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Chemical (HE and WL) and electrochemical (PDP and EIS) measurements were applied to evaluate the potential of aqueous extract of Hibiscus sabdariffa leaves (AEHSL) for inhibiting the corrosion of Al in 0.5 M NaOH. It was found that the inhibition efficiency increases with the increase of AEHSL concentration. Electrochemical measurements revealed that AEHSL acts as a mixedtype inhibitor with inhibition category that belongs to geometric blocking. Adsorption of inhibitor species was found to follow Langmuir and Dubinin-Radushkevich isotherm models and the ability of AEHSL species to be adsorbed physically on the Al surface was illustrated by Dubinin-Radushkevich isotherm parameters. The data obtained from chemical and electrochemical measurements are in reasonably good agreement. Physical adsorption mechanism of AEHSl species on Al surface in 0.5 M NaOH becomes clear cut by following the trend of inhibitor adsorption with solution temperature. Good correlation between AEHSL water-soluble constituents and the suggested physical adsorption mechanism was obtained. Moreover, at a certain concentration of AEHSL (1.00 g L -1 ), the Al surface coverage increases with the increase of NaOH concentration up to 0.5 M after which limited decrease was obtained with further increase in NaOH concentration.

Five selected S-alkylisothisothiouronium iodides have been studied as acid corrosion inhibitors at 30°C for steel in 0.5 M H 2 SO 4 using gasometry, mass loss, and direct current (DC) polarization techniques. All of the data reveal that the compounds act as inhibitors in the acid environments; furthermore, polarization curves show that the compounds act as mixed-type inhibitors. It was found that the inhibition efficiency increases with the increase of the length of the additive alkyl chain. Langmuir's adsorption isotherms fit the experimental data for the studied compounds. Thermodynamic parameters were obtained from experimental data of the temperature studies of the inhibition process at five temperatures ranging from 30 to 70°C. It was observed that the activation energy is slightly increased with the increase of the additive alkyl chain. On the other hand, the sudden large increase of the inhibition behavior of Shexylisothiouronium iodide was attributed to a different adsorption process.

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The inhibitory action of Red onion seeds and peels extracts (ROSE & ROPE) was testing and comparing on the corrosion of steel in 0.75 M H 3 PO 4 using chemical measurements (hydrogen evolution, HE and mass loss, ML) and SEM technique. The effect of temperature on the corrosion of steel in 0.75 M H 3 PO 4 without and with certain concentration of each extract was studied in the temperature range of 303-333 K. Generally, the inhibition efficiency (i.e. surface coverage) increases with extracts concentration and fits the Langmuir adsorption isotherm. The results of HE, ML and SEM showed that ROSE has a capability to protect steel surface in 0.75 M H 3 PO 4 more than ROPE. Good agreement between HE and ML measurements was obtained. Temperature coefficients without and with each extract revealed somewhat chemisorptive and physiosorptive behavior of ROSE and ROPE, respectively. Good correlation between the major effective constituents of each extract and the inhibition mechanism was obtained.