Water-soluble carboxymethylchitosan used as corrosion inhibitor for carbon steel in saline medium

Biodegradability and ecotoxicity of products used in oil industry are of great relevance and corrosion inhibitor could not be an exception. In earlier reports, chitosan and some derivatives were evaluated as corrosion inhibitors at acid pH, mainly due to polymer solubility.

Contents lists available atScienceDirect

Carbohydrate Polymers journal homepage:www.elsevier.com/locate/carbpol

Water-soluble carboxymethylchitosan used as corrosion inhibitor for carbon

steel in saline medium

Ruza Gabriela Medeiros de Araújo Macedoa, Nívia do Nascimento Marquesa, Josealdo Tonholob,

Rosangela de Carvalho Balabana,⁎

a Universidade Federal do Rio Grande do Norte – UFRN, Natal, RN, Brazil

b Universidade Federal de Alagoas – UFAL, Maceió, AL, Brazil

A R T I C L E I N F O

Keywords:

Carboxymethylchitosan

Corrosion inhibitor

Carbon steel

Chlorides

Polarization

EIS

A B S T R A C T Biodegradability and ecotoxicity of products used in oil industry are of great relevance and corrosion inhibitor could not be an exception In earlier reports, chitosan and some derivatives were evaluated as corrosion in-hibitors at acid pH, mainly due to polymer solubility An eco-friendly corrosion inhibitor with water solubility in all pH range should be ideal, as well as that could act under the high salinity of oilfield environment Thus, herein is presented the performance of a water-soluble carboxymethylchitosan (CMC) as corrosion inhibitor in presence of chlorides (3.5% NaCl) in 1020 carbon steel, without any addition of acid or base CMC showed good properties as corrosion inhibitor in media containing Cl−, and behaved as an anodic inhibitor CMC exhibited inhibitory efficiency of about 80% and 67%, according to Tafel curve and electrochemical impedance, respec-tively, which was attributed to chemisorption mechanism (ΔGads≈ −45 kJ/mol)

1 Introduction

Crude oil, a complex mixture of hydrocarbons, is the basis for

en-ergy savings around the world In many cases, the presence of these

liquid hydrocarbons may help to reduce corrosion as a result of their

ability to form highly adherent films on the metal surface (Heakal,

Fouda, & Radwan, 2011) However, in other cases, the presence of

impurities such as H2S, CO2, naphthenic acids and chlorides can

pro-mote the corrosion of steel pipes and equipment used in the

explora-tion, producexplora-tion, transportation and petroleum refining process

(Ghassem Mahjani, Neshati, Parvaneh Masiha, & Jafarian, 2007;Heakal

et al., 2011) The main cause of corrosion in carbon steel by pitting is

related to the presence of chloride ions (Ghassem Mahjani et al., 2007),

due to its aggressive nature that is attributed to its small ionic radius,

which allows a greater diffusion between the monolayers formed in the

metal surface Corrosion in the petroleum industry can be generally

mitigated by the use of inhibitors, which are chemicals that retard the

rate of corrosion of the metal (Heakal et al., 2011)

In general, the chemicals used as inhibitors contain in their

struc-ture phosphonates and sulfonates, which, although efficient, have low

biodegradability (Frenier & Ziauddin, 2008) Therefore, frequent use

can result in damage to the environment One of the great challenges of

the industry has been to develop chemicals that are environmentally

safe and effective in inhibiting corrosion under the conditions of each well The efficiency of these organic inhibitors of corrosion is related to the presence of polar functions containing S, O or N atoms, which are centers for the adsorption process In addition to these compounds, there are also polymers or macromolecules that function as good cor-rosion inhibitors, not only by the presence of functional groups (eOH, eCOOH, eNH2, etc.) but also by the size of the polymer chains, which favors the adsorption in the surface (Benchikh, Aitout, Makhloufi, Benhaddad, & Saidani, 2009; Darmokoesoemo, Suyanto, Anggara, Amenaghawon, & Kusuma, 2018; Eduok, Ohaeri, & Szpunar, 2018; Mobin & Rizvi, 2017; Sun, Wang, Wang, & Yan, 2018; Umoren, AlAhmary, Gasem, & Solomon, 2018)

