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| S. no. | Inhibitors used | Active constituents | Inhibition efficiency (%) | Remarks |
| (1) | Lawsonia |  | 95.0 | The aqueous extract of the leaves of henna (lawsonia) as the corrosion inhibitor was reported in C steel, nickel and zinc in acidic, neutral and alkaline solutions, using the polarization technique [1] |
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| (2) | Fenugreek |  | 92.2 | The temperature effects were investigated on mild steel corrosion in 2.0 M of HCl and H2SO4 in the absence and presence of aqueous extract of fenugreek leaves (AEFLs) with the help of gravimetric method [2] |
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| (3) | Olea europaea |  | 93.0 | The inhibitive action of the aqueous extract of olive leaves was reported towards the corrosion of C-steel in 2 M HCl solution using weight loss measurements, Tafel polarization, and cyclic voltammetry [3] |
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| (4) | Cotula cinerea, Retama retam, and Artemisia herba | Anagyrine, cytisine | 67.0 | Plant extracts were investigated on the corrosion of X52 mild steel in aqueous 20% (2.3 M) sulphuric acid. Weight loss determinations and electrochemical measurements were also performed [4] |
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| (5) | Eclipta alba |  | 99.6 | The inhibition effect of Eclipta alba in 1 N hydrochloric acid on corrosion of mild steel was investigated by weight loss, potentiodynamic polarization, and impedance methods, and the extracts of Eclipta alba were found to be effective corrosion pickling inhibitor [5] |
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| (6) | Rauvolfia serpentina | Reserpine, ajmalicine, ajmaline, isoajmaline, ajmalinine, chandrine | 94.0 | Rauvolfia serpentina was tested as the corrosion inhibitor for mild steel in 1 M HCl and H2SO4 using weight loss method at three different temperatures, namely, 303, 313, and 323 K. Potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning electron microscope (SEM) studies were also performed [4] |
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| (7) | Lupinus albus |  | 86.5 | The behaviour of the inhibitive effect of lupine (Lupinus albus L.) extract on the corrosion of steel in aqueous solution of 1 M sulphuric, and 2 M hydrochloric acid was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques [6] |
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| (8) | Solanum tuberosum |  | 91.3 | The acid extracts of Solanum tuberosum were studied as the corrosion inhibitor for mild steel in 1 M HCl and H2SO4 medium using different techniques. It was found to be a good corrosion inhibitor [7] |
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| (9) | Nauclea latifolia | Monoterpene, triterpene indole alkaloid, saponins | 76.0 | The inhibitive action of ethanol extracts from leaves (LV), bark (BK), and roots (RT) of Nauclea latifolia on mild steel corrosion in H2SO4 solutions at 30° and 60°C was studied using weight loss and gasometric techniques [8] |
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| (10) | Sida rhombifolia |  | 97.4 | The efficacy of an acid extracts of leaves of Sida rhombifolia L. as the corrosion inhibitor for mild steel in 1 M phosphoric acid medium using weight loss measurements, polarization, and electrochemical impedance spectral studies were investigated. It was found to be an effective corrosion inhibitor [9] |
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| (11) | Ammi visnaga |  | 99.3 | The inhibitive effect of the extract of Khillah (Ammi visnaga) seeds, on the corrosion of SX 316 steel in HCl solution using weight loss measurements as well as potentiostatic technique, was assessed. Negative values were calculated for the energy of adsorption indicating the spontaneity of the adsorption process [10] |
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| (12) | Embilica uflicianalis, Terminalia chebula and Terminalia bellirica | Emblicanin A&B, puniglucanin, pedunculagin, tannic acid, chebulinic acid, and gallic acid | 80% | Extracts were used in 5% (w/v) commercial hydrochloric acid as corrosion inhibitors of mild steel exposed into 5% (w/v) hydrochloric acid at 328 K on mild steel. Both Tafel polarization and linear polarization resistance techniques were used. Remarkable decrease in corrosion current and increase in linear polarization resistance values were observed in the presence of the acid extracts [11] |
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| (13) | Carica papaya and Azadirachta indica | Papain, carpaine, chymopapain, azadirachtin, salannin, gedunin, and azadirone | 87% | Extracts were used as corrosion inhibitors for corrosion of mild steel. The percentage inhibition of efficiency was found to increase with the increase in concentration of both inhibitors [12] |
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| (14) | Mentha pulegium | Pulegone | 80% | Natural oil extracted from pennyroyal mint (Mentha pulegium, PM) was evaluated as the corrosion inhibitor of steel in molar hydrochloric using weight loss measurements, electrochemical polarisation, and EIS methods. PM oil acted as an efficient cathodic inhibitor [13] |
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| (15) | Zanthoxylum alatum | Terpineol, isoxazolidine, and imidazolinedione | 85% | The inhibition effect of Zanthoxylum alatum plant extracts on the corrosion of mild steel in 5% and 15% aqueous hydrochloric acid solution was investigated by weight loss and electrochemical impedance spectroscopy (EIS) methods. The effect of temperature on the corrosion behaviour of mild steel in 5% and 15% HCl with the addition of plant extracts was studied in the temperature range 50–80°C. Surface analysis (SEM, XPS and FT-IR) was also carried out to establish the corrosion inhibitive property of this plant extract in HCl solution [14] |
