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The present work studies the effect of fly ash content (0–25 wt.%), pH (8, 12.5), and steel type (316L, 304L) on the cyclic polarization of stainless steel rebars in electrolytes, simulating fresh concrete exposed to acid rain and corroded concrete cover that has exposed the reinforcement to direct acid rain attack. At the same time, it tries to elucidate the corrosion inhibition activities of a Greek lignite fly ash that is a high-Ca fly ash with a questionable effect on the corrosion resistance of concrete. A higher pH results in lower corrosion rates for both steels and all fly ash content. However, different passivity trends are noted for the two steels as a function of pH. The partial replacement of Ca(OH)2 with fly ash up to 20 wt.% has a beneficial effect on the electrochemical behavior of the stainless steel rebars, in terms of both uniform and localized corrosion resistance. However, this trend is reversed at 25 wt.% FA. The reasons for such trends are explored via microstructural examinations of the steels after polarization and XRD analysis of fly ash, as well as reinforced concrete containing fly ash.

The corrosion of steel reinforcement is the most significant factor responsible for the premature deterioration of the durability and seismic resistance of reinforced concrete structures. As such, the main objective of the present study is the investigation of the effect of fly ash (FA) as a corrosion inhibitor and pH on the electrochemical behavior of 316L and 304L concrete reinforcement in a simulating concrete pore solution exposed to aggressive environments, i.e., acid rain. The corrosion performance of 316L and 304L stainless steel rebars was examined by means of cyclic (reverse) polarization in order to evaluate the susceptibility of the rebars to localized corrosion. Two types of electrolyte were used. The first electrolyte was a highly alkaline solution simulating fresh concrete exposed to acid rain (pH ≈ 12), while the second electrolyte was a mildly alkaline solution simulating corroded concrete cover that exposed the reinforcement to direct acid rain attack (pH ≈ 8). Both solutions contained Ca(OH)2, an acid-rain-simulating solution, and FA (replacing 0–25 wt.% of Ca(OH)2) as a corrosion inhibitor. In both electrolytes, the beneficial effect of FA replacing up to 20 wt.% of Ca(OH)2 on the corrosion resistance of both 316L and 304L rebars was manifested. However, this trend was reversed at the 25 wt.% replacement of Ca(OH)2 due to the localized presence of agglomerates of FA on the surface of the steel. The above finding was confirmed by SEM/EDX examination of cross-sections after cyclic polarization. An important conclusion of this study was the feasibility of replacing 316L stainless steel with 304L (at pH ≈ 12) in critical applications, such as the restoration of ancient monuments, provided that FA is included in the concrete mixture, even at low contents (10 or 15 wt.% of cement).