INTRODUCTION:

Mild steel is one of the frequently used structural materials for storage tanks, reaction vessels, pipelines etc., in chemical and allied industries. During certain operations like cleaning, pickling, descaling or even transportation it may come in the contact with sulfuric acid and get severely corroded.Several organic compounds as inhibitors have been reported for mild steel corrosion in acidic environments^[1-11]. Acidization of a petroleum oil well is one of the important stimulation techniques for enhancing oil production. It is commonly brought about by forcing a solution of 15% to 28% hydrochloric acid into the well to open up near bore channels in the formation and hence to increase the flow of oil. To reduce the aggressive attack of the acid on tubing and casing materials (N-80 steel), inhibitors are added to the acid solution during the acidifying process. The effective acidizing inhibitors that are usually found in commercial formulations are acetylenic alcohols, alkenyl phenones, aromatic aldehydes, nitrogen-containing heterocyclics, quaternary salts and condensation products of carbonyls and amines^12-17. However, these inhibitors suffer from drawbacks , they are effective only at high concentrations and they are harmful to the environment due to their toxicity, so it is important to search for new nontoxic and effective organic corrosion inhibitors for N-80 steel – 15% hydrochloric acid system.

In this regard, imidazoline derivatives have a promising alternative for the design of eco-friendly corrosion inhibitors which will satisfy the environmental requirements.

Thus, it was considered interesting to prepare imidazoline compounds like 1-(2-hydroxy ethyl)-2-methylimidazoline (Inh I) and 1-(2-hydroxyethyl)-2-decylimidazoline (Inh II) and to assess their inhibitive properties for oil-well tubular steel (N-80) in 15% hydrochloric acid.

MATERIALS AND METHODS:

Materials:

The working electrode and specimens for weight loss experiments were prepared from oil-well N-80 steel sheets having the following percentage composition:

C	Mn	Si	P	S	Cr	Fe
0.31	0.92	0.19	0.01	0.008	0.20	Remainder

Weight measurements:

The specimens for the weight loss experiments were of the size 3cm X 3cm and those for electrochemical studies the size of the electrodes was 1cmX1cm with a 4 cm long tag for electrochemical contact. Both sides of the specimens were exposed for both the techniques. The specimens were mechanically polished successively with 1/0, 2/0, 3/0 and 4/0 grade emery papers. After polishing with the paper of each grade, the surface was thoroughly washed with soap, running tap water, distilled water and finally was degreased with acetone. The samples were dried and stored in a vacuum dessicator before immersing in the test solution. For weight loss experiments 300 mL of 15% hydrochloric acid was taken in 500 mL glass beakers with lids. The inhibition efficiencies were evaluated after a pre-optimized time interval of 6 h using 10, 20, 50, 100 and 150 ppm of inhibitors. The specimens were removed from the electrolyte, washed thoroughly with distilled water, dried and weighed. The inhibition efficiencies were evaluated using the formula

where, w is weight loss in absence of inhibitor and w_iis weight loss in presence of inhibitor.

Electrochemical measurements:

The electrochemical experiments were carried out in a three necked glass assembly containing 150 mL of the electrolyte with different concentrations of inhibitors (from 10 ppm to 150 ppm by weight) dissolved in it. The potentiodynamic polarization studies were carried out with N80 steel strips having an exposed area of 1cm². A conventional three electrode cell consisting of N80 steel as working electrode, Platinum as counter electrode and a saturated calomel electrode as reference electrode was used. Polarisation studies were carried out using VoltaLab 10 electrochemical analyser and data was analysed using Voltamaster 4.0 software. The potential sweep rate was 10 mVs^-1. All experiments were performed at 25 ± 0.2°C in an electronically controlled air thermostat. For calculating %IE by electrochemical polarization method we use the formula-

Where, I₀ = Corrosion current in absence of inhibitor

I_inh = Corrosion current in presence of inhibitor

Synthesis of inhibitors:

The inhibitors were synthesized in the laboratory by reaction of fatty acids with mono-substituted ethylenediamine ¹⁸.

