phytochemicals as green corrosion inhibitors in various corrosive

PHYTOCHEMICALS AS GREEN CORROSION INHIBITORS IN

VARIOUS CORROSIVE MEDIA: A REVIEW

J BuchweishaijaChemistry Department, College of Natural and Applied Sciences, University of Dar es Salaam

P.O. Box 35061, Dar es Salaam, Tanzania.

ABSTRACTThere is an intensive effort underway to develop new plant origin corrosion inhibitors for metalsubjected to various environmental conditions. These efforts have been motivated by the desire toreplace toxic inhibitors used for mitigation of corrosion of various metals and alloys in aqueoussolutions. Plants represent a class of interesting source of compounds currently being exploredfor use in metal corrosion protection in most systems, as possible replacement of toxic syntheticinhibitors. In this review article, research results on the use of eco-friendly phytochemicals ascorrosion inhibitors have been summarized. A general introduction to the topic of corrosionmitigation by inhibitors is presented followed by extensive literature survey on the use of naturalinhibitors for corrosion control of metals and alloys in different corrosive media.

INTRODUCTIONMetallic materials are still the most widelyused group of materials particularly in bothmechanical engineering a n d thetransportation industry. In addition, metalsare commonly used in electronics andincreasingly also in the constructionindustry. However the usefulness of metalsand alloys is constrained by one commonproblem known as corrosion. Corrosion is anaturally occurring phenomenon commonlydefined as deterioration of metal surfacescaused by the reaction with the surroundingenvironmental conditions. Corrosion cancause disastrous damage to metal and alloystructures causing economic consequences interms of repair, replacement, product losses,safety and environmental pollution. Due tothese harmful effects, corrosion is anundesirable phenomenon that ought to beprevented.

There are several ways of preventingcorrosion and the rates at which it canpropagate with a view of improving thelifetime of metallic and alloy materials. Theuse of inhibitors for the control of corrosionof metals and alloys which are in contactwith aggressive environment is one amongthe acceptable practices used to reduce and/orprevent corrosion. A corrosion inhibitor is a

substance which when added in smallconcentration to an environment, effectivelyreduces the corrosion rate of a metal exposedto that environment. Corrosion inhibitorscan be divided into two broad categoriesnamely those that enhance the formation of aprotective oxide film through an oxidizingeffect and those that inhibit corrosion byselectively adsorbing on the metal surfaceand creating a barrier that prevents access ofcorrosive agents to the metal surface. Largenumbers of organic compounds have beenstudied and analysed to investigate theirpotential as corrosion inhibitors (Riggs1973, Raja and Sethuraman 2008). Most ofthese studies reveal that almost all organicmolecules containing heteroatoms such asnitrogen, sulphur, phosphorous and oxygenshow significant inhibition efficiency (Riggs1973, Buchweishaija 1997, Raja andSethuraman 2008). Despite of thesepromising findings about possible corrosioninhibitors, most of these substances are notonly expensive but also toxic (Raja andSethuraman 2008) non-biodegradable thuscausing pollution problems. Hence, thesedeficiencies have prompted the search fortheir replacement.

Plants have been recognized as sources ofnaturally occurring compounds, some with

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rather complex molecular structures andhaving varying of physical, chemical andbiological properties (Farooqi et al . 1997,Mukherjee et al. 1997, Philip 2001,Mutasingwa 2004, Magufuli 2009). Most ofthe compounds extracted from plants areenjoying the use in traditional applicationssuch as pharmaceuticals and biofuels(Nkunya 2002). Furthermore, the use ofnaturally occurring compounds are ofinterest, because of their cost effectiveness,abundant availability and more importantlytheir environmentally acceptability. Due tothese advantages, extracts of some commonplants and plant products have been tried ascorrosion inhibitors for metals and alloysunder different environment (Abdel-Gaber etal. 2006, Ebenso and Ekpe 1996, Ebenso etal. 2004, Ekpe et al. 1994, Kliskic et al.2000). The above attributes have madeplants become an important source of a widerange of eco-friendly (green) corrosioninhibitors.

In this review paper, it is intended tohighlight the work conducted in the searchfor alternative metal corrosion protectionusing plant resources. The paper thereforesummarizes some work on the research andthe use of green inhibitors reported inliterature as part of contribution to thegrowing interest of exploring greencorrosion inhibitors.

