"N, N-bis (phosphorylmethyl) glycine, Zn2 + and tartrate" -- a new ternary corrosion inhibitor formula for corrosion control of carbon steel
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https://www.eduzhai.net International Journal of M aterials and Chemistry 2013, 3(2): 17-27 DOI: 10.5923/j.ijmc.20130302.01 “N,N-Bis(phosphonomethyl) Glycine, Zn2+ and Tartrate” – A New Ternary Inhibitor Formulation for Corrosion Control of Carbon Steel B. V. Appa Rao1, M. Venkateswara Rao2, S. Srinivasa Rao3,*, B. Sreedhar4 1Department of Chemistry, National Institute of Technology, Warangal, 506004, India 2Department of Chemistry, Andhra Loyola College, Vijayawada, 520008, India 3Department of Chemistry, V. R. Siddhartha Engineering College, Vijayawada, 520007, India 3Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad, 500007, India Abstract Studies on inhibition of corrosion of carbon steel in aqueous environment using N,N -Bis(phosphonomethyl) glycine (BPM G), zinc ions and tartrate are presented. The investigations revealed that tartrate acts as an exce llent synergist in corrosion inhibition. Optimu m concentrations of all the three co mponents of the ternary formu lation are established by gravimetric studies. Potentiodynamic polarization studies indicate that the new ternary systemis a mixed inhib itor. Results of the impedance studies show that a protective film is formed on the metal surface in presence of the inhibitor formu lation. Analysis of the protective film us ing X-ray photoelectron spectroscopy (XPS) and reflect ion absorption Fourier transform infrared (FTIR) spectroscopy infer the presence of Zn(OH)2, o xides and hydroxides of iron and the inhibitor mo lecules in the surface film probably in the form of a co mplex, [Zn(II)-BPM G-tartrate]. A plausible mechanism o f corrosion inhibit ion is p ro p os ed . Keywords Corrosion Inhibitor, Carbon Steel, Phosphonated Glycine, Tartaric Acid, Synergism 1. Introduction Carbon steel is an important material of construction for cooling water systems and heat exchangers in industries. In order to control corrosion of carbon steel in such systems, application of corrosion inhib itors is an effective practice. A fter th e exit o f fo rmu lat ions b ased o n ch ro mates , phosphates, etc., phosphon ate b ased fo rmu lat ions are introduced and they have been in use as corrosion inhibitors s ince th e p ast t h ree decad es [1-8]. Ph osp hon ates in combination with zinc ions proved to be effect ive corros ion inh ibito rs for carbon steel[1-8]. Howev er, they requ ire higher concentrations of both phosphonate and zinc ions for an effective inhib ition. An efficient technique to reduce the concentrations of both phosphonate and toxic zinc ions in t h e in h ib it o r fo r mu lat io n s is t o ad d a n o n -t o xic organic/inorganic co mpound as a synergist. A few of such ternary inh ib ito r formu lat ions contain ing phosphonate, zincions and organic/inorganic addit ive were reported in lit eratu re[9-13]. Th ese repo rts su ggest that th e h igh inhibit ion efficiency exh ibited by the ternary formu lations * Corresponding author: email@example.com (S. Srinivasa Rao) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved is due to synergism existing among the inhib itor components of the formulat ions. In this background, the present study was carried out using an environmentally friendly organic compound namely tartaric acid as a synergist to the binary formu lation containing N,N-Bis(phosphonomethyl) g lycine (BPM G) and zinc ions for corrosion control of carbon steel. It was reported in a prev ious study that BPM G-Zn2+ binary formulat ion acts as an effective corrosion inhibitor for carbon steel. Tartaric acid is chosen as the synergist for the present study due to presence of two carboxy l groups and two hydroxy l groups in its structure, wh ich provide complexing ability with metal ions. The main objective of the present study is to investigate the inhibitory effects of the new ternary inhibitor formu lation containing BPM G, Zn2+ and tartrate in corrosion inhibition of carbon steel in low ch loride aqueous environment. Also, it is of interest to study the synergistic effect of tartrate in combination with BPM G and Zn2+ on corrosion inhibition. For the present study, 200 pp m of NaCl solution has been chosen as control because of the following reason. The water used in cooling water systems is generally either demineralized water or unpolluted surface water. In either case, the aggressiveness of the water will never e xceed that of 200 ppm of Na Cl. 2. Experimental Section 18 B. V. Appa Rao et al.: “N,N-Bis(phosphonomethyl) glycine, Zn2+ and Tartrate” – A New Ternary Inhibitor Formulation for Corrosion Control of Carbon Steel 2.1. Solutions and S peci mens For all the studies, the specimens taken from a single sheet of carbon steel of the following composition we re chosen. C – 0.1 to 0.2 %, P – 0.03 to 0.08 %, S – 0.02 to 0.03%, Mn – 