Study of the electrochemical characteristics of cathodically protected 2205 duplex stainless steel in saline environment

Abstract : Duplex stainless steels particularly the 2205 Duplex stainless steels are widely used in the petroleum industry, in oil and gas production, buried pipeline system due to their good mechanical properties, however when they are used in environments that comprise high salinity, they suffer degradation as a result of pitting corrosion that occurs due to the presence of chloride ions. A good approach to solve the pitting corrosion problem is to apply cathodic protection method (CP) on 2205 Duplex stainless steel in order to improve its corrosion resistance. Cathodic protection it is a widely used method to prevent corrosion of metals used in corrosive environments. It can be achieved by two applications such as use of impressed current and sacrificial anode. The research focused on the use of impressed current to reduce corrosion rate of 2205 Duplex stainless steel in saline environment. In achieving the objectives, different electrochemical corrosion measurements; Open Circuit potential, Potentiodynamic polarization and Chronoamperometry were carried out. Characterization of the as received 2205 Duplex stainless steel before and after corrosion test works was conducted using SEM and Optical microscope. Electrochemical corrosion measurements were carried out using the electrochemical cell made up of; Prince Applied Research (VersaSTAT 4) Potentiostat, a Ag/AgCl reference electrode, graphite counter electrode and 2205 Duplex stainless steel specimen. Simulated saline soil solutions of 1% NaCl and 3.5% NaCl were used as electrolytes. 10mm × 10mm specimens were cold mounted except for the exposed surface area of 10mm2. The exposed surface area was ground and polished using Diamaxx 6μm poly suspension. All the measurements were conducted at room temperature (±25 ºC). The electrochemical parameters were calculated by analysis of Tafel region using the VersaSTAT 4 software. Open circuit potential was conducted on 2205 Duplex stainless steel specimens in solutions of 1% NaCl and 3.5% NaCl to measure the free potential with respect to the reference electrode when no current was applied. Polarization measurements were carried out using a ASTM standard scan rate of 0.16667mV/s at potential initiated from -1.75V to 1.5V to determine the corrosion rate of 2205 Duplex stainless steel in 1% NaCl and 3.5% NaCl. Chronoamperometry measurements were conducted to study the current response with time when potentials of ( -1.3V, -0.8v, -0.3V) and ( -1.2V, -1.4V, -1.6) were applied to 2205 Duplex stainless steel in 1% NaCl and 3.5% NaCl. The results show that 2205 Duplex stainless steel immersed in 1% NaCl stabilized at higher potentials of 0.7V whereas when immersed in 3.5% NaCl it stabilized at lower potentials of 0.1V signifying that 2205 Duplex stainless steel in 3.5% NaCl has higher tendency to corrode in comparison to when immersed in 1% NaCl. High corrosion rates were observed for 2205 Duplex stainless steel in 3.5% NaCl than in 1% NaCl. Increasing the concentration of NaCl increased the corrosion current densities (Icorr) and decreased the corrosion resistance. Furthermore, applying a more negative potential produced high current densities with time which accelerated the pitting corrosion on the surface of 2205 Duplex stainless steel. This is supported by the SEM and Optical micrographs obtained. It is recommended that potential of -0.3V and -1.2V should be used as cathodic protection potentials of 2205 Duplex stainless steel in 1% NaCl and 3.5% NaCl.M.Ing. (Chemical Engineering