Galvanising

Uses of hydrogen chloride—Hydrogen chloride is sometimes used in the preparation of an ester, for example ethyl benzoate, where it acts as both an acid catalyst and a dehydrating agent. Hydrochloric acid is used primarily to produce chlorides, for example ammonium chloride. It is extensively used in the manufacture of anilme dyes, and for cleaning iron before galvanising and tin-plating. 

Because of its resistance to corrosion, zinc may be used to coat iron. This may be done by dipping the iron into molten zinc or by spraying zinc on the iron articles, for example iron sheets. This is known as galvanising. Smaller iron articles may be coated by heating with zinc dust, a process known as sherardising, or suspensions of zinc may be used in paints. 

Sheets of galvanised iron are used for roofing, guttering and the like. Alloys of zinc, notably brass, are used extensively. The metal is used m wet and dry Leclanche batteries. 

Fig. 2. Friction factors for cylindrical pipe where line A represents drawn tubing line B, commercial steel C, galvanised iron and lines D, E, and F,...

Fig. 4. Submersible oil well pump cable. A, SoHd copper conductor B, EPR-based insulation C, chemical barrier D, lead sheath E, filler E, galvanised...

How does galvanising work As Fig. 24.4 shows, the galvanising process leaves a thin layer of zinc on the surface of the steel. This acts as a barrier between the steel and the atmosphere and although the driving voltage for the corrosion of zinc is greater than that for steel (see Fig. 23.3) in fact zinc corrodes quite slowly in a normal urban atmosphere because of the barrier effect of its oxide film. The loss in thickness is typically 0.1 mm in 20 years. 

Fig. 24.4. Galvanised steel is protected by a sacrificial layer of zinc.

At first sight, the answer would seem to be to increase the thickness of the zinc layer. This is not easily done, however, because the hot dipping process used for galvanising is not sufficiently adjustable and electroplating the zinc onto the steel sheet increases the production cost considerably. Painting the sheet (for example, with a bituminous paint) helps to reduce the loss of zinc considerably, but at the same time should vastly decrease the area available for the cathodic protection of the steel and if a scratch penetrates both the paint and the zinc, the exposed steel may corrode through much more quickly than before. 

A note of caution about roof fasteners. A common mistake is to fix a galvanised or aluminium roof in place with nails or screws of a different metal copper or brass, for instance. The copper acts as cathode, and the zinc or aluminium corrodes away rapidly near to the fastening. A similar sort of goof has been known to occur when copper roofing sheet has been secured with steel nails. As Fig. 24.6 shows, this sort of situation leads to catastrophically rapid corrosion not only because the iron is anodic, but because it is so easy for the electrons generated by the anodic corrosion to get away to the large copper cathode. 

Under aggressive corrosion conditions it is estimated that the maximum corrosion current density in a galvanised steel sheet will be 6 X 10 A m . Estimate the thickness of the galvanised layer needed to give a rust-free life of at least 5 years. The density of zinc is 7.13 Mg m , and its atomic weight is 65.4. Assume that the zinc corrodes to give Zn " ions. 

Artefacts Galvanised steel sheet, new and old anodised Al polymeric roofing material corroded exhaust system. 

A whole science, called metallography, is devoted to this. The oldest method is to cut the alloy in half, polish the cut faces, etch them in acid to colour the phases differently, and look at them in the light microscope. But you don t even need a microscope to see some grains. Look at any galvanised steel fire-escape or cast brass door knob and you will see the grains, etched by acid rain or the salts from people s hands. 

The metals most commonly used for water systems are iron and steel. These metals often have some sort of applied protective coating galvanised steel, for example, relies on a thin layer of zinc, which is anodic to the steel except at high temperatures. Many systems, however, contain a wide variety of other metals and the effect of various water constituents on these must be considered. The more usual are copper, brasses, bronzes, lead, aluminium, stainless steel and solder. 

Zinc coatings on steel (galvanised) are attacked in the same way Jis iron, but usually more slowly. Very alkaline waters are usually aggressive to zinc and will often remove galvanised coatings the corrosion products consist of basic zinc carbonate or other basic compounds and may take the form of a thick creamy deposit or hard abrjisive particles. 

An important aspect of design is to predict the lines of run-off of surface water. This is because the water will contain minute particles of brown rust, especially in the pre-stabilisation period, that will stain some surfaces. Matt, porous surfaces stain particularly easily and run-off should not be over concrete, stucco, galvanised steel, unglazed brick or stone. 

Aluminium may accelerate attack on zinc alloys this is particularly noticeable when there is an unfavourable area ratio, as with galvanised fittings in aluminium sheets. In alkaline solutions, however, the aluminium may be preferentially attacked. 

This type of corrosion can take place on any new surface of zinc and is best prevented by storing the metal in a dry, airy place until a protective layer has been formed. Zinc which has been properly aged in this way is safe against white-rust formation. Various methods are employed to prevent white rust. A chromate treatment is widely used for zinc-plated articles and for galvanised sheet, and occasionally for zinc die castings. Fatty substances, such as oils or lanolin, are sometimes used to protect larger items. 

The effect of pH on the corrosion of zinc has already been mentioned (p. 4.170). In the range of pH values from 5 -5 to 12, zinc is quite stable, and since most natural waters come within this range little difficulty is encountered in respect of pH. The pH does, however, affect the scale-forming properties of hard water (see Section 2.3 for a discussion of the Langelier index). If the pH is below the value at which the water is in equilibrium with calcium carbonate, the calcium carbonate will tend to dissolve rather than form a scale. The same effect is produced in the presence of considerable amounts of carbon dioxide, which also favours the dissolution of calcium carbonate. In addition, it is important to note that small amounts of metallic impurities (particularly copper) in the water can cause quite severe corrosion, and as little as 0-05 p.p.m. of copper in a domestic water system can be a source of considerable trouble with galvanised tanks and pipes. 

BISRA tests on galvanised steel pipe buried for five years at five different sites are described by Hudson and Acock The galvanised pipes resisted corrosion rather better than steel at all sites. Galvanised pipes of small diameter are frequently used to provide underground water services in farms and similar establishments, and little trouble is experienced. 

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