Role of phosphorus as micro alloying element and its effect on corrosion characteristics of steel rebars in concrete environment

This communication reports the effect of phosphorus (P) added in micro concentration range in steel on kinetics, mechanism and growth of passive film in contact of chloride contaminated concrete. Electrochemical impedance spectroscopy, direct-current polarization, mass loss and Raman spectroscopic techniques were used to arrive at the findings. The results showed that an intentional addition of P in steel (0.064%) makes it more prone to uniform and localized corrosion (about 1.1 and 1.7 times) than the steel having low phosphorus (< 0.016%, present as tramp element) exposed under wet/dry conditions in simulated pore solution added with chloride and in the absence of this ion. A similar effect is also noted for the rebars embedded in mortars. Identification of corrosion products formed on steel rebars surface by Raman spectroscopy reveals thermodynamically stable maghemite and goethite phases on the surface of low P content steel. Unstable phase of lepidocrocite is recorded on the surface of higher phosphorus steel rebars. The findings are discussed with experimental evidence and taking clues from the published literature to arrive at plausible mechanism for this behaviour.