Online Monitoring Systems of Microbiologically Influenced Corrosion on Cu-10% Ni Alloy in Chlorinated, Brackish Water

Alloy Cu-10% Ni (ASTM B-111, UNS C70600) has been used extensively for condenser and heat exchanger tubes in power stations. However, there have been cases of severe localized corrosion in this alloy. This paper presents the evaluation of Cu-10% Ni in a dynamic online monitoring system with one-way circulation using alkaline (pH 7.2 to pH 8.8) brackish water with and without chlorination for 3-, 8-, and 12-month exposure periods. Electrochemical laboratory tests—open-circuit potential (OCP), linear polarization resistance (LPR), and cyclic polarization (CP)—also were run. Before chlorination, the microbiological water analysis indicated microbial development with planktonic and sessile sulfate-reducing bacteria (SRB) on the order of 10⁷ cells/mL and 10⁵ cells/mL, respectively, with sessile SRB at 10 cells/mL after chlorination. Prechlorination scanning electron microscopy (SEM)/ energy dispersive x-ray (EDX) analysis/x-ray diffraction (XRD) analysis after 3 months revealed very few cuprite (Cu₂O) crystals and bacterial cells, whereas dense cell populations associated with hemispheric holes typical of microbiologically influenced corrosion (MIC) were found after 8 months. Postchlorination SEM/EDX/XRD analysis helped establish a definition of how chlorine increases the Cu-10% Ni corrosion rate. Initially, a dense layer of star-shaped Cu₂O crystals was observed. It later was removed partly because it quickly oxidized into nonprotective, nonadherent secondary corrosion products, which again exposed the material to the hypochlorite ions in the corrosive medium, consequently forming a new layer of Cu₂O. The formation of different corrosion products and redeposited copper with severe localized corrosion below the deposits and severe general corrosion finally were shown by the analysis of a tube that was in service for 3 years in the same chlorinated, brackish water. CP curves indicated that chlorine content increased the corrosion current from 2.5 μA/cm² per 0.0 ppm Cl₂ to 5.0 μA/cm² per 0.3 ppm Cl₂, and that Cu-10% Ni is not passivated in this brackish water. All these results suggest that this alloy is not corrosion resistant in brackish water, and chlorine treatment accelerates corrosion even more.