Electrochemical and anticorrosion properties of 5 hydroxytryptophan on mild steel in simulated well acidizing fluid Accepted Manuscript Title Electrochemical and anticorrosion properties of 5 hydroxyt[.]
Accepted Manuscript Title: Electrochemical and anticorrosion properties of 5-hydroxytryptophan on mild steel in simulated well acidizing fluid Authors: Ekemini Ituen, Onyewuchi Akaranta, Abosede James PII: DOI: Reference: S1658-3655(17)30008-0 http://dx.doi.org/doi:10.1016/j.jtusci.2017.01.005 JTUSCI 353 To appear in: Received date: Revised date: Accepted date: 19-9-2016 12-12-2016 3-1-2017 Please cite this article as: Ekemini Ituen, Onyewuchi Akaranta, Abosede James, Electrochemical and anticorrosion properties of 5-hydroxytryptophan on mild steel in simulated well acidizing fluid, http://dx.doi.org/10.1016/j.jtusci.2017.01.005 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Electrochemical and anticorrosion properties of 5-hydroxytryptophan on mild steel in simulated well acidizing fluid Ekemini Ituen1,2* Onyewuchi Akaranta2,3 Abosede James Materials Physics and Chemistry Research Laboratory, China University of Petroleum, Qingdao, East China African Centre of Excellence in Oilfield Chemicals Research, Institute of Petroleum Studies, University of Port Harcourt, Nigeria Department of Pure and Industrial Chemistry, University of Port Harcourt, Nigeria *Corresponding author’s email: ebituen@gmail.com Abstract Anticorrosion effect of 5-hydroxytryptophan (5-HTP) on mild steel (MS) was investigated by gravimetric and electrochemical techniques Two different concentrations (1 M and 15 %) of hydrochloric acid were used to simulate well acidizing fluid Results show that in 10 x 10-5 M 5-HTP is 96.1% efficient in M HCl and 78.1% efficient in 15 % HCl at 30 oC The efficiency decreases as temperature increases, reaching 66.9 % and 39.8 % in M and 15 % HCl respectively at 90 oC When 5-HTP is blended with potassium iodide and glutathione, the efficiency increases to values above 88 % and 78 % in M and 15 % HCl respectively at 90 oC Increase in 5-HTP concentrations decreases the double layer capacitance and increases charge transfer resistance 5-HTP behaves as mixed type corrosion inhibitor with anodic predominance and is spontaneously adsorbed on the steel surface Physisorption of 5-HTP is best described by Langmuir adsorption model and is also exothermic with resultant decrease in entropy of the bulk solution Results of SEM/EDAX, FTIR and UV-VIS studies support that a protective film forms 5-HTP and MS facilitated by O, N and C=C functionalities Keywords: 5-hydroxytryptophan; well acidizing; corrosion inhibitor; EIS; SEM/EDAX, acid corrosion Introduction When existing wells deplete and their natural pressure declines, the use of chemistry to maintain production becomes very essential This is achieved through well stimulation, fracturing, secondary and enhanced oil recovery operations Well acidizing is a common field practice that involve forcing acid through the well bore at high pressure to dissolve formation rocks, enlarge existing flow channels and open new ones It can also be used for removal or clearing of scales During these procedures, corrosion of metal structural materials occurs because of contact with acid Such materials include line pipes, casings and tubings, as well as storage facilities, which are usually constructed from steel The consequences of corrosion include rupturing of these materials, spills, failure and loss of integrity of materials and flow problems [1, 2] To avoid the high cost of managing/cleaning spills or the down time for shutting down of plant for maintenance or damage to employee and company integrity, prevention of corrosion by use of corrosion inhibitors (CI) becomes essential Therefore, huge amounts are spent on CIs, being a simple, practical and cost effective means of reducing corrosion CIs are substances which are added in small amounts to the corroding fluid in order to retard the speed of corrosive attack on the metal surface it contacts [3] They offer surface protection by adsorption of their active functionalities on metal surfaces [4] A thin protective film which acts as a blanket on the metal surface and protects it from the aggressive medium is formed The inhibitor effectiveness varies with properties like its chemical composition, concentration and operational temperature [5] Many compounds