0.4 to 0.5 % and the rest iron. Prior to the tests, the specimens were polished to mirror fin ish with 1/0, 2/0, 3/0 and 4/0 emery polishing papers respectively, washed with distilled water, degreased with acetone and dried. For gravimetric measurements, the polished specimens of the d imensions, 3.5 cm x 1.5 cm x 0.2 cm, were used while for other studies, the dimensions of the specimens are 1.0 cm x 1.0 cm x 0.1 cm. BPM G (C4H11P2O8N) obtained fro m A ldrich Chemical Co mpany Inc., USA was used in the present study. Zinc sulphate (ZnSO4.7H2O), potassium sodiu m tartrate tetrahydrate (C4H4NaKO6.4H2O) and other reagents were analytical grade chemicals. All the solutions were prepared by using triple distilled water. The pH values of the solutions were ad justed by using 0.01 N NaOH and 0.01 N H2SO4 solutions. An aqueous solution consisting of 200 ppm of NaCl has been used as the control throughout the study . 2.2. Gravi metric Measurements In all the gravimetric experiments, the polished specimens were weighed and immersed in duplicate, in 100 mL control solution in the absence and presence of inhib itor formulat ions of different concentrations, for a period of seven days. Then the specimens were reweighed after washing, degreasing and drying. During the studies, only those results were taken into consideration, in which the difference in the weight-loss of the two specimens immersed in the same solution did not exceed 0.1 mg. Accuracy in weighing upto 0.01 mg and in surface area measured upto 0.1 cm2, as recommended by ASTM G31, was followed. The immersion period of seven days was fixed in view of the considerable magnitude of the corrosion rate obtained in the absence of any inhib itor after this immersion period. The immersion period was maintained accurately upto 0.1 h in view of the lengthy immersion time of 168 h. Under these conditions of accuracy, the relat ive standard error in corrosion rate determinations is of the order of 2 % or less for an immersion time of 168 h. Corrosion rates (CR) of carbon steel in the absence and presence of various inhibitor formu lations are exp ressed in mmpy. Inhibit ion efficiencies (IEg) of the inhib itor formulat ions were calcu lated by using the formula, IEg (%) = 100 [(CR)o – (CR)I] / (CR)o (1) where (CR)o and (CR)I are the corrosion rates in the absence and presence of inhibitor respectively. Gravimetric studies of the ternary formulations containing BPM G (20-30 ppm), Zn2+ (20-30 ppm) and tartrate (0-150 ppm) were carried out at pH 7. The selected concentration ranges of both BPM G and Zn2+ in these studies are based on the results of gravimetric studies of the binary inhib itor system, BPM G-Zn2+. The influence of pH on inhibition efficiency of the ternary inhib itor formulat ion was also studied in the pH range, 5-9. Gravimetric experiments were also conducted using the specimens covered by the protective film in the ternary inhibitor formu lation, in order to decide the required minimu m dosage of each of the components for maintenance of the protective film in the chosen corrosive environment. 2.3. Electrochemical Studies Both the potentiodynamic polarizat ion studies and electrochemical impedance spectroscopic (EIS) studies were carried out using the Electrochemical Workstation Model IM6e, Zahner-elekt rik, GmbH, Germany and the experimental data were analy zed by using the Thales software. The measurements were conducted in a conventional three electrode cylindrical glass cell with platinum electrode as auxiliary electrode and saturated calo mel e lectrode (SCE) as reference electrode. The working electrode was carbon steel embedded in epoxy resin of polytetrafluoroethylene so that the flat surface of 1 cm2 was the only surface exposed to the electrolyte. The three-electrode set up was immersed in control solution of volume 500 mL both in the absence and presence of various inhibitor formu lations and allowed to attain a stable open circuit potential (OCP). The p H values of the solutions were adjusted to 7.0 and the solutions were unstirred during the e xperiments . Polarization curves were recorded in the potential range of –750 to –150 mV with a resolution of 2 mV. The curves were recorded in the dyna mic scan mode with a scan rate of 2 mV s-1 in the current range of –20 mA to +20 mA. The oh mic drop compensation has been made during the studies. The corrosion potential (Ecorr), corrosion current (Icorr), anodic Tafel slope (βa) and cathodic Tafel slope (βc) were obtained by extrapolation of anodic and cathodic regions of the Tafel plots. The inhibition efficiency (IEp) values were calculated fro m Icorr values using the equation, IEp (%) = [1- (I'corr / Icorr)] x 100 (2) where Icorr and I'corr are the corrosion current densities in case of control and inhibited solutions respectively. Electrochemical impedance spectra in the form of Nyquist plots were recorded at OCP in the frequency range from 60 kHz to 10 mHz with 4 to 10 steps per decade. A sine wave, with 10 mV amp litude, was used to perturb the system. The impedance parameters viz., charge transfer resistance (Rct), constant phase element (CPE) and CPE exponent (n) were obtained from the Nyquist plots. The inhibition efficiencies (IEi) were calculated using the equation, IEi (%) = 100 [1 – (Rct / R'ct)] (3) where Rct and R'ct are the charge transfer resistance values in the absence and presence of the inhibitor respectively. 