Free «Accorrosion prevention techniques and analysis» Essay Sample

The physical and chemical effects leading to corrosion were recognized long ago. Recently attention has also been attracted to biological elements also causing corrosion in steel and cement. While research data have not yet been consolidated into a comprehensive body of knowledge, there are many individual reports on biological influences. Biological along with chemical and physical effects cause various forms of degradation to buildings (including corrosion), and some are also harmful to health. This paper, by referring to a number of scholarly articles and sources, analyzes the nature of corrosion, discussing techniques and methods used to prevent corrosion in the modern industries.

A study by the UK Building Research Establishment (BRE, 1992) identified the following groups of biological growths on exterior building materials that contribute or cause corrosion: algae, lichens, mosses, liverworts, moulds, and bacteria. Some of these also grow on internal surfaces. Algae, lichen and moss are commonly found growing on the external surfaces of buildings that are subjected to frequent wetting. Liverworts are typically leafy, close-growing, green-coloured plants that are usually found on the surfaces of stone walls where soil and dirt have accumulated.

Certain bacteria cause the deterioration of stone, bricks, concrete and metals. Moulds develop on damp and dirty external or internal surfaces. Mould growth has increased in recent years, partly because of energy conservation measures. Growth can begin at a relative humidity of 80% in indoor air, and so the requirement that the partial vapour pressure in the air should remain under saturation point is not enough. A more stringent requirement aimed at maintaining a lower level of humidity in indoor air should be introduced (Fargus and Hepworth, 1994).

The American Hotel and Motel Association has estimated that problems caused by mould and mildew cost the industry about US$68 million in lost revenues and repairs every year. As well as in-room mould, problems can also include mould behind low-permeability wallpaper, for example. Mould behind wallpaper can occur in hot and humid climates during the summer when outdoor moisture is transferred into prefabricated wall construction by way of diffusion and air infiltration.

If the wallpaper has a high water vapour-resistance, moisture accumulates behind it, providing an environment conducive to mould and mildew growth (Burch, 1993). Surface biocides and fungicidal paints can provide short-term protection against mould, but only an improvement in conditions through better ventilation and heating, appropriate arrangement of wall layers and correct indoor-air pressure can permanently prevent mould growth.

As early as 1900, corrosion of concrete was described in the sewer network of Los Angeles. At the time it was considered to be a purely chemical process and became known as hydrogen sulphide corrosion (Sand and Bock, 1984), but later research by chemists, civil engineers and microbiologists in collaboration, has demonstrated the active part played by bacteria in this form of corrosion, which is now termed biogenic sulphuric acid corrosion. “Sulphate-reducing bacteria can corrode steel machinery” as well as steel pipes for water and gas (Postgate, 1988, pp. 59-60).

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A quite different cause of stone and concrete corrosion has been described by German scientists (Bock and Sand, 1986). At the Cologne and Regensburg cathedrals, among others, they found that bacteria that produced nitric and sulphuric acid, together with fungi and algae, are deleterious to stone. “Degradation of asbestos cement slabs has also been found to be caused by bacteria that produce nitrates and nitrites” (Sterling, Bieva, and Collett, 1993, p. 18). Air conditioning can cause fungal, viral and bacterial infections, bronchial allergies and asthma. Humidifier fever and Legionnaire’s disease can both be caused by bacteria in air conditioning and hot water systems. They are rare, but precautions are necessary. It is safest if the temperature of warm water is kept above a certain level (Blanc, McEvoy, and Plank, 2002). The disinfection of cooling water by ultraviolet light and the biocide treatment of evaporative condensers are two methods of controlling microbial growth in wet cooling systems.

Scientists have long been trying to prevent corrosion using improved materials and technologies. Portland cement and reinforced concrete were invented in the nineteenth century (Blanc et al., 2002). During the twentieth century they became important building materials and were developed into many different types such as lightweight, pre-stressed, high-strength and decorative concretes. A number of additives and chemicals are used to modify and improve their properties to make the materials corrosion resistant.

High alumina cement concrete was developed in the First World War. It has the advantage of setting rapidly, but its other properties led to loss of strength and even collapse. This type of concrete is therefore used infrequently or not at all, and attention is now devoted to inspection and regular monitoring of existing high alumina cement structures (Easterling and Roddis, 2000). Concrete and reinforced concrete have a number of special applications for specific performance requirements, such as in the oil industry (fast setting), in the nuclear power industry (heavy concretes for radiation shielding), bridges, tunnels, chimneys, silos, caisson foundations and offshore structures.

