Cathodic behaviour of stainless steel in coastal Indian seawater: calcareous deposits overwhelm biofilms

Abstract

Type-316 stainless steel (SS) was investigated as the cathode in galvanic couples in full-strength seawater from the Gulf of Mannar on the southeast coast of India. Tests were devised to examine the impact of SS cathodes on anode materials with or without the accrual of marine biofilms. Biofilmed SS cathodes significantly enhanced the rate of corrosion of nickel, causing noble shifts in the couple potentials. With mild steel and zinc as the anodes, calcareous deposits developed quite rapidly on the SS cathodes and led to a significant reduction of bacterial numbers. The calcareous deposits also caused substantial reduction of galvanic corrosion rates for mild steel, whereas there was no difference for zinc. The deposits were identified by XRD as essentially carbonates, oxides and hydroxides of calcium and magnesium. Potentiodynamic polarization performed on the actual couples after disconnection and equilibration provided reasonable interpretations of the galvanic corrosion trends. Data from this work suggest that a potential of about -0.70 V vs. saturated calomel electrode (SCE) should provide optimum protection of SS in warmer, full-strength seawater that supports the precipitation of calcareous deposits. The criterion commonly recommended for temperate conditions of lower water temperature and estuarine waters of lower alkalinity is -1.0 V (SCE).

Evolution of Calcareous Deposits and Passive Film on 304 Stainless Steel with Cathodic Polarization in Sea Water

“…This result is consistent with that of Eashwar [31], that calcareous deposit can be precipitated if the potential is more negative than −0.67 V on stainless steel. that the deposit is thin at 20 h of cathodic polarization.…”

“…For the samples with polarization at −0.50 V, only elements of Fe, C, Cr, and Ni were found on the surface, implying that calcareous deposit cannot be formed at −0.50 V for the whole test period. This result is consistent with that of Eashwar [31], that calcareous deposit can be precipitated if the potential is more negative than −0.67 V on stainless steel. …”

“…EIS is a very valuable technique for the indirect characterization of the passive film and the calcareous deposits on the surface of stainless steel [26,[30][31][32]38]. Figure 3 shows the EIS spectra of 304 SS after polarization at −0.80 V in sea water for different times.…”

“…The precipitated mechanism and the protective property of calcareous deposits on carbon steel substrate have been thoroughly studied [18,22,30]. However, only few investigations have focused on the surface of stainless steels under cathodic protection [1,26,31,32]. The cathodic polarization should affect the oxide film, change the interfacial environment and cause the development of calcareous deposits on the surface, which make the evolution of surface conditions of stainless steel in sea water much more complicated than that of carbon steel.…”

“…The formation of calcareous deposits is influenced by many factors, such as temperature, hydrostatic pressure (depth), velocity, chemistry of sea water; current density, potential and period of cathodic polarization; biofilms; substrate materials and surface preparation; and so on [26,[28][29][30][31][32]. The precipitated mechanism and the protective property of calcareous deposits on carbon steel substrate have been thoroughly studied [18,22,30].…”