CuNi 70/30 is widely used in marine applications, mainly for the tubes of seawater cooled steam condensers in power stations. Its good corrosion protection behaviour is attributed to the formation of a slim, dense and adherent corrosion protective layer, film which forms naturally and quickly on exposure to clean seawater, creating a barrier between the alloy’s surface and the cooler. Even though this alloy is resistant against the chemical aggressiveness of seawater, microorganisms living in marine environment are able to modify the chemical and physical characteristics of the surface and facilitate the development of corrosion processes, introducing an adverse resistance to the heat exchange as well as drastically reducing the operative life of equipments. The widest employed technique for the prevention of biofouling and biocorrosion is the chlorination, in spite of the strong present restriction to the chlorine release into natural bodies (generally, ≤ 0.2 mg/l is allowed). The oxidant effect of low chlorine concentration influences the formation of the protective layer other than inhibit the biofilm growth by mechanisms that need to be deeper investigated. Another important mechanism involved in the microbial corrosion of copper alloys, to be investigated, is the negative biofilm action, destroying/inhibiting the protective layer by the occurrence of acidification of the metal-biofilm interface. Based on these assumptions, specific experimentations in lab and in field are in progress aiming to clarify the interaction of the different mechanisms involved. The results obtained by electrochemical and morphological investigation on corrosion products of CuNi70, during laboratory experiments conducted with natural filtered seawater, at different pH’s are reported. Linear polarization curves, impedance and surface analyses (XPS and ToF-SIMS) are presented.