Report concerning the corrosion behaviour of CuNi10Fe in seawater, based on investigations carried out at Royal Naval College of the Royal Netherlands Navy. The testing program included exposition to flowing seawater, tests aimed at determining the erosion behaviour, electrochemical measurement of polarization behaviour, corrosion potential and polarization resistance, and electron microscopic investigations into the structure and composition of protective layers. The tests were carried out with natural and synthetic seawater in the laboratory and under conditions encountered on ships. Studies made with the electron microprobe reveal that the protective layers are of a very complex structure and that the formation of a really protecting layer requires two weeks' time. The protective action is due to concentration polarization because of the reduction of oxygen and oxidation of copper (due to concentration phenomena in pores) and to a reduction of the active surface.