Contribution of Post-Rolling Surface Treatment to Corrosion Performance of Duplex and Austenitic Stainless Steel Reinforcing Bar

This study elucidates the complex relationship between stainless steel (SS) rebar surface treatment and corrosion resistance in high chloride environments using microscopy, analytical, and conventional electrochemical and novel corrosion microscopy techniques in a short- and long-term assessment. Previous studies on carbon steel reported that millscale defects (cracks and porosity) compromise corrosion resistance by allowing chloride ions to reach the base steel. This study examining three SS grades with four different surface type shows that their high Cr content improves compactness and reduces defectiveness of the inner (Fe-Cr) millscale layer, resulting in better corrosion resistance which varied with surface treatment methods, alloy composition, concrete composition, and chloride exposure method. Despite its defective outer (Fe) millscale layer, as-rolled (AR) bars showed comparable or better corrosion resistance than bars subjected to post-rolling treatment. Treating SS surface with mechanical blasting or acid pickling alone deteriorates corrosion performance due to embedment of blasting particles into rebar surface and/or the presence of loose residual millscale. Employing both shotblasting and acid pickling (SBPK) causes micropits to develop on the bar surface, which deteriorates corrosion resistance.

Additionally, this study also show that pore solution tests do not adequately reflect the corrosion performance of SS millscales because such test uniformly exposes chloride ions to millscales, causing high corrosion rate resulting from uniform millscale dissolution and quicker exposure of underlying substrate. This behaviour is different from that encountered in concrete where non-uniform chloride exposures causes non-uniform millscale dissolution and lower corrosion of underlying substrate. Overall, this work shows that achieving consistent post-roll treatment or pristine SBPK surface between manufacturers and batches is a bigger challenge than achieving homogeneous millscale, making AR SS rebar a cost-effective corrosion resistant option for construction.