2.4. Surface Anal ysis By X-Ray Photoelectron Spectroscopy (XPS) XPS measurements of the surface film was carried out with Kratos analytical photoelectron spectrometer (A XIS 165) with monochromated Al Kα X-ray source (1486.6 eV) operated at 100 W. The spectra were collected at an electron International Journal of M aterials and Chemistry 2013, 3(2): 17-27 19 take-off angle of 90o. Analyser pass energy was 80 eV, with a step of 1 eV. Deconvolution spectra were recorded with analyser pass energy of 80 eV, with a step of 0.1 eV for the elements of interest namely Fe 2p, O 1s, P 2p, C 1s , N 1s and Zn 2p. Binding energies for the deconvolution spectra were corrected individually for each measurement set, based on a value of 285.0 eV for the C– C co mponent of C 1s. 2.5. Fourier Transform Infrared (FTIR) Spectroscopic Studies FTIR spectra were recorded using FTIR spectrophotometer fro m Thermo Electron Corporation, USA, model Nexus 670 with a resolving power of 0.125 cm-1. The detector is temperature stabilised DTGS (KBr window) and liquid n itrogen cooled MCT-A and the beam splitter is XT-KBr. FTIR spectra of pure BPM G and pure potassium sodium tartrate were recorded using the KBr pellet method. The reflection absorption FTIR spectra of the surface films were recorded in the wave nu mber range of 4000– 400 cm-1. The measurements were made at a g razing angle of 85o. of tartrate ions viz., 100 pp m and above in the bulk o f the solution, more tartrate is reaching the surface of the metal at the cost of the required optimu m concentrations of BPM G and Zn2+ on the surface and therefore the necessary optimu m concentration of BPM G and Zn2+ required for the formation of protective film, is not available on the surface. Hence, the decrease in inhib ition efficiency is observed at concentrations of tartrate ions ≥ 100 ppm. It may be mentioned here that the molar rat io of BPM G : Zn2+ : tartrate is 1.0 : 5.4 : 2.0 to exh ibit excellent synergism. The role of each synergist is explained under mechanistic aspects of corrosion inhibition. 3. Results and Discussion 3.1. Gravi metric Measurements Figure 1 shows the inhibition efficiency of the ternary system, BPM G-Zn2+-tartrate, as a function of concentration of tartrate at d ifferent concentrations of BPM G and Zn2+. Fro m the figure, it can be observed that in case of all these ternary formu lations at pH 7, as the concentration of tartrate is increased, the inhibit ion efficiency increases, reaches a maximu m at an optimu m concentration of tartrate and then decreases. In order to achieve an inhibit ion efficiency > 90 %, the required minimu m concentrations of BPM G and Zn2+ are 20 pp m and 30 pp m respectively in presence of tartrate. While the binary system consisting of 20 ppm BPM G and 30 ppm Zn2+ accelerates corrosion, with the addition of just 25 ppm of tartrate, the inhibit ion efficiency of the ternary formulat ion is 92 %. Further increase in the concentration of tartrate ions reduced the inhibition efficiency reach ing 5 9 % at 150 pp m of tartrate. In case of the binary system containing 30 ppm each of BPM G and Zn2+, the inhib ition efficiency is 68 %, wh ich is increased to 95 % by the addition of 50 pp m of tart rate. On further increase in the concentration of tartrate, say to 100 pp m and 150 pp m, the inhibit ion efficiency is reduced to 88 % and 68 % respectively. Fro m these results, it can be inferred that in case of the ternary system consisting of BPM G, Zn2+ and tartrate, the formu lation containing optimu m concentration of each of the co mponents gives the highest inhibition efficiency. In other words , optimu m amounts of each of the three components must be adsorbed on the surface of the metal, so that each one of them p lays its own role in the formation of protective film either through complex formation or through formation of Zn(OH)2 on cathodic sites, covering the entire metal surface. At higher concentrations Figure 1. Corrosion inhibition efficiency of the ternary inhibitor formulation containiang BPMG, Zn2+ and tartrate as a function of concentration of tartrate, (●) BPMG (30 ppm) + Zn2+ (20 ppm) + tartrate, (▲)BPMG (20 ppm) + Zn2+ (30 ppm) + tartrate, (■)BPMG (30 ppm) + Zn2+ (30 ppm) + tart rat e The influence of pH on inhibition efficiencies of the two ternary inhibitor formu lations namely, BPM G (20 ppm) + Zn2+ (30 pp m) + tartrate (25 pp m) and BPM G (30 ppm) + Zn2+ (30 pp m) + tartrate (50 pp m), in the pH range, 5.0 to 9.0 is shown in Figure 2. Figure 2. Corrosion