The presence of deleterious Al4C3 phase has been for long a major challenge in the manufacture of Al matrix composites reinforced with carbide phases. In this contribution, the hydrophilic Al4C3 phase has been successfully eliminated from the surface of B4C/Al composites prepared by pressureless infiltration, using a post-processing approach by the hybrid system chemical vapor deposition (HYSY-CVD) route. B4C/Al composites containing intentionally formed Al4C3 were first prepared by pressureless infiltration and subsequently treated by HYSY-CVD, varying temperature (400 and 500°C), time (30 and 60min) and amount of solid precursor Na2SiF6 (10 and 20g). In conformity with reaction thermodynamic predictions using the HSC 6.1® and FactSage® programs and databases, analysis by XRD and SEM confirms the superficial elimination of Al4C3 and the seedless formation of passivating AlF3 and Na5Al3F14. Optimal conditions to maximize the amount of AlF3 deposited were established by ANOVA and the effectiveness of the fluoride phases was evaluated in hardness and corrosion tests, confirming the stability of fluoride phases and the integrity of the composites. Microstructure characterization and hardness tests of specimens after three years substantiate the efficacy of AlF3 and Na5Al3F14 phases. A hydrophobic and impermeable fluoride coating multiphase layer is thus obtained. Microstructure evolution of B4C/Al composites before and after post-processing. Display omitted •The hygroscopic and deleterious Al4C3 phase was successfully eliminated by a post-processing approach.•A hydrophobic and impermeable fluoride coating multiphase layer is deposited on the composites.•The stability of fluoride phases and the integrity of the composites are confirmed.•Optimal conditions to maximize the amount of AlF3 deposited were established.