Microscale investigation of the corrosion performances of low-carbon and stainless steels in highly alkaline concretes

Low-carbon steel shows good stability with respect to corrosion when embedded in ordinary portland cement concrete. This is due to the high alkaline content of the concrete pore solution favoring the formation of an iron oxide film that naturally keeps the steel in a passive state. With the rise of new types of concretes, based on different chemistries, the durability of reinforcements made out of low-carbon steel is at stake. Among the new concrete types, inorganic polymer concretes are characterized by an alkalinity of their pore solution at an early age that is higher than the one found in ordinary portland cement (OPC) concrete. The impact of these higher alkalinities on the stability of the passivity layer needs to be investigated. Low-carbon steel coupons are immersed for 2 days in four different solutions with hydroxide concentrations spanning from 0.1 mol/L to 3.7 mol/L. These solutions simulate the alkalinity of the pore solutions found in OPC concrete and three different types of inorganic polymer concretes. Stainless steels are often employed when the corrosion performances of low-carbon steel are insufficient. Whether duplex and ferritic stainless steels perform better in these environments, and thanks to which mechanism, is analyzed following the same procedure.This work carries a multiscale approach of the corrosion performances of low-carbon and stainless steels in various highly alkaline environments. First, the few-nanometer-thick passive film is analyzed by combining traditional electrochemical tests with x-ray photoelectron spectroscopy (XPS). The corrosion performance of the passive films is measured by the polarization resistance method, and the stability of the iron oxides is evaluated by the cyclic potentiodynamic polarization method. XPS measures both the oxide composition of the film, and its thickness. Then, once the passive film breaks down, the corrosion of steel embedded in cement paste is studied at the micrometer scale, and the formation of corrosion products is monitored in-situ by micro-computed tomography (µCT). The acquisition of three-dimensional images is then crucial in understanding the mechanism of corrosion of steel in concrete.Results show that when exposed to alkalinities higher than OPC pore solutions, the film growing on low-carbon steel (ASTM A36) is thinner and poorer in iron(III)-oxide. This goes in parallel with a decrease of the corrosion performances. Synchrotron-based XPS depth profiling show that the film formed after immersion in the 0.1 mol/L solution for 2 days is 4.1 nm-thick, with a 1.6 nm-thick inner-layer of Fe3O4, and an iron(III)-oxide outer-layer. On duplex stainless steel (UNS S32101), the iron oxide film grows thinner and poorer in iron(III)-oxide, but it gets enriched in chromium oxide - up to 4 times - as the alkalinity increases. This enrichment stabilizes the corrosion performances at higher alkalinities. The XPS depth profile shows that the film grown for 2 days in the 0.1 mol/L solution is 2.8nm-thick and contains chromium(III)-oxide in its outer-layer.On a subsequent µCT study of steel embedded in cement paste, the passive film is broken down, and active corrosion induced by applying a galvanostatic anodic current of +5 mA/cm2 to a steel electrode embedded in cement paste. Low-carbon steel (ASTM A36) is homogeneously etched away and a homogeneous layer of corrosion products forms at the interface steel/cement paste with a thickness of 40 µm after 3 hours. The induced pressure eventually fractures the cement paste cover, and corrosion products migrate out of the cracks. In the case of ferritic stainless steel (UNS S41000), the film depassivates locally, pits are formed, and concentrate the formation of corrosion products. Over 3 hours, 73 µm of corrosion products accumulates and their localized expansion creates more extensive damage on the cement paste cover.While low-carbon steel shows considerably lower corrosion performances at higher alkalinities, stainless steels can postpone the onset of corrosion by forming a stronger passive film. However, local breakdown of the passive film formed on stainless steels can induce greater damages to the reinforced concrete structure once active corrosion starts