A Master of Science Thesis in Chemical Engineering submitted by Alaa Shanableh entitled, "Studies on Natural Extracts as Inhibitors of Mild Steel Corrosion in 1M HC1 Solution," submitted in January 2011. Available are both soft and hard copies of the thesis.
The inhibitive action of the aqueous plant leaves extracts and their mixtures toward the corrosion of mild steel in 1M HCl solution is investigated using different standard corrosion measurements. Inhibitors used are extracted from Figs, Olives, Rosemary and Cypress plants. Corrosion weight loss technique is applied on mild steel plates to evaluate inhibition efficiency in the presence of these plant extracts as corrosion inhibitors. Electrochemical techniques including Linear Polarization Resistance (LPR), Electrochemical Impedance Spectroscopy (EIS) and Cyclic Sweep are used in order to validate and support inhibition efficiencies from the weight loss technique and confirm the inhibition properties and mechanisms of the extracted plants. A detailed study of the experimental results is reported for each of the tests conducted. Moreover, the adsorption film isotherm is determined from the experimental data obtained and the kinetics are found to follow Langmuir isotherm. The results showed that the extracts serve as excellent corrosion inhibitors for the tested system. It was observed in all tests that the inhibition efficiency increases as the plant extract concentration increases. The electrochemical analysis also confirmed that the corrosion current density decreases as the concentration of the extract increases, causing a reduction in the corrosion rate of the mild steel specimen. It was also found that the inhibitors explored in this study act as mixed-type inhibitors. Moreover, Nyquist plots from EIS also proved that as the concentration of plant extracts increases, the charge transfer resistance increases and the double layer capacitance decreases. The inhibitive action of the plant extracts demonstrates that the adsorption of plant extracts is spontaneous, and the physical adsorption follows Langmuir adsorption isotherm. The inhibition efficiency of these plant extracts is explained thoroughly in this study. The evaluation looks at inhibition performances under different test conditions, especially the more relevant oil and gas production conditions. The inhibition of the plant extracts is studied at 25oC as well as elevated temperatures of 45oC and 55oC. The inhibition efficiency decreases as the temperature increases causing an increase in the corrosion rate of the mild steel sample. The decrease in the inhibition efficiency with the rise in temperature and activation energy, in presence and absence of inhibitor, suggests the formation of an adsorption film of a physical nature. The maximum inhibition efficiency was obtained when using pure fig inhibitor extract. However, the result changes for conditions of elevated temperatures. A comparison is made between some commercial inhibitors and the studied plant extracts. Electrochemical analysis is applied at 25oC and at concentrations of 400 and 1000 ppm in 1M HCl solution. The study showed that the inhibition efficiencies of commercial inhibitors are comparable to the one's obtained by natural inhibitors. This study demonstrates high inhibitive action of the plant extracts used. All tests performed proved similar trends in inhibitive action of the extracts. An insight about the formation of a protective film on the mild steel specimen is clarified by electrochemical analysis. The film formation caused by adsorption causes the reduction of the corrosion rate in such media.