inhibition efficiencies of the ternary inhibitor formulations as a function of pH, (▲)BPMG (20 ppm) + Zn2+ (30 ppm) + tartrate (25 ppm), (●)BPMG (30 ppm) + Zn2+ (30 ppm) +tartrate (50 ppm) There is decrease in inhibition efficiency of these formulat ions with a decrease of pH fro m neutral to acidic i.e. fro m pH 7.0 to 5.0. Similarly, increase in pH fro m 7.0 to 9.0, decreased the inhibition efficiency of both the ternary inhibitor formu lations. Fro m these results, it may be 20 B. V. Appa Rao et al.: “N,N-Bis(phosphonomethyl) glycine, Zn2+ and Tartrate” – A New Ternary Inhibitor Formulation for Corrosion Control of Carbon Steel concluded that the ternary system, BPM G-Zn2+-tartrate, is effective in the neutral as well as slightly acidic and slightly basic environments. Once, the protective film is formed on the metal surface in contact with the ternary inhibitor system consisting of BPM G (20 pp m), Zn2+ (30 ppm) and tartrate (25 pp m), the concentrations of each of these components in the inhibitor could be even less in order to maintain the protective film. The results of gravimetric studies carried out in order to determine the minimu m concentrations of the formulat ion for maintenance of the protective film are shown in Table 1. These results show that the inhibitor mixtu re containing only 10 ppm of BPM G, 10 pp m of Zn2+ and 20 ppm of tartrate could ma intain the protective film. The molar ratio of BPM G : Zn2+ : tartrate in the maintenance dosage is 1.0 : 3.6 : 3.2. cathodic partial react ions of the corrosion process and hence act as mixed type inhibitor. This is in agreement with several reports of literature indicat ing that phosphonate-based inhibitor formulat ions are mixed type inhibitors. Pech-Canul and Chi-Canul investigated the inhibitive effect of N-phosphonomethyl glycine (NPM G) on the corrosion of carbon steel in neutral solutions using electrochemical techniques. Fro m the results of polarization studies, they inferred that NPM G-Zn2+ system acts as a mixed type inhibitor. Table 1. Result s of Gravimet ric St udies of t he Inhibitor Formulat ions for Maintenance of the Protective Film Inhibitor formulation for maintenance of the film, ppm BPMG Zn2+ Tartrat e - - - 20 30 25 20 25 25 20 20 25 20 15 25 20 10 25 20 5 25 15 10 25 10 10 25 5 10 25 10 10 20 10 10 15 Corrosion rate, mmpy 0.08108 0.00636 0.00641 0.00640 0.00647 0.00705 0.02063 0.00721 0.00725 0.02778 0.00767 0.04530 Inh ibit ion efficiency, % 92.16 92.09 92.10 92.02 91.30 74.55 91.10 91.05 65.73 90.54 44.13 3.2. Potenti odynamic Polarization Studies The potentiodynamic polarization curves of carbon steel electrode in 200 pp m of ch loride solution at pH 7 in the absence and presence of various inhib itor formu lations are shown in Figure 3. The corrosion parameters are listed in Table 2. The Tafel potential (Ecorr) in case of the control solution is –328 mV vs. SCE. It is shifted to more cathodic side in case of all the inhibitor formu lations considered for polarization studies. The corrosion current density (Icorr) in case of the control solution is 12.84 A.c m–2. In presence of all the inhib itor systems, the decrease in corrosion current is observed. The significant reduction in Icorr value in presence of the ternary inhibitor formulations indicates the decrease in corrosion rate in presence of the ternary inhibitor systems. An examination of the Tafel slopes reveals that th e addition of 20 pp m of BPM G, 30 pp m of Zn2+ and 25 ppm of tartrate to the control solution shifts the anodic Tafel slope to an extent of 33 mV.dec-1, wh ile the shift in cathodic Tafel slope is 46 mV.dec-1. Similarly, in presence of 30 ppm of BPM G, 30 pp m of Zn2+ and 50 pp m of tartrate, the shift in anodic Tafe l slope is 20 mV.dec-1 and that in cathodic Tafe l slope is 30 mV.dec-1. These shifts indicate that the ternary inhibitor system under study retards both the anodic and Figure 3. Potentiodynamic polarization curves for carbon steel in 200 ppm NaCl environment in the absence and presence of various inhibitor components, (a) Control (No inhibitor), (b) BPMG (20 ppm) + Zn2+ (30 ppm), (c) BPMG (30 ppm) + Zn2+ (30 ppm), (d) BPMG (20 ppm) + Zn2+ (30 ppm) + tartrate (25 ppm), (e) BPMG (30 ppm) + Zn2+ (30 ppm) + tartrate (50 ppm) Table 2. Tafel Parameters for Carbon Steel in 200 ppm NaCl Environment in the Absence and Presence of the Inhibitor Formulations Inhibitor formulation, ppm BPMG Zn2+ Tartr ate - - - Tafel parameters Ecorr, mV vs. SCE –328 Icorr, A. cm–2 12.84 a, mV. dec-1 12 c, mV. dec-1 132 IEp, % - 20 30 - –358 12.30 32 80 4.20 30 30 - –383 4.64 46 163 63.86 20 30 25 –427 1.93 45 86 84.97 30 30 50 –398 1.14 32 102 91.12 3.3. Electrochemical Impedance Studies The impedance spectra in the form of Nyquist plots for carbon steel immersed in various environments at pH 7 are given in Figure 4. These plots are discussed to throw light on the nature of the protective film. The p lots are depressed semicircles with the centre