Metal Corrosion Inhibitors of PlantOriginThe use of phytochemicals as corrosioninhibitors can be traced back to 1960’s whentannins and their derivatives were used toprotect corrosion of steel, iron and othertools. In 1972, El Hosary et al. reported theuse of common plant extracts as corrosioninhibitors. Plant extracts of tobacco fromstems, twigs as well as leaves have beenreported to show significant corrosioninhibition of aluminum and steel in bothsaline solutions and strong pickling acids(Davis et al. 2001). Extracts from leaveswere investigated and found to be effectivecorrosion inhibitors for mild steel in 2 M

HCl solutions. The authors reportedmaximum inhibition efficiency of 96% withonly 0.01% tobacco concentration (100ppm). Tobacco extracts are reported tocontain high concentrations of chemicalcompounds including terpenes, alcohols,polyphenols, carboxylic acids, nitrogencontaining compounds and alkaloids thatmay exhibit electrochemical activity such ascorrosion inhibition (Davis et a l . 2001).Some of their results are shown in Figure 1.

In 1981, Srivastava and Srivastavainvestigated the inhibition effect of tobacco,black pepper, castor seed, Acacia gum andlignin and found to be good corrosioninhibitors for steel in acidic media. Tobacco,lignin and black pepper were also found tobe effective inhibitors for aluminium in acidmedium. Further research on castor seed,black pepper and lignin on carbon steelcorrosion in 5% HCl solution obtained 60 –70% inhibition efficiency (Quraishi 2004).

Saleh and his co-workers (Saleh et al. 1982)carried out an intensive study on theinhibition effect of aqueous extract ofOpuntia ficus indica, Aloe eru leaves andpeels of orange, mango and pomegranatefruits on the corrosion of steel, aluminium,zinc and copper in both HCl and H2SO4 acidsolutions using gravimetric and polarizationmeasurement techniques. From theirinvestigations, it was reported that themango peel extract was the most effectivecorrosion inhibitor for Al and Zn whereaspomegranate fruit shells extract was mostsuitable for Cu. It was further reported that,all the extracts were more efficientlycorrosion inhibitors in HCl solution ascompared to H2SO4 solution.

Pravinar et al. (1993) reported the inhibitiveeffects of aqueous extract of Eucalyptusleaves on the corrosion of mild steel andcopper in 1 M HCl solution. The inhibitionefficiency has been investigated bygalvanostatic polarization, mass lossmeasurements and surface characterizationtechniques. The inhibition efficiency was

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found to increase with increase inconcentration of extract and decrease withincrease in temperature. The extract wasfound to be a mixed type inhibitor (i.e.

inhibits both cathodic and anodic reactions)predominantly of cathodic control.

Figure 1: Inhibition efficiency of tobacco extracts for steel/Cu, Al/Cu, and steel/Al galvaniccouples in 3.5% NaCl solution as measured by a zero resistance ammeter (ZRA)(Davis et al. 2001)

The inhibitive effects of pomegranatealkaloids on acid corrosion of mild steel inH2SO4 were also investigated by AymenHussein and Singh (1991) usinggalvanostatic polarization and mass lossmeasurements at different temperatures. Itwas found that pomegranate alkaloids have agood efficiency at low temperatures. Theyexplained the observed efficiency to be dueto the metal additive complex formation.

El-Etre (2003) has investigated inhibiting ofaluminum corrosion using Opuntia extract.The inhibitive action of the extracts towardacid corrosion of aluminum was tested usingmass loss, thermometry, hydrogen evolution

and polarization techniques. It was foundthat the extract acts as a good corrosioninhibitor for aluminum corrosion in 2 MHCl solution. The inhibition efficiency wasreported to increase as the extractconcentration increases. The inhibitionaction of the extract was found to followLangmuir adsorption isotherm. The authoralso reported that the Opuntia extractprovides a good protection to aluminumagainst pitting corrosion in chloride ioncontaining solutions.