The most common corrosion inhibitor polymers are derivatives of polyamines, polyvinylamides, polyaspartates and other amino acids, polyaniline, polycarboxylates/polycarboxylic acids and poly-saccharides (Tiu & Advincula, 2015) In general, corrosion inhibitor selection criteria are not limited to their chemical structure but also to their environmental impact, which should be low Due to the world-wide interest in environmental safety, the use of toxic chemicals and the operations in which they are generated have been minimized For this reason, natural products such as organic acids, vitamins, plant extract and natural water-soluble polymers have been studied as cor-rosion inhibitors (Bello et al., 2010;El-Haddad, 2014;Jenkins & Harris,

https://doi.org/10.1016/j.carbpol.2018.10.081

Received 15 June 2018; Received in revised form 23 October 2018; Accepted 23 October 2018

⁎Corresponding author

E-mail address:balaban@supercabo.com.br(R.d.C Balaban)

Available online 26 October 2018

0144-8617/ © 2018 Elsevier Ltd All rights reserved

T

2011;Umoren & Eduok, 2016).

In particular, many papers have been describing the performance of

chitosan or its derivatives as corrosion inhibitors and the obtained

re-sults have been considered very promising (Cheng, Chen, Liu, Chang, &

Yin, 2007;Eduok et al., 2018;El-Haddad, 2013;Giuliani et al., 2018;

Menaka & Subhashini, 2017; Sangeetha, Meenakshi, & Sundaram,

2016;Srivastava et al., 2018;Umoren et al., 2018;Wan, Feng, Hou, &

Li, 2016) Chitosan is a polymer generally obtained by desacetylation of

chitin, a polysaccharide extracted from the shells of crustaceans,

exoskeleton of many arthropods and some fungi Mainly due to polymer

solubility, the studies have been restricted to acid-induced conditions,

mostly in sweet water However, in order to avoid poor solubility or

precipitation, which would lead to inefficient inhibition, financial loss

and reduction of petroleum production, oil industry requires a

corro-sion inhibitor water-soluble in all pH range and that could be applied

under typical high salinity encountered in thefield

Thus, the main objective of this work is to evaluate the behavior of a

water-soluble carboxymethylchitosan as a preventive inhibitor of the

corrosion processes in pipelines used in the oil well installations of

Brazil, considering the high salinity of 3.5% NaCl in the medium

2 Experimental

2.1 Chemical and materials

The water-soluble carboxymethylchitosan (M¯v= 2.28 × 104g/mol

and degree of carboxymethylation = 0.55) was prepared as described

byChen and Park (2003), however, with some changes The

tempera-ture wasfixed at 10 °C and the water/isopropanol ratio used was 2/8

(Fig 1) The chemical composition of carbon steel used in the study is

the following (weight %) C– 0.18, S – 0.05, P – 0.04, Mn – 0.85 The

coupon was embedded in epoxy resin in a glass tube and the electrical

count was performed through a copper wire Prior to all measurements,

the exposed surface area of the electrode (0.308 cm2) was abraded with

series of emery papers up to 1200 grade, rinsed with double distilled

water, ethanol and dried air This was used as the working electrode

during the electrochemical methods The aggressive solution used was

3.5% NaCl diluted in double distilled water Stock solution of

carbox-ymethylchitosan (1 g/L) was prepared in double distilled water The

concentration range of CMC used in this work was 10–80 ppm

2.2 Electrochemical measurements

A conventional three-electrode cell, composed of a working

elec-trode carbon steel 1020, Ag/AgCl reference elecelec-trode and

contra-elec-trode was used for electrochemical measures The experiments were

performed in a Metrom Instrument Autolab PGSTAT 30 Potentiostat/

Galvanostat with FRA– Frequency Response Analyzer, which were held

the following electrochemical techniques: Linear Voltammetry and

Electrochemical Impedance Spectroscopy Initially, the electrode was

preconditioned on open circuit potential (OCP) for 30 min at a

tem-perature of ± 25 °C for all trials Potentiodynamic polarization

mea-surements were performed in a range of ± 100 mV of OCP with

near-stationary scanning of 1 mVs−1 From the polarization curve, were

calculated the corrosion current density and corrosion rate EIS mea-surements were carried out using AC signals of amplitude 10 mV peak

to peak at the open circuit potential in the frequency range 10 kHz a

100 mHz The electrical equivalent circuit was estimated from the EIS Spectrum Analyser software, which uses the method of complex non-linear least squares to approximate the theoretical data of experimental The quality of the treatment of experimental data was evaluated through a parameter set to Chi-square, x2, which indicates a good ap-proximation the smaller its value For this study, all the approaches were estimated with values in the order