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| (16) | Thyme, Coriander, Hibiscus, Anis, Black Cumin and Garden Cress. | Thymol, malic acid, salicin, glutamic acid, leucine, and methionine | 85% | Electrochemical impedance spectroscopy has been successfully used to evaluate the performance of these compounds. The ac measurements showed that the dissolution process is activation controlled. Potentiodynamic polarization curves indicate that the studied compounds are mixed-type inhibitors. Thyme, which contained the powerful antiseptic thymol as the active ingredient, offers excellent protection for steel surface [15] |
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| (17) | Phoenix dactylifera, Lawsonia inermis, and Zea mays | Lawsone, esculetin, fraxetin, allantoin, sterols, and hordenine | 90% | Extracts were used as corrosion inhibitors for steel, aluminum, copper, and brass in acid chloride and sodium hydroxide solutions using weight loss, solution analysis, and potential measurements. Only, Phoenix dactylifera, Lawsonia inermis extracts were found highly effective in reducing corrosion rate of steel in acid chloride solutions and aluminum in sodium hydroxide solutions [16] |
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| (18) | Datura metel | Scopolamine, b-sitosterol, daturadiol, tropine, and daturilin | 86% | Acid extract of the D. metel was studied for its corrosion inhibitive effect by electrochemical and weight loss methods. The results of AC impedance and polarisation studies correlate well with the weight loss studies [17] |
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| (19) | Ricinus communis | Ricinoleic or ricinic acid, ricinolein, and palmitin | 84% | The corrosion behaviour of plant extract (Ricinus communis) was studied by means of electrochemical polarization, and impedance measurements. Results of study from polarization and electrochemical impedance measurements indicated that Ricinus communis might alleviate the corrosion process in mild steel [18] |
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| (20) | Mentha pulegium | Pugelone, alpha-pinene, limonene, methone, and piperitone | 80% | Mentha was used as the corrosion inhibitor of steel in molar hydrochloric using weight loss measurements, electrochemical polarisation and EIS methods. The increase in temperature leads to an increase in the inhibition efficiency of the natural substance [19] |
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| (21) | Carica papaya | Chymopapain, pectin, carposide, carpaine, pseudocarpaine, dehydrocarpines, carotenoids, cryptoglavine, cis-violaxanthin, and antheraxanthin. | 92% | Acid extracts of the different parts of Carica papaya were used as inhibitors in various corrosion tests. Gravimetric and gasometric techniques were used to characterize the mechanism of inhibition [20] |
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| (22) | Acacia seyal | Catechu, dimethyltryptamine (DMT) | 95% | The inhibitive effect of the gum exudate from Acacia seyal var. seyal was studied on the corrosion of mild steel in drinking water using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The corrosion rates of steel and inhibition efficiencies of the gum exudates obtained from impedance and polarization measurements were in good agreement [21] |
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| (23) | Calotropis procera | a-and b-Amyrins, cyanidin-3-rhamnoglucoside, cycloart-23-en-3b, 25-diol, cyclosadol | 89% | Extract of the C. procera was studied for its corrosion inhibitive effect by weight loss, electrochemical, SEM, and UV methods. Using weight loss measurement data, mechanism of inhibitive action is probed by fitting in the adsorption isotherm [22] |
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| (24) | Centella asiatica | Centellin, asiaticin, and centellicin | 86% | Centella asiatica was studied as the corrosion inhibitor on mild steel in 1 N hydrochloric acid by weight loss method, gasometric method, potentiodynamic polarization method and AC impedance method [23] |
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| (25) | Allium sativum, Juglans regia and Pogostemon cablin | Allyl cysteine sulfoxide, methyl allyl thiosulfinate, allicin, diallyl disulfide, diallyl trisulfide, ajoene, pogostone, friedelin, epifriedelinol, pachypodol, retusine, and oleanolic acid | 94% | Plant extracts on the corrosion of steel in aqueous solution of I N sulphuric acid were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques [24] |
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| (26) | Combretum bracteosum | Tannic acid | 83% | Mature leaves of Combretum bracteosum were used for the corrosion inhibition of mild steel in H2SO4. Inhibition efficiency increases with the plant extracts concentration and decreases with temperature [25] |
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| (27) | Phyllanthus amarus | Alkaloids, flavonoids, geraniin, hypophyllanthin, and phyllanthin | | The inhibitive action of leaves (LV), seeds (SD), and a combination of leaves and seeds (LVSD) extracts of Phyllanthus amarus on mild steel corrosion in HCl and H2SO4 solutions was studied using weight loss and gasometric techniques. The results indicated that the extracts functioned as a good inhibitor in both environments and inhibition efficiency increased with extracts concentration. Temperature studies revealed an increase in inhibition efficiency with the rise in temperature, and activation energies decreased in the presence of the extract [26] |
