Inhibiting Properties of Morpholine as Corrosion Inhibitor for Mild Steel in 2 N Sulphuric Acid and Phosphoric Acid Medium

The inhibition effect of morpholine on the corrosion of mild steel in 2N sulphuric acid and phosphoric acid has been studied by mass loss and polarization techniques between 302K and 333K.The inhibition efficiency increased with increase in concentration. The corrosion rate increased with increase in temperature and decreased with increase in concentration of inhibitor compared to blank. The adsorption of inhibitor on the mild steel surface has been found to obey Temkin’s adsorption isotherm. Potentiostatic polarization results reveal that morpholine act as mixed type inhibitor. The values of activation energy (Ea), free energy of adsorption (ΔGads), enthalpy of adsorption (ΔH), and entropy of adsorption (ΔS) were also calculated.


Introduction
Phosphoric acid is a major chemical product, which has many important uses, especially in the production of fertilizers.Most of the acid is produced from phosphate rock by wet process.Generally nickel-base alloys and stainless steel are frequently used in many parts of the wet process and a considerable quantity of data has been published about the resistance of these materials to corrosion by phosphoric acid solution [1][2][3][4] .Most of the previous studies were focused on the inhibition of stainless steel or chromium-nickel steel in hydrochloric acid or sulphuric acid solutions using organic compounds containing nitrogen, sulphur and oxygen atoms as corrosion inhibitors 5,6 .Organic inhibitors are widely used in various industries.Among them, heterocyclic compounds comprise a potential class of inhibitors.There is wide consideration in the literature regarding corrosion inhibition studies by nitrogen containing heterocyclies [7][8][9][10] .Heterocyclic compounds containing nitrogen and sulphur atoms are of particular importance as they often provide excellent inhibition compared with compounds containing only nitrogen or sulphur [11][12] .The corrosion inhibiting property of these compounds is attributed to their molecular structure.These compounds contain  electrons and heteroatom, which induce greater adsorption of the inhibition molecules onto the mild steel surface.So, in this investigation, the corrosion of mild steel in 2N phosphoric acid and 2N sulphuric acid solutions in the absence and presence of morpholine at 302 K to 333K has been studied by mass loss method and polarization techniques.It is aimed to predict the corrosion rate, inhibition efficiency on mild steel corrosion and the thermodynamic feasibility of inhibition via surface coverage on mild steel by adsorbed morpholine at various temperatures.The adsorption characteristic of morpholine was studied in order to access the adsorption isotherm(s).

Mass loss measurement
Mild steel specimens were cut to size of 5 cm x 1 cm from the mild steel sheets having the following percentage composition as shown below.The surface of specimens were polished with emery papers ranging from 110 to 410 grades and degreased with trichloroethylene specimens were dried and stored in vacuum desiccators containing siligagel and then initially weighed in an electronic balance.After that the specimens were suspended with the help of PTFE, threads and glass rod in 100ml beaker containing acid in the presence and absence of inhibitors.The specimens were removed after 4 hours exposure period, washed with water to remove any corrosion products and finally washed with acetone.After that they were dried and reweighed.Mass loss measurements were carried out in 2N phosphoric acid and 2N sulphuric acid with Morpholine in the concentration range of 1 % to 5% as inhibitors and the temperature between 302 K and 333 K for an immersion period of 4 hours.All the solutions were prepared with AR grade chemicals in double distilled water.Mass loss measurements were performed as per ASTM method described previously [13][14][15] .

Potentiostatic Polarization measurements
Polarization measurements were carried out in a conventional three-electrode cell.Mild steel strips coated with lacquer except for an exposed area of 1 cm 2 were used as the working electrode.The saturated calomel electrode and the platinum foil were used as reference and counter electrodes respectively.The potentiostatic polarization measurement was carried out using BAS -100 a model instrument.The potential of the test electrode was measured with respect to SCE and platinum electrode was used as auxiliary electrode and the experiment were carried out at 302K to333K.

