Due　to　the　dense　microstructure　and　high　resistance　to　chloride　ion　erosion,　engineered　cementitious　composite　(ECC)　possesses　of　a　potential　to　be　applied　in　the　chloride-contaminated　environment.　However,　the　passivation　mechanism　of　embedded　in　ECC　with　endogenous　chloride　is　still　unclear.　This　study　investigates　the　microstructure　of　ECC　using　scanning　electron　microscopy　(SEM)　and　nitrogen　adsorption-desorption　(NAD)　measurements,　while　evaluating　the　passivation　behavior　of　embedded　steel　via　electrochemical　workstation　testing.　The　results　show　that　the　synergistic　effect　of　fly　ash　and　polyvinyl　alcohol　(PVA)　fiber　significantly　refines　the　pore　structure　of　ECC,　which　is　conducive　to　the　early　formation　of　dense　matrix.　This　microstructure　effectively　impedes　harmful　ion　penetration　and　preserves　structural　integrity,　even　under　endogenous　chloride　exposure.　At　endogenous　chloride　levels　≤　1.25　%,　steel　reinforcement　in　ECC　maintains　passivated　after　28　days　(Ecorr　＞　−126　mV,　icorr　−2),　which　is　attributed　to　the　dense　microstructure　of　ECC　as　an　ion　barrier.　Electrochemical　impedance　spectroscopy　reveals　stable　passivation　film　formation　in　ECC　(except　at　1.25　%　chloride),　evidenced　by　small-radius　capacitive　arcs　in　Nyquist　plots　and　high-stable　phase　angles　in　Bode　plots.　However,　at　chloride　concentrations　＞　1.00　%,　the　charge　transfer　resistance　(Rct)　decreased　sharply　while　the　electric　double　layer　capacitance　(CPEdl)　increased,　signaling　rising　depassivation　risks　and　behavior.　Cyclic　polarization　curves　further　confirmed　dynamic　stability　of　passivation　films　at　≤　1.00　%　chloride,　whereas　destabilization　and　localized　pitting　tendencies　emerged　at　1.25　%　chloride.　©　2025　Elsevier　Ltd