below the real axis. This kind of phenomenon is known as dispers ing effect. When the complex p lane impedance contains a depressed semicircle with the centre belo w the real axis, which is characteristic for solid electrodes, it is often attributed to roughness and inhomogeneities of the solid surface. It is also attributed to the distribution of active sites, adsorption of inhibitor mo lecules and format ion of porous layers. In such cases, the parallel network charge transfer resistance -double layer capacitance (Rct-Cdl) is a poor approximat ion especially for systems where an efficient inhibitor is present. Considering that the impedance of a double layer did not behave as an International Journal of M aterials and Chemistry 2013, 3(2): 17-27 21 ideal capacitor in presence of the d ispersing effect, a constant phase element (CPE) is substituted for the capacitor to fit the depressed semicircles more exactly. The admittance and impedance of a CPE are, respectively defined as YCPE = Yo (j)n and ZCPE = A (j)-n, where Yo is the modulus, ω is the angular frequency, n is the CPE exponent and A is a proportional factor mathe matica lly the recip rocal o f modulus[22,23]. For a highly polished electrode, the value of n is close to 1.0. The lower the value of n, the rougher is the electrode. It can be seen that when n = 1, the element CPE is an ideal capacitor and A is equal to the capacitance, C. Figure 4 indicates that the Nyquist plots are characterized by a single time constant. The experimental data obtained fro m these plots are fitted by the equivalent electrical circu it shown in Figure 5. Such an equivalent circuit was also discussed by several authors[22,23], who obtained similar depressed semicircles with a single t ime constant. The impedance parameters obtained fro m Nyquist plots and the inhibit ion efficiencies (IEi) calcu lated fro m these parameters are shown in Table 3. ternary inhib itor formu lations. It also infers the synergistic effect operating between BPM G, Zn2+ and tartrate. Pech-Canul and Chi-Canul investigated the inhibitive effect of N-phosphonomethyl glycine (NPM G) on the corrosion of carbon steel in neutral solutions using electrochemical techniques. They reported the fit parameters in their study. Based on these parameters, they interpreted that the 3D p rotective layer developed in presence of the inhibitor was less permeable than that formed in the blank s o lu tio n . Table 3. Impedance Parameters for Carbon Steel in 200 ppm NaCl Environment in the Absence and Presence of the Inhibitor Formulations Inhibitor formulation, ppm BPMG Zn2+ Tartrate - - - 20 30 - 30 30 - 20 30 25 30 30 50 Impedance parameters Rct, Ω.cm2 CP E, F. cm–2 n 2575 13.41 0.592 1390 15.10 0.679 3830 10.09 0.811 12460 4.19 0.913 21870 3.75 0.922 IEi, % 32.76 79.33 88.22 Fi gure 4. Nyquist plot s for carbon steel in 200 ppm NaCl environment in the absence and presence of various inhibitor components, (+) Control (No inhibitor), (■) BPMG (20 ppm) + Zn2+ (30 ppm), (*) BPMG (30 ppm) + Zn2+ (30 ppm), (▲) BPMG (20 ppm) + Zn2+ (30 ppm) + t artrat e (25 ppm), (●)BPMG (30 ppm) + Zn2+ (30 ppm) + t art rate (50 ppm), (lines represent fitted curves) The inference given above for the increase of charge transfer resistance is supported by the decrease in the value of CPE in presence of the ternary inhibitor formu lations. The CPE value in case of the control solution is 13.41 F.cm-2. In presence of the ternary inhibitor formu lations, the CPE value is significantly decreased. The value of n, i.e., CPE exponent is found to be nearer to 1.0 in case of the ternary inhib itor systems. It indicates that the surface film formed in presence of these formulat ions is ho mogeneous. The increase in charge transfer resistance and decrease in CPE value in presence of the ternary inhibitor formu lations indicate that there is format ion of a highly protective surface film on the metal surface. It also infers that tartrate ions play an excellent role in the synergistic effect. A significant observation related to the inhibit ion efficiency values is to be noted. If the inhibit ion efficiency values obtained fro m g ravimetric (IEg), polarization (IEp) and EIS (IEi) studies are compared, slight differences are observed. It is suggested that the inhibition efficiency values obtained from various methods may not be strictly compared when the immersion times used in these methods are not same. Figure 5. Equivalent circuit used to fit the impedance data The value of Rct in case of the control solution is 2575 . In presence of the binary formu lation namely BPM G (20 ppm) + Zn2+ (30 pp m), the value of Rct is decreased while in case of the binary system containing BPM G (30 pp m) + Zn2+ (30 ppm), the value of Rct is increased to 3830 . Addition of tartrate to these binary systems increased the value of Rct enormously to 12460 and 21870 respectively. The large increase in charge transfer resistance in presence of the ternary inhibitor