The　use　of　recycled　aggregate　concrete　(RAC)　in　engineering　structures　is　a　cornerstone　of　sustainable　development　within　the　construction　industry.　Nonetheless,　the　intricate　microstructural　characteristics　of　RAC,　such　as　the　prevalence　of　multiple　mortars　and　the　complex　nature　of　the　interfacial　transition　zones　(ITZ),　pose　significant　challenges　to　understanding　the　corrosion　features　of　steel　reinforcement,　especially　under　chloride　ion　attack.　Based　on　this,　the　present　study　investigates　the　corrosion　behavior　of　rebar　in　RAC　subjected　to　admixed　chloride　ion　contamination,　with　a　focus　on　the　effects　of　different　water-to-cement　(w/c)　ratios　and　varying　levels　of　recycled　aggregate　replacement　rate,　denoted　as　φRA.　Electrochemical　methods　were　employed　to　macroscopically　assess　the　corrosion　process　of　the　specimens.　The　results　indicate　that　as　the　w/c　ratio　and　φRA　increase,　rebar　corrosion　occurs　earlier　and　the　corrosion　rate　accelerates.　Furthermore,　microscopic　testing　methods　were　used　to　provide　a　detailed　description　of　rebar　corrosion　behavior　in　RAC.　The　findings　show　that　with　increasing　w/c　ratio　and　φRA,　the　rust-filled　area　significantly　enlarges,　while　the　corrosion　layer　thickness　remains　relatively　stable.　The　porosity　of　the　surrounding　concrete　matrix　is　identified　as　the　primary　factor　influencing　the　development　of　the　rust　filling,　which　suggests　the　potential　for　mitigating　rust　expansion　cracks　in　RAC　than　conventional　concrete.　The　impact　of　the　microstructural　characteristics　of　the　multiple　ITZ　and　mortar　phases　in　RAC　on　rust　filling　behavior　depends　on　the　relative　compactness　of　each　material　phase,　as　reflected　by　their　pore　structure　features　and　microhardness.　Although　the　new　mortar　phase　serves　as　the　main　rust-filled　area　in　RAC,　accounting　for　72　%-87　%,　the　rust　filling　sites　provided　by　the　interfacial　transition　zone　should　not　be　overlooked,　due　to　the　rust-filled　preference　induced　by　its　high　porosity.　©　2025　Elsevier　Ltd