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I m : s n i ) ^ OF CORROSION INHIBITION O F PYRIDINE AND ITS I > I : R I \ A I T V K S ON STI EL USIN(; E L E C T R O C H E M I C A L M E T H O D S
H o a n g Van H u n g , D a n g T h i Tuoi a n d N g u y e n N g o c H a I acuity oj Chemisii y. Hanoi National University ofEduealitm Vhslract. Two iidv;incc(l methods in clcctrochcmisiry have been employed lo study conosum inhibitnin of pyridine (PY) and ils methyl derivatives: 2-methyl pyridine |2MI''i I. .1-nictliyl pyridine (3MPY) and 4-methyl pyridine (4MPY) on sicci surface in acidic media of a I M HCI solution. The obtained results show that all mclhyl derivatives of pyridine arc more efficient than pyridine in inhibiting corrosion. The obiduicd results are also in good agreemenl with our theoretical results which were previously published The relationship between the electronic structure of pyridine and ils derivatives and corrosion inhibition efficiency has been explained referring to theoretical and experimental results.
Keywords Pyridine, impedance spectroscopy, polarization measurements, corrosion inhibition.
1. Introduction
Acid solutions arc generally used in seseral processes: industrial cleaning, acid descaling and in petrochemical processes. Hydrochloric acid is widely used in cleaning and pickling processing of metals where the use of inhibitors is one of the most practical methods used to protect metals from corrosion 111,
Organic substances containing heteroatoms such as nitrogen, sulfur and oxygen have frequently been used as corrosion inhibitors of metals in acidic media for the last few decades [1-3]. These substances are adsorbed onto the metallic surface and prevent the degradation of the metal in a corrosive media 14]. Among heteroatom-containing compounds, nitrogen-containing compounds function more effectively in hydrochloric solutions, whereas compounds containing sulfur are preferred for sulfuric acid solutions.
Many experimental studies have been carried out which have revealed the corrosion Received May 28, 2012. Accepted October I, 2012.
Contact Hoang Van Hung, e-mail address: [email protected] 28
The .study of eorrosicm inhibition tfpyndinr and ii\ dcrivaliws on steel...
efficiency of chemical compounds containing heteroatoms in .icidic media. Il was revealed that pyridine compounds arc effective coirosion inhibiiors for slccl in acidic solutions. Replacing hydrogen aloms in pyridine rings with sc\cral substituted groups has shown improved corrosion inhibition elTicienc> | 5 . 6|.
In ordci to examine the systematic rclalionship IILMWCCII the electronic properties of heteroaloms and atoms in Ihc molecules of inhibiiors. the present work made use of pyridine and its derivatives lo study corrosion-mhibiiion efficiency on steel surfaces in a i M HCI medium using a combination of advanced thcoreiical and electrochemical melhods. Theoretical calculations were carried (Hit using the density functional theory (DFT) mclhod and experiments weic done using two advanced electrochemical methods, polarization measurements and electrochemical impedance speclioscopy. Our theoretical calculations ha\c been published in a previous article | 7 | . In this ariiclc. we present the results of an experiment iiucsligaling corrosion inhibition effectiveness of pyridine and its methyl derivatives in a I M HCI medium on steel.
2. Content
2.1. Materials and methods
^ Materials
Pyridine, 1-methyl pyridine, 2-methyl pyridine, 3-methyl pyridine and 4-niethyl pyridine were purchased from the MERK chemical company An aggressive 1 M HCI solution was prepared using analytical grade 37% HCI and bi-dislilled water. All acidic solutions were used direclly with no deaerating procedure
For electrochemical measurements, steel samples (C = 0.08%, Mn = 2.00%, P = 0.045%, S = 0.030%, Si = 0.75%, Cr = 18,00%, Ni = 8.00%. N = 0.107c and remainder iron) were made in the form of disc electrodes, sealed by epoxy rcsin, with an exposed surface diameter of 2.0 mm.
* Methods
Cathodic and anodic polarization curves were potentiodynamically recorded wilh the scan rate of 5mVs"^ using a conventional three-electrode cell connected lo an Autolab potentiostal/galvanostat instrument supplied by Melrohm AUTOLAB B, V, Netherland.
The steel disc electrode with a diameter of 2.0 mm was used as a working electrode (WE), a platinum wire was the counter electrode and a saturated calomel electrode (SCE) served as the reference electrode. All experiments were performed al (25 C). The working electrode (WE) was polished mechanically with different emery papers up to 1000 grade.
