Threshold chloride concentrations of selected corrosion resistant rebar materials compared to carbon steel

The threshold chloride concentration for solid 316LN stainless steel, 316L stainless steel clad, 2101 LDX, MMFX-2, and carbon steel rebar was investigated through potentiodynamic and potentiostatic current monitoring techniques in saturated Ca(OH)₂ + NaCI solutions. There is general consensus in this study and the literature that the chloride threshold for carbon steel is less than a CI^-/OH^- molar ratio of 1. Solid 316LN stainless steel rebar was found to have a much higher chloride threshold (i.e., threshold CI^-/OH^- ratio > 20) than carbon steel (0.25 < CI^-/OH^- < 0.34). 316L stainless steel clad rebar possessed a chloride threshold CI^-/OH^- ratio of 4.9 when cladding was intact. However, surface preparation, test method, duration of period exposed to a passivating condition prior to introduction of chloride, and presence of cladding defects all affected the threshold chloride concentration obtained. For instance, the presence of mill scale on any of the more corrosion-resistant materials reduced the chloride threshold to approximately that of carbon steel. The chloride threshold for 316L clad rebar was highly dependent on any defects that exposed the carbon steel core. At best, it was similar to that of solid stainless steel and when defective, it was equal to that of carbon steel rebar. A model was implemented to predict the extension of time until corrosion initiation afforded by use of rebar materials with a higher corrosion resistance than carbon steel. Simulation results confirmed that corrosion-resistant rebar materials in a pickled condition may increase time until chloride-induced breakdown of passivity and onset of corrosion to 100 years or more.