Inhibition of Acidic Corrosion of Iron by Some Carrageenan Compounds

The corrosion inhibition of some sulfated water soluble natural polymer (carrageenan) compounds on the corrosion of iron in 1M HCl was studied using weight loss and galvanostatic polarization measurements. The percentage inhibition efficiency was found to increase with increasing concentration of inhibitor and with decreasing temperature. The inhibitive action of these compounds was discussed in terms of blocking the electrode surface by adsorption of the molecules. The adsorption process follows a Langmuir adsorption isotherm. The effect of temperature on the rate of corrosion in the absence and presence of these compounds was also, studied. Some activation thermodynamic parameters were calculated.

The aim of the present work is to study the inhibition action of some naturally occurring compounds known as carrageenans which produced from certain species of sea weeds to inhibit the corrosion of iron electrode in 1M HCl solution using weight loss and galvanostatic polarization technique.The effect of temperature on the dissolution of iron electrode in 1M HCl containing 500 ppm of the inhibitor used was also studied and some thermodynamic parameters were computed.

EXPERIMENTAL
The test electrode was made of iron obtained from Saudi iron and steel company and having the following chemical compositions (%) (C, 0.052, Mn 0.189, S 0.011, P 0.008, Si 0.011, Al 0.039, N 0.001, Cr 0.0128, Cu 0.04, Mo 0.024, Ni 0.0293 and remainder is Fe).
Iron sheets with dimension 2x2x0.1 cm were used for weight loss measurements.For galvanostatic polarization technique a cylindrical rod embedded in araldite with exposed surface area of 0.5 cm 2 was employed.Prior to each experiment, the surface of iron specimens were mechanically polished with different grades of emery paper, degreased with acetone and rinsed by distilled water.
For weight loss measurements, the cleaned Fe sheet was weighed before and after immersion in 50 ml of the test solution for a period of time up to 3 hours.The average weight loss for each two identical experiments was taken and expressed in mg cm -2 , the temperature was adjusted to 25 ± 0.1 °C using air thermostat.The corrosion rate R corr was calculated from the following equation [13] : ...2 where, S is the surface area (cm 2 ).t is the time (min), after 150 min.
The percentage inhibition efficiency (%I.E) of the selected carrageenans compounds were calculated using the following equation where, R corr.add and R corr.free are the corrosion rates of iron in the absence and presence of the carrageenan compounds, respectively.
For galvanostatic polarization measurements were carried out using Mensberger potentiostat galvanostatic PS6 with controlling measurements of potential and current density.Three compartments electrode (SCE) and a platinum foil auxiliary electrode was used.
The percentage inhibition efficiency (%I.E) of the selected carrageenan compounds were calculated using the following equation.
%I.E = ...4 where, I free and I add are the corrosion current densities in the absence and presence of the inhibitors, respectively.

RESULTS AND DISCUSSION
Weight loss measurements Fig. 2 represents the relation between time and weight losses of iron coupons in 1M HCl solution devoid of and containing different concentrations of compounds III as an example.Similar curves were also obtained for other two compounds (not shown).Inspection of this figures reveals that, as the concentration of these compounds increases, the loss in weight of iron compounds decreases.This reflects that, these compounds acts as an inhibitor.The linear variation of weight loss with time in uninhibited and inhibited 1M HCl solution indicates the absence of insoluble surface film during corrosion i.e. the inhibitor are first adsorbed on the metal surface and there after impede corrosion either by merely blocking the reaction sites (anodic and cathodic) or by uttering the mechanism of the anodic and cathodic processes.
The calculated values of inhibition efficiencies obtained from weight loss are listed in Table 1.It is obvious that the I.E increases with increasing the inhibitor concentrations, whereas decreases in the following order.

