Electrochemical processes occurring on previously cathodically polarized AISI 316L stainless steel electrodes during emersion in marine atmosphere

Abstract

Carbon and low alloy steel structures immersed in seawater are commonly protected against corrosion by cathodic protection. In some complex structures, even though care is taken to avoid galvanic coupling, other metallic materials may be in contact with the protected carbon/low alloy steel and thus cathodically polarized too. The present study deals with complex structures that undergo immersion / emersion cycles. It is focused on the behavior of AISI 316L stainless steel parts affected by cathodic protection and aims to provide information about the electrochemical behavior of stainless steel during emersion periods. For that purpose, AISI 316L electrodes were polarized at -850 mV/_Ag-AgCl-sw or -1050 mV/_Ag-AgCl-sw during two months in natural seawater at 10°C and subsequently left at open circuit potential in a marine atmosphere for two weeks. µ-Raman spectroscopy analysis showed that the deposits were in any case mainly composed of aragonite CaCO₃ but XRD analysis revealed the presence of brucite Mg(OH)₂ at -1050 mV/_Ag-AgCl-sw. Linear polarization resistance and electrochemical impedance spectroscopy measurements were carried out during the emersion period and showed that the calcareous deposit formed at -1050 mV/_Ag-AgCl-sw retained more water during emersion than the deposit formed at -850 mV/_Ag-AgCl-sw. Moreover, it delayed the increase in OCP associated with the interruption of cathodic protection. These differences are attributed to the underlying brucite film only present at -1050 mV/_Ag-AgCl-sw that tends to dissolve when cathodic protection is interrupted, thus maintaining a higher pH at the metal/seawater interface.