Leguminous seeds which are rich source ofamino acids have also been studied for theircorrosion inhibition potential. Subhashini


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(2004) studied the inhibition effect of theseeds extracts of Alfa alfa (Aa),Adenanthera pavonina (A p ), Phaseoluslunatus (Pl), Psophocarpus tetragonolobus(Pt) and Sesbania grandiflora (Sg). Theseed extracts were tested as corrosioninhibitors of mild steel in 1 M HCl and 0.5M H2SO4 with various immersion time andconcentrations. Inhibition performances wereassessed using mass loss, polarization andelectrochemical impedance spectroscopy.The surfaces of mild steel tested wereanalyzed using Fourier transform infraredspectroscopy and optical microscope. Theresults clearly indicated the decrease incorrosion rate with increase in concentrationof the extract as well as with increase inimmersion time. All the extractsinvestigated have shown maximumefficiency at 0.7% extract concentration. Thecorrosion inhibition performance of theextracts decreased in the following order:S g > A a > P t > A p > P l in H C l andPt>Aa>Pl>Sg>Aa in H2SO4. However,these extracts showed a better inhibitionperformance in HCl than in H2SO4. Thiswas explained to be due to the adsorption ofchloride ions on the metal surface thansulphate ions preferential.

The aqueous extract of the leaves of Henna(Lawsonia) has been tested as corrosioninhibitor of carbon steel, nickel, and zinc inacidic, neutral and alkaline solutions, usingthe polarization technique (El-Etre et al.2005). Lawsonia is cultivated in Africa andAsia for medicinal and dyeing purposes (El-Etre et al. 2005). It was found that theextract acts as a good corrosion inhibitor forthe three metals in all tested media. Theauthors (El-Etre et al. 2005) postulated thatthe degree of inhibition depended on thenature of metal and the type of the medium.For steel and nickel, the inhibitionefficiency increased in the order: alkaline <neutral < acidic, while in case of zinc, itincreased in the order: acid < alkaline <

neutral, thereby reconciling with theobserved concept of the Lawsonia extractbeing a mixed inhibitor. The inhibitiveaction of the extract was discussed in viewof adsorption of the complex Lawsoniamolecules onto the metal surface. El-Etre etal. 2005 found that this adsorption followedLangmuir adsorption isotherm in all testedsystems. They also proposed that theformation of a complex between the metalcations and Lawsone (2-hydroxy-1,4-naphthoquinone) was an additionalinhibition mechanism of steel and nickelcorrosion.

In 2006, El-Etre and El-Tantawy reportedthe inhibitive action of Ficus extract towardsgeneral and pitting corrosion of carbon steel,nickel and zinc in different aqueous media.The study was performed using weight lossmeasurements, potentiostat ic andpotentiodynamic polarization techniques. Itwas found that the presence of Ficus extractin the corrosive media (acidic, neutral oralkaline) decreased the corrosion rates of thethree tested metals significantly. The extractwas reported to contain friedelin,epifriedelanol, nitidol (a triterpene, C10H50O)and a mixture of two sterols (Figure 2).

The extract of H e n n a leaves asenvironmentally friendly corrosioninhibitors of metals was also investigated byAl-Sehaibani 2000. The water extracts ofHenna, Lawsonia inermis leaves powderwere evaluated as corrosion inhibitor forsteel and commercial aluminium in saline,acid and alkaline water. The maximumefficiency was attained by just 20 g/L of theextract. The inhibition efficiency of mildsteel corrosion in HCl by the extract was96% and that of aluminium in NaOH wasup to 99.8% and observed no inhibition forsteel and aluminium in NaCl solution.Some of the results are shown in Figure 3.


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Figure 2: Chemical structures of some compounds contained in Ficus extract (El-Etre and El-Tantawy 2006).

Figure 3:(a): Inhibition percentage of corrosion of steel 37 in 0.1 M HCl solutions in thepresence of extracts of various brands of henna (Al-Sehaibani 2000).


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Figure 3:(b): Inhibition percentage of corrosion of aluminium in NaOH solutions in the presenceof extracts of various brands of Henna (Al-Sehaibani 2000)

Chaieb et al. 2005, investigated the effect ofeugenol and its derivative (acetyleugenol)extracted from the nail of giroflier (Eugeniacaryophyllata) on the corrosion inhibition ofsteel in 1 M HCl solution. They used theweight loss measurements, electrochemical

polarization and electrochemical impedancespectroscopy methods. Eugenol andacetyleugenol belong to a class ofcompounds called vanilloids and theirchemical structures are shown in Figure 4.