After polishing, the electrode was washed several times with bi-distilled water, degreased with ethanol and dried.
The electrochemical impedance spectroscopy (EIS) measurements were carried out using an electrochemical system and instruments similar to that used in polarization 29
1 ui.ii|.' \,iii Ilung, Dang Thi Tuoi and Nguyen Ngoc Ha
iiuMMHi'iiRM' \Mili iiHululaiion aiiipiiludc of 10 mV in the frequency range between I 11/ 1,1 MK) k l l ' 1 \.iliMiioii nf the impedance dala was performed assuming equivalent (, iiLiiii w iili N< )\ A sdllware version 1,5.
2.2. Resiills a m i di.scussion
2.2.1. tOnccnliiilion (kpiiidtiUT ut (he inliihition efficiency of pyridine Pnlaii/atioii curves m the 'fafcl form of steel in a 1.0 M HCI .solution with and without the adiliiion of PY al dilfcienl concentrations arc shown in Figure 2, The clcctiochemical paiaineiers associalcd with polarization mea.surcments. for instance polarization resistances R,, and conosion current densities i,„,, arc listed in Table 1.
The \ahics oi coirosion current density and polarization resislance R,, were obtained by extrapolating Tafcl plots and using the SlcnvCicaiy equation, respectively, [S. 9] with the help of NOVA 1,5 software.
/ ^ • ^ 1
/?,, — • (Stern-Geary equation) '' 2,.3():!(/-.,+ /v) ',,.u A
The corrosion inhibition efiiciencics were calculated making use ofthe following equation [10]:
,,(%) = ' " - ' ' : " ' ° ^ '
where b„ and b, are Tafel slopes and A is the surface area of the steel electrode, i.^rr and ''orT(iniubttor) ^^^ the corrosion current densities for uninhibited and inhibited solutions respectively.
(•) (b) (c) (d) Figure I. Structures of pyridine (a), 2M-Py (b), 3M-Py (c) and 4M-Py (d) From electrochemical polarization measurements, it is clear that the addition of the inhibitor caused a decrease in current density. The values of corrosion current density of steel in inhibited solutions were smaller than those for the inhibitor free solution.
The Tafel plots remain almost unchanged indicating that the presence of these inhibitors 30
The study ofeorrasion inhibition cf pyridine and iis derivatives on steel...
has no effect on the mechanism of the dissolution process of steel and the adsorbed molecules mechanically .screen the coated part of electrode and thciefore protect the steel surface from corrosive media [11]. In general, (he presence of inhibilois will prevent the degradation of a metal surface that is caused by the attack of oxidants present in the electrolyte. These inhibitors arc adsorbed onto the metallic surfaces lo form a barrier isolating the metal surface from attacking species | I 2 | The iiihibilion efficiency of inhibitors depends on their concentration in solulion (sec Tabic I). lu the case of PY.
we found that inhibition efficiency reaches a maximum value at the concentration of 10"^' M (the highest value of polarization lesistaiicc. 181 ^'). In order to compare the inhibition efficiencies of inhibitors conveniently, the conccntiation of 10"' M was employed for all inhibitors in this study.
Figure 2. Tafel plot for the steel electi-ode in LOM HCI with and without pyridine at different concentrations
Table 1. Electi-ochemical parameters of steel in various concentrations ofPY in LO M HCI Cjv;/niol L '
5.10-=" M l O - ' M S . l O - i M
1 0 - ' ' M S.IO-^^M
10-= M 5.10-" M
blank
i,„rrili^ em '^
26.1 29.3 31.1 28.9 27.3 25.1 29.8 33.4
R,.
175 156 148 158 165 181 153 136
MM.II)^: V.iii 11 IIII;', D.iiigTIii Tuoi and Nguyen Ngoc Ha
IIC.I I 1 I (1 /cd II
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• niiiiihci
11 laic 1 R, I
uy pol MCI '
•spomls ((
in r l r c i i o t hi'iiiicai im|ic{.laiicc measurements arc similar to iiou uicasutvmenis, I-igurc 3 shows N y q u i s t plots o f steel ,111(1 w i l h o u l Ihc a d d i t i o n o l p y r i d i n e . T h e impedance dala ilcui t i i c i i i l coinposctl o f a dotiblc layer capacitance, C/>/,, . and s o l i i i m j i icsisiaiu c. K s . as in Figure 4 . In general, the loycd c q i i i v a l c n l c i i c u i l depends ou cach particular system.
and oihci c l c m c n l s needed l o l u l l y describe the impedance ion o l a t i l w i t h i i u n i m a l d e v i a t i o n between measured and tin laie is inversely p r o j i o i t i o n a l t o the value o f R, / and a ) a l o w t n i i o s M i n rale 1131.