Mass loss Studies
Table 1 shows the value of inhibition efficiency [IE%] surface coverage (θ) and corrosion rate obtained at different concentration of the inhibitors in 1N hydrochloric acid solutions for an immersion period of 3 hours.From the mass value, the inhibition efficiency [IE%] and surface coverage (θ) were calculated using the following equation [16][17][18] . Temp Where Wu and Wi are the corrosion rates for mild steel in the absence and presence of inhibitor respectively at the same temperature.
It could be seen from the table that the addition of inhibitor to the acid has reduced the corrosion rate.The inhibition efficiency increased with increase in concentration of inhibitors and increased with temperature from 302 K to 313 K and then decreased in 2N sulphuric acid.The inhibition efficiency increased with increase in concentration of inhibitors and decreased with temperature from 313 K to 333 K in 2N phosphoric acid .The values of the corrosion rate and inhibition efficiency of the inhibitors are known to depend on the molecular structure of the inhibitors.
The maximum inhibition efficiency of morpholine was found to be 86.93% and 96.86% in 2N sulphuric acid and 2N phosphoric acid with 5% of inhibitor concentration respectively at 313K and 302K and then decreased.Inhibition efficiency of morpholine in 2N phosphoric acid solution was found to be greater than 2Nsulphuric acid solution because in 2N phosphoric acid solution phosphate ions probably form a bridge between the protanated molecules of inhibitors and facilitate the adsorption of inhibitor molecules, while in 2Nsulphuric acid this bridge is not formed due to Low charge to mass ratio, as such that adsorption of molecules on the surface of the metal is mostly physical and gives low inhibition efficiency [19][20] Thermodynamic Consideration Table 2 shows that the calculated values of activation energy (Ea) and free energy of absorption (ΔG ads ) for mild steel corrosion in 2N sulphuric acid and 2N phosphoric acid with and without inhibitors at 313K to 333K.Energy of activation (Ea) was calculated from the slopes of plots of log p versus 1/T in Fig. 1&2 and also calculated from Arrhenius equation [21][22] .
where P 1 and P 2 are the corrosion rates at temperatures T 1 and T 2 respectively.Ea value was found to be 41.73KJ/mole and 21.86 in 2N sulphuric acid and 2N phosphoric acid at 313K to 333 K .The Ea values for 2N sulphuric acid and 2N phosphoric acid containing inhibitors are found to be higher than that of without inhibitors.These higher values of Ea indicate the physical adsorption of the inhibitors on metal surface [23][24] and also indicate that besides, adsorption of these inhibitors increases the activation energy of the corrosion process.The Ea values are calculated from the slopes of Arrhenius plot and by using equation-3 are approximately almost similar.The free energy of adsorption (ΔG ads ) at different temperatures was calculated from the following equation 25 .
ΔG ( ads ) = -RT In (55.5 K) [4]   where K is given by where θ is surface coverage on the metal surface, C is concentration of inhibitor in mole/lit and K is equilibrium constant.55.5 is concentration of water (mol/lit).The negative values of (ΔG ads ) indicated the spontaneous adsorption of the inhibitors.This is usually characteristic of strong interaction with the metal surface.It is found that the ΔG ads values are more positive than (-40 KJ/mole -1 ) indicating that inhibitors is physically adsorbed on the metal surface [26][27] .
The free energy of adsorption (ΔG ads ) of Morpholine in 2N sulphuric acid and 2N phosphoric acid can be calculated from the equation ( 4) at 303K to 333K, while the enthalpy of adsorption (ΔH) and entropy of adsorption (ΔS) were also calculated from the following equations (28).ΔH 0 = E a -RT [6]   ΔG 0 = ΔH 0 -T ΔS 0 [7]   Table 2 shows thermodynamic data obtained in this study .It could be seen from the table the activation energy increases linearly with increasing efficiency of the inhibitor.
Ideally, a corrosion inhibitor is a substance that greatly increases the activation energy of corrosion.The negative values of (ΔG ads ) indicate the spontaneous adsorption of the inhibitor on the surface of mild steel.It's also observed that (ΔS) increases with increasing efficiency of the inhibitors.This is opposite to that we expect, since the adsorption is an exothermic process and is always accompanied by a decrease in entropy.Ateya et.Al. 29 have described this situation as due to the adsorption of the organic compound, which is accompanied by desorption of water molecules from the surface.While the adsorption process is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent.Therefore, this gain in entropy that accompanied the substitutional adsorption process is attributed to the increase in solvent entropy.

Adsorption isotherms
The electrochemical process on the metal surface are likely to be closely to the adsorption of the inhibitors 30 and the adsorption is known to depend on the chemical structure of the inhibitors 31,32 The adsorption of the inhibitors molecules from aqueous solutions can be regarded as quasi-substitution process 33 between the organic compound in the aqueous phase, org ( aq ) and water molecules at the electrode surface, H2O ( s ).
Where x (the size ratio) is the number of water molecules displaced by one molecule of inhibitor.
Adsorption isotherms are very important in determining the mechanism of organoelectrochemical reactions.The most frequently used are those of Langmuir, Frumkin, Parsons, Temkin, Flory -huggins and Bockris -Swinkels 34,35 .All these isotherms are of the general form: Where f (θ,x) is the configurational factor that depends essentially on the physical model and assumptions underlying the derivation of the isotherm 36 .
The mechanism of inhibition of corrosion is generally believed to be due to the formation and maintenance of a protective film on the metal surface.The plot of surface coverage (θ) obtained by mass loss method versus log C at different concentrations of the inhibitors shows a straight line indicating that the adsorption of the inhibitor from acid on mild steel surface follows the Temkin's adsorption isotherm 25 .This also points out that the corrosion inhibition by these compounds being a result of their adsorption on the metal surface.