OH

OCH3

CH2CH CH2

OCOCH3

OCH3

CH2CH CH2

(a) (b)

Figure 4: Molecular structures of chemicals extracted from Eugenia caryophyllata: (a) Eugenoland (b) Acetyleugenol (Chaieb et al. 2005).

These compounds are known to have someantioxidant properties and diets containingthese antioxidants can reduce the risk ofdiseases such as cancer and possibly malaria,AIDS and ageing effects (Chaieb et al.2005). It was observed that the extractsreduce the corrosion rate of steel in 1 M HClsignificantly. Their inhibition efficiencies

were found to increase with eugenol andacetyleugenol extract concentrations andattained 80 and 91% at concentration of0.173 g/L, respectively. This implies thatacetyleugenol is more active to the surface ascompared to eugenol due to the presence ofthe carbonyl group.


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Similarly, the effect of temperature was alsoinvestigated at the maximum concentrationof both eugenol and acetyleugenol at 0.173g/L. The results indicated that, as thetemperature increases the inhibitorperformance also increases. The percentageinhibitor performance changed from 64% at298 K to 87% at 328 K. The adsorption ofthese extracts on the metal surface wasreported to follow the Langmuir adsorptionisotherm.

Some essentials oils have also beeninvestigated as green corrosion inhibitors.Good examples are Ginger, Henna, Jojobaoil and Artemisia oil, reported to have veryefficient corrosion inhibition for iron andsteel in acidic media (Chetouani et al. 2004,Bouyanzer and Hommouti 2004).

The effect of natural occurring extract ofArtemisia on the corrosion of steel in 0.5 MH2SO4 in the temperature range 298 – 353 Kwas studied using weight loss,electrochemical polarization and linear

polarization methods (Bouklah andHammouti 2006). The results obtainedreveal that the extract reduces the corrosionrate quite significantly. The inhibitionefficiency increases with the increase ofArtemisia content at 10g/L to reach 95% and99% at 298 and 353 K, respectively. Theinhibition efficiency increased withtemperature and it was found that theadsorption of Artemisia extract on the steelsurface follows Langmuir adsorptionisotherm. Similar results were reported onthe Artemisia oil on the corrosion of steel inHCl (Bendahou et al, 2006, Bouklah andHammouti 2006) and H3PO4 (Bouyanzer andHammouti 2004). Artemisia has received aconsiderable attention as a promising andpotent antimalarial drug and davanone(Figure 5) has been found to be its majorconstituent (Benjilali et al. 1982). Sincedavanone is a diketone compound, theinhibitory action may be interpreted by theformation of Fe(II)-davanone complex.

Figure 5: (a) Molecular structural of Artemisia extract – davonone, (b) Tautomeric equilibrium ofdavonone (Bouklah and Hammouti 2006).

Rehan (2003) conducted a research on theeffect of water extracts from dry leaves ofeconomic plants , date palm (Phoenixdectylifera), Henna (Lawsonia inermis) and

corn (Zea mays) on the corrosion inhibitionof commercial grade metals; steel,aluminum, and copper in acidic chloride andsodium hydroxide solution using weight


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loss, solution analysis and potentialmeasurements. The inhibition action wasfound to critically depend on the metal typeand solution composition. Only date palmand henna extract were found highlyeffective in reducing corrosion rate of steelin acidic chloride solution and aluminum insodium hydroxide solutions. The inhibitionefficiency increased with increasing theconcentration of the extract. The inhibitionwas interpreted in terms of chemisorption of

some active ingredients in the leavesaccording to Temkin isotherm. The resultsare summarized in Table 1.

The leaves of date palm and corn aregenerally by-products and used chiefly inBasketry and animal feeding, respectively.Henna leaves are used as hair dyestuff andused in shampoo industry due to thepleasant dermatological effect (Rehan 2003).

Table 1: Weight loss, $W, together with calculated inhibition performance, I%, fordifferent metals in the absence and presence of extracts from leaves of threeeconomic plants at 25

oC (Rehan 2003).

