Electrochemical Measurements on Copper Nickel Alloy CuNi 90/10 in Artificial Seawater - Influence of Flow Conditions and Chlorine Concentration

Over several decades the copper-nickel alloy (UNS C70600, CuNi 90/10) has been used on different marine engineering structures, mainly as piping for seawater handling and heat exchangers. In spite of the marcofouling resistance of CuNi 90/10, the chlorination of seawater is often applied. Since, there is only limited information on the effect of chlorination on the corrosion behaviour of copper-nickel, a systematic approach is required to understand the effect of free chlorine on the corrosion behaviour of this alloy. Therefore, the first aim of this work is the characterisation of the electrochemical behaviour of CuNi 90/10 in chlorinated seawater under stagnating conditions. The results were obtained in synthetic seawater containing up to 5 ppm free chlorine using fresh and pre-exposed samples. For pre-exposure, the samples have been stored for six weeks in artifical sea water. The free corrosion potential and the polarisation resistance has been measured as a function of time. Polarisation curves have been recorded in cathodic and anodic direction separately. Gravimetric and metallographic examinations have been taken to proof the electrochemical results and to examine the type of corrosion attack. The obtained data did not reveal a distinct dependence of the corrosion behaviour and obtained corrosion rates on the free chlorine concentration. With increasing formation of oxide layers, the corrosion rate decreases. The gained data is used as the bases for the studies under erosive conditions to compare it with the reported data and validate the current practical recommendations [1]. The results of the second part presented in this paper evaluate the effect of flow on the corrosion behaviour of fresh and pre-exposed CuNi 90/10 samples. Testing involved, evaluation of the effect of free chlorine on critical shear stress using rotating cage and submersed jet apparatus under turbulent flow conditions. Furthermore the effect of laminar flow was investigated by rotating disc-electrodes [2]. Under laminar flow conditions the corrosion rate increases with higher hypochlorite concentrations, which can be determined from polarization curves. Pre-exposed samples did show lower corrosion rates. Polarisation curves are displayed in fig. 1. During the jet testing, the electrochemical noise measurements by means of CoulCount-diagnostic were used for the monitoring of corrosion process. The data is being used to validate the current practical service recommendations. Fig.1: CuNi 90/10after pre exposure for 6 weeks at 1000 RPM in ASTM seawater with different concentrations of NaOCl