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Study of the corrosive effects of water

Corrosion is defined as the destruction of a material under chemical or electrochemical action by its surrounding environment. The reactions and transformations involved in this process are due to the thermodynamic instability in the materials of which the surrounding environment is made up, giving rise to products with properties which are different from those initially.

When analysing material used for piping water, one of the first consequences of corrosion is a weight loss of the material, with corresponding variations of the initial measurements. The various mechanisms of corrosion are important when considering which material to select for its design and its use over an acceptable period of time. Corrosion, due to the effects of water, follows an electrochemical mechanism; the metal together with oxygen and water forming corrosion products. In the case of steel, the corrosion products formed are ferrous hydroxide, commonly known as rust. Rust formation is very common, but unfortunately does not have the property to form a protective coating that is sufficiently compact to diminish the corrosion reaction. Thus, the products formed in the corrosive process influence the behaviour of the materials in that particular environment. If the corrosion products generated are very compact and adherent, they tend to form a film that impedes subsequent attack on the material by the surrounding environment, as it happens in the case of zinc (galvanised steel). Regarding copper, it is commonly used for the supply of drinking water because of its mechanical properties and resistance to corrosion, due to its ability to form a protective coating. Corrosion of copper only appears with certain kinds of water, appearing in the form of pitting. The work undertaken in the laboratory was based on the study of the corrosiveness of different materials immersed in water under constant conditions of composition and subjected to variations in temperature and concentration of oxygen. The temperatures which the materials were subjected to were: 10, 22 and 50º C which more or less emulate the temperature changes to which the water supply pipes are subjected to over the different seasons of the year and in the hot water boiler and radiator pipes in households. At the same time and for each temperature, the behaviour of each material, in the presence and in the absence of oxygen, was studied. Moreover, electrochemical means, such as the resistance to polarisation, were used to investigate the corrosion processes of metallic surfaces. The method involved applying a small voltage sweep and as a consequence, an electrical imbalance was produced which was translated into a measurable current. From this measurement, the rate of corrosion of a metal in an environment may be determined. The advantage of this method lies in the fact that the sweep of potential applied is so small that the metal remains practically unaltered during the measurement process. It is also important to pay attention to the use of the different materials used in these water supply networks, if, e.g. the copper and galvanised steel are used in the same circuit, it is not sufficient to connect them using insulation material. The fact that dissolved copper accelerates the corrosion of zinc has also to be considered. This copper with the remaining zinc, forms a galvanic cell resulting in the zinc functioning as an anode and thus, undergoing accelerated corrosion. This is why circulation in galvanised steel pipes is recommendable. Moreover, the presence of dissolved copper in the water running through iron pipes turns it into copper metal thus producing corrosion in the iron pipes. This is why it is recommended, as a measure of protection, that water is first run through iron pipes and subsequently through copper ones in order to avoid the re-circulation of water in systems with mixed materials. The study began last July when the Mancomunidad de AGUAS DEL AÑARBE (Association of Municipal Councils supplied with water from the Añarbe reservoir) contracted the CIDETEC Research Centre (both from the Basque Country) to carry out a study of the corrosive capacity of the water supply treated at the AÑARBE reservoir purification plant and supplied to households in pipes made of various materials.

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