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Pyoverdine, an yellowish-green, water-soluble fluorescent siderophore, was predominantly produced by the species of Pseudomonas, P. aeruginosa, is used as a corrosion inhibitor for mild steel in 0.5 M HCl, and its efficiency was studied by gravimetric method, electrochemical impedance spectroscopy and polarization techniques at temperatures of 303, 308 and 313 K. Pyoverdine being a strong iron chelator shows excellent inhibition efficiency at all these temperature regions. The adsorption of pyoverdine on mild steel surface increases the charge transfer resistance and activation energy of metal corrosion. 97.55 % of protection efficiency was reported for 10 ppm inhibitor at 303 K; the bacterial biofilm can protect the metal even at 308 and 313 K, and efficiency reduced to only 73 and 68 %. Pyoverdine inhibited both anodic and cathodic reactions and hence worked as a mixed type inhibitor. The results of electrochemical studies also support this observation. The surface morphology of mild steel was examined with scanning electron microscopy and atomic force microscopy. The inhibitor got adsorbed on the metal surface, and the process of adsorption follows Langmuir and Temkin isotherms.

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The corrosion inhibition behavior of Ruta graveolens extract (RGE) for mild steel in 1 M HCl is studied using gravimetric and electrochemical techniques. 1-4 V/V% RGE is added to the corrosive media and analyzed. Gravimetric measurements conducted at different time intervals reveal the potential activity of RGE in the system investigated. RGE exhibits 95 % inhibition efficiency in the investigated system for 7 days of exposure time. Electrochemical impedance spectroscopic studies show that mild steel is protected 98 % from corrosion in 1 M HCl in the presence of RGE. Potentiodynamic polarization study agrees this observation showing 99 % inhibition efficiency and also reveals the mixed type inhibition behavior of RGE. One of the active constituents of Ruta graveolens, Rutin is also monitored for its inhibition behavior and is found to be active. The theoretical parameters obtained for rutin also stands for the good inhibition potential of the extract. Both RGE and Rutin show a decrease in inhibition efficiency as the temperature raised. Adsorption behavior of RGE is according to Langmuir isotherm and that of Rutin obeys Temkin isotherm.

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