Control of corrosion

A.2.1 Basic methods. There are four main methods

for preventing or controlling corrosion. a) Treating the environment.

b) Insulating the steel from the environment by means of a protective coating.

c) Cathodic protection.

d) Attention to design (see A.3).

A.2.2 Relative corrosion resistance of different types

of iron and steel. In some cases, the danger or severity of corrosion can be reduced by the choice of a more resistant metal. The characteristics of the main types of ferrous metals in this respect are summarized below.

A.2.2.1 Structural steel. Structural steels

manufactured by different processes corrode at substantially the same rate. The minor variations in their carbon content, analysis and microstructure have little effect.

The mill-scale on hot rolled steel is a potential source of corrosion and is injurious to the performance of protective coatings; it should be removed.

A.2.2.2 Cast iron. The casting skin formed on cast

iron in the mould is more adherent than mill-scale and has some protective properties. If it is removed, unalloyed cast irons corrode at substantially the same rate as descaled mild steels but there is an important difference in the effects of corrosion on the two materials.

Ordinary grey cast irons corrode in a peculiar way. They contain about 8 % of elements other than iron, mainly carbon in the form of graphite, phosphorus as iron phosphide, and silicon. The graphite and iron phosphide are inert and remain in situ after the iron has corroded away. The silicon becomes oxidized to silica or silicates, which help to bind the other constituents. Thus, the corrosion process leaves behind a non-metallic “graphitic corrosion residue”, which largely retains the appearance and shape of the original iron, although its mechanical strength is negligible.

A.2.2.3 Low-alloy steel12). Low-alloy steels corrode

more slowly than mild steel when freely exposed outdoors (11). In time their corrosion rate falls to a low value and they are designed to be used

unprotected. These steels have been widely used in the bare condition in both rural and industrial environments in North America and on a small scale in the United Kingdom.

A.2.2.4 Stainless steel. The resistance of stainless

steels to atmospheric corrosion far surpasses that of ordinary or low-alloy steels. The best of them, e.g. the 18/10/3 Cr-Ni-Mo type, are virtually incorrodible in dust-free air. They owe their immunity from corrosion to a thin protective oxide film, which reforms spontaneously if it is damaged but anything that interferes with this film

jeopardizes the corrosion resistance. Since chlorides tend to break it down, even Cr-Ni-Mo stainless steels may become slightly rust-spotted in marine environments. The passivity of the film is also impaired if supply of oxygen to it is cut off by dust settling on the surface. Consequently, the

performance of stainless steels in all types of atmosphere is greatly improved by regular cleaning, say at monthly intervals.

For the same reason, stainless steels are liable to pit severely in soil, in wet timbers or in water. For example, a sheet 2 mm thick became holed

within 6 months when immersed in the sea, where barnacles had settled and cut off the oxygen supply. Some stainless steels become vulnerable to stress corrosion cracking in sea water after certain methods of heat treatment. Where such a set of conditions can be foreseen the advice of a metallurgist should be sought during design.

A.2.3 Protection against atmospheric corrosion.

Rusting in atmospheric environments is best prevented by means of protective coatings, as has been fully discussed in the code.

Dehumidification of the air (12) is an alternative or a useful adjunct in some cases (see A.3.2.2).

Cathodic protection is impracticable, because of the absence of an electrolyte.

A.2.4 Protection against soil corrosion. The degree of

protection should match the corrosiveness of the soil.

The most effective system is cathodic protection BS 7361-1 plus a protective coating. Bitumen sheathing, about 6 mm thick, or a thick coal tar pitch/epoxide coating should prove adequate in moderately corrosive soils. So should hot-dip galvanized coatings.

It may sometimes be useful to replace an aggressive soil by a non-corrosive backfill, e.g. limestone or clean washed sand.

12) _{Those intending to use low-alloy steel should consult the British Steel Corporation.}

102 © BSI 11-1998

A.2.5 Cathodic protection. Cathodic protection is a

system in which an electric potential is applied from an external source to oppose the natural flow of electric current that gives rise to electrochemical corrosion. This can be done by either the sacrificial anode method or the impressed-current method. In the first case the steel is connected to a metal of lower potential, e.g. zinc, so that the zinc becomes the anode and corrodes, leaving the steel as the uncorroded cathode. In the second system the current is applied from an external d.c. source. Again the steel becomes the cathode and usually graphite is used as a permanent anode.

Cathodic protection operates only in the presence of an electrolyte, and is often used to protect steel immersed in sea water and in corrosive soils. To reduce current requirements this method is generally used in conjunction with protective coatings on the steel. Only a small current is then required to protect breaks in the coating.

Detailed recommendations on cathodic protection are given in BS 7361-1, but further advice should always be obtained from a specialist.