that contain functionalities like nitrogen, oxygen, multiple bonds, conjugated double bond systems, heteroatoms and aromatic rings have been found to be efficient CIs for different metals in various media [6-15] However, some of them are toxic or very expensive hence the need to source CIs from cheap, sustainable and non-toxic materials 5-HTP is not toxic and can be locally sourced in large quantity [16, 17] or cheaply synthesized [18, 19] It also contains electron rich functionalities earlier mentioned in its molecular structure (Fig 1) These motivated us to investigate 5-HTP as alternative CI for MS Fig Chemical structure of 5-HTP Hydrochloric acid is widely used in the field for acidizing purposes; therefore the well acidizing fluid used in this study is simulated using M HCl and 15 % HCl [20, 21] Mild steel coupons are used to simulate steel pipelines, casings, etc Standard techniques like Thermogravimetry or weight-loss (TG), Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP) are used to assess the inhibitor efficiency The morphology of the metal specimen is checked by SEM to observe possible difference with and without the inhibitor Other techniques like FTIR, UV-VIS, EDAX are employed to further characterize the inhibition phenomenon The 5-HTP was also blended with some compounds to improve its efficiency at high temperature Adsorption, kinetic, and thermodynamic models are employed to further explain the inhibitor interaction with metal surface Experimental 2.1 Preparation of steel specimens MS sheet was purchased from Building and Construction Materials Market in Uyo, Akwa Ibom state It was mechanically press-cut into coupons of dimension cm x cm for TG experiments; 1cm x cm for electrochemical studies and cm x cm for surface analysis The surface was treated as provided by NACE Recommended Practice RP-0775 and ASTM G-1 & G-4 for surface finishing and cleaning of coupons for weight-loss In addition, the mild steel coupons for electrochemical studies were abraded to mirror surface with CC-22F P2000 grade silicon carbide paper All the prepared specimens were enclosed in sealed water-proof bags and stored in moisture free desiccator prior to use The chemical compositions (wt %) of MS was C (0.13), Si (0.18), Mn (0.39), P (0.40), S (0.04), Cu (0.025), Fe (balance) 2.2 Preparation of inhibitor solutions Analytical grade HCl was diluted to concentrations of M and 15% using double- distilled water Powdered 5-hydroxytryptophan (HPLC 99.9 %, extracted from seeds of Griffonia simplicifolia) was supplied by Shaanxi Kanglai Ecology Agriculture Co Ltd., China and was prepared (as received) to five different concentrations (1x10-5, 3x10-5, 5x10-5, 8x10-5 and 10x105 M) in both M and 15 % HCl solutions 2.3 Preparation of inhibitor blends The additives used in this study were: (i) PEG-4000 (Industrial grade) supplied by Richest group Ltd., Shangai, China (ii) Sodium gluconate (Analytical grade) supplied by Wuhan Yuancheng Gongchuang Technology Co Ltd., China (iii) Potassium chloride (Analytical grade) supplied by Meyer Chemical Technology Co Lt., Shangai, China (iv) Glutathione (Industrial grade) supplied by Wuhan Yuancheng Gongchuang Technology Co Ltd., China Each compound (as received) was prepared to a concentration of 1x10-6 M [3] in the respective acid solutions and blended with the 5-HTP at the ration 1:1, followed by vigorous stirring 2.3 Thermo-gravimetric (weight loss) technique These measurements were conducted according to ASTM standard method explained in literature [20] Sartorius CPA225D analytical balance with sensitivity = ±0.00001 g was used for weight measurement Pre-weighed steel coupons were immersed in the acid solutions without and with the test solutions for five hours maintained at 30 oC in a water bath Retrieved coupons were cleaned in 20 % NaOH solution containing about 200 g/L of zinc dust, dried in air after rinsing in acetone and weighed to determine the weight loss Experiments were carried out in triplicates and the mean values of the weight losses (g) were used for computation Corrosion rate (CR), Inhibition efficiency (ℇ ) and degree of surface coverage (θ) were calculated as follows [22]: = ℇ ∆ (1) = 100( = 0.01ℇ where ∆ ) (2) (3) is the mean weight loss of MS, absence and presence of the inhibitor, and are the corrosion rates (cmh -1) in the is the density of iron, is the average surface area (cm2) of the metal