RESULTS AND DISCUSSION:

Weight loss study:

The percentage inhibition efficiencies (% IEs) of 10, 20, 50, 100 and 150 ppm Inh I and Inh II have been evaluated by weight loss technique at 25C and the results are summarized in Table 1. It is evident from these values that both the inhibitors are significantly effective even at low concentrations like 10 ppm and there is a linear increase in %IE in the whole range of concentrations studied. The structure of the inhibitors are given below-

Inh I Inh II

Table 1: Percentage inhibition efficiency (% IE) values calculated by weight loss technique for Inh I and Inh II at 25^oC

Concentration	Inh I (%IE)	Inh II (%IE)
0	-	-
10	52.25	63.24
20	60.36	71.45
50	68.46	80.35
100	75.16	88.18
150	77.18	90.32

On the basis of observed IE values the inhibitors follow the order Inh II > Inh I

The above order may be explained well considering adsorption of the inhibitor molecules on the metal surface through the probable sites of adsorption, N, O and imidazoline ring. In the acid solution both the inhibitors exist as protonated species which could adsorb on the cathodic sites of N80 steel and decrease the evolution of hydrogen .The inhibitor Inh II gives higher inhibition efficiency than Inh I because due to presence of long alkyl group, Inh II accumulate quickly on the N80 steel surface due to the fast chemical reaction of the imidazoline ring, its large size and van der Waals force of the alkyl group. The fast film formation is an important factor for the corrosion protection because a thick and dense complex film is believed to act as a protective barrier for the N80 steel corrosion.

Figure 1: Electrochemical polarization curves for N80 steel in 15% hydrochloric acid without (1), with 10 ppm (2), 20 ppm (3), 50 ppm (4) 100 ppm (5) and 150 ppm (6) of Inh I at 25^oC

Electrochemical studies:

Electrochemical polarisation behaviour of Inh I and Inh II for N80 steel in 15% hydrochloric acid at 25^oC is shown in figures 1 to 2 and various parameters obtained are given in tables 2 to 3.

Table 2 : Corrosion parameters obtained from polarisation curves shown in Figure. 1 for N80 steel in 15% hydrochloric acid in the absence and presence of inhibitor Inh I.

Concentration (ppm)	E_corr (mV)	I_corr (µA cm^-2)	b_c (mV dec^-1)	b_a (mVdec^-1)	% IE
0	- 470	575	150	130	-
10	- 469	282	166	154	51.64
20	- 467	224	172	156	61.12
50	- 465	178	174	158	69.32
100	- 464	138	1775	160	76.18
150	- 462	126	176	161	78.12

Table 3 : Corrosion parameters obtained from polarisation curves shown in Figure. 2 for N80 steel in 15% hydrochloric acid in the absence and presence of inhibitor Inh II.

Concentration (ppm)	E_corr (mV)	I_corr (µA cm^-2)	b_c (mV dec^-1)	b_a (mV dec^-1)	% IE
0	- 470	575	150	130	-
10	- 468	207	162	155	64.38
20	- 466	155	164	158	73.16
50	- 463	109	168	162	81.25
100	- 460	63	170	164	89.12
1500	- 459	52	174	167	91.62

The inhibition efficiencies calculated from polarization data are in good agreement with those obtained from weight loss measurement.

Figure 2: Electrochemical polarization curves for N80 steel in 15% hydrochloric acid without (1), with 10 ppm (2), 20 ppm (3), 50 ppm (4) 100 ppm (5) and 150 ppm (6) of Inh II at 25^oC

The curves in figures 1 and 2, illustrate that the nature of the curve remains almost same even after the addition of the inhibitors and also on increasing the concentration of the inhibitor. However, on increasing the concentration of the inhibitor the curves are gradually shifted towards lower current density. The corrosion potential also slightly shifted towards negative direction on increasing the concentration of the inhibitors.

It is realized from these observations that the inhibitors molecules retard the corrosion process without changing the mechanism of corrosion process in the medium of investigation. The magnitude of the shift in current density is directly proportional to the concentration of the inhibitors, indicating that the inhibitive property of the inhibitor is concentration dependent. It is clear from the polarization curves of the inhibitors that the shift in current density towards lower current density for anodic as well as cathodic curve increases on increasing the concentration of the inhibitor. The positive shift in the E_corr in presence of inhibitors on increasing the concentration of the inhibitors is due to the decrease in the rate of anodic reaction. Moreover, the increase in the cathodic and anodic Tafel slopes (β_c and β_a) are related to the decrease in both the cathodic and anodic currents. Both the inhibitors affect both, the anodic as well as cathodic sites, so these are mixed inhibitors.

