Effect of pH and Salinity on Polarization Behavior of Cathodically Protected HY80 Steel, Inconel 625 and Nickel-Aluminum Bronze in Mexican Gulf Seawater

Abstract

The effect of salinity and pH on current demand reduction on cathodically protected 625 Inconel, NAB and HY80 have been investigated. The bias of -1 V vs Ag/AgCl reference electrode was applied for 24 h to working electrodes immersed in (1) natural seawater, (2) seawater diluted to 0.5 and 3.5 ppt salinity levels or (3) pH-buffered seawater with pH of 5, 6 and 7 or. For 625 Inconel, reduction of cathodic current demand was found to be dependent on salinity and independent on pH. The amount of cathodic current demand reduction increased with salinity. This was presumed to be related to calcareous deposit sensitivity related to lower pH required for precipitation of CaCO3 and Mg(OH)2 from solutions of higher salinity. Reduction of cathodic current demand was not affected by pH likely due to (1) a negative shift of potential of zero charge in the presence of organic buffers or (2) generation of a sufficient amount of OH- at the surface to overwhelm the buffer capacity. Both these factors could cause a local rise in pH sufficient to cause mineral deposition, which would further degrade the ability of the buffer to reach the surface.