Potentiostatic Polarization studies
The Polarization behavior of mild steel functioning as cathode as well as anode in the test solution is shown in Fig. 5 and 6 for 2N sulphuric acid and 2N phosphoric acid with Morpholine at room temperature (302K).The electrochemical data obtained are shown in Table 3.It is evident that Morpholine bring about considerable polarization of cathode as well as anode.It was therefore inferred that the inhibitive action is of a mixed type and the cathodic and anodic Tafel slopes increased with increasing inhibitor concentration and the increase is predominant in the case of the former indicating that the cathodic inhibition is dominating through the inhibitive action is of mixed nature.The non-constancy of Tafel slopes for different inhibitor concentration reveals that the inhibitor act through their interference in the mechanism of the corrosion processes at the cathode as well as anode.
The corrosion Parameters deduced from Tafel polarization such as corrosion current I corr, corrosion potential E corr, Tafel constant b a and -b c and inhibition efficiency are given in Table 3.The i corr values were decreased with increasing concentration of the inhibitors.
The inhibition efficiency values were determined from the values of corrosion current.

Reasons for inhibition
It has already been reported by many workers [37][38][39][40][41] that most of the organic inhibitors work by way of adsorption on the metal surface as has been established in the present work.Adsorption bond strength depends upon the electron density available at the point of adsorption.The point of adsorption may be any electron-donating element such as N, O, S, P , etc. present in the molecular structure of the inhibitor .If there is more than one electron donating elements present then which atom will have a higher tendency to become point of adsorption depends upon the distribution of electrons on the over all structure of the molecule.
In the presence of an inhibitor, a thin black film has always been observed on the surface of the specimens.This shows that the inhibition is due to the formation of some complex film with the metal ions.The organic compounds used as inhibitors like Morpholine have the following structure.
This inhibitors act as a proton acceptor in an acidic medium.It forms an organo metallic complex layer with the metal ions on surface of the metal, thus inhibiting corrosion.The N and O atoms act as the reaction centers in the complexation reaction with the metal ions.The adsorption of the inhibitors over mild steel surface may be through N and O. which is supposed to be active center for adsorption of inhibitor molecule due to the presence of charge on it.

Conclusion
Corrosion rates of mild steel in 2N sulphuric acid and 2N phosphoric acid decreased with increasing concentration of Morpholine.The inhibition efficiency increased with respect to the concentration of inhibitors as it is assumed that the inhibition efficiency is equal to surface coverage.The inhibition efficiency of Morpholine in 2N sulphuric acid and 2N phosphoric acid increased with rise in temperature respectively up to 313K and then decreased.The adsorption of Morpholine on mild steel surface from the acid solution follows Temkin's adsorption isotherm.The low and negative value of ΔG ads indicated that the Morpholine is physically adsorbed and spontaneous adsorption of inhibitor on the surface of mild steel.The inhibition efficiency obtained from mass loss studies and polarization measurement shows fairly good agreement.Morpholine shows the better inhibition in 2N phosphoric acid than 2N sulphuric acid.It is found that the Morpholine acting as mixed type inhibitor.Energy of activation (Ea) values indicates physical adsorption of the inhibitor on metal surface.

Figure 2 .
Figure 2. Arrhenius Plot for Corrosion in 2N Phosphoric acid with Morpholine.

Fig.3 and 4 Figure 3 .
Figure 3.Tempkin's adsorption isotherm for corrosion behaviour of mild steel in 2N Sulphuric acid with Morpholine.

Figure 4 .
Figure 4.Temkin's adsorption isotherm for corrosion behavior of mild steel in 2N Phosphoric acid with Morpholine.

Table 1
Corrosion parameters of Morpholine on mild steel in 2N sulphuric acid and 2N phosphoric acid by weight loss method.

Table 2 .
Thermodynamic parameters for mild steel in 2N sulphuric acid and 2N phosphoric acid with Morpholine.

Table 3 .
Polarization parameters for the corrosion behaviour of mild steel in 2N sulphuric acid and 2N phosphoric acid with and without Morpholine.