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Effect of chlorination on the corrosion behavior of CuNi 70/30 condenser tubing

pubblicazioni - Articolo

Effect of chlorination on the corrosion behavior of CuNi 70/30 condenser tubing

condenser tubing Pierangela Cristiani*, Giorgio Perboni**, Alessandro De Benedetti*** BIOCORYS 2007 International Conference on Biocorrosion of Materials Prigi, 11-14 Giugno 2007 * CESI RICERCA ** CESI SpA *** CNR – IENI Microbial activity influences negatively the process of passivation of copper alloys and chlorination is a traditional method to control biological activity in industrial cooling water circuits. If no chlorination or insufficient biocide treatments are carried during the start-up period, severe and early failures induced by localised corrosion could affect the exchanger equipments made of copper alloys. In the light of case histories noticed in the last years in power plants, some experimental trials have been performed with 70/30 Copper-Nickel alloy exposed to sea water, with the aim to demonstrate the positive role played by short chlorination treatments to the stability and passivity of the layers growing on the metal surface. Innovative electrochemical sensors integrated with traditional probes and data acquisition system have been used to monitor on-line: • the biofilm growth (electrochemical on-line BIOX sensor) • the corrosion rate (electrochemical LPR technique probe) • the chlorination treatment (electrochemical on-line BIOX sensor) • physical-chemical parameters of the water (flow, temperature, turbidity). Weight loss determination, chemical and metallographic analyses have been completed the characterisation of samples exposed to sea water before and after the tests. The monitoring have been conducted both in power plants full scale circuits and in pilot circuits. All the data collected confirmed the occurrence of severe localised corrosion on the surface of the CuNi 70/30 samples in case of event causing the enrichment of biological active mud in sea water. An effective chlorination treatment of the water is able to decrease the corrosion rate during the event of mud or in case of stagnant condition occurring. A regular short treatment, in the range of 0.4 – 1 mg/l of free oxidant for 0.5 – 1 hour, few time a day, promote the development of a different and more “passive layer” on the CuNi 70/30 surface. When well stabilised, the “passive layer” is able to prevents the development of new localised corrosion phenomena also in case of chlorination interruption, for a long time. (*) Work financed by the Italian Ministry of Economic Development with the Research Fund for the Italian Electrical System under the Contract Agreement established with the Ministry Decree of march 23, 2006.

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