Low-carbon concretes, designed by replacing Portland clinker with supplementary cementitious additions can exhibit lower carbonation resistance. Consequently, rebar corrosion risk influenced by concrete cover quality must be assessed. This study investigated the corrosion risk of three low-carbon slag-based concretes (low-clinker high filler LCHF, alkali-activated AAS, and a supersulfated SSC one) compared to standard CEM II/A-L and CEM III/B concretes belonging to the same C25/30 strength class. Carbonation and corrosion rates were assessed under exposure classes XC1 to XC4 according to NF EN 206+A2/CN:2022. A corrosion risk map and a service life approach, separating initiation and propagation phases, were used to predict the time to cover cracking. According to the choice of our models, most of the concretes exceeded the total service life of 50 years when complying with European standard cover depth. In the case of improper cover, the service life is based on the corrosion rate during propagation. Concerning the propagation period alone, the likelihood of cracking onset by 50 years was assessed as negligible in most of the XC1 concretes and low once exposed to XC2. Concerning outdoor exposure classes, under Lyon’s climate (weather data 2021–2024), the risk in XC3 was found comparable to the risk in XC1, and may increase in XC4. Since rainfall, relative humidity, and temperature impact wetting time at the steel-concrete interface, further research is needed to link hygroscopic properties to corrosion risk for low-carbon reinforced concretes in other real climates.