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Synthesis and Application of 3-(4-Hydroxyphenylimino)indolin-2-one as Corrosion Inhibitor.

Byline: Hua-Rui Hao, Wei Zhao, Jie Zhang, Yan-Qing Miao and Zhi-Fang Zhang

Summary: A isatin Schiff base, 3-(4-hydroxyphenylimino)indolin-2-one, was synthesized and analyzed by NMR and MS. The inhibition and the mechanism of the title compound on the corrosion of high protective Q235A steel in HCl solution were screened and discussed. The results indicated that it can inhibit the corrosion with moderate inhibition efficiency in different conditions, and the inhibition mechanism of the corrosion inhibiting may be mainly contributed to the adsorption. Potentiodynamic polarization studies indicate that extracts are mixed-type inhibitors. While the title compound was companied with hexamethylenetetramine (HMTA) and 1,4-dihydroxy-2-butyne (BOZ), the inhibition efficiency was enhanced effectively.

Keywords: Isatin; Schiff base; Synthesis; Corrosion inhibition; Adsorption; Synergist.

Introduction

About one third of the world production of crude oil from carbonate reservoirs, most of which have natural permeabilities below 10 mD, and the stimulation of the production is achieved through acidification and/or fracturing operations. During the acidification operation, hydrochloric acidic fluid is pumped into wells, which etches the fracture walls irregularly to create highly conductive channels [1]. So it is a challenge for the metal instruments involved. Q235A (A3) steel is a basic material for its resistance properties against corrosion in the gas and oil field. However, there is a need to prolong their lifetime, because the steel is susceptible to pitting corrosion in the presence of in acidic media, especially in high concentration of HCl under high temperature [2, 3]. Great attentions have been drowning to heat treatment and changing chemical composition to improve its corrosion resistance.

But, for the aggressiveness of acidic media, the use of corrosion inhibitors is considered as the most effective method for the protection of many metals against such acid attack [2-6]. Compounds, containing functional electronegative groups and p and/or p-electron in triple or conjugated double bonds, are found to be efficient as inhibitors against metal corrosion [7, 8]. It has been commonly recognized that an organic inhibitor usually promotes formation of a chelate on a metal surface, by transferring p and/or p-electrons from the organic compounds to the metal and forming a coordinate covalent bond during the chemical adsorption [9-11]. Organic compounds, containing heteroatoms, such as sulfur, phosphorus, nitrogen and oxygen, together with aromatic rings in their structure are the major adsorption centers, and the Schiff bases, a condensation product of an amine and a ketone/aldehyde, are such typical molecules [3-8, 12-14].

Some polydentate Schiff base compounds (PSCs) have been reported as effective corrosion inhibitors for various metals in acid media [13, 15-17].

Several isatin derivatives have been reported as inhibitors in 1M HCl [18-20], the aim of this work is to screen the inhibitory action of an isatin Schiff base (3-(4-Hydroxyphenylimino)indolin-2-one) for the corrosion of mild steel in high concentrated HCl solution. Effects of concentration and the additives on the inhibition efficiencies will also be studied systematically.

Experimental

Synthesis of Isatin Schiff Bases

3-(4-hydroxyphenylimino)indolin-2-one was synthesized according to published methods (as shown in Scheme-1) [21, 22]. Isatin (1 mmol) was dissolved in methanol (20 ml) and a methanol solution of 1.2 mmol 4-aminophenol (10 ml) was added dropwise, until the disappearance of isatin, as evidenced by thin-layer chromatography. The solvent was removed in vacuo and the residue was separated by column chromatography (silica gel, petroleum ether/ethyl acetate = 1:1 v/v), to give the title compound. Red single crystals of the title compound suitable for X-ray analysis were obtained on slow evaporation of a methanol solution (30 ml) of the title compound (30 mg) over a period of 4 d. 1H-NMR (D6-DMSO, 400 MHz): 10.92 (1H, s), 9.56 (1H, s), 7.32 (2H, m), 6.86 (4H, m), 6.74 (3H, m); MS (EI) m/z: 238 (M+).

Scheme-1: Synthesis of 3-(4-hydroxyphenylimino) indolin-2-one by condensation reaction.

Gravimetric Measurements

The corrosion tests were performed on Q235A with a composition (in wt.%) C: 0.22, P: 0.045, Si: 0.35, S: 0.05, Mn: 1.40, and Fe balance. The electrolyte solution was 3M HCl, prepared from analytical grade 38% HCl and distilled water. The concentration range of 3-(4-hydroxyphenylimino)indolin-2-one was employed as 0.05g/L and 0.10g/L. All tests have been performed in deaerated solutions and at 60 0.5. The gravimetric tests were carried out according to the People's Republic of China Standard of Petroleum and Natural Gas Industry (Evaluation method for behavior of corrosion inhibitor for produced water of oilfield, SY/T5273-2000) with a few modifications. Each test was done with three specimens at the same time to give reproducible results.