specimens and is the immersion time (h) The obtained corrosion rate values were converted to another unit (mmpy) using conversion factors explained in literature [20] This procedure was repeated at other temperatures namely 45 oC, 60 oC, 75 oC and 90 oC maintained in a water bath 2.4 Electrochemical measurements Gamry ZRA REF 600-18042 potentiostat/galvanostat was used for electrochemical measurement The conventional three electrode set up was used which is made up of saturated calomel electrode (SCE) as reference electrode, platinum as counter electrode and the different steel coupons as working electrode Only three concentrations (1x10-5, 5x10-5 and 10x10-5 M) of the inhibitor were tested The EIS was conducted at frequency of 10 kHz to 10 mHz for open circuit immersion time of 1800 s at 30 oC The voltage ranged from -0.15 V to +0.15 V vs EOC at scan rate of 0.2 mV/s for PDP measurements [24] EChem Analyst was used for data analyses/fitting Charge transfer resistance was used to compute the inhibition efficiency according to Eq The inhibition efficiency from PDP was calculated from the corrosion current densities using Eq ℇ and where = 100 (4) are charge transfer resistances in the absence and presence of inhibitor respectively ℇ where = 100(1 − ) (5) are the corrosion current densities in the absence and presence of the and inhibitor respectively The magnitude of the double layer capacitance ( ) of the adsorbed film was calculated from constant phase element (CPE) constant ( ) and charge transfer resistance ( ) using Eq =( ) (6) where n is a constant showing degree of roughness of the metal surface obtained from the phase angle given that ( 2.5 = −1) and = 2α⁄(π ) UV-vis study UV-vis spectral data was obtained using 756PG Spectrum (Shanghai Spectrum Instruments Co., Ltd) The UV-vis spectrum was first obtained using the solution containing 10x10 -5 M 5-HTP prior to immersion of the steel Another spectrum was obtained with the solution after immersing the steel for 24 hours Only the spectral profiles in M HCl were compared and discussed 2.6 FTIR study FTIR spectrum of the pure sample and that of the 5-HTP film formed on mild steel surface after immersion (both mixed with potassium bromide) were recorded The spectra were measured using TENSOR II FTIR Spectrophotometer 2.7 SEM/EDAX study SEM images were recorded using AMETEX S4800 EDAX TSL in the vacuum mode before and after immersion in HCl This was repeated with a coupon immersed in HCl containing 10x10-5 M 5-HTP solution The instrument was operated at kV Also, EDAX profiles of the steel surface ab initio and the corrosion products in the inhibited and uninhibited solutions were recorded Results and discussion 3.1 Thermo-gravimetric study The corrosion rate, inhibition efficiency and fractional surface coverage obtained from weight loss measurements for the corrosion of MS in both M and 15 % HCl containing the different concentrations of 5-HTP are presented in Table 3.1.1 Effect of inhibitor and acid concentration Inhibition efficiency obtained increases as concentration of 5-HTP increases at constant temperature (Table 1), similar to trends reported in literature [23] Higher inhibition efficiency could be obtained if 5-HTP concentration is further increased However, the efficiency of 5-HTP decreases as acid concentration increases At 30 oC, the inhibition efficiency decreased from 96.01 % in M HCl to 78.14 % in 15 % (about 4.4 M) HCl which represents about 18.61 % decrease in inhibition efficiency on 340 % increase in acid concentration Efficiency of 78 % is still reasonable for a bio-based material and may be improved if blended synergistic additives Table Corrosion rate, inhibition efficiency and fractional surface coverage data for the inhibition of mild steel corrosion in M and 15 % HCl using different concentrations of 5-HTP at 30 oC Concentration (M) Blank solution (mmpy) 26.72 In M HCl θ (%) - (mmpy) 153.94 In 15 % HCl θ (%) - x 10-5 x 10-5 x 10-5 x 10-5 10 x 10-5 3.1.2 4.26 3.39 2.59 2.25 1.07 84.07 87.32 90.32 91.58 96.01 0.84 0.87 0.90 0.92 0.96 54.16 49.35 41.49 37.84 33.65 64.82 67.94 73.05 75.42 78.14 0.65 0.68 0.73 0.75 0.78 Effect of temperature and intensifier blends Temperature is one of the major factors that affect the performance of corrosion inhibitors [25] Since industries have recently ventured more into production and recovery of hydrocarbons from deep pay