Copper alloys are widely used in marine environments due to their corrosion resistance, mechanical workability, excellent electrical and thermal conductivity and resistance to macrofouling. Copper was believed to be immune to MIC because its toxicity to the biological organisms, but it is well known that this material suffers of this kind of corrosion. Biofilms modifies the local environment at the biofilm/alloy interface; the local environment can be radically different from that of the bulk medium in terms of pH, dissolved oxygen, and species concentrations. It is well established that some microbial species cause a decrease of local pH at the biofilm interface; the steady state current density increases due to the dissolution of the protective film formed on the copper alloy. We investigate the effect of pH on CuNi 70:30 samples covered with a biofilm. Experiments were carried out in the laboratory and in the field. The methodology consisted in measuring the coupling current between two electrodes, one covered by a biofilm and the other clean, each one immersed in a different compartment of an electrochemical dual-cell (ionically connected), filled with natural seawater. Zero Voltage was imposed between the electrodes (allows to keep the two samples at the same potential). For interfaces between the two samples different, a current will be detected between the two electrodes (biofouled vs. clean samples); the current will become gradually smaller as the pH in the cell compartment with clean sample is decreased. Experience was carried out in field, with fouled samples and under real condenser tubes conditions (pH, temperature, flow…). A specific dual-cell under natural seawater flow was used, suited for long time samples exposition.The results showed that decreasing the pH in the cell compartment containing the clean sample has a significant impact on the coupling current, as it is expected, but it was not possible to reach a null current between the two electrodes even at low pH (2.5). This will indicate that Cu/Ni passive film under the biofilm samples is very different to the passive film formed on the clean samples and that pH is not the only variable affecting the corrosion formed under biofilm. Because the current does not reach zero we can assume that the passive film under the biofilm went dissolution or breakdown, or the passive film composition had changes. Because the use of an hydraulic line under seawater flow, crevices can be present already on the biofilm samples. The possible complex roles of the pH on the cathodic and anodic reactions are discussed.