Inhibitive and adsorption behaviour of carboxymethyl cellulose on mild steel

Inhibitive and adsorption behaviour of carboxymethyl cellulose on mild steel corrosion in sulphuric acid solutionM.M.Solomon,S.A.Umoren *,I.I.Udosoro,A.P.UdohDepartment of Chemistry,Faculty of Science,University of Uyo,P.M.B,1017Uyo,Nigeriaa r t i c l e i n f o Article history:Received 30August 2009Accepted 28November 2009Available online 5December 2009Keywords:A.AcidA.Carboxymethyl cellulose d steelC.Corrosion inhibition C.Adsorptiona b s t r a c tChemical methods were used to assess the inhibitive and adsorption behaviour of carboxymethyl cellu-lose (CMC)for mild steel in H 2SO 4solution at 30–60°C.Results obtained show that CMC act as inhibitor for mild steel in H 2SO 4.The inhibition efficiency was found to increase with increase in CMC concentra-tion but decreased with rise in temperature,which is suggestive of physical adsorption mechanism.The adsorption of the CMC onto the mild steel surface was found to follow Langmuir and Dubinin–Radushke-vich adsorption isotherm models.The inhibition mechanism was further corroborated by the values of activation parameters obtained from the experimental data.Ó2009Elsevier Ltd.All rights reserved.1.IntroductionIn chemical industries,aggressive acids are frequently used for the removal of mill scale and rust from iron and steel.The use of sulphuric acid predominates.To mitigate the excessive loss of me-tal usually incurred from this practice,acid inhibitors are always added to the pickling bath.A number of organic compounds con-taining polar functions in their molecules have been reported as effective corrosion inhibitors for mild steel in acidic media [1–5].The toxic nature and high cost of inorganic and organic acid inhib-itors have necessitated research activities in recent times toward finding alternative acid corrosion inhibitors.Naturally occurring substances of both plant and animal origin have been found to sat-isfy this need.Raja and Sethuraman [6]has given comprehensive review of natural products as corrosion inhibitors for metals in acidic media.The interest in naturally occurring substances stems from the fact that they are non-toxic,ecofriendly,cheap and a renewable source of materials.Although synthetic polymers [7–13]have been reported as corrosion inhibitors for metals in differ-ent media,reports on their naturally occurring counterpart are very scanty [14].In our laboratory we have reported on the use of gum Arabic and exudates gums from Raphia hookeri and Pachlo-bus edulis for inhibition of mild steel and aluminium in acidic med-ium [15–18].Carboxymethyl cellulose which is the subject of this present study is water soluble polymer derived from cellulose.It has wide-spread applications as a binder,thickener,stabilizer,suspension and water retaining agent in food industry,pharmaceutical,cos-metic,paper and other industrial areas [19,20].Carboxymethyl cel-lulose (CMC)has been reported to inhibit the corrosion of cadmium [21]and mild steel in HCl solution [22].To our knowl-edge,there has been no report concerning the corrosion inhibition of mild steel in sulphuric acid solution using CMC as an parative studies of corrosion inhibition of organic species in chloride (HCl)and sulphate (H 2SO 4)containing solutions has shown that the inhibiting effect of organic species in HCl is much higher than that in H 2SO 4acid solutions [23–28].It has been sug-gested that chloride ions being less hydrated than sulphate ions has a stronger tendency to adsorb than do sulphate ions on the me-tal by creating an excess negative charge towards the solution phase,which favours synergistic adsorption on the metal surface [27]may be the reason for an increased protective effect in the chloride containing solution.It will be of interest to know the inhibitive potential and adsorption behaviour of CMC in H 2SO 4solution since previous works were focused on its inhibition ability in HCl solution.The present work is aimed at investigating the inhibitive ability of CMC on the mild steel corrosion in sulphuric acid medium in the temperature range of 30–60°C using weight loss,hydrogen evolu-tion and thermometric methods.2.Experimental detailsA freshly prepared sheet of mild steel 0.09cm in thickness obtained from God’s Desire Nig Enterprise,Uyo,Nigeria was used0010-938X/$-see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.corsci.2009.11.041*Corresponding