Effect of temperature
The effect of temperature on the corrosion rate of iron electrode in 1M HCl containing and devoid of 500 ppm of the studied these compounds was tested by weight loss measurements over a temperature range from 30-60°C.Fig. 3 shows the weight loss-time curves of iron electrode in 1 M HCl containing 500 ppm of the compound III, at different temperatures.Similar curves were obtained for other two compounds I & II (not shown).It is clear that as the temperature increases the weight loss increases, the rate of corrosion increases and hence the inhibition efficiency decreases.This is due to desorption which is aided by increasing the temperature.
The activation energy (E a ) of the corrosion process was calculated using Arrhenius eq. 14 ...5 and the logarithmic form ...6 where, R corr is the rate of corrosion from weight loss, A, is Arrhenius constant , R is the gas constant and T is the absolute temperature.Inspection of Table -2 it is obvious that the values of E a increases in the presence of the inhibitor.This is attributed to an appreciable decreases in the adsorption process of the inhibitors on the metal surface with increase of temperature and a corresponding increase in the reaction rate because of the greater area of the metal that is exposed to the acid 15 .
The enthalpy change of activation (ΔH * ) and the entropy change of activation (ΔS * ) for dissolution of iron electrode in 1M HCl in presence and absence of 500 ppm of each used compound were obtained by applying the transition state equation 14 .
...7 where N is Avogadro number, h is Planck's constant.A plot of log (R corr /T) vs (1/T) (Fig. 5) should give straight line with a slope of (-ΔH * /2.303 R) and an intercept of [log (R/Nh)-( ΔS * /2.303R)].The values of DH *---and DS * are listed in Table -2.Inspection of Table it is clear that , the values of ΔH * is positive.This reflect that the process of adsorption of the inhibitors on the iron surface is endothermic process.The values of ΔS * is the presence and absence of the inhibitors is negative.This implies that the activated complex is the rate determining step and represents association rather than dissociation indicating that a decrease in disorder takes place on going from reactants to the activated complex 16 .

Adsorption isotherm
The values of surface coverage θ for different concentrations of the studied compounds (I-III) at 25°C have been used to explain the best isotherm to determine the adsorption process.The adsorption of organic adsorbate on the surface of iron electrode is regarded as substitutional adsorption process between the organic compound in the aqueous phase ( where, X is the size ratio, that is the number of water molecules replaced by one organic molecules.Attempts were made to fit è values to various isotherm including Furmkin, Temkin, Freundilch and Langmuir isotherm.Plotting C/θ against C gave a straight line with unit slope value (Fig. 6) indicating the adsorption of carrageenan compounds on iron surface follows Langmuir adsorption isotherm.
From these results one can postulates that there is no interaction between the adsorbed species.

Galvanostatic polarization
The effect of carrageenan compounds on the anodic and cathodic polarization curves of iron electrode in 1 M HCl was studied.Fig. 7 shows the effect of increasing concentrations of compound III as an example.However, similar curves were obtained for the other two compounds (not shown).
Inspection of Fig. 7 reveals that, the presence of increasing concentrations of carrageenan compound causes a marked decreases in corrosion rate i.e. shifts the anodic curves to more positive potentials and the cathodic curves to more negative potentials.This may be ascribed to the adsorption of the carrageenan compounds over the metal surface.The corrosion current density (I corr ) was determined from the intersection of the linear parts of the cathodic curves with the stationary corrosion potential (E corr ).
The electrochemical parameters such as, corrosion potential (E corr ), corrosion current density (I corr ) anodic and cathodic Tafel slopes (b a &b c ) and inhibition efficiency (I.E) were calculated and given in Table 3. Inspection of the data cited in this Table reveals that , as the concentration of the additives   The corrosion of iron electrode in 1M HCl is inhibited by addition of some carrageenan compounds.

2-
The inhibition efficiency increases with increasing of the inhibitor concentrations and decreases with increasing of temperatures.

3-
The inhibitive effect of these compounds is attributed to the adsorption on the metal surface.

5-
The inhibition efficiency from weight loss measurements agrees with those obtained from polarization measurements.6-Investigated carrageenan compounds are anodic inhibitors reducing the anodic reaction.

Fig. - 1 :
Fig. -1: Chemical structure of Carrageenan compounds The loss in weight of metal in the corrosive solution ΔW = W B -W A ...1 where ΔW = Loss in mass of metal in the corrosive solution.W B = Weight of metal before exposure to the corrosive solution.W A = Weight of metal after exposure to the corrosive solution.

Fig. 4 represents
Fig.4represents Arrhenius plot (log R corr vs. I/T) for uninhibited and inhibited 1M HCl containing 500 ppm of the studied compounds.The values of E a can be obtained from the slope of the straight lines and have been given inTable -2.

Fig. - 5 :Fig. 6 :
Fig. -5: Transition state plot of the corrosion rate for iron electrode in 1M HCl in the absence and presence of the studied compounds.a) free b) 1) compound I 2) compound II 3) compound III