zones, this study moved to elucidate the effect of temperature in 5HTP efficiency Down the well, the difference in temperature per unit well length has been described as geothermal gradient An average geothermal gradient of about 25 °C/km (1°F per 70 feet of depth) is believed to be universal [26] while 28 oC/km is for Niger delta fields [27-29] Table Effect of temperature on the inhibition efficiency of 10 x 10-5 M 5-HTP as corrosion inhibitor for mild steel in acidic solutions T(oC) 30 45 60 75 90 M HCl 96.01 91.05 84.24 78.08 66.95 15 % HCl 78.14 74.36 67.14 52.38 39.88 Results obtained using the 10x10-5 M 5-HTP can be seen in Table The inhibition efficiency decreases as temperature increases In M HCl solution, efficiency reduces from 96.01 % at 30 oC to 66.95 % at 90 oC The obtained efficiencies are even lower in 15 % HCl compared to M HCl as temperature increased This implies that if 5-HTP is used as corrosion inhibitor in acidizing fluid, it will perform effectively in surface than down hole pipes To improve on this performance, 5-HTP may be blended with some intensifiers Intensifiers are desirable because corrosion inhibitors frequently cannot provide adequate protection to steels at high temperatures and long exposure time [3] The intensifiers used in this study are potassium iodide (KI), polyethylene glycol (PEG), sodium gluconate (NaG), and glutathione (GLU) Inhibition efficiency at 90 oC improved with the blends containing PEG, KI and GLU to 88.5, 89.0 and 90.2 % respectively in M HCl and 78.4, 80.7, and 82.5 % respectively in 15 % HCl (see Table 3-4) This demonstrates that the inhibitor blends could be suitable for various oilfield acidizing procedures associated with high temperature operations Table Effect of Intensifiers on inhibition efficiency (%) of 10 x 10-5 M 5-HTP on mild steel in M HCl solution T(oC) 5-HTP Only 96.0 91.1 84.2 78.0 66.0 30 45 60 75 90 5-HTP + KI 99.3 99.1 98.8 94.4 89.0 5-HTP +PEG 99.9 99.0 98.7 92.5 88.5 5-HTP +NaG 97.2 96.2 94.1 88.3 73.1 5-HTP + GLU 99.9 99.6 98.9 96.3 90.2 Table Effect of Intensifiers on inhibition efficiency (%) of 10 x 10-5 M 5-HTP on mild steel in 15 % HCl solution T(oC) 30 45 60 75 90 3.1.3 5-HTP Only 78.1 74.3 67.1 52.4 39.9 5-HTP + KI 93.9 92.1 90.0 85.0 80.7 5-HTP +PEG 95.1 91.0 90.9 85.7 78.4 5-HTP +NaG 88.4 85.2 80.0 74.2 65.2 5-HTP + GLU 96.3 95.0 90.4 85.6 82.5 Adsorption study Adsorption of corrosion inhibitors can occur through electrostatic interactions between charged species of the inhibitor and metal surface (physisorption) or by actual chemical interaction between inhibitor functional groups and metal orbitals (chemisorption) [30] To predict which mechanism 5-HTP was adsorbed on MS, surface coverage (θ) data were fitted into different adsorption isotherm models namely Langmuir, Temkin, Freundlich, Flory Huggins and El-Awady et al The best fit was obtained with Langmuir isotherm (Fig 2) with 0.99985 The expression for the model is given in Eq = + (7) ≥ is the concentration of inhibitor (M) in the acid solution and where is the adsorption- desorption equilibrium constant (M-1) which is used to determine the free energy change of adsorption (∆ ∆ ) according to Eq =− ln(55.5 ) (8) where 55.5 represents the concentration of water in the solution, and is the absolute temperature The values of Usually, the values of and ∆ is the universal gas constant obtained are given in Table relates to the strength of inhibitor-metal surface binding [31] The obtained values decrease as temperature increases, implying that the binding strength of 5-HTP decreases as temperature increases due to desorption of its molecules from the surface The ∆ values are between -12.67 kJmol-1 to -13.53 kJmol-1 and within the range assigned to physical adsorption mechanism [32] Therefore, the adsorptive film has an electrostatic character The negative values of ∆ also indicate that 5-HTP was spontaneously adsorbed on MS at all temperatures and acid concentrations 16 (i) 12 20 10 Linear Linear Linear Linear Linear fit fit fit fit fit for for for for for o 30 C o 45 C o 60 C o 75 C o 90 C -5 C/ (x10 M) -5 C/ (x10 M) (ii) 25 14 15 10 Linear Linear Linear Linear Linear -5 C (x10 M) 10 C (x10-5 M) fit fit fit fit fit for for for for for 10 Fig Langmuir adsorption isotherms for the inhibition of mild steel corrosion in (i) M and (ii) 15% HCl by different concentrations of 5-HTP Table Parameters deduced from