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| (28) | Azadirachta indica | azadirachtin, azadirone, gedunin, nimbin, nimbandiol, nimbinene, nimbolide, nimonol, nimbolin, salannin,margolone, melianol, vilasanin, and flavanoids | 80% | The inhibitive action of leaves (LV), root (RT), and seeds (SD) extracts of Azadirachta indica on mildsteel corrosion in H2SO4 solutions was studied using weight loss and gasometric techniques. The results obtained indicate that the extracts functioned as good inhibitors in H2SO4 solutions. Inhibition efficiency was found to increase with extracts concentration and temperature and followed the trend: SD > RT > LV. A mechanism of chemical adsorption of the phytochemical components of the plant extracts on the surface of the metal is proposed for the inhibition behaviour. The experimental data fitted into the Freundlich adsorption isotherm [27] |
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| (29) | Musa sapientum and banana peels | Gallocatechin and dopamine | 71% | The inhibition of the corrosion of mild steel by ethanol extract of Musa sapientum peels in H2SO4 was studied using gasometric and thermometric methods. The results of the study reveal that the different concentrations of ethanol extract of M. sapientum peels inhibit mild steel corrosion [28] |
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| (30) | Murraya koenigii | | 80% | The inhibitive action of extract of curry leaves (Murraya koenigii) on carbon steel in 1N HCl was studied using weight loss, gasometric studies electrochemical polarization, and AC impedance measurements [29] |
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| (31) | Medicago Sativa | biotin, cytidine, inosine, guanine, guanosine, and riboflavin | 90% | The inhibitive effect of water and alcoholic extracts of Medicago Sativa (MS) on the corrosion of steel in 2.0 M H2SO4 containing 10% EtOH has been studied using chemical (weight loss (ML), hydrogen evolution (HE)), electrochemical (potentiodynamic polarization (PDP) and impedance spectroscopy (EIS)) techniques [30] |
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| (32) | Oxandra asbeckii | Liriodenine, azafluorenones alkaloids | 86% | The inhibition effect of alkaloids extract from Oxandra asbeckii plant (OAPE) on the corrosion of C38 steel in 1 M hydrochloric acid solution was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The corrosion inhibition efficiency increases on increasing plant extracts concentration. Cathodic and anodic polarization curves showed that OAPE is a mixed-type inhibitor [31] |
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| (33) | Adhatoda vasica, Eclipta alba, and Centella asiatica | Vasicine, vasicinone, asiaticoside, wedelolactone, β-sitosterol, and stigmasterol | 99% | The inhibitive action of the extracts of Adhatoda vasica, Eclipta alba, and Centella asiatica on the corrosion of mild steel in 1N HCl was studied using weight loss method, electrochemical methods, and hydrogen permeation method. Polarization method indicated that the plant extracts are under mixed control, that is, promoting retardation of both anodic and cathodic reactions [32] |
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| (34) | Ocimum sanctum, Aegle marmelos, and Solanum trilobatum | | 99% | A comparative study of the inhibitory effect of plant extracts, Ocimum sanctum, Aegle marmelos, and Solanum trilobatum, on the Corrosion of mild steel in 1 N HCl medium was investigated using weight loss method, electrochemical methods, and hydrogen permeation method. Polarization method indicated that plant extracts behaved as mixed-type inhibitor [33] |
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| (35) | Anna squamosa | Liriodenine and oxoanalobine | 84% | Alkaloids extract from Annona squamosa plant has been studied as possible corrosion inhibitor for C38 steel in molar hydrochloric acid (1 M HCl). Potentiodynamic polarization and AC impedance methods have been used. The corrosion inhibition efficiency increases on increasing plant extract concentration [34] |
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| (36) | Heinsia crinita | | | The paper provides information on the use of ethanol extract of Heinsia crinita as a corrosion inhibitor. Electrochemical studies such as polarisation and AC impedance spectra will throw more light on the mechanistic aspects of the corrosion inhibition [35] |
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| (37) | Dacryodis edulis | | | The inhibition of low-carbon-steel corrosion in 1 M HCl and 0.5 M H2SO4 by extracts of Dacryodis edulis (DE) was investigated using gravimetric and electrochemical techniques. DE extract was found to inhibit the uniform and localized corrosion of carbon steel in the acidic media, affecting both the cathodic and anodic partial reactions [36] |
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| (38) | Emblica officinalis | | 87% | Corrosion inhibition efficiency of acid extract of dry Emblica officinalis leaves for mild steel in 1 N HCl medium was investigated. Experimental methods include weight loss, potentiodynamic polarization, and impedance studies [37] |
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| (39) | Cyamopsis tetragonoloba | 3-epikatonic acid 7-o-beta-(2-rhamnosyl-glucosyl) myricetin, ash, astragalin, caffeic acid, and chlorogenic acid | 92% | The role of seed extract of Cyamopsis tetragonoloba on corrosion mitigation of mild steel in 1 M HCl was investigated by weight loss method and potentiodynamic polarization technique. Experimental results were fitted into Langmuir and Temkin adsorption isotherm to study the process of inhibition [38] |
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