Electrochemical Measurements

The electrodes were mechanically abraded with a series of emery papers (800 and 1,200 grades), then rinsed in acetone and double-distilled water before their immersion in the experimental solution. Electrochemical measurements were conducted in a conventional three-electrode thermostated cell. The electrode was inserted into a Teflon tube and isolated with polyester so that only its section (0.5 cm2) was allowed to contact the aggressive solutions. A platinum disk as counter electrode and standard calomel electrode (SCE) as the reference electrode have been used in the electrochemical studies.

The potentiodynamic curves were recorded using a CS350 system connected to a personal computer. The working electrode was first immersed in the test solution for 60 min to establish a steady state open circuit potential. After measuring the open circuit, potential dynamic polarization curves were obtained with a scan rate of 0.5 mV/s. Corrosion rates (corrosion current densities) were obtained from the polarization curves by linear extrapolation of the anodic and cathodic branches of the Tafel plots at points 100 mV more positive and more negative than the Ecorr.

CS350 electrochemical workstation hardware parameters:

Potentiostat potential control: 10V; Current Control Range: 2.0A; Potential control precision: 0.1% A- full scale reading 1mV; Current control accuracy: 0.1% A- full scale reading; Potential resolution: 10V (greater than 100Hz), 2V (less than 10Hz); Current resolution: less than 10pA; Potential rise time: less than 1S (less than 10mA), less than 10S (less than 2A); Auxiliary 24-bit data acquisition-10KHz, 20bit-1KHz; Reference electrode input impedance: 1012 ohms || 20pF; Current range 2A-100nA, a total of 8 files; Tank pressure: 21V; CV and LSV scan rate: 0.01-20000mV/s; CA and CC pulse width: 0.0001-1000s; Potential scan potential incremental: 0.1mV-1V/mS; SWV frequency: 0.001-100KHz; DPV and NPV pulse width: 0.0001-1000s; AD data acquisition: 16 bits-1MHz, 24bit-100Hz; Minimum potential increment CV: 0.075mV.

Results and Discussion

Inhibitor Properties and Mechanism

Q235A (A3) steel is a widely used material in the gas and oil field. Although it has been treated by heat and changing chemical composition to improve the corrosion resistance, for the aggressiveness of acidic media, it is also a great challenge especially in high concentration of HCl under high temperature. The use of corrosion inhibitors has been considered as the most effective method for the protection against such acid attack. Some inhibitors, such as imidazoline, Mannich base, Schiff base and some other heterocyclic compounds, have been employed in this process, but the concentration or the price is too high to be acceptable. In the following work, the performance of the title compound as an inhibiter with the concentration from 100 to 2000 mg/L in 1M and 2M HCl, and the results were summarized in Table-1.

From the table, it was found that almost all the IE increases along with the concentration of inhibiter, but there is difference in the two concentrations and different temperatures, and it reaches to 67.64% with the concentration of 500 mg/L in 2M HCl solution, further increase of the inhibitor does not increase the IE.

Table-1: The corrosion inhibition efficiency of 3-(4-hydroxyphenylimino)indolin-2-one.

Concentration###HCl###Temperature###Inhibition

###(mg/L)###Concentration###(C)###Efficiency

###(M)###(%)

###100###1###30###12.23

###100###1###45###22.49

###100###1###60###17.40

###100###2###30###12.74

###100###2###45###10.77

###100###2###60###19.30

###200###1###30###26.18

###200###1###45###18.01

###200###1###60###16.81

###200###2###30###16.51

###200###2###45###15.09

###200###2###60###18.92

###500###1###30###14.81

###500###1###45###77.63

###500###1###60###62.90

###500###2###30###40.12

###500###2###45###27.00

###500###2###60###67.64

###1000###1###30###63.94

###1000###1###45###27.24

###1000###1###60###23.67

###1000###2###30###58.68

###1000###2###45###13.94

###1000###2###60###30.80

###2000###1###30###11.90

###2000###1###45###9.88

###2000###1###60###36.95

###2000###2###30###21.71

###2000###2###45###19.96

###2000###2###60###28.91

The inhibition mechanism of the corrosion inhibiting may be mainly contributed to the adsorption. The process of adsorption is governed by the chemical structure of these inhibitors. The presence of N, O, S atoms and conjugated bonds in the structures makes the formation of p-d bonds resulting form the overlap of p electrons to the 3d vacant orbital of Fe atoms, which conforms the adsorption of the compounds on the metal surface [21]. The inhibitive performance of 3-(4-hydroxyphenylimino)indolin- 2-one can be explained on the presence of polydentate Schiff base in the molecular structure which rises the possibility of transferring the unshared electron of this molecule to iron [22]. The molecules may absorb on the ion surface by the coordinate covalent bonds with the 3d vacant orbital of Fe atoms in the manner described in Fig. 1 to form the protective film.

