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

Abstract

Metal structures submerged in seawater are subject to corrosion. Corrosion protection is usually implemented by applying negative potentials between -0.8 and -1.0 V vs Ag/AgCl reference electrode [1]. At these potentials, anodic metal dissolution is expected to be suppressed, and the typical dominant reactions that govern electron transfer on the protected surface in seawater are oxygen reduction reaction (ORR) and hydrogen evolution from water. The ORR in basic environments (pH~ 8.4) result in the generation of hydroxyl ions, eliciting an increase in the carbonate (CO3 2-) ions from the carbonate buffer system. When concentrations of CO3 2- and OH- near the electrode surface become sufficient to exceed solubility constants of CaCO3 and Mg(OH)2 compounds, they precipitate on the surface, decreasing the electroactive surface area, and making cathodic protection more efficient [2]. Formation of calcareous deposits on metal alloys is affected by applied potential, DO level, salinity, pH and temperature. Although the influence of DO, pH and temperature on the formation of calcareous deposits have been previously reported [3-5], the effect of salinity level has been overlooked. The effect of pH has been explored only minimally for a limited range of pH (usually ~ 7). Here, we explore polarization behavior of three cathodically protected alloys (Inconel 625, HY 80 Steel, and C-632 Nickel-Aluminum Bronze used in marine environment as a function of seawater salinity and pH. Electrochemical experiments were conducted in a three-electrode electrochemical cell (BioLogic, Inc). Freshly polished coupons of three different alloys served as working electrodes (WEs), while platinum mesh and Ag/AgCl in 3M NaCl (BioLogic, Inc) were used as a counter and reference electrodes, respectively. WEs held at -1.0 V were immersed into 0.5, 3.5 or 35 ppt seawater for 24 h, and polarization curves were recorded at 0.6 V/h prior and following the potential hold. Acetic acid, MES and MOPS organic buffers (Sigma Andrich) were used for maintaining pH levels of 5, 6, and 7, respectively. Seawater from Mexican Gulf was collected at Key West, FL. The influence of salinity and pH on polarization behavior of HY 80 steel is shown in Figures 1 and 2, respectively. The presentation will discuss the reasoning behind the changes in HY 80 Steel, Inconel 625 and C-632 Nickel-Aluminum Bronze polarization behavior under different environmental conditions.