Threshold Chloride Concentrations of Selected Corrosion-Resistant Rebar Materials Compared to Carbon Steel

The threshold chloride concentration for solid Type 316LN (UNS S31653) stainless steel, Type 316L (UNS S31603) stainless steel clad, 2101 (UNS S32101), Fe-9%Cr, and carbon steel rebar (ordinary ASTM A 615M) was investigated using potentiodynamic and potentiostatic current monitoring techniques in saturated calcium hydroxide (Ca[OH]₂ + sodium chloride (NaCl) solutions. There is general consensus in this study and the literature that the chloride threshold for carbon steel is less than a chloride to hydroxl (Cl^-/OH⁻) molar ratio of 1. Solid Type 316LN stainless steel rebar was found to have a much higher chloride threshold (i.e., threshold Cl⁻/OH⁻ ratio > 20) than carbon steel (0.25 < Cl⁻/OH⁻< 0.34). Type 316L stainless steel clad rebar possessed a chloride threshold expressed as a Cl⁻/OH⁻ ratio of 4.9 when cladding was intact. However, surface preparation, test method, duration of period exposed to a passivating condition prior to the introduction of chloride, and the 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 Type 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. However, when defective, it was equal to that of carbon steel rebar in the potentiostatic method used here. A model was implemented to predict the extension of the Cl⁻ diffusion time period until corrosion initiation would be expected using rebar materials with a higher chloride threshold concentration than carbon steel. Model 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.