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Reinforced concrete deterioration due to acidification of the environment from microbial activity in view of steel performance is seldom reported and still a debate. An initial scrutiny of several inhibitors indicated methylene blue dye and trisodium-phosphate as the most promising candidates for mild steel protection in diluted H2SO4. Such compounds were combined together into two organic/inorganic hybrids composed of hydroxyapatite (HAP) or vaterite porous matrixes impregnated with methylene blue dye. The novel hybrid systems were characterized by means of scanning electron microscopy, X-ray diffraction, and Brunauer–Emmett–Teller analysis. The electrochemical response of steel specimens in a simulated environment containing loaded and empty HAP host was monitored by means of linear polarization resistance and electrochemical impedance spectroscopy. The results confirmed the inhibitive properties of the chosen compounds in acidic medium, pointing out a synergistic effect resulting from the release of the organic compound and the dissolution of the inorganic matrix.

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Although much less investigated than that induced by chlorides, the corrosion of steel reinforcing bars due to bacteria metabolic products is recognised as a serious issue, primarily for concrete pipes in sewer network. In order to overcome the complications due to the preparation of concrete samples the investigation was performed using simulating solutions. The passivation of the metallic specimens was obtained by immersion in Ca (OH)2 sat and monitored through several different electrochemical techniques. The depassivation was induced by either sulphuric acid or sulphides in order to simulate different bacterial metabolites. Anodic polarization curves and optical microscopy examination revealed a significant corrosion in the former case, while a competitive interaction among the sulphide and the alkalinity was hypothesized in sulphide-containing solutions.