author.Tel.:+2348023144384.E-mail address:saviourumoren@ (S.A.Umoren).Corrosion Science 52(2010)1317–1325Contents lists available at ScienceDirectCorrosion Sciencej o ur na l h om e pa ge :w w w.e lse v ie r.c om /lo c at e /c or s cifor this work.The composition of the mild steel sheet used is: C=0.13%;Si=0.18%;Mn=0.39%;P=0.40%;S=0.04%; Cu=0.025%;and the balance Fe.The mild steel was mechanically press-cut into coupons of5cmÂ4cm(surface area=20cm2) dimension for weight loss study and3cmÂ3cm dimension for thermometric and hydrogen evolution methods.These coupons were used without mechanical polishing,but were degreased in absolute ethanol,dried in acetone and stored in moisture-free des-iccators prior to use.The inhibitor,carboxymethyl cellulose(CMC) (Sigma–Aldrich)with chemical structure of the repeat unit given in Fig.1was used as obtained in the concentration range of0.1–0.5g/ l.The concentration of the blank corrodent was2M prepared from AR grade98%H2SO4(Fluka).All preparations were made using doubly distilled water.Weight loss measurements were conducted under total immer-sion using250mL capacity beakers containing200mL test solu-tions at30–60°C maintained in a thermostated water bath.The mild steel coupons were weighed and suspended in the beaker with the help of rod and hook.The coupons were retrieved at2h interval progressively for10h,washed thoroughly in20%NaOH solution containing200g/l of zinc dust[29]with bristle brush, rinsed severally in deionised water,cleaned,dried in acetone, and re-weighed.The weight loss,in grammes,was taken as the dif-ference in the weight of the mild steel coupons before and after immersion in different test solutions.The corrosion rate(g/cm2h)in the absence and presence of CMC was determined using Eq.(1):q¼D WAtð1Þwhere q is the corrosion rate,D W is the weight loss of the mild steel coupon in gramme,A is the sectional area of the mild steel coupon in cm2and t is the exposure time of the mild steel coupon in hour.The inhibition efficiency of CMC was evaluated from the follow-ing equation:%I¼q1Àq2q1Â100ð2Þwhere q1and q2are the corrosion rates of the mild steel coupons in the absence and presence of inhibitor,respectively.Hydrogen evolution experiments were undertaken using gaso-metric assembly similar that described in the literature[30–32]. The assembly is essentially an apparatus that measures the volume of gas evolved from a reaction system.It consists of essentially two-necked round bottomflask which serves as the reaction med-ium containing the corrodent and the metal coupons.Others are a separating funnel,a burettefitted with taps and an outer glass jacket that serves as a water condenser.In this technique, 100mL of different test solutions were introduced into a reaction vessel which was connected to a burette through a delivery tube. The initial volume of air in the burette was d steel coupons of dimensions3cmÂ3cm were carefully dropped into the test solution and the reaction vessel quickly closed to avoid any escape of hydrogen gas.The volume of hydrogen gas evolved from the corrosion reaction was monitored by the depression(in cm3)in the paraffin oil level.This depression was monitored at fixed time interval.From the volume of hydrogen gas evolved, the inhibition efficiency(%I)was calculated,using the equation: %I¼1ÀV1HtV oHt!Â100ð3Þwhere V1Htis the volume of hydrogen evolved at time‘t’for inhibitedsolution and V oHtis the volume of hydrogen evolved at time‘t’for uninhibited solutionAn innovation was introduced in the conduct of thermometric measurements.Here a digital pH meter having a separate probe for temperature measurement was used.The temperature probe was inserted into a two-necked round bottomflask containing 2M d steel coupon of dimension3Â3cm was carefully dropped into the solution through one of the neck and was quickly corked.The temperature variation as displayed by the pH meter was monitored at1min interval until the maximum temperature was reached.The experiment was repeated in the presence of dif-ferent concentrations of the inhibitor(CMC).This method allowed the computation of reaction number(R)defined as[17]:R C minÀ1¼T mÀT itð4Þwhere T m is the maximum temperature,T i is the initial temperature of the systems and‘t’is time(min)taken to reach the maximum temperature.The inhibition efficiency(%I)were computed from the percent-age reduction in reaction number as follows:%I¼R aqÀR wiaqÂ100ð5Þwhere R