This paper presents the observed grain boundary structure of Cu:Ni 70:30 alloy tubes before and after exposure to seawater. The results show that the thin platelet precipitates along grain boundaries of the alloys nucleated in the process of heat treatment and coarsened when they were exposed to seawater, indicating that the coarsening was induced by seawater corrosion. The diffraction image of the precipitate is fringe structure and possessed its own wave lengths. The fine scale structure is susceptible to intergranular corrosion, which could be confirmed by corrosion morphology observation of specimens after a long-time seawater exposure. The experiments were carried out by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical metallography (OM) and tensile testing etc., detecting corrosion morphology, microstructure and the mechanical property change due to the precipitation coarsening induced at grain boundaries of the specimens during exposure in seawater.
