamel, corrosion

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. 

PROTECTIVE COATING. A film or thin layer of metal glass of paint applied to a substrate primarily to inhibit corrosion, and secondarily for decorative purposes, Metals such as nickel, chromium, copper, and tin are electrodeposited on the base metal paints may be sprayed or brushed on. Vitreous enamel coatings are also used these require baking. Zinc coating are applied by continuous bath process in which a strip of ferrous metal is passed through molten zinc. 

Ni or vitreous enamel. This construction was costly, and complete protection from corrosion was not always obtained by lining. These difficulties were overcome in 1912 when Prof S.W.Parr developed a bomb made of an alloy "Ilium , comparatively inert to acids formed as result of combustion. This type of bomb is now manufd by the Parr Instrument Co, Moline, Illinois and the alloy used for its body consists of (approx)tNi 29, Cr 20, Mo 2, Cu 3, Si 1 Fe 45%. The alloy is fundamentally similar to the original "Ilium . 

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... 

Page, M., 1980, Vitreous enamels provide cost effective corrosion protection. Corr. Prevention and Control 17, February. 

Low-melting-point vitreous enamels for aluminium utilise as much as 30-40% P2O5 in their formulations [67]. The formation of protective films on metal surfaces (Section 12.7) in some cases involves the formation, in part, of glassy phosphate phases. These can be for corrosion protection or electrical insulation [68]. 

Vitreous and porcelain enamels—Corrosion tests in closed systems... 

Colour Retention. The colour retention of red, orange and yellow vitreous enamels containing cadmium sulphide, or sulphoselenide, when exposed to atmospheric corrosion is measured by treating the enamel surface with cupric sulphate solution details of the test are given in ASTM - C538. 

ISO 2746 Vitreous and porcelain enamels Enameled articles for service under highly corrosive conditions High voltage test... 

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