Study of effect of pollutant additions on corrosion behaviour of Al-brass in sea water

The corrosion behaviour of Al-brass was studied in stagnant and flowing sea water in the absence and presence of 5 ppm Mn and 4 ppm Cl. The addition of Mn or Cl to stagnant sea water had little effect on the free corrosion potential. Inflowing sea water, Mn caused a shift in the corrosion potential to more noble values at a flowrate of 0.1 m s^-1 and to more active values at 2.2 m s^-1. Addition of Cl caused an electropositive shift at both flow velocities. Under all testing conditions, the linear polarisation resistance decreased in the presence of Mn and increased in the presence of Cl. Potentiodynamic polarisation tests indicated that the presence of either pollutant in stagnant sea water causes cathodic depolarisation, electropositive shift in corrosion potential, and elimination of active-passive transition. Microscopic examination revealed that the film formed in the presence of Mn was thicker and more porous than that formed in clean sea water. In the presence of Cl, the film was smooth and thick but contained scattered nodules of basic Cu chlorides, particularly at high velocity. These nodules were found to cause crevice corrosion and their presence was assumed to be responsible for the occurrence of erosion corrosion at high flow velocity, resulting in an increase in corrosion rate.