3 Results and discussion

3.1 Electrochemical measurements 3.1.1 Potentiondynamic polarization Potentiondynamic polarization curves for 1020 C-steel in 3.5% NaCl

in absence and presence of different concentrations of CMC at 25 °C are shown in Fig 2 The percentage inhibition efficiency (ε %) and the degree of surface coverage (θ), were calculated from the Eq.(1)(Wang, Liu, & Xin, 2004;Zhang, Gong, Yu, & Du, 2011)

⎣

⎦

I

(%) * 100 corr corr * 100

corr

0 0

(1) Where I°corrand Icorrare the corrosion current densities in the absence and the presence of the inhibitor, respectively InFig 1is possible to observe a displacement in the corrosion potential by the addition of the carboxymethylchitosan to the medium, indicating that the polymer has

a strong potential for inhibition of corrosion by chloride The corrosion potential of NaCl 3.5% was −501 mV When 10 ppm of CMC was added, this potential was shifted to−484 mV And at each concentra-tion increase, that potential was shifted to more positive regions,

Fig 1 Synthesis of carboxymethylchitosan

Fig 2 (a) Polarization curves obtained with the 1020 C-steel electrode in 3.5% NaCl in the presence and absence of different concentrations of the CMC in-hibitor at 25 °C andυ = 1 mVs−1; (b) Curve of corrosion potential as a function

of CMC concentration

reaching−444 mV at the concentration of 80 ppm This result suggests

an anodic behavior From these curves, it was possible to estimate,

through the extrapolation of the Tafel curve, the electrochemical

parameters related to this system, such as current density and corrosion

rate, which are presented inTable 1 The obtained data indicated that

the corrosion rate reduces with increasing concentration of CMC in the

system, reaching 8 times less than pure brine for the maximum

con-centration of inhibitor The corrosion current is also reduced with

in-creasing inhibitor concentration, thus suggesting the formation of a

protective layer that hinders the permeation of ions through the electric

double layer in order to reach the metal surface Therefore, it is

sug-gested that CMC shows a good performance as a corrosion inhibitor

Table 1shows the electrochemical parameters that give evidences

that the degree of coverage of the metal rise with increasing

con-centration of the CMC, suggesting a possible chemical adsorption on

metal surface

3.1.2 Electrochemical impedance spectroscopy (EIS)

The impedance responses of 1020 c-steel in solutions of 3.5% NaCl

in absence and presence of CMC in various concentrations, at 25 °C, are

represented inFig 3, at Nyquist and Bode plots The Nyquist diagram is

typical for a system of carbon steel corrosion in 3.5% NaCl solution (

El-Haddad, 2014) Although it is not formed a perfect semi-circle, can be

seen that the addition of CMC causes the formation of a bow more

capacitive, compared to the curve of 3.5% NaCl solution, which induces

an increase in diameter of the arc The intersection of the curve with the

x-axis (Zr) provides data of polarization resistance, it can be said that

the relationship between diameter and increase resistance to

polariza-tion is direct It is observed that the polarizapolariza-tion resistance increases

with the increase of CMC concentration

The Bode curves represented inFig 3(c) show that the addition of

the inhibitor did not cause a significant displacement to a region of

greater impedance when compared to the curve of the electrolyte,

however a slight increase in the polarization resistance of 1020 carbon

steel was observed with the increase of CMC concentration, with the

highest displacement reached when using the polymer concentration of

80 ppm, which suggests a higher adsorption on the metal surface

The cross of the line, which determines the slope of the curve, with

Logω = 0 gives the electric double layer capacitance through Eq.(2)

(Umoren, Obot, Madhankumar, & Gasem, 2015;Wang, Liu, Bin, & Xin,

2004)