Adsorption Isotherms

The adsorption of inhibitor molecules on the surface of the corroding metal has been considered as the root cause of corrosion inhibition. Assuming that the percentage area covered by the inhibitors is directly proportional to retardation in the corrosion rate, the compounds should obey Langmuir adsorption isotherm¹⁹.

Figure.3 : Langmuir plots

Langmuir isotherm may be written as

where, A and Q are temperature independent constant and heat of adsorption respectively. The validity of Langmuir isotherm is confirmed from the linearity of the log vs log C plot having the slope value to be unity. The plots of log vs log C for the investigated inhibitors at 25⁰C are shown in Fig. 3. It is observed that although these plots are linear, the gradient are never unity, contrary to what is expected for the ideal Langmuir adsorption isotherm equation. The departure in the values of the slopes of Langmuir plots from unity may be advocated to be due to the mutual interaction between adsorbed molecules in a close vicinity ²⁰. Organic molecules and metal complexes having polar atoms or groups which are adsorbed on the metal surface may interact by mutual repulsion or attraction and hence may affect the heat of adsorption. All the inhibitors follow the Langmuir adsorption isotherm, indicating that the adsorption of inhibitors at the surface of N-80 is the root cause of corrosion inhibition.

Microscopic Study:

Figure 4 (a, b, c,) show the microphotographs for N80 steel in 15% hydrochloric acid in the absence and presence of 150 ppm of Inh I at 200 X magnification. On comparing these micrographs, it appears that in the presence of inhibitor, the surface of the test material has improved remarkably with respect to its smoothness. The smoothening of the surface would have been caused by the deposition of inhibitor molecules on it and thus, the surface is fully covered.

(a)

(b)

(c)

Figure 4: SEM of (a) Polished sample (b) Sample in presence of 15 % hydrochloric acid (c) Sample in presence of 150ppm of Inh I.

CONCLUSIONS:

(i) Both the inhibitors Inh I and Inh II act as efficient corrosion inhibitor for N-80 steel in 15% HCl solution.

(ii) Inh II shows appreciably higher efficiency than the Inh I due to its larger size as compared to the inhibitor Inh I.

(iii) Both the the inhibitors act as mixed inhibitors.

(iv) It is suggested from the results obtained from SEM and Langmuir adsorption isotherm that the mechanism of corrosion inhibition is occurring through adsorption process.

REFERENCES:

1. Aljourani J, Raeissi K and Golozar M A. Corrs.Sci. 51 :2009: 1836.

2. Bentiss F. et al. Corros. Sci. 51:2009:.1628.

3. Bentiss F. et al. Mater. Chem. Phys. 87:2004: 18.

4. Emregal Kaan C. and Hatvali Mustfa. Corros. Sci. 48:2006: 797.

5. Popova A. et al. Corros Sci. 49:2007: 2131.

6. Acharya Ganesha et al. Mater. Chem. Phys. 107: 2008: 44.

7. Quraishi M.A. et al. Mater. Chem. Phys. 71: 2001: 309.

8. Moretti G. and Zingales A. Electrochimica Acta 41 : 1996:1971.

9. Bastidas J.M. and Torres C.L. Journal of Materials Science 35: 2000 :. 2637 .

10. Anwar I. A. and Danish S. Corros. Sci. 7: 1967:579 .

11. Ozcan M. et al. Appl. Surf. Sci. 236: 2004 : 155 .

12. Poling G W. J. Electrochem. Soc. 114 : 1967: 209.

13. Neemla K D et al. Corros. Prev. Cont. 6: 1992:.69.

14. Frenier W et al. Corrosion. 44: 1988 :.590.

15. Frenier W W. Eur. Pat. 04 : 1972 : 7400.

16. Cizek A, U.S. Pat. 1991. 4997040 .

17. Monroe R F et al. U.S. Pat. 1963. 3007454.

18. Bodanszky M. Peptide chemistry:A practical text book, Springer-Verlog, Berline.1988. 53

19. Singh M M et al. Corrosion 50 : 1994 : 620.

20. Singh M.M. et al. Bull. Electrochem. 12 : 1996 : 26.

Received on 01.07.2011 Modified on 13.07.2011

Asian J. Research Chem. 4(9): Sept, 2011; Page 1404-1407

Corrosion Inhibitors for Acidization of Petroleum Oil Well

M. Yadav* and Usha Sharma

Remainder

Synthesis of inhibitors: