Chloride threshold levels in clad 316L and Solid 316LN stainless steel rebar

Initiation of corrosion on reinforcing bars occurs when a critical chloride concentration is achieved. A universal value for the chloride concentration that causes depassivation has not been agreed upon for carbon steels due to a variety of experimental factors. The chloride threshold depends on the rebar material, rebar surface condition, testing environment, method of detection, definition of depassivation, and method of expression of results. However, most data for carbon steel points to a Cl^-/OH^- molar ratio <1. The chloride threshold for stainless steel clad rebar is unknown. The chloride threshold for solid 316LN stainless steel, 316L stainless steel clad, and carbon steel rebar was investigated through current monitoring during potentiostic polarization with comparison to open circuit potentials (in a saturated Ca(OH)₂ + NaCl in glass sand bead mixture). Solid 316LN stainless steel rebar was found to have a much higher chloride threshold (i.e. threshold Cl^-/OH^- ratio >24) than carbon steel at all potentials investigated (-200,0.00, and +200 mV vs. SCE). The chloride threshold for 316L clad rebar was highly dependent on defects and the method used to seal the cut end, which exposes the carbon steel core. At best, it was similar to that of solid stainless steel and at worst, it was less than that of carbon steel rebar, owing to elevation in potential due to galvanic coupling between a defective clad layer and the exposed carbon steel core.