Tafel Polarisation Measurements

The anodic and cathodic polarization curves for a mild steel electrode in 2 M HCl in absence and compound at 298 K are shown in Fig. 2.

Table-2 shows the electrochemical corrosion kinetic parameters, i.e., corrosion potential (Ecorr), cathodic and anodic Tafel slopes (ba, bc) and corrosion current density Icorr obtained by extrapolation of the Tafel lines. The IE (%) is also calculated from the following equation:

(Equation)

where Icorr and Icorr(i) are corrosion current densities obtained in the absence and presence of inhibitors, respectively. As it was expected both anodic and cathodic reactions of mild steel electrode corrosion were inhibited by the increase of the title compound. This result suggests that the addition of the title compound reduces anodic dissolution and also retards the hydrogen evolution reaction [23]. It can be seen that the corrosion rate is decreased and inhibition efficiency IE is increased by increasing inhibitor concentration [23]. With a concentration of 100 mg/L, title compound exhibits maximum IE of 69.4%. Increasing IE with increasing concentration of the title compound shows that the inhibition actions are due to its adsorption on the steel surface.

The IEs obtained from potentiodynamic polarization were quite different from those calculated from weight-loss measurements, which is attributable to the fact that the weight-loss method gives average corrosion rate, whereas electrochemical method gives instantaneous corrosion rates. These differences may arise frequently because of the difference in the time required to form an adsorbed layer of inhibitors on metal surface [24].

Table-2: Potentiodynamic polarization parameters for the corrosion of the A3 steel in the HCl solution containing inhibitor

Concentration###Eo(V)###Io###Ba(mV)###Bc(mV)###Corrosion###IE (%)

(mg/L)###(A/cm2)###rate(mm/a)

###0###-0.4545 2.9237A-10-4###203.06###130.79###3.4391

100###-0.4467 1.91945A-10-4###179.60###109.40###2.8577###16.8

200###-0.4905 1.8548A-10-4###254.00###130.65###1.9818###42.8

500###-0.4820 1.6710A-10-4###257.30###132.30###1.4357###58.1

1000###-0.4800 1.0400A-10-4###236.81###109.49###1.0641###69.4

Optimizing of the Formulation

Hexamethylenetetramine (HMTA, shown in Fig. 3) is known as urotropine. The molecule has a symmetric tetrahedral cage-like structure, similar to adamantane, whose four "corners" are nitrogen atoms and "edges" are methylene groups. The molecule behaves like an amine base, undergoing protonation and N-alkylation. With these features, it has been used as corrosion inhibitor.

Beside, 1,4-dihydroxy-2-butyne (BOZ) (shown in Figure 3) contains hydroxyl groups and p-electron in triple bonds, which makes it is suitable used as a corrosion inhibitor in many conditions. But it was found that the inhibition efficiencies are relative poor, or much quantity is needed to obtain higher inhibition efficiency (about 1-3%), so the two are only used as additives in some corrosion inhibitor formulations [25, 26]. In the following work, HMTA and BOZ were investigated as synergists for the isatin Schiff base (IS) in 2 M HCl, and the results were shown in Table-3.

Table-3: The corrosion rate inhibition efficiency of some Schiff base derivatives companioning with HMTA or DHBY

###Temperature###Inhibition Efficiency

###Formulation

###(C)###(%)

###HMTA###30###23.38

###HMTA###45###18.95

###HMTA###60###15.32

###BOZ###30###19.25

###BOZ###45###23.69

###BOZ###60###20.18

###IS + HMTA###30###60.15

###IS + HMTA###45###44.78

###IS + HMTA###60###82.56

###IS + BOZ###30###55.37

###IS + BOZ###45###50.50

###IS + BOZ###60###78.26

###IS + HMTA + BOZ###30###81.50

###IS + HMTA + BOZ###45###75.39

###IS + HMTA + BOZ###60###85.33

From the table, it was found that HMTA and BOZ are not efficient inhibiters, but they can enhance the IE of the title compound effectively, and the highest IE reaches to 85.33%. The reason might lie on the fruitful p-electrons of N, O and triple bond, which can form covalent bonds between the molecules and the ion surface, capture H to release the acidity and even join the isatin Schiff base molecules as "bridges" to conform the protective film on the ion surface.

Conclusions

3-(4-hydroxyphenylimino)indolin-2-one was synthesized and the its inhibition and the mechanism on the corrosion of high protective Q235A steel in HCl solution were screened and discussed. The title compound can inhibit the corrosion with moderate inhibition efficiency in different conditions and the highest reaches to 77.63%, as it was companied with hexamethylenetetramine (HMTA) and 1,4-dihydroxy-2-butyne (BOZ), the inhibition efficiency was enhanced to 85.33%. Potentiodynamic polarization studies indicate that extracts are mixed-type inhibitors.

Acknowledgments

This work was financially supported by the grants from National Science Foundation of China (50874092) and Scientific Research Program Funded by Shaanxi Provincial Education Department (15JF035).

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