aq is the reaction number of the aqueous2M H2SO4and R wi is the reaction number in the presence of the inhibitor(CMC).3.Results and discussion3.1.Weight loss measurements3.1.1.Weight loss,corrosion rates and inhibition efficiencyThe anodic dissolution of iron in acidic media and the corre-sponding cathodic reaction has been reported to proceed as follows [33,34]:Fe!Fe2þþ2eÀð6Þ2Hþþ2eÀ!2H ads!H2ð7ÞAs a result of these reactions,including the high solubility of the corrosion products,the metal loses weight in the solution.Fig.2(a–d)shows plots of weight loss versus time for mild steel in2M H2SO4without and with different concentrations of CMC at30–60°C,respectively.From the plots it is very clear that the weight loss of mild steel in the different test solutions increases with time. The non-uniformity and non-linearity of the curves of the weight loss plot may be attributed to the presence of mill scale on the mild steel surface(as they were used in unpolished state).It may also suggest that the mild steel corrosion by H2SO4is a heterogeneous process involving several steps.Similar observations have been re-ported for mild steel corrosion by other authors[35,36].It is also observed from the plots that weight loss of mild steel decreases on introduction of CMC into the corrodent indicating CMC func-tioned as an inhibitor isolating the metal from attack bythe Fig.1.Chemical structure of CMC repeat unit.1318M.M.Solomon et al./Corrosion Science52(2010)1317–1325aggressive anions present in the solution.Further inspection of the figure revealed that weight loss of mild steel in the absence and presence of inhibitor increases with increase in temperature.Also the decrease in weight loss of the metal coupons on addition of CMC was concentration dependent,that is,weight loss decreases as the concentration of CMC increases.The various parameters derived from weight loss measure-ments,regarding the corrosion of mild steel in 2M H 2SO 4solution without and with various concentrations of CMC are summarized in Table 1.A comparison of the corrosion rate of mild steel in 2M H 2SO 4in the absence and presence of different concentrations of CMC reveals that corrosion rate was reduced in the presence of different concentrations of CMC with the lowest value obtained at the highest concentration (0.5g/l)of CMC used at all the tempera-tures studied.Further inspection of the table revealed that corro-sion rate of mild steel in the presence of CMC increases with rise in temperature.Results presented in the table show that corrosion rates de-creases in the presence of CMC corresponding to increase in inhibi-tion efficiency.It can be seen from the table that the inhibition efficiency increases with increase in CMC concentration.This could be attributed to the adsorption of CMC onto the mild steel surface leading to corrosion inhibition phenomenon.Corrosion inhibition is initiated by the displacement of adsorbed water molecules by the inhibitor species leading to specific adsorption of the inhibitor on the metal surface [31].The inhibition efficiency was also found to decrease with increase in temperature.However,some discrep-ancies are observed in some instances where inhibition efficiency is near constancy with increase in temperature .The reason for this is not clear.However,as suggested by Norr [37]the apparent constancy of %I with temperature increase may be due to the formation of dimeric film on the surface of mild steel.In this case,increase in temperature leads to gradual desorption of the outer layer of the dimeric inhibitor film and at the same timetheFig.2.Variation of weight loss against time for mild steel corrosion in 2M,H 2SO 4in the presence of different concentrations of CMC at (a)30(b)40(c)50and (d)60°C.M.M.Solomon et al./Corrosion Science 52(2010)1317–13251319underlying layer effectively protects the metal surface from acid attack showing constant%I values.Furthermore,adsorption pro-cess which has been generally accepted to be the mechanism responsible for the inhibitory action of organic corrosion inhibitors can affect the corrosion rate in two ways[38]:(i)by decreasing the available reaction area;the so called geometric blocking effect and (ii)by modifying the activation energy of the cathodic and/or ano-dic reactions occurring in the inhibitor free metal in the course of the inhibited corrosion process.The near constancy values of inhi-bition efficiency with increase in temperature observed may be attributed to any of these factors or both although it is difficult task to determine which aspects of the inhibiting effect are connected to the geometric blocking effect and which are connected to the energy action.The complex effect of temperature on the inhibited acid–metal reaction which usually results in many changes in the metal surface such as rapid etching,desorption of inhibitor itself and the inhibitor undergoing decomposition and/or rearrangement may also contribute to the observed effect.The decrease in inhibi-tion efficiency with increase in temperature;an indication of phys-isorption may be explained by the fact that the time lag between the process of adsorption and desorption of inhibitor molecules over the metal surface is becoming shorter with increase in tem-perature.Hence,the metal surface remains exposed to the acid environment for longer period thereby increasing the rate of corro-sion with increase in temperature and therefore the inhibition effi-ciency falls at elevated temperature[35].In addition,it has been reported that,the substitution process in CMC is slightly coopera-tive rather than random,giving slightly higher than expected unsubstituted and trisubstituted areas[39].The molecules are therefore most extended(rod-like)at low temperatures but at higher temperatures,the molecules overlap and coil up and then, entangle to become a thermoreversible gel.This behaviour at high-er temperatures may be responsible for the low inhibition effi-ciency of CMC observed as the thermoreversible gel nature may not appreciably cover the mild steel surface.Insight into the possible mechanism of corrosion inhibition of mild steel in H2SO4can be gained by considering the molecular structure of CMC(Fig.1).CMC contains carboxyl functional group (A COOH)in addition to the hydroxyl functional group(A OH)in its molecule.In acid solution,the carbonyl oxygen(C@O)may be protonated and the molecule exists as a polycation.However,in H2SO4solution,steel is assumed to be positively charged and having hydrated ions of sulphate being weakly adsorbed on the surface thereby given rise to negatively charged steel surface [40].The formation of positively charged protonated species facilitates adsorption of the compound on the metal surface through electrostatic interaction between the CMC molecule and the mild steel surface(physisorption).It is worthy to mention that the inhibition efficiency of CMC in H2SO4obtained in this study is lower than that obtained in HCl by Bayol et al.[22]which seem to confirm earlier reports of higher inhibiting effect of organic species in chloride containing solution than in sulphate containing solution.Experimental results ob-tained seem to suggest that CMC inhibits mild steel corrosion in H2SO4by simple blocking effect mechanism.Some authors[41–44]have reported that organic inhibitors that functioned by the simple blocking effect are mixed type inhibitors.Earlier studies [22]using potentidynamic polarization technique involving corro-sion inhibition and adsorption behaviour of CMC for mild steel in HCl revealed that the mechanism of inhibition was by simple blocking effect and that CMC functioned as a mixed type inhibitor. It is therefore pertinent,by extension to assume that in H2SO4 medium CMC also behave as a mixed type inhibitor.3.2.Hydrogen evolution methodThe accompanied iron dissolution reaction is the cathodic reac-tion to consume the electrons generated in the anodic reaction as shown in Eq.(7).At acidic pH,the H ads,atomic hydrogen adsorbed on the metal surface reacts by combining with other adsorbed H atom to form H2gas,which bubbles from the surface.The corro-sion behaviour of mild steel in2M H2SO4in the absence and pres-ence of CMC was,therefore evaluated by monitoring the volume of hydrogen gas evolved.The relative rapidity and effectiveness of the hydrogen evolution method as well as its suitability for monitoring in situ,any interruption by an inhibitor with regard to gas evolu-tion in metal/corrodent systems have been established in earlier reports[45–47].Fig.3(a–d)depicts the plots of volume of H2 evolved against time for mild steel corrosion in2M H2SO4without and with different concentrations of CMC at30–60°C,respectively. Inspection of thefigure revealed that the volume of H2evolved vary linearly with immersion time according to the relation:V¼ktð8Þwhere V is the volume of H2gas evolved at time‘t’and‘k’is the rate constant of the H2evolution reaction.It is also observed from the figures that lesser H2gas was liberated from the mild steel surface on introduction of CMC compared to the blank solution showing that CMC actually inhibits the corrosion of mild steel in H2SO4solu-tion.The volume of H2evolved was found to increase with increase in temperature and time but decreased with increase in the concen-tration of CMC.The values of hydrogen evolution rate(ml/min)evaluated from the slopes of the volume–time plots of Fig.3and inhibition effi-ciency for mild steel corrosion in2M H2SO4in the absence and presence of CMC at different temperatures are presented in Table2. From the table,it can be seen clearly that hydrogen evolution rate decreases in the presence of CMC compared to the blank solution. The reduction in hydrogen evolution rate of the mild steel was seen to be dependent upon the concentration of the inhibitor(CMC). However,hydrogen evolution rate was found to increase with rise in temperature.The values of inhibition efficiency(%I)follows the same trend as observed from the weight loss method.It decreases with rise in temperature but increased with increase in the concen-tration of CMC.Table1Calculated values of corrosion rates(g/cm2h)and inhibition efficiency(%I)for mild steel corrosion in the absence and presence of different concentrations of CMC at different temperatures from weight loss measurements.System/concentration Corrosion rate(g/cm2h)Â10À3Inhibition efficiency(%I)30°C40°C50°C60°C30°C40°C50°C60°C2.0M H2SO4 1.79 4.7215.6623.60––––0.1g/l CMC0.86 2.608.6513.1251.944.944.844.40.2g/l CMC0.84 2.347.9312.3053.150.449.447.90.3g/l CMC0.75 1.99 6.7011.9258.157.257.849.50.4g/l CMC0.71 1.90 6.3210.9560.359.859.653.60.5g/l CMC0.63 1.84 6.129.2664.861.060.960.8 1320M.M.Solomon et al./Corrosion Science52(2010)1317–1325It is seen from the results displayed in Tables 1and 2for the weight loss and hydrogen evolution methods respectively that the values of corrosion rate (for weight loss measurements)and hydrogen evolution rates (for hydrogen evolution measurements)as well as inhibition efficiency are different with the ones from hydrogen evolution technique being lower.This may be attributed to the fact that the metal is naturally covered with mill scale,the oxide layer will react with the acid first followed by the attack of the bare metal by the aggressive anions of the acid leading to corrosion in the case of weight loss measurements while hydrogen evolution can only occur if the metal is corroded.It is therefore pertinent to conclude that the non-linearity and non-uniformity of the weight loss plots (Fig.2)as well as the differences in inhibition efficiency from the two techniques is due to the pres-ence of mill scale on the mild steel specimen used in the study.The corrosion rate from weight loss measurements includebothFig.3.Variation of volume of H 2evolved with time for mild steel corrosion in 2M,H 2SO 4in the presence of different concentrations of CMC at (a)30(b)40(c)50and (d)60°C.Table 2Calculated values of hydrogen evolution rates (ml/min)and inhibition efficiency (%I )for mild steel corrosion in 2M H 2SO 4in the absence and presence of different concentrations of CMC at different temperature from H 2evolution measurements.System/concentrationHydrogen evolution rate (ml/min)Inhibition efficiency (%I )30°C40°C 50°C 60°C 30°C 40°C 50°C 60°C 2.0M H 2SO 40.570.77 1.44 2.67––––0.1g/l CMC 0.270.410.86 1.7748.243.938.732.90.2g/l CMC 0.260.390.84 1.7550.045.940.233.70.3g/l CMC 0.240.380.81 1.7252.747.342.034.70.4g/l CMC 0.220.360.79 1.6955.449.343.535.70.5g/l CMC0.220.340.761.6656.351.045.436.9M.M.Solomon et al./Corrosion Science 52(2010)1317–13251321the chemical dissolution of the mill scale and the metal exposed whereas the dissolution of the mill scale being chemical and not electrochemical in nature cannot evolve hydrogen hence hydrogen evolution rate is mainly due to the reaction of the metal with acid.3.3.Thermometric methodThe corrosion reaction of mild steel in2M H2SO4in the absence and presence of CMC was also investigated using thermometric method.This method,according to Oza and Singha[48]has proved to be of significant importance and help in evaluating corrosion behaviour of a number of metals and alloys in various corroding environments.The method also gain application in studying theinhibitor efficiency of a number of surface-active agents plots of temperature as a function of time in the absence and presence of the different concentration of CMC is shown in Fig.4.It is observed from the plots that temperature decreases with time after reaching their maximum values.The reason for this observation is not still clear,however,El-Etre[49]has attributed it to decreasing corro-dent concentration with increasing reaction time which in turn de-creases the corrosion rate;hence a decrease in the quantity of heat evolved.Maximum temperature was attained at a very short time by the blank corrodent solution and this corresponds to a reaction number(R)of0.082°C/min(Table3).Introduction of different con-centrations of the inhibitor(CMC)leads to reduction in the maxi-mum temperature attained and an increase in the time needed to attain it.This is an indication that CMC inhibits the corrosion of mild steel in the acidic medium probably by adsorption on the metal surface.The extent of inhibition depends on the degree of surface coverage of the metal by the adsorbed molecules.Strong adsorption is noted for the highest concentration of CMC(0.5g/l) studied as depicted for a large decrease in R value at this concen-tration as shown in Table3.The calculated values of reaction number and percentage reduc-tion in reaction number(inhibition efficiency)for mild steel corro-sion in2M H2SO4without and with varying concentration of CMC is presented in Table3.Results in the table show a decrease in reaction number of the various concentrations of CMC compared to the blank solution(0.0822°C/min).Also inhibition efficiency in-creased with increasing concentration of the inhibitor.The inhibi-tion efficiency values obtained from the three independent methods as shown in Tables1–3follow the same trend.3.4.Adsorption considerationsA direct relationship between inhibition efficiency(%I)and the degree of surface coverage(h)[%I=100Âh]can be assumed for the different concentrations of the inhibitor.The degree of surface coverage(h)for the different concentrations of CMC has been eval-uated from the weight loss measurements in2M H2SO4at30–60°C for10h of immersion period.The data were tested graphi-cally byfitting to various adsorption isotherms including Freund-lich,Temkin,Flory–Huggins,Bockris–Swinkles,Langmuir and Frumkin isotherms.The correlation coefficient(R2)was used to determine the bestfit isotherm which was obtained for Langmuir. According to this isotherm,h is related to the inhibitor concentra-tion by the following equation[50]:Ch¼1K adsþCð9Þwhere h is the surface coverage,C is the concentration,K ads is the equilibrium constant of adsorption process.K ads is related to the free energy of adsorption D G°ads by the equation[43]:log K ads¼Àlog C H2OÀD Gads2:303RTð10Þwhere C H2O is the concentration of water expressed in g/l(the same as that of inhibitor concentration),R is the molar gasconstantFig.4.Variation of temperature with time for mild steel corrosion in2M H2SO4inthe absence and presence of different concentrations of CMC.Table3Calculated values of reaction number and percent reduction in reaction number(inhibition efficiency)for mild steel corrosion in2M H2SO4containing CMC fromthermometric measurements.System/concentration Reaction number,RN(°C/min)Inhibitionefficiency(%I)2.0M H2SO40.082–0.1g/l CMC0.03755.20.2g/l CMC0.03558.00.3g/l CMC0.03260.80.4g/l CMC0.03063.50.5g/l CMC0.02866.2ngmuir adsorption isotherm model for mild steel in2M H2SO4containingCMC at different temperatures.1322M.M.Solomon et al./Corrosion Science52(2010)1317–1325(kJ mol À1K À1)and T is the absolute temperature (K).Fig.5shows the plot of C /h vs.C and linear plots were obtained for the different temperatures indicating that the adsorption of CMC followed Lang-muir isotherm.The various adsorption parameters obtained from this isotherm are listed in Table 4.It is seen from the table that the correlation coefficients are very good and K values decreases with increasing temperature showing that the molecules of the inhibitor was physically adsorbed on the mild steel surface.Although the plots are linear as depicted by R 2values (0.99)how-ever,the slopes deviates from the value of unity as expected from the ideal Langmuir adsorption equation.This deviation may be ex-plained on the basis of the interaction among adsorbed species on the surface of the metal.It has been postulated in the derivation of Langmuir isotherm equation that adsorbed molecules do not interact with one another,but this is not true in the case of large or-ganic molecules (such as CMC)having polar atoms or groups which can adsorbed on the cathodic and anodic sites of the metal surface.Such adsorbed species interact by mutual repulsion or attraction.It is also possible that the inhibitor studied can be adsorbed on the anodic and cathodic sites resulting in deviation from unit gradient.Similar observation has been reported by Sethi et al.[28]and other authors [51,52].The values of the free energy of adsorption,D G °ads was obtained from Eq.(9)and are given in Table 4.Results presented in the table indicate that the values of D G °ads are negative in all cases and lies between À17.13and À16.78kJ/mol.The negative values signify a spontaneous adsorption of the inhibitor molecules via physical adsorption mechanism.It is also seen that the values of D G °ads de-creased with an increase in temperature,a phenomenon which indicates that the adsorption of the inhibitor onto mild steel sur-face was unfavourable with increasing experimental temperatures as a result of desorption of adsorbed inhibitor from the metal sur-face.Generally,values of D G °ads 6À20kJ/mol (as obtained in this study)signify physisorption,and values more negative than À40kJ/mol signify chemisorption [53,54],physisorption is consis-tent with electrostatic interaction between charged molecules and a charged metal while chemisorption is consistent with charge sharing or transfer from the inhibitor components to the metal sur-face to form a co-ordinate type of bond.Experimental results obtained in this study were further fitted into Dubinin–Radushkevich isotherm model (D–RIM).This model was initially used to distinguish between physical and chemical adsorption for removal of some pollutants from aqueous solutions by adsorption on various adsorbents [55,56].Recently,Noor [50]has applied this model in explaining the mechanism of adsorption of corrosion inhibitor onto a metal surface in acidic medium.The equation for the isotherm model can be expressed as:ln h ¼ln h max Àa d 2ð11Þwhere h max is the maximum surface coverage and d (Polany poten-tial)can be correlated as:d ¼RT ln 1þ1C inhð12Þwhere R is the universal gas constant,T is the absolute temperature and C inh is the concentration of the inhibitor.The constant a givesthe mean adsorption energy,E ,which is the transfer energy of 1mol of adsorbate from infinity (bulk solution)to the surface of the adsorbent.E ¼1ffiffiffiffiffiffi2ap ð13ÞThe magnitude of E gives information about the type of adsorp-tion.Values of E less than 8kJ/mol indicates physical adsorption [50].Fig.6shows the relationship between ln h and d 2for the data obtained for the different methods at 30°C and the corresponding parameters from the regression analysis are listed in Table 5.It is clear from the table that the numerical values of E reflect physical adsorption mechanism.The values of h max (maximum surface cov-erage)for the three methods are in good agreement which further corroborates the results obtained for the inhibition efficiency values.3.5.Effect of temperature and activation parameters of inhibition processTemperature is an important parameter in studies of metal dis-solution .To verify the effect of temperature on the corrosion and corrosion inhibition process for mild steel in 2M H 2SO 4in the ab-sence and presence of different concentrations of CMC,weight loss experiments were undertaken in this study in the temperature range of 30–60°C.Results obtained indicate that corrosion rate in-creases with increase in temperature both in the absence and pres-Table 4Some parameters from Langmuir isotherm model for mild steel in 2M H 2SO 4.InhibitorLangmuir Temperature (°C)D G °ads (kJ/mol)Slope K ads (g/l)R 2CMC30À16.8 1.4514.10.9940À16.9 1.4711.80.9950À17.2 1.4610.90.9960À17.11.508.80.99Table 5Some parameters of linear regression from Dubinin–Radushkevich isotherm model obtained from different methods for CMC adsorption on mild steel surface in 2M H 2SO 4at 30°C.InhibitorAdsorption parameters Methodsh max a (mol 2k J À1)E (kJ/mol)R 2CMCWEM 0.990.42 1.090.87HEM 0.990.55 1.280.95TEM0.990.391.770.96WEM,weight loss method;HEM,hydrogen evolution method;TEM,thermometricmethod.Fig.6.Dubinin–Radushkevich adsorption isotherm model for mild steel in 2M H 2SO 4containing CMC for the different methods.M.M.Solomon et al./Corrosion Science 52(2010)1317–13251323。