=

Z

C

| | 1

It is possible to extract from this curve the data concerning the

re-sistance of the electrolytic solution (Re) and resistance to polarization

(Rp) The difference between these two provides the load transfer

re-sistance (Rtc) The load transfer resistance (Rtc) and the electric double

layer capacitance (Cdl) obtained experimentally after treatment of the

data with specific software are presented inTable 2 It is evidenced that

the addition of CMC causes an increase in the transfer resistance of

charge and consequently a reduction in the capacitance of the double

electric layer These low Cdlvalues may be associated with an increase

in the thickness of the electric double layer (Zhang et al., 2011),

Table 1

Electrochemical parameters acquired from Tafel extrapolation for 1020 C-steel

processes in the absence and presence of carboxymethylchitosan, at 25 °C

CMC concentration

(ppm)

E corr

(mV)

I corr

(μA/cm 2 )

T (mm/year)

θ ε (%)

10 −484 2.102 4.070e−3 0.507 50.71

20 −472 1.579 3.024e−3 0.521 52.07

40 −467 0.468 1.501e−3 0.818 81.82

80 −444 0.458 1.191e−3 0.855 85.57

Fig 3 Nyquist plots (a) and Bode plots (b,c) of 1020 c-steel in uninhibited and inhibited 3.5% NaCl solutions containing various concentrations of CMC, at

25 °C

Table 2 Data of the load transfer resistance (Rtc) and the electric double layer capaci-tance (Cdl) for CMC after treatment of the experimental data

Inhibitor Concentration (ppm) R tc (Ωcm 2 ) C dl (μF/cm 2 )

suggesting that the inhibitor molecules adsorb at the metal/solution

interface

In the Bode (Log ω vs phase angle) curves for CMC at different

concentrations is possible to observe the existence of two phase

con-stants when CMC is added because the diagram shows two distinct

peaks Thefirst peak, located in the low frequency region, can be

at-tributed to the metal/electrolyte interaction It is possible to observe

that the increase in the concentration of CMC increases the phase angle,

which contributes to a better protection of the metal The second peak,

a region of high frequency, can indicate a higher resistance of the steel,

associated to the process of adsorption of thefilm The maximum point

of the curve is related to the polarization resistance, according to Eq.(3)

(Umoren et al., 2015;Wang et al., 2004) By the Eq.(3), if Rp

(polar-ization resistance) is increased and Re (solution resistance) is kept

constant,θmaxwill increase Thus, it is observed that the concentration

of 80 ppm presents greater polarization resistance and probably higher

adsorption power, compared to the other concentrations, thus making

the steel more protected

=

+

m x

p

á

(3) Inhibition efficiency was computed from the electrochemical

im-pedance spectroscopy measurements using the Eqs (4) and (5)

R

1 tc

tc

0

(4)

=

ε θ

Where, Rtc° and Rtccorrespond to the charge transfer resistance without

and with inhibitor, respectively

The impedance spectra were analyzed by fitting an equivalent

electric circuit using the EIS Spectrum Analyzer Software, as shown in

Fig 4 CMC followed the same mechanism of action for all

concentra-tions studied In this way, we have that R1 represents the resistance of

the solution, CPE1 is the electric double layer capacitance, R2 is the

charge transfer resistance and Ws1 is the semi-finite linear diffusion

resistance (Warburg)

3.2 Adsorption isotherm

To obtain more information on the mode of adsorption of CMC on

the metal surface, the data acquired from the extrapolation of the Tafel

curve and electrochemical impedance spectroscopy were tested by

several models, and the best correlation was obtained with the

Langmuir isotherm According to this isotherm, θ is related to

con-centration through Eqs (6) and (7) (Abdallah, El-Etre, Soliman, &

Mabrouk, 2006;El-Haddad, 2014;Wang et al., 2004) The values of C

and C/θ are represented inTable 3for each technique studied

θ

Rearranging,

C

1

Where C is the concentration of the inhibitor andθ is the fraction of the surface covered

Eq.(7)provides a linearity between the values of C/θ and C Since

ΔGads is the free energy of adsorption and kads the equilibrium ad-sorption constant, it is possible to calculate this energy through Eq.(8) (Umoren & Eduok, 2016;Umoren et al., 2015) These parameters were calculated and are described inTable 4

55,5

Δ

ads

(8) Fig 5shows the adsorption isotherms according to the Langmuir model obtained by the two experimental techniques used in the study According to theFig 4, it is possible to observe linearity between C/θ and C for both techniques, with correlation values of 0.9902 and 0.9925 for Tafel and EIS, respectively, indicating an optimal correlation For a corrosion inhibitor to exhibit good adsorption and thus pro-mote good inhibition, it must be adhered spontaneously to the metal surface As the spontaneity is determined by the value ofΔGads, which

