Metals galvanisation

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. 

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. 

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. 

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. 

In its resistance to liquid metals, titanium shows variable behaviour, the rate of attack often depending upon temperature and increasing with rise in temperature. By thickening the surface film of oxide, resistance to attack is enhanced, and, for example, repeated repair of the surface film renders titanium resistant, on a limited-time basis, to molten zinc in galvanising baths. A surface-oxide thickening technique also enables titanium to be employed in contact with molten aluminium. Titanium equipment is also used in applications involving lead-tin solders, and it is resistant to mercury, at least up to 150 C. 

Probably the most familiar coated steel sheets are the ubiquitous galvanised corrugated roofing and cladding sheets which have been used for many years, particularly for farm buildings, either painted or unpainted. In addition to zinc other metallic coatings are available, e.g. hot dip aluminium and hot dip aluminium-zinc alloys. 

The characteristics of the various metals commonly used for seawater systems, chiefly, nickel and titanium alloys, galvanised steel and to a lesser extent aluminium alloys and stainless steels, are fully described in their respective sections. Reference here will be confined to mentioning some of the advantages and limitations of clad and non-metallic piping. 

The faying surface of the steel should be given a metal coating of either zinc (by spraying or galvanising) or aluminium spray. 

Canister anodes consist of a spirally wound galvanised steel outer casing containing a carbonaceous based extender which surrounds the primary anode element which may be graphite, silicon iron, magnetite, platinised titanium, mixed metal oxide-coated titanium or platinised niobium, etc. 

Zinc diffusion sherardisingY " is mainly used for protection of ferrous metals against atmospheric corrosion. It has, in some respects, properties related to other types of zinc coating such as galvanising, but owing to the small dimensional change involved, it is of particular value for the treatment of machined parts, bolts, nuts, etc. 

Characteristics of coating Hot-dip galvanising Metal spraying Plating Sherardising Zinc dust painting... 

Etch primers are widely used. They are mostly based on polyvinyl butyral and contain chromates and phosphoric acid. They are said to act both as primers and as etching solutions because it is believed that the chromates and phosphoric acid form an inorganic him, which provides adhesion, while oxidised polyvinyl butyral provides an organic him. For direct application to new galvanised steel, the best known primers are based on calcium orthoplumbate pigment and metallic lead, but these are now less used for environmental reasons. Zinc-dust paints and zinc-phosphate pigmented paints are also used, but the trend is to use pretreatments to assure good adhesion. 

Zinc phosphate is now probably the most important pigment in anticorrosive paints. The selection of the correct binder for use with these pigments is very important and can dramatically affect their performance. Red lead is likely to accelerate the corrosion of non-ferrous metals, but calcium plumbate is unique in providing adhesion to newly galvanised surfaces in the absence of pretreatment, and is claimed to behave similarly on other metals in this group. 

Polyolefin copolymers Although there is a wide variety of these available, the only one currently commercially available as a compounded powder is saponified EVA. This is reported to have good weatherability and will not suffer from ESC. One major advantage this coating has is that it can be applied by the fluidised bed process at low temperatures and this offers the possibility of coating temperature-sensitive metals such as galvanised steel. 

In many cases there will be a need to test metal-coated specimens, e.g. galvanised steel, tin-plated copper, nickel-plated zinc, etc. It will then be necessary to test specimens in the completely coated condition and also with the coating damaged so that the basis metal is exposed. The latter condition will provide the conditions for galvanic action between the coating and the basis metal. With sheet specimens this condition is most readily achieved by leaving cut edges exposed to the test environment. 

Certain British Standards, e.g. for galvanising or metal spraying", contain detailed sections on surface preparation. Others are concerned with acid pickling but the chief method of preparing iron and steel for a long-life protective system is by blast cleaning see Section 12.4). 

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