Continuous corrosion monitoring in desalination plants

Multiflash evaporation is the most widely used process in large desalination plants. Acid injection has been the main anti-scaling treatment in these plants for many years but recently the anti-scaling agents have been used more frequently, probably because it is generally assumed that they allow a better corrosion control than acid. Indeed the use of anti-scalings is not completely safe because of the release of carbon dioxide and oxygen in the first two or three flash chambers. According to Authors opinion the acid treatment is not only cheeper than anti-scalings, but is also reliable provided that the corrosivity of the circulating water is controlled in several points of the plant. This result can be easily achieved by means of a suitable on-line corrosion monitoring system. The results of two series of corrosion rate measurements, both related to acid treated multi-flash desalination plants are reported in the paper. The data recorded during the tests prove once more the usefulness of on-line corrosion monitoring which allows to control the variations of very important parameters as oxygen concentration and pH, and shows sensitivity to hydrodynamic conditions. The reliability of on-line monitoring has been also proved by comparing the values of corrosion rates measured by means of LPR (Linear Polarization Resistance) probes and the ion concentrations contemporaneously found in the process fluid.