Langmuir adsorption isotherm T (oC) 30 45 60 Slope 1.43 1.23 1.15 (M-1) 3.879 2.943 2.226 ∆ (kJmol-1) -13.53 -13.47 -13.34 o 30 C o 45 C o 60 C o 75 C o 90 C 75 90 3.1.4 1.06 1.02 1.435 1.417 -12.67 -13.17 Kinetic and Thermodynamic studies Corrosion rate data were fitted into Arrhenius kinetic model (Eq 9) to determine the activation energy Linear plots of log (units in mmpy) against reciprocal of temperature (Fig 3) were constructed log = log − (9) where Ea is the activation energy, A the Arrhenius pre-exponential factor or frequency factor, R Is the universal gas constant and T is absolute temperature Activation energy deduced was higher with the inhibitor (depending on inhibitor concentration) than the free acid solutions (see Table 6) From the concept of activation and collision theory, it can be considered that for collision of the acid molecules with MS to result in corrosive attack, corrodes the metal surface, molecules of the acid must collide with the metal molecules on the surface The acid molecules should possess energy up to a minimum threshold called the activation energy This implies that the acid molecules must acquire extra (higher) energy in the inhibited solution for corrosion to occur, hence corrosion inhibition Therefore, addition of 5-HTP increases the activation energy and reduces tendency of the acid to collide and attack the metal surface The activation energy was lower in 15 % HCl than M HCl implying molecules of 15 % HCl solution require less energy barrier to cross the activated complex and form corrosion products with mild steel than those of M HCl The increase in activation energy in the presence of inhibitor in both acids is consistent with trends reported in literature and is associated with physical adsorption mechanism [31] The other activation parameters in Table were derived from the transition state equation (Eq 10) Linear plots of log( ) (in mmpy per Kelvin) against reciprocal of temperature (Fig 4) log( ) = log + ∆ ∗ − ∆ ∗ (10) where ∆ ∗ ∆ ∗ and ∆ ∗ are the enthalpy and entropy change of activation respectively The values of are all negative which indicates that adsorption of 5-HTP on MS is exothermic and involves evolution of heat Negative ∆ ∗ values show that at the rate activated complex, there is association of inhibitor molecules with steel surface which through adsorption [34] 2.0 (I) MS-1M HCl with 5-HTP-Arrhenius (II) MS-15% HCl with 5-HTP-Arrhenius 1.8 2.0 1.6 1.4 1.8 log CR log CR 1.2 1.0 1.6 0.8 0.6 1M HCl -5 1x10 M -5 5x10 M -5 10x10 M 0.4 0.2 15% HCl -5 1x10 M -5 5x10 M -5 10x10 M 1.4 1.2 0.0 2.7 2.8 2.9 3.0 3.1 3.2 1000/T (K-1) 3.3 2.7 2.8 2.9 3.0 3.1 3.2 1000/T (K-1) 3.3 Fig Arrhenius plot for the inhibition of corrosion of mild steel in (i) M and (ii) 15% HCl using different concentrations of 5-HTP -0.6 -0.6 (I) MS-1M HCl with 5-HTP-Transition state -0.8 (IV) MS-15% HCl with 5-HTP-Transition state -0.8 -1.0 -1.2 -1.0 log (CR/T) log (CR/T) -1.4 -1.2 -1.6 -1.8 -1.4 1M HCl -5 1x10 M -5 5x10 M -5 10x10 M -2.0 -2.2 15% HCl -5 1x10 M -5 5x10 M -5 10x10 M -1.6 -2.4 -1.8 -2.6 2.7 2.8 2.9 3.0 3.1 -1 1000/T (K ) 3.2 3.3 2.7 2.8 2.9 3.0 3.1 -1 1000/T (K ) 3.2 3.3 Fig Transition state plot for the corrosion of mild steel in (i) M and (ii) 15 % HCl in the absence and presence of different concentrations of 5-HTP Table Activation parameters for corrosion of mild steel in both M and 15 % HCl containing different concentrations of 5-HTP Conc (x10-5M) (kJmol-1) 17.04 ∆ M HCl ∗(kJmol-1) -14.36 ∆ ∗ (kJmol-1) -0.19 (kJmol-1) 11.25 ∆ 15 % HCl ∗ (kJmol-1) -22.43 ∆ ∗(kJmol-1) -0.12 5 10 3.2 35.42 36.34 38.29 42.03 48.44 -32.74 -33.09 -35.81 -39.07 -45.74 -0.16 -0.15 -0.14 -0.12 -0.11 18.32 23.92 27.45 31.17 34.33 -37.88 -44.01 -53.55 -60.18 -72.18 -0.13 -0.11 -0.09 -0.90 -0.07 Electrochemical impedance spectroscopy The open circuit potential scanned for 1800 s afforded a fairly steady potential after 600 s (Fig 5a) It can also be observed from the figure that open circuit voltage (OCP) is more stable at about -480 ± mV whereas in the presence of the inhibitors, the OCP shift to more negative values The shift is in OCP with 5-HTP from M HCl is not up to -85 mV, therefore the inhibitor cannot be categorized as cathodic or anodic type [35] Nyquist and Bode modulus/Phase angle plots shown in Fig 5b were obtained for the different concentrations of 5HTP in M HCl The semicircles in the Nyquist plots are imperfect