Fig 4 (a) Overlap of experimental and theoreticalfitting of Nyquist curves, at 10, 40 and 80 ppm of CMC, and (b) pro-posed equivalent electric circuit for the validation of the im-pedance curves of the system under study R1 = solution re-sistance, CPE1 = electric double layer capacitance, R2 = charge transfer resistance and Ws1 = semi-finite linear diffusion resistance

Table 3 Data on efficiency (ε) and degree of coverage (θ) of the CMC inhibitor obtained

by extrapolation of the Tafel curve and Electrochemical Impedance, at 25 °C

Inhibitor C (ppm) C (M) θ C (M)/θ

10 4.386e−7 0.507 8.6508e−7

20 8.772e−7 0.521 1.6836e−6

40 1.754e−6 0.818 2.1447e−6

80 3.508e−6 0.855 4.1038e−6 EIS

10 4.386e−7 0.384 1.1421e−6

20 8.772e−7 0.423 2.0737e−6

40 1.754e−6 0.536 3.2731e−6

80 3.508e−6 0.678 5.1751e−6

Table 4 Data of adsorption constant (kads) and free energy of Gibbs (ΔGads) for the in-hibitor CMC in NaCl 3.5% obtained by extrapolation of the Tafel curve and electrochemical impedance, at 25 °C

Electrochemical technique Inhibitor k ads

(M−1)

ΔG ads

(KJ/mol)

must be less than zero, it can be concluded that the CMC adsorbs

spontaneously on the metal surface TheΔG values by the Tafel and EIS

techniques were−45.666 KJ/mol and −44.661 KJ/mol, respectively

The presence ofeCOO−andeNH2 groups in the CMC chemical

structure may have favored interactions with the metal ions (Fe2+),

promoting strong adsorption of the inhibitor on the metal surface

(Benchikh et al., 2009;Darmokoesoemo et al., 2018;Eduok et al., 2018;

Mobin & Rizvi, 2017; Sun et al., 2018; Umoren et al., 2018) and,

consequently, a corrosion inhibition efficiency of 85%, for the 80 ppm

concentration of the inhibitor, since it has a high value of kads

(1.8219e6)

According to Hu, Zhang, Li, and Hou, (2010) and Wang et al

(2004), when the absolute value of ΔGads is below 20 kJmol−1, a

physisorption process occurs; and an absolute value of ΔGads above

40 kJ mol−1indicates a chemisorption process, and between the two

values there are two processes In this way, it can be inferred that the

CMC follows the mechanism of chemisorption, since the values ofΔGads

are greater than |40| kJmol−1 Thus, it is suggested a sharing or transfer

of organic molecules to the metal surface forming a coordinate-like

bond This phenomenon was confirmed by both techniques studied

Literature states that the chemisorption force derives from the

inter-action between the lone electron pairs of nitrogen atoms of amino

groups or oxygen atoms in the hydroxyl and carboxyl groups and the empty 3d orbitals of iron atoms on the metallic surface (Fig 6) (Yoo, Kim, Chung, Kim, & Kim, 2013;Wan et al., 2016) This result indicates that the inhibitor molecules can form a protective layer at the metal/ solution interface

4 Conclusions The water-soluble CMC is an excellent corrosion inhibitor in media

of 3.5% NaCl, since the efficiency results were 85%, determined by the extrapolation of the Tafel curve, at the maximum concentration studied (80 ppm) According to data from the extrapolation of the Tafel curve, the CMC behaves as an anodic inhibitor, as the polarization curves were shifted to a region of more positive potentials

The values ofΔGadsobtained by the extrapolation techniques of the Tafel curve and electrochemical impedance are consistent and suggest a mechanism of chemisorption, since this energy is greater than

|40| kJmol−1, which suggests a strong adsorption of the organic mo-lecule, carboxymethylchitosan, on the metallic surface The perfor-mance exhibited under NaCl medium, coupled to the water solubility of CMC, indicates that it has great potential as corrosion inhibitor of carbon steel in oilfield environment

Acknowledgement This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Brasil (CAPES) -Finance Code 001

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