formu lations indicates that there is formation of an insulating protective film in presence of the 3.4. Interpretation of X-Ray Photoelectron S pectra The X-ray photoelectron spectrum of the surface film formed on carbon steel immersed in the environment containing BPM G (20 pp m) + Zn2+ (30 ppm) + tartrate (25 ppm) is shown in Figure 6. The XPS spectrum shows binding energy peaks due to electrons of various elements present in the surface film. The corresponding computer deconvolution spectra of the individual ele ments are shown in Figures 7 and 8. The interpretation of all the spectra is done with the help of the data of the elemental b inding energies reported in literature and also with the help of reports published on the 22 B. V. Appa Rao et al.: “N,N-Bis(phosphonomethyl) glycine, Zn2+ and Tartrate” – A New Ternary Inhibitor Formulation for Corrosion Control of Carbon Steel analysis of XPS spectra of the surface films formed on carbon steel. Figure 6. XPS survey spectrum of the surface film formed in presen ce of the ternary inhibitor formulation Figure 7. XPS deconvolution spectra of the elements in the surface film, a) Fe 2p and b) Zn 2p The XPS spectrum of Fe 2p in case of the inhibitor formulat ion is shown in Figure 7a. It shows two peaks, one with lower binding energy value corresponding to Fe 2p 3/2 electron and the other one with h igher bind ing energy value corresponding to Fe 2p1/2 electron. The peak due to Fe 2p3/2 is interpreted for the determination of che mica l state of iron in the surface film. This peak is observed at 710.9 eV. The characteristic elemental b inding energy of Fe 2p3/2 electron is 707.0 eV. That means the shift is to the extent of 3.9 eV. Such large shift suggests that iron is present in Fe3+ state in the surface film. Th is shift may be ascribed due to the presence of iron in Fe3+ state in the form of Fe (OH)3, FeOOH, -Fe2O3 and [Fe(III)-BPM G-tartrate] co mplex [25-28]. The binding energy of Fe2+ state in iron o xides is reported to be around 708.5 eV. The absence of any peak in this region in the present study also supports that iron does not exist in Fe2+, but in Fe3+ state in the surface film. The XPS spectrum of zinc is presented in Figure 7b. The spectrum shows two peaks, one corresponding to Zn 2p 3/2 and the other one corresponding to Zn 2p1/2. It can be observed that the intensity of Zn 2p3/2 peak is far greater than the intensity of Zn 2p1/2 peak. Zn 2p3/2 is normally interpreted. This peak is observed at 1021.7 e V. The h igh intensity peak of Zn 2p3/2 may be ascribed to the involvement of Zn2+ in the complex formation with BPM G as well as tartrate and also to the presence of Zn(OH)2 in the surface film. It was reported in the literature by Aramaki [30,31] that the Zn 2p3/2 peak at 1021.7 e V was due to the presence of Zn(OH)2 in the surface film. Felhosi et a l. interpreted fro m the XPS analysis that there is format ion of [Zn-HEDP] co mplex on the mild steel surface when immersed in a solution consisting of a mixtu re of Zn2+ and 1-hydro xyethane- 1,1-diphosphonic aicd (HEDP). The XPS spectrum o f phosphorus is shown in Fig8a. The figure shows two peaks, one corresponding to P 2p3/2 and the other one corresponding to P 2p1/2. The peaks for P 2p3/2 and P 2p1/2 are observed at 132.3 and 133.2 eV respectively. These are shifted fro m their characteristic elemental binding energies of 130.0 eV for P 2p3/2 and 131.0 eV for P 2p1/2 . Felhosi et al. observed a P 2p peak at 132.1 e V in the XPS of the surface film formed on carbon steel when immersed in a solution containing Zn2+ and HEDP. They interpreted this peak due to the presence of [Zn-HEDP] co mplex in the surface film. Ochoa et al. in their studies on the mixtures of salts of phosphono-carboxy lic acids and fatty amines as inhibitors for corrosion of carbon steel reported the P 2p peak at 132.1 eV and interpreted it due to the presence of phosphonate group in the surface film. In the literature, it was reported that the P 2p peak could be observed in the range of 132.9 to 133.8 eV, for iron or steels immersed in the solutions containing phosphonates, orthophosphates or polyphosphates. In the light of these reports, the P 2p peaks observed in the present study suggest the presence of BPM G in the surface film in the form of a co mp lex with Fe (III) and Zn (II). Figure 8b shows the N 1s peak in the XPS spectrum of the surface film observed at 399.6 eV. Th is peak is shifted fro m the characteristic elemental b inding energy value of 398.0 eV. This shift may be attributed to the presence of BPM G mo lecules in the surface film in the form of a complex with Fe (III) and Zn (II). It was reported in the literature that N 1s peak observed at 399.7 eV could be assigned to the presence of (= N –) in the molecule adsorbed on the metal surface. Meneguzzi et al. reported that the peak at 399.9 eV could be attributed to the neutral imine (– N =) and amine (– N–H) nitrogen atoms. The XPS spectrum