and the sizes of diameters are influenced in the presence of 5-HTP from that of the free acid solution The imperfection in the shape of the semicircle can be attributed to surface roughness of the mild steel while the difference in sizes of their diameters demonstrates that 5-HTP has influence on corrosion rate due to inhibition The diameter increases as inhibitor concentration increases following the same trend as inhibition efficiency The single capacitive loop obtained indicates that the mechanism of corrosion is controlled mainly by charge transfer process [36] The shapes of the plots were similar in both inhibited and free acid solution indicating that the mechanism of steel corrosion is not influenced by introduction of 5-HTP The equivalent circuit shown in Fig provides a best fit for analyses of experimental data with goodness of fits less than 0.35x10-3 Some associated parameters obtained are given in Table Surface roughness of the steel was compensated by introduction of a non-integer element dependent on frequency called constant phase element, CPE which can be estimated using and , related to impedance by: = ( ) ( where ) (11) is the impedance of the CPE, an imaginary complex number, ( is the CPE constant, is the angular frequency, is = −1) is the phase angle of CPE Value of indicates deviation of the CPE and can be used to predict the degree of roughness or inhomogeneity of the mild steel surface This value decreased on addition of the extracts suggesting that the surface roughness of the mild steel is increased by adsorption of inhibitor molecules on steel surface active sites [37] It also indicates that there is relative and/or integrated influence on the CPE: not just a single resistance, capacitance or inductive element Decrease in n on addition inhibitors can also be associated with insulation of the metal/solution interface by formation of a surface film responsible for the increase in charge transfer resistance Charge transfer resistance increases with increase in inhibitor concentration showing that the ‘blanketing’ property of the film improves as inhibitor concentration increases The inhibition efficiency also increases with increase in inhibitor concentration, following the same trend as result from gravimetric studies This capacitive response shown by increase in peak heights in Bode plot suggests formation of an electrochemical double layer with a capacitance ( ) estimated using Eq The values decreases in the presence of inhibitors, similar to results reported in literature [38], attributed to decrease in the local dielectric or an increase in the thickness of the double layer or both, caused by the adsorbed protective film of the inhibitors -0.45 M HCl -5 1x10 M -5 5x10 M -5 10x10 M EOC (V vs SCE) -0.46 Fairly stable potential obtained after 600 s -0.47 -0.48 -0.49 -0.50 -0.51 300 600 900 1200 1500 1800 T (s) Fig a Plot of OCP against time for the inhibition of MS corrosion in M HCl 00 20 (I) N yqu is t-5-H T P -1M H C l in M S HCl -5 1x1 M -5 5x1 M -5 10 x10 M 00 cm ) HCl -5 1x M -5 5x M -5 10 x M 00 -2 -4 00 -6 00 -8 00 -1 0 10 00 30 00 Z ( cm ) 50 00 70 -1 lo g f ( H z ) Phase angle (deg) 00 Fig b Nyquist and Bode modulus/Phase angle plots for inhibition of mild steel corrosion in M HCl using different concentrations of 5-HTP Fig Electrochemical equivalent circuit ( ( ∅ )) model used for data fitting/analyses Table Some parameters obtained from EIS technique used to monitor the inhibition of mild steel corrosion in M HCl containing different concentrations of 5-HTP EIS Parameters R (Ωcm2) R (Ωcm2) Y (μΩ s ) Fit(x10 -6) n C (μFcm-2) ε (%) 3.3 HCl 102.3 1.035 157.7 351.8 0.872 127.7 - Test solutions 1x10-5 5x10-5 732.1 878.5 0.902 0.907 191.1 155.9 218.2 143.8 0.857 0.835 53.1 10.4 86.03 88.36 10x10-5 1013.7 0.947 146.4 124.8 0.828 6.0 89.91 Potentiodynamic polarization During corrosion of mild steel in HCl, at least one oxidation and one reduction process takes place Typical reactions involving iron at the electrodes are: Anode: Cathode: ( ) → ( ) ( ) + → +2 (12) (13) ( ) The sum total of cathodic and anodic processes contributes to the compromise or free corrosion potential ( ) and the corresponding current density ( ) obtained through analyses of Tafel plot obtained (Fig 7) Tafel cathodic and anodic constants ( and ) were also obtained from the slope of the plots Some of the PDP parameters determined are