Metal $Wo

Henna Date Palm Corn$W I% $W I% $W I%

0.2 M HClSteel 5.40

(5.51)0.75

(0.81)86.1

(85.3)1.65

(1.21)78.7

(78.0)4.88

(4.99)9.6

(10.2)

Aluminium 1.32(1.14)

1.14(1.22)

*(*)

0.89(0.75)

32.6(34.2)

1.43(1.26)

*(*)

Copper 1.29(1.31)

0.88(0.92)

31.8(29.8)

1.01(1.07)

21.7(18.3)

1.07(1.10)

17.2(16.1)

Brass 0.89(1.03)

0.71(0.85)

19.1(17.5)

0.71(0.83)

20.2(19.4)

0.84(0.96)

5.8(7.1)

0.2 M KOHAluminium 31.70

(32.30)1.27

(1.31)96.0

(95.9)9.32

(10.1)70.6

(68.7)30.50

(31.21)3.8

(3.4)

* No inhibition; The figures in the blackest represents results from solution metal ion analysiswith atomic absorption

Over the years, our research groupinvestigated Cashew Nut Shell Liquid(CNSL), an extract from the Cashew NutShells of the cashew tree (Anacadiumaccidentale Linn) for its inhibitive effects asgreen inhibitor for corrosion of carbon steelin carbon dioxide media using gravimetric,potentiodynamic polarizat ion andelectrochemical impedance techniques(Mutasingwa 2004, Philip 2001 and Philipet al. 2001). The molecular structures of theCNSL constituents are shown in Figure 6.The major constituent of this oil depends onthe extraction method, whereby anacardic

acid and cardanol are major constituents forcold extraction and thermo extraction,respectively. The tests were performed bothunder static and dynamic conditions. It wasfound that CNSL reduces the extent ofelectrochemical processes taking place oncarbon steel undergoing corrosion. Theinhibition efficiency increased with theincrease of the extract concentration.Adsorption of CNSL was found to followthe Langmuir’s adsorption isotherm. The oilwas also found to function as an inhibitor ofmixed type acting by blocking the activesites on both cathodic and anodic regions.


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Figure 6: Chemical structures of the components of natural CNSL (cold extracted) (Philip etal. 2001).

Further investigation of CNSL greeninhibitor on the effect of temperature on itscorrosion protection performance was carriedout by Mutasingwa 2004. Temperatures upto 80

oC were studied using an optimal

concentration of the extract under dynamiccondition. The results showed that theinhibition performance decreases as thetemperature increases.

One among the crucial factors for thedetermination of the inhibition mechanismas well as the performance of the corrosioninhibitor is the solution pH. Most of theinhibitors are pH selective which depends onthe molecular structure of the inhibitor, themetal corroding, the active species present inthe solution and the composition of theinhibitor.

Philip et al. (Philip et al. 2002) investigatedfurther on the solution pH effects of CO2

saturated 3% NaCl solutions on theperformance of CNSL using technicalextracted CNSL at room temperature. Thesolution pHs investigated were 4.0, 5.5, and6.0. It was found that as the solution pHchanges from 4 to 6, a significant drop incurrent densities as well as anodic shifts ofthe open circuit potential was observed. Thissuggested a significant reduction ofcorrosion rate as the solution pH changes

from 4 to 6.0. This observation made theauthor to demonstrate that CNSL performsmore efficiently in solutions of higher pHs% 6.0. Further, Philip et al. investigated theinhibition mechanism of this extract oncarbon steel in CO2 saturated 3% NaClsolutions using weight loss, UV/VIS andelectrochemical techniques. It was found thatthe phenoxide ions from the CNSL werefound to be responsible for the inhibition ofcorrosion of mild steel in CO2 medium. Itwas also found that the surface charge ofcarbon steel is positive in solutioncontaining CNSL. The authors suggestedthe mechanism of CNSL inhibitoradsorption to involve electrostatic attractionbetween the positively charged metal surfaceand negatively charged phenoxide ions asschematically shown in Figure 7.

Bendahou et al. (2006) evaluated the effectof natural Rosemary oil as green inhibitor onthe corrosion of steel in H3PO4 media atvarious temperatures. Various techniqueswere used including gravimetric andelectrochemical methods to characterize thecorrosion mechanisms. Bendahou et al.(2006) demonstrated good agreementbetween the various methods explored forcorrosion inhibition analysis. Thepolarization measurements showed thatRosemary oil acted essentially as a cathodic


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inhibitor. The efficiency of corrosioninhibition of the oil increased with theconcentration of extract to attain 73% at

10g/L, but decreased with the rise oftemperature from 25 to 75

oC range.

