Steel Reinforcement in Slag Containing Binders and Its Susceptibility to Chloride-Induced Corrosion

Abstract

AbstractThe construction industry has moved from using plain Portland cement (PC) to binders with high proportions of supplementary cementitious materials such as blast furnace slags such as CEM II, CEM III A/B, and is increasingly targeting alkali-activated slags (AAS). The reducing nature of the blast furnace is retained by the slag, which contains ~1–3 wt.% sulfur (expressed as SO3), mostly in a reduced state and available to dissolve when mixed with water or an alkaline solution. The pore solution chemistry of many modern ‘alternative’ construction materials can be characterised as being rich in reduced sulfur species, and highly alkaline. There remain many open questions about the influence of such alkaline-sulfide solutions on the passive film formed on common mild steel reinforcement, and thus the susceptibility of this steel to chloride-induced corrosion. This study focusses on the influence of reduced sulfur species on mechanisms of passivation of steel and the phenomena of localised corrosion due to chlorides in highly alkaline electrolyte solutions with containing varying concentrations of reduced sulfur species, via electrochemical and spectroscopic techniques. The presence of HS− in alkaline electrolytes alters not only the passivation behaviour of mild steel, but also the mechanism of chloride-induced corrosion. In alkaline solutions containing sulfide, the competitive adsorption of [OH−] and [HS−] inhibits and retards the formation of a passive film, conventionally a Fe (III) oxide, and instead forms a surface film on the reinforcement composed of an assemblage of Fe(OH)2 and Fe-S complexes. In alkaline-sulfide solutions, the critical chloride concentration to induce corrosion was found to be dependent on the concentration of sulfide (i.e. the reducing capability of the electrolyte), and on the time that steel specimens were exposed to the electrolyte, consistent with the progressive formation of a sulfidic layer on the steel. Additionally, it has been shown that in the presence of high concentrations of HS−, the onset of chloride induced corrosion cannot be easily detected by conventional electrochemical measurements of OCP, Rp or icorr, as these tend to be highly influenced by the chemistry of the pore solution. Therefore, interpretation of electrochemical data obtained for aqueous systems containing HS− based on standard guidelines that assume Fe to be the sole redox-active species would result in misleading conclusions regarding whether the steel is in the passive or the active state.KeywordsSulfideMild steelCorrosionHigh alkalinityCement