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This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) Table 35.1 Bimetallic corrosion and structural steelwork Effect of the environment 1033 Anodic end (more prone to corrosion) Magnesium Tendency to inhibit Zinc corrosion of structural Aluminium steels Cadmium Carbon and low alloy (structural) steels Cast irons Lead Tendency to accelerate Tin corrosion of structural Copper steels Brasses Bronzes Nickel (passive) Titanium Stainless steels 430/304/316 (passive) Cathodic end (less prone to corrosion) (1) Time of wetness. This is the proportion of total time during which the surface is wet, due to rainfall condensation, etc. It follows, therefore, that for unprotected steel in dry environments, e.g. inside heated buildings, corrosion will be negligible due to the low availability of water. (2) Atmospheric pollution. The type and amount of atmospheric pollution and contaminants, e.g. sulphur dioxide chlorides, dust, etc. (3) Sulphates. These originate from sulphur dioxide gas, which is produced during the combustion of fossil fuels, e.g. sulphur-bearing oils and coal. The sulphur dioxide gas reacts with water or moisture in the atmosphere to form sulphurous and sulphuric acids. Industrial environments are a prime source of sulphur dioxide. (4) Chlorides. These are mainly present in marine environments. The highest concentrations of chlorides are to be found in coastal regions, and there is a rapid reduction moving inland. Within a given local environment corrosion rates can vary markedly due to the effects of sheltering and prevailing winds. It is therefore the ‘microclimate’ immediately surrounding the structure which determines corrosion rates for practical purposes. 35.2 Effect of the environment Corrosion rate data cannot be generalized; however, environments can be broadly classified and corresponding corrosion rates provide a useful indication. A range of
This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) 1034 Corrosion and corrosion prevention UK environments and steel corrosion rates are considered as follows: (Note: Corrosion rates are usually expressed as mm/year; 1 mm = 0.001mm.) (1) Rural atmospheric – essentially inland, unpolluted environments; steel corrosion rates tend to be low, usually less than 50 mm/year (2) Industrial atmospheric – inland, polluted environments; corrosion rates are usually between 40 and 80 mm/year, depending upon level of SO2 (3) Marine atmospheric – in the UK a 2km strip around the coast is broadly considered as being in a marine environment; corrosion rates are usually between 50 and 100mm/year, largely dependent upon proximity to the sea (4) Marine/industrial atmospheric – polluted coastal environments which produce the highest corrosion rates e.g. between 50 and 150 mm/year (5) Sea-water immersion – in tidal waters four vertical zones are usually encountered: (a) the splash zone, immediately above the high-tide level, is usually the most corrosive zone with a mean corrosion rate of about 75 mm/year (b) the tidal zone, between high-tide and low-tide levels, is often covered with marine growths and exhibits low corrosion rates e.g. 35 mm/year (c) the low-water zone, a narrow band just below the low-water level, exhibits corrosion rates similar to the splash zone (d) the permanent immersion zone, from the low-water level down to bed level, exhibits low corrosion rates e.g. 35 mm/year (6) Fresh-water immersion – corrosion rates are lower in fresh water than in salt water e.g. 30–50 mm/year (7) Soils – the corrosion process is complex and very variable; various methods are used to assess the corrosivity of soils: (a) resistivity; generally high-resistance soils are least corrosive (b) redox potential; to assess the soil’s capability of anaerobic bacterial corrosion (c) pH; highly acidic soils (e.g. pH less than 4.0) can be corrosive (d) water content; corrosion depends upon the presence of moisture in the soil, the position of the water-table has an important bearing. Long buried steel structures, e.g. pipelines, are most susceptible to corrosion. Steel piles driven into undisturbed soils are much less susceptible due to the low availability of oxygen. 35.3 Design and corrosion In external or wet environments design can have an important bearing on the corrosion of steel structures. The prevention of corrosion should therefore be taken into account during the design stage of a project. The main points to be considered are:
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