- # ^5 .•- s III • \ f S 10 \ 1 Ml \ ( i n ' \ | 'V •; I d ' M
Mi \ I blank Filling
Figure 3. Nyquist plots oj steel eleeirotle in LO M HCI with and without the addition of inhibitttrs
Ra R,
Figure 4. Eqaimlenee circuit for steel electrode in I.O M HCI with and without the addition of inhibitors
2.2,2, I n h i b i t i o n efTiciency of p y r i d i n e a n d its methyl derivatives
Figure 5 shows a Tafel plot for steel electrode in 1.0 M HCI with the addition of different inhibitors at a concentration of I Q - ' M. It is clear that when a hydrogen atom 32
The sludy of corrositm inhihititm of pyridine and ils derivatives tm steel...
at any position in a pyridine ring is replaced by a methyl group, inhibition cITiciency will increase (see Table 2), However, the increase in inhibition efficiency depends on the position of the substituted group. The inhibition efficiency of 2Me-PY ((lO 5'/>) and 4Me-PY (63.2%) is higher than Ihat of 3Me-PY (55,7%) and PY (24.8%), There ai e some parameters which can affect the inhibition efficiency ttf the inhibitors. In the case of PY and its methyl-derivatives, efficiency depends much on their electronic and geometric structures. When a methyl group is al position number 2 or 4, (he donating effect (or inductive effect) of the methyl gioup and the conjugation effect creates an increase in electron density at the N atom which is much more greater in comparison lo the electron density ofthe N atom in pyridine and ^Mc-I'Y This incrca.scs the bond strength between the steel surface and the inhibitor molecules which leads to an increase in inhibition efficiency of the inhibitors. In 3Me-P> the donating effect of Ihc methyl group docs nol have as much affect as il docs in 2 and 4Mc-PY and therefore inhibition efficiency is less in 3Me-PY than it is in 2 and 4Mc-PY.
•
^
It 'i
•
PY - - 2Me-PY
3Me-PY - -4Me-PY
HCI
Figure 5. Tafel plots for the steel electrode in 1.0 M HCI with different inhibitors at a concentration of 10'" M
Table 2. Electrochemical parameters of steel at concentration of 10 ^ M ofPY and its derivatives in 1.0 M HCI
Parameter l„rlpA cm-^
Rp/oj RCTIII^
r,(%)
HCI 33.4 136 127
-
HCI + PY 25.1
181 156 24.8
HCI + 2Me-PY 13.2 348 336 60.5
HCI + 3Me-PY 14.8 299 253 55.7
HCI + 4Me-PY 12.3 368 299 63.2
33
I liiiiiL' Daug Tlli Tuoi and Nguyen Ngoc Ha
Figure <i Nytpiisl plots fttr the steel electritde in 1.0 M HCI with different inhibitors at concentration of 10 .M
The restilis obtained from cxpcrinicnlal work arc in good agreement with the theoretical results published elsewhere | 7 | and with experimenlal study on an aluminum surface of PY and ils derivatives The polarization icsistances calculated from Tafel plots (R/') arc almost in agreement with Ihc values of charge transfer resislance ( R ( T ) calculated from impedance data. The relatively small difference between these two values at the same electrode is expected because al frequency zero the sum of all ohmic components of the impedance is equal lo polarization resistance. The relatively small contribution of (he solulion resistance, Rs. and the double lavcr results in the present case in fairly good agreement between R,. and RCT 114|,
3. Conclusion
Corrosion inhibition efficiencies of pyridine and its methyl derivatives were studied experimentally using electrochemical melhods. The results obtained reveal the relationship between electronic structure and the inhibition effect of inhibitors.
The experimenlal results are in good agrecmeni with the theoretical calculation and experimental study on an aluminum surface. These results can be applied to corrosion study which combines theoretical calculation and experimental study.
Acknowledgements. The authors would like to acknowledge the financial support received from the Ministry of Education and Training through the B2010-17-240 Project
The sludy ofeorrasion itthihiliott of pyridine end us derivalivcs nit .\lct'l. .
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