shown in Table The values decrease with increase in inhibitor concentration due to formation of adsorbed protective film A shift in values to more positive values with the inhibited solutions compared to the free acid solution was observed Since anodic inhibitors shift the corrosion potential in the positive direction [37], it therefore implies that 5-HTP has dominant influence on the partial anodic reaction However, the highest shift from that of the free acid ) was -47 mV which is less than -85 mV required to categorize the inhibitor as either ( cathodic or anodic type Similar shift in values has been reported in literature and the inhibitor was regarded as mixed type inhibitors with anodic predominance [38] This implies that 5-HTP inhibits both the iron dissolution and hydrogen evolution processes but more actively inhibiting anodic iron oxidation reaction The values of and obtained change with concentration of inhibitor without a definite trend Olasunkanmi et al also obtained similar trend and inferred mixed type inhibitors with cathodic predominance because the magnitude of the net change was higher in to the free acid solution [39] In this study, larger changes in than when compared values were obtained than supporting that the inhibitors shows anodic predominance The calculated inhibition efficiency also increased with increase in concentration of the inhibitor, similar to weight loss and EIS results Tafel - 5HTP - MS in1 M HCl -2 -2 logI (Acm ) -3 -4 -5 1M HCl -5 1x10 M -5 5x10 M -5 10x10 M -6 -7 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 E (V vs SCE) Fig Tafel plots for inhibition of mild steel corrosion in M HCl by 5-HTP at 30 oC Table Some parameters obtained from PDP technique used to monitor the inhibition of mild steel corrosion by 5-HTP at 30oC PDP parameters β (mV/decade) β (mV/decade) HCl 95.4 64.7 1x10 -5 54.8 91.2 5x10 -5 57.6 97.2 10x10 -5 57.8 100.9 I 3.4 (μAcm-2) E (mV) ε (%) 969.3 -499 - 128.8 -452 86.7 49.7 -454 94.75 26.5 -453 97.25 Uv-visible spectroscopy The absorption spectra shown in Fig reveal that after immersion of the mild steel coupon, the band obtained in the UV-visible region shifted to lower absorbance values This behavior may be ascribed to possible interaction between Fe 2+ and inhibitor compounds in the inhibited solution [39] This could have been due to some electronic transitions like ∗ → (involving non-bonding electrons of O and N) or → ∗ → or n (involving multiple bonds and conjugated system) and formation of a surface complex between the inhibitor and metal surface This complex acts as the adsorbed protective film that reduces direct acid attack on the metal surface, hence corrosion inhibition 100 Absorbance (%) 80 60 Before MS immersion After MS immersion 40 20 350 400 Wavelength (nm) 450 500 Fig UV-vis spectra of M HCl containing 10x10-5 M HCl before and after immersion of mild steel 3.5 FTIR study This technique was employed to predict the functional group(s) in the inhibitor involved in the adsorption process Before immersion of the metal, the prominent spectral peaks provide information on the functional groups in the compound After immersion, some of the peaks are either lost or less prominent due to involvement of the corresponding functional group(s) in adsorption The FTIR spectra 5-HTP obtained can be seen in Fig Those around 3100-3300 cm-1, 1600-1700 cm-1 and 1200-1300 cm-1 are worthy of note on the black line The peak around 3100-3300 cm-1 can be assigned to either –OH or –NH vibrations: –OH vibrations could be broad in the presence of hydrogen bonding while –NH2 is characterized by a single peak These functional groups are available in the molecular structure of 5-HTP which has been embedded in Fig The peak around 1600-1700 cm-1 is popularly assigned to C=O of an acid group which is available in the molecule The peak around 1200-1300 cm-1 can be assigned to C-O group which is also present in the molecule The disappearance of these peaks after immersion of mild steel (see the red line) demonstrates that these functional groups could have been actively involved in the adsorption process 1.2 Before immersion After immersion FTIR-5HTP-1M HCl Transmittance (%) 1.0 0.8 0.6 0.4 0.2 1000 1500 2000 2500 3000 -1 3500 4000 Wavelength number (cm ) Fig FTIR spectra for 5-HTP in M HCl before and after immersion of mild steel 3.6 SEM/EDAX studies SEM micrographs of abraded mild steel