Effect of service conditions on corrosion performance of alloy CN 108 tubes in MSF desalinators

Results of tests performed on CN 108 alloy tubes in a desalinator and in a test loop, showed that the main corrosion cause has to be ascribed to incorrect service conditions. Both high chlorination and no chlorination cause a rapid increase of the alloy corrosion potential from the first days and pitting initiation of tube samples with different superficial finish (as received and pickled). Moderate chlorine dosage (free residual chlorine ≤ 0.5 ppm) is effective to avoid that rapid increase of Ecorr and pitting initiation. The presence of a thermal Mn-rich oxide film is not harmful by itself; however, in concurrence with incorrect service conditions it can contribute to the pitting corrosion initiation.