Figure 7: The schematic diagram for the cardanol adsorption mechanism on carbon steelsurface (Philip et al. 2002).

The Hibiscus sabdariffa plant popularlyknown as “Roselle” which belongs to theMalvaceae family has basically been used invarious foods and medicines. Its extract, thecalyx of Hibiscus sabdariffa has been testedon the inhibition of mild steel corrosion in2 M HCl and 1 M H2SO4 solution bygasometric technique (Oguzie, 2008). Theresults obtained showed that the corrosioninhibition efficiency of the extract calyx atconcentration of 50% was 93% in 1 MH2SO4 and 90.4% in 2 M HCl. The authorobserved no effect on the inhibitionperformance as temperature changes in 1 MH2SO4.

Raja and Sethuraman (2008) studied thecorrosion inhibitive effect of the extract ofblack paper on mild steel in 1M H2SO4

solutions. The rate of corrosion attack on themetal surface was assessed by conventionalweight loss, Tafel polarization, impedancespectroscopy and Scanning Electronmicroscopy (SEM) techniques. Results ofthe weight loss study revealed that blackpepper extract acts as a good inhibitor even

at high temperatures. Polarization curvesrevealed the mixed mode of inhibition ofblack pepper extract. Analysis of impedancedata has been made with equivalent circuitwith constant phase angle element forcalculation of the double layer capacitancevalue. SEM studies provided theconfirmatory evidence for the protection ofmild steel by the inhibitor.

Abdel-Gaber et al. 2008 studied theinhibition of aluminum corrosion in 2 Msodium hydroxide solution in the presenceand absence of 0.5 M sodium chloride usingDamsissa (Ambrosia maritime, L ) extractemploying different chemical andelectrochemical techniques. Chemicalgasometry technique showed that addition ofchloride ions or Damsissa extract to sodiumhydroxide solution decreases the volume ofhydrogen gas evolved suggesting a decreasein the metal corrosion. Potentiodynamicresults manifested that chloride ion retardthe anodic dissolution of aluminum belowthe pitting potential in sodium hydroxidesolution. Damsissa extract in the presence or


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absence of chlorine ion, influenced both theanodic dissolution of aluminium and thegenerated hydrogen gas at the cathodeindicating that the extract behaves as amixed type inhibitor. The decrease in theobserved limiting current with increasingDamsissa extract concentration indicatedthat the anodic process is controlled bydiffusion. The impedance results alsoshowed that the Damsissa extract couldserve as an effective inhibitor for thecorrosion of aluminium in alkalinesolutions. The impedance measurementsverified the remarkable stability of theextracts during storage up to 35 days.Damsissa extract was found more effectivein the presence of chloride ions than in theabsence. Inhibition was found to increasewith increasing concentration of the extractbut decreases with increasing temperature.

Buchweishaija and Mhinzi (2008)investigated the inhibition effect of gumexudates from Acacia seyal var seyal on thecorrosion of mild steel in drinking waterusing electrochemical techniques (i.e.potentiodynamic polarizat ion andelectrochemical impedance spectroscopy).These Acacia gum exudates are obtainedfrom the stems and branches of sub-SaharanLeguminosae trees which grow extensivelyin central parts of Tanzania. These exudatesparticularly from senegal are permitted food

additive (JECFA/FAO 1990, Glicksman1983, Anderson and Stoddart 1966). It wasfound that Acacia seyal var seyal could serveas an effective green corrosion inhibitor forthe mild steel in drinking water systems.The percentage inhibition efficiency wasfound to increase with increasingconcentration of the gum. The inhibitionefficiency was almost unaffected by thechange of solution temperature.

The application of the acid extract of leavesof Citrus aurantifolia plant on the corrosioninhibition of mild steel in 1 M HClsolution was investigated using weight lossmeasurement and electrochemical studies(Saratha et al. 2009). The results obtainedshow that the extract served as an effectiveinhibitor for the corrosion protection of mildsteel in 1 M HCl medium. The inhibitiveaction was discussed based on theadsorption isotherms and was found to fitall the models tested i.e. Langmuir,Temkin, Freundlich, Frumkin and Flory-Huggins. The polarization curves revealedthat this extract acts as a mixed typeinhibitor. The authors reported theinhibition efficiency of up to 97.5%. Theinhibition mechanism was explained on thebasis of adsorption of the phytochemicalconstituents (Figure 8) present in the extractthrough oxygen atoms.