Concrete protects the steel reinforcement from corrosion if the concrete has high cement content, is sufficiently compacted and if the reinforcement is covered to an adequate thickness by the concrete. Recently, there has been concern about the risks of steel corrosion, which can be caused by inadequate care in design and execution and by certain additives such as calcium chloride, which is used to protect fresh concrete from frost and to de-ice motorways and bridges. The production of crack-free concrete and the use of new corrosion-resistant reinforcing bars are new ways to prevent the corrosion of the reinforcement (Easterling and Roddis, 2000).

The introduction of cold-forming opened up new possibilities. Steel sheet and sections can be cold-formed by means of roll-forming machines or press brakes (Toma et al., 1992). The most frequently used thin sheets have a thickness of 0.4-3.0 mm, although heavier gauges can also be produced. Manufactured from very thin material, slender sections with different shapes are possible. For special purposes such as claddings, roofs and composite floors, specific profiles may be used. Cold-rolling is the most productive method and is therefore mainly used for standardized profiles. Composite floors are made with concrete cast on top of the steel sheet decking with possible additional reinforcement primarily for taking the shear and bending stresses. Buckling of thin steel members was found to be a frequent cause of failure (Davies, 1994).

New means have been developed for jointing and fastening thin cold-formed steel sheets, e.g. blind rivets, self-tapping screws, gluing and seam locking. Thin metal sheets must be well protected against corrosion. Steel sheet is usually galvanized or, for visible applications, additionally plastic-coated. Coil-coating is the most productive plastic-coating of thin steel (or aluminium) sheets. Sections can be manufactured from coated sheet, or be galvanized and coated after forming. Important uses for galvanized cold-formed corrugated steel or aluminium sheets (coated or uncoated) are frames of low-rise or medium-rise buildings, supports for partitions, roof deckings, claddings and floors. Codes for the use of cold-formed steel were created later than those for hot-rolled steel. The 1974 code of the Canadian Standards Association was still based on permissible stress design, although with a limit state option. The 1984 revised code was already based entirely on limit state design principles (referred to as load and resistance factor design). The plastic design approach was introduced simultaneously in Europe and the USA. Eurocode 4 contains the principles for composite structures (Easterling and Roddis, 2000).

Steel and concrete are continuously in competition for use as the basic structural building material. Concrete gained ground in the first half of the twentieth century, but steel has made a comeback in recent years.

As already mentioned, their combined use in composite structures is now widespread. Both steel and concrete are used in heavy construction (bridges, tall buildings, long-span structures, etc.), and steel has been increasingly used in lightweight structures (Blanc, et al., 2002).

While steel and reinforced concrete are the most widely used methods to prevent corrosion, the use of aluminium as the anti-corrosive device has also increased in a number of fields recently.

Aluminium was first produced between 1800 and 1830, but its industrial production only started between 1830 and 1870. The first European Recommendations for Aluminium Alloy Structures (ERAAS) were published in 1978 (Toma, 2000). The specific weight of aluminium is only one-third that of steel, and its corrosion resistance is good. Both are cogent reasons for its use, but on the other hand, its strength is inferior to that of steel and its cost is much higher. Structural shapes can be obtained by extrusion, casting and building up by various means into combined sections. The shaping has an impact on the form, which is why aluminium is used in different shapes from steel.

Aluminium can be lacquered or coil-coated, and there is also a surface treatment developed specifically for aluminium, anodic oxidation technology. This gives the material a hard and coloured surface, and provides good protection against corrosion. The colour can be produced either in a bath containing a solution of metal salts, or by direct colouring (self-colouring) in a bath containing aromatic sulphonic acid and certain additives (Toma, 2000). The colours depend on the chemicals used and include gold, bronze, brown, grey and various combinations.

As one can tell, corrosion prevention is an important task in the number of fields, ranging from construction to ship-building and marine industries. A number of methods to prevent corrosion were devised such as adding of special chemicals to reinforce the concrete structure, cold-forming technologies of steel production, and widespread usage of aluminium, renown for is anticorrosive characteristics.


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