Study of the corrosive effects of the water treated in the AÑARBE reservoir purification plant

Last July the Mancomunidad de AGUAS DEL AÑARBE (Association of Municipal Councils supplied with water from the Añarbe reservoir) contracted the CIDETEC Research Centre 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. The Mancomunidad de Aguas del Añarbe is made up of Donostia-San Sebastian City Council and the following Town Councils; Rentería, Pasaia, Hernani, Lasarte, Oiartzun, Usurbil, Lezo, Urnieta and Astigarraga; it is the authority responsible for water supply to all these districts.

As is known, 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 corrosion 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 of the material initially. One of the first consequences of the corrosion is a weight loss of the material, with corresponding variations in the initial dimensions of the material used for piping the water. The various mechanisms of corrosion are important when considering which material to be selected 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 corrosion reaction. Thus, the products formed in the corrosive process influence the behaviour of the materials in that 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 happens in the case of zinc (galvanised steel). As regards copper, it is commonly used for the supply of drinking water because of its good 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, moreover, 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, used in this study, 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.

Apart from all this, it is important to pay attention to the use of the different materials employed in these water supply networks, given the fact that, for example, if 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 the 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 first in galvanised steel pipes is recommendable. Moreover, the presence of dissolved copper in the water running through iron pipes makes this dissolved copper become 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.