Figure 8: The phytochemical constituents present in acid extract of leaves of Citrusaurantifolia plant (Saratha et al. 2009)


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Saratha and Vasudha (2009) investigated theacid extracts of Nyctanthes arbortristisleaves on the corrosion inhibition of mildsteel in aerated 1 N H2SO4 solution. Theresults indicated the extract to be a goodcorrosion inhibitor. The phytoconstituentsof leaf extracts were reported to containalkaloidal principle named Nyctanthine. Theleaves also contain mannitol, astringentprinciples, resinous tannic acid, flavanoidsand iridoid glucosides (Saratha and Vasudha2009). The presence of these heterocycliccompounds enhanced the adsorption of thisacid extract on the metal surface leading tothe effective inhibition of mild steelcorrosion in sulfuric acid. The maximuminhibition efficiency of about 90% was

recorded at the inhibitor concentration of 1%v/v.

James and Akaranta (2009) studied theinhibiting action of acetone extract of redonion skin on the corrosion of zinc inhydrochloric acid solution using weight lossmethod. The results of the study revealedthat different concentrations of extractinhibit zinc corrosion. Inhibition efficiencyis found to vary with concentration andtemperature. The inhibition efficiency greaterthan 90% was attained at concentration of0.08 g/L of red onion skin extract whosemajor constituent chemical structure is givenin Figure 9.

Figure 9: The molecular structure of quercetin extracted from red onion (James and Akaranta2009)

Kumar et al. 2009, have studied thealcoholic extracts of leaves, latex and fruitfrom the Calotropis procera and Calotropisgigantea as corrosion inhibitors for mildsteel in basic solution. The extracts werereported to reduce the corrosion rate of mildsteel in basic solution and gave up to 81%efficiency. The corrosion inhibition potentialof Calotropis procera on mild steel insulphuric acid medium was also tested byweight loss, electrochemical, SEM and UVmethods. Calotropis procera was found to

show significant corrosion inhibitive effecton mild steel in sulphuric acid medium(Raja and Sethuraman 2009). Basically,Calotropis is used as a traditional medicinalplant with unique properties to treatcommon disease. The Calotropis extractcontains uscharin (Figure 10) as a majorconstituent. The nitrogen, sulphur andoxygen of uscharin were reported to beresponsible for its adsorption on the metalsurface.


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Figure 10: The chemical molecular structure of major constituents of Calotropisextract–Uscharin (Raja and Sethuraman 2009).

The corrosion inhibition performance inalmost all the sited plant extracts could bedue to the presence of surface activeconstituents which normally enhance thefilm formation over the metal surface, thusmitigating corrosion. Inspection of thechemical structures of some of theconstituents of the plants extracts reveal thatall molecules are long chain hydrocarbonscarrying a polar group(s) at one or eitherends. The polar groups normally containoxygen, nitrogen or sulphur atoms. Theseare in accordance to Riggs 1973, narrationabout molecular properties of organicinhibitors which include molecular size andits geometrical structure, bonding type,carbon chain length, type of atoms andcharacteristic molecular groups which arepresent in the molecule, molecular ability toform continuous layer on the metal surfaceor cross link, ability to react and formcomplex with metal atoms, metal ions orwith corrosion products and the bondingstrength to the metal surface.

CONCLUSIONThe diverse set of research summarized anddiscussed indicates ongoing intensiveresearch being carried out to tackle theproblem of metal corrosion. Although it isrealized that the preceding discussions arenot infinitive the literature provided in thisreview reveal concerted efforts directed at the

search for more green inhibitors asalternatives to the fossil origin toxiccorrosion inhibitors. One main drawback iseven with the growing interest in the searchfor green inhibitors the amount of researchbeing undertaken is not significant comparedto the effect of corrosion to the economygiven the current consumption of mild steel.However, regardless of the drawbackmentioned, this review has shown that theuse of green corrosion inhibitors is the wayforward in the search for safer andenvironmentally secure protection againstmetal corrosion. The use of green inhibitorsalso has the potential of being cost effectivedue to the renewability of its resources.

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