of the surface film for carbon is shown in Figure 8c. The spectrumcontains three peaks of which one possesses high intensity relative to the other two peaks. The high intensity peak is observed at 284.3 eV. Ochoa et al. in their studies on the mixtures of salts of phosphonocarboxylic acids and fatty amines as inhibitors for corrosion of carbon steel reported the C 1s peak at 284.5 e V. This signal was accounted for by the p resence of the inhib itor mo lecules on the steel surface. BPM G consists of three different carbon environ ments namely C– C–OH, N– C–C International Journal of M aterials and Chemistry 2013, 3(2): 17-27 23 and N–C–P and tartrate ion also possesses two different carbon environments. Hence, the three peaks observed in case of carbon in the present study may be attributed to the presence of BPM G and tartrate in the surface film. Meneguzzi et al. observed in their studies a less intense peak at 286.8 eV and ascribed it to C–O or C– N or C=N. The XPS spectrum of O 1s is presented in Figure 8d. The O 1s peak of very h igh intensity is observed at 531.2 eV. Karman et al. studied the role of oxide layer formation during corrosion inhibition of mild steel in neutral aqueous med ia by a mixtu re of HEDP and Ca2+ ions. Fro m the XPS studies, they interpreted that the peak at 531 eV is due to the presence of OH- on the surface. Fang et al. ascribed the O 1s peak observed at 531.3 eV to the complex fo rmed between iron and phosphonate. Pech-Canul and Bartolo -Perez observed the O 1s peak at 531.3 eV, wh ich was ascribed to OH- from hydrous iron oxides and to the complex formed between iron and phsophonate group. It was also mentioned in their paper that such hydrous ferric o xides consist of Fe(OH)3 and FeOOH. Felhosi et a l. studied the effects of bivalent cations on corrosion inhibition of steel by HEDP. They mentioned that the O 1s peak at 531.4 e V is due to HO–Fe bond. Asami et al. observed O 1s peak at 531.5 eV in their study and attributed it to o xygen with a kind of Fe–O–H bond. In the light of these results and interpretations reported in literature, the O 1s peaks of high intensity observed in the present study may be interpreted as follows. The XPS spectrum of surface film (cf. Figure 6) shows that besides oxygen, there is presence of carbon, nitrogen, phosphorus, iron and zinc in the surface film. That means BPM G is present on the surface. Zinc is present as Zn2+. The interpretation g iven above in case of Fe 2p indicates that iron is present in the form of Fe3+. Hence, O 1s peak can be ascribed to the presence of Fe(OH)3, FeOOH, Zn(OH)2 and oxygen of phosphonate and tartrate ion in the surface film. Fro m the XPS spectrum (cf. Figure 6) of the surface film, it is clear that the intensity of Fe 2p peak is low when compared to that of Zn 2p. It can be exp lained as follows. In the surface film, iron exists as Fe3+ in the form of its o xides and hydroxides as well as in the form of co mp lex with BPM G and tartrate. These Fe3+ ions are present on the surface due to initial corrosion. But after the formation of protective surface film, there is no further corrosion and hence the Fe 2p peak intensity is less. It is interesting to note that the intensity of Zn 2p peak is far greater than that of Fe 2p peak. Due to sufficient concentration of Zn2+ in the bulk of the solution, these ions diffuse to the metal surface and exist in the form of Zn(OH)2 as well as in the fo rm of complex with BPM G as well as tartrate. Hence the intensity of Zn 2p peak is very high. After consolidating all the inferences drawn fro m the XPS of the individual elements present in the surface film, it is suggested that the surface film consists of Fe(OH)3, FeOOH, -Fe2O3, Zn(OH)2 and [Fe(III),Zn (II)–BPM G–tartrate] hetero-polynuclear co mplex. These compounds and the complex all together make the film highly protective. Figure 8. XPS deconvolution spectra of the elements in the surface film, a) P 2p, b) N 1s, c) C 1s and d) O 1s 3.5. Interpretation FTIR S pectra 24 B. V. Appa Rao et al.: “N,N-Bis(phosphonomethyl) glycine, Zn2+ and Tartrate” – A New Ternary Inhibitor Formulation for Corrosion Control of Carbon Steel The reflection absorption FTIR spectrum of the surface film formed on carbon steel in presence of the ternary inhibitor formu lation, BPM G (20 pp m) + Zn2+ (30 pp m) + tartrate (25 ppm), is shown in Figure 9. Th is spectrum is compared with the FTIR spectra of pure BPM G and pure potassium sodium tartrate tetrahydrate in KBr pellet. The spectra of pure compounds are not included in this paper. The reflection absorption FTIR spectrum of the surface film can be interpreted as follows. The peak due to P–OH stretching vibrations is observed at 1140 cm-1. Th is result can be interpreted due to interaction of free P – O- present in the phosphonate with metallic species, viz., Fe(III) and Zn(II) to form P– O–Metal bonds[4,36]. Raman et al. reported that a peak due to P–OH stretching vibrations appear in the range of 1100-1200 cm-1. It infers that there is presence of BPM G in the surface film. The two peaks appeared at 1680 cm-1 and 1520 cm-1 in the spectrum of the surface film may be due to the carbonyl group of BPM G and due to carbonyl group of tartrate ions. These are shifted fro m 1732 cm-1 and 1740 cm-1 observed for carbonyl group of BPM G and cabonyl group of tartrate ion respectively in the spectra of pure compounds. The two peaks due to carbonyl groups in the spectrum of the surface film are observed to be at lower frequencies when compared to the corresponding peaks of pure compounds. It is therefore inferred that BPM G and tartrate ion are involved in co mp lex fo rmation with Fe(III) as well as Zn(II) and the presence of [Fe(III), Zn(II)–BPM G–tartrate] hetero-polynuclear co mplex is suggested. The small peak observed at 1320 cm-1 is due to the presence of Zn(OH)2. A broad band appeared around 3280 cm-1 can be interpreted due to O–H stretching vibrations. The hydro xy l group is present in BPM G, tartrate ions, Zn(OH)2 and Fe(OH)3. Hence this peak supports the presence of these compounds within the protective film. The peak at 2360 cm-1 can be attributed to strong hydrogen bond, due to which the tartrate exists as a dimer or trimer. In case of pure potassium sodium tartrate, this peak is observed at 2500 cm-1. The peaks observed at 570 cm-1 and 630 cm-1 in the spectrum of the surface film can be assigned to amorphous oxides of Fe2O3 and Fe3O4. Fro m the reflection absorption FTIR spectrum of the surface film, it is inferred that apart from [Fe(III),Zn (II)–BPM G–tartrate] hetero-polynuclear co mplex, the protective film consists of Zn(OH)2 and oxides and hydroxides of iron (III). Figure 9. Reflection absorption FTIR spectrum of the surface film formed in presence of the ternary inhibitor formulation 3.6. Mechanism of Corrosion Inhi biti on In order to e xpla in all the e xperimental results, a plausible mechanis m of corrosion inhibit ion is proposed as follows: The mechanis m of corrosion of carbon steel in nearly neutral aqueous media is well established. The well-known reactions are mentioned below. Fe Fe2+ + 2e– (4) Fe2+ further undergoes oxidation in the presence of oxygen available in the aqueous solution. Fe2+ Fe3+ + e– (5) The corresponding reduction reaction at cathodic sites in neutral and alka line media is O2 + 2H2O + 4e– 4 OH– (6) Fe3+ ions produced at anodic areas and OH– ions produced at cathodic areas combine to form Fe(OH)3, (Fe2O3.H2O) which gets precipitated on the surface of the metal due to its very low solubility product. When BPM G, Zn2+ and tartrate ions are added to the aqueous solution, both BPM G and tartrate react with Zn2+ to form a ternary co mplex, [Zn2+-BPM G-tart rate]. This comple x d iffuses to the metal surface and b inds with Fe (III) ions available on the metal surface. A dense polymeric network structure is constituted on the surface by high degree of cross-linkage and reorganization. The polynuclear mu ltiligand complex, [Fe(III), Zn(II)-BPM G-tartrate] covers the anodic sites and controls the anodic reaction of the corrosion process. Felhosi et al. showed that iron can be passivated by simple immersion of it in aqueous solutions of 1,7-diphosphonoheptane. The formation of protective layer by the phosphonate consists of a fast adsorption step and subsequent slower process, which is supposed to be due to the organization. According to the authors, this effect is due to the format ion of self-assembling layer of co mp lexes of the organic inhibitor with iron. [Zn(II)-BPM G-tartrate] + Fe3+ [Fe(III), Zn(II)-BPM G-tartrate] (7) Free Zn2+ ions are available in the bulk of the solution because of relat ively higher molar concentration of Zn 2+ in the inhibitor mixture. These Zn2+ ions diffuse to the metal surface and react with OH– ions produced at the cathodic sites to form a precipitate of Zn(OH)2. Zn2+ + 2 OH– Zn(OH)2 (8) The precipitate of Zn(OH)2 gets deposited on the cathodic sites and controls the cathodic partial react ion of corrosion p ro cess . The decrease in inhibition efficiency when the bulk concentration of tartrate is higher than the optimu m value can be exp lained as follows. When the bulk concentration of tartrate is higher, the nature and composition of the complex, [Zn(II)-BPM G- tartrate] may be entirely different, with mo re tartrate than BPM G. Such a co mplex may not be protective. Secondly, higher concentration of free tartrate is available in the bulk of the solution. This free tartrate diffuses to the steel surface and gets chemisorbed on the metal surface. To that extent, the protective [Zn(II)-BPM G- tartrate] co mplex will not be available on the meta l surface. The ternary inhibitor formu lation is effective in the pH range, 6-8. At pH 9, h igher concentration of OH– ions are available both in the bulk of the solution and on the surface. International Journal of M aterials and Chemistry 2013, 3(2): 17-27 25 