Localized Corrosion of High-Grade Stainless Steels in Chlorinated Seawater

Chlorination is widely used in seawater systems to avoid fouling. Free chlorine is a strong oxidizing agent that prevents the biofilm formation on immersed surfaces, when used above a certain content. However, the presence of residual chlorine associated with the high chloride content in seawater, significantly increases the risk of localized corrosion for most stainless steels. In the present study, a module initially developed to quantify the formation of electroactive biofilms on stainless steels has been used to assess the corrosivity of chlorinated seawater. Both the electrochemical potential and the cathodic current were measured on super-duplex stainless steel as a function of residual chlorine levels and seawater temperatures. In parallel, long term localized corrosion tests have been performed in simulated environments to assess the environmental limits for safe use of high-grade stainless steels in chlorinated seawater. It includes crevice corrosion exposure tests using adapted ISO18070:2015 crevice formers and internal tube pitting corrosion exposure tests in model tube heat exchangers simulating heat flux from 35°C to 170°C. The synergetic effect of residual chlorine content and temperature on the risk of localized corrosion has been quantified. Corrosion resistance properties are correlated to the electrochemical monitoring data, and the environmental limits of selected stainless steels have been established for duplex stainless steel UNS S32205, super-duplex stainless steel UNS S32750, hyper-duplex stainless steel UNS S32707 and the high-grade austenitic stainless steel UNS S31266.