Numerous studies have been already carried out to study the chloride threshold value for depassivation of the steel embedded in concrete. Most of them were based on natural penetration of chlorides, whose main disadvantage is that it is very time consuming and makes necessary a long time to obtain reliable data. Other methods used were potentiostatic tests that, depending on the potential applied, may also be very long. In present paper, a new accelerated method to determine the threshold value of corrosion of steel is tried on a mortar specimen with an embedded rebar. The procedure is based on the use of the mechanism of migration to force the penetration of chlorides into the specimen, but switching off the external voltage and measuring the resulting threshold value in quasi-natural conditions. In spite of some considerations regarding the fact of the acceleration, the thresholds found (total, free Cl− and Cl−/OH− ratio) are in good enough accordance with those found for a twin specimen in natural conditions and within the range reported for mortars in the literature. In addition, this method has the advantage of enabling the simultaneous determination of the non-stationary chloride diffusion coefficient, Dns from the migration data, which has also been validated in natural conditions.