coupons prior to immersion and those immersed in M HCl with and without 5-HTP for 24 hours were recorded by SEM Result reveals that the surface of the abraded steel coupon (Fig 10, top left) was considerably smooth with minimal undulation or pitting The surface of the coupon immersed in the free acid solution (Fig 10, top middle) was severely pitted by acid corrosion However, the coupon immersed in the solution containing 5HTP (Fig 10, top right) was relatively smoother compared to the free acid solution This demonstrates that addition of inhibitor reduces corrosive pitting which occurs in the free acid solution The protective layer composed of some cracks which may be due to uneven distribution of 5-HTP molecules over the steel surface It is also possible that the active sites on the mild steel surface are not be equivalent or not possess similar affinity for active molecules of the inhibitors In that case, the adsorption of some molecules on a portion of the surface differentially blankets the acid from attacking that portion by steric hindrance or micelles-like conformation of adsorbed molecules as described by [40] To further establish the reliability of inferences drawn from FTIR results, EDS profiles of the surfaces of mild steel was obtained without and with formation of adsorbed film The spectral profile of pure mild steel surface before immersion (Fig 10, bottom left) shows presence of Fe, C and small O On immersion in M HCl, Fe slightly decreased, O slightly increased and Cl ions (Fig 10, bottom middle) The profile in the solution containing 5-HTP also shows increased O and N atoms (Fig 10, bottom right) The EDAX data obtained is summarized in Table Thus, adsorption of the inhibitor on the surface involved mainly N and O atoms, and results in decreased iron exposure due to protective effect of the inhibitors This result is in agreement with observations made from FTIR studies Fig 10 SEM/EDAX profiles of abraded mild steel surface (Left), MS immersed in M HCl without (middle) and with 10x10-5 M 5-HTP Table Summary of atoms obtained on pure MS surface (case I), corroded MS surface (case II), and MS corrosion inhibited by 5-HTP surface (case IV) investigated by EDAX Element Fe C O N Cl 3.7 Case I Wt % At % 89.85 64.17 09.07 29.90 01.04 04.91 - Case II Wt % At % 80.24 49.28 09.79 30.69 07.96 17.07 02.01 02.96 Case IV Wt % At % 69.74 42.10 10.78 30.17 12.31 14.88 05.73 11.03 01.44 01.82 Mechanism of inhibition Corrosion inhibitors are believed to act by adsorption on metal surfaces by either physical or chemical means [31] Based on results obtained from this study, two mechanisms are proposed for the inhibition of MS corrosion by 5-HTP molecules First, adsorption of 5-HTP on MS surface may result in electrostatic interactions between charged –COO- group 5-HTP species and iron ions Second, it is also possible that since 5-HTP can easily be protonated, the Cl- on the surface forms a protective film by interaction with protonated group (-NH3+) in 5-HTP The proposed mechanisms are supported by results obtained from functional group analyses Conclusion Corrosion of MS was monitored in laboratory simulated well acidizing fluid with and without different concentrations of 5-HTP to determine its anticorrosive effect The 5-HTP functioned as effective corrosion inhibitor with efficiency increasing with increase in concentration but decreasing with temperature increase Potassium iodide, PEG and GLU can improve the efficiency of 5-HTP at high temperature 5-HTP is spontaneously physically adsorbed on MS surface and obeys Langmuir adsorption Active functional groups involved in the adsorption process are O, N and C=C 5-HTP and its blends can serve as ecofriendly alternative anticorrosive oilfield chemical additives for well acidizing procedure Acknowledgements ... the inhibition of mild steel corrosion in M and 15 % HCl using different concentrations of 5- HTP at 30 oC Concentration (M) Blank solution (mmpy) 26.72 In M HCl θ (%) - (mmpy) 153 .94 In 15 %.. .Electrochemical and anticorrosion properties of 5- hydroxytryptophan on mild steel in simulated well acidizing fluid Ekemini Ituen1,2* Onyewuchi Akaranta2,3 Abosede James Materials Physics and. .. Transition state plot for the corrosion of mild steel in (i) M and (ii) 15 % HCl in the absence and presence of different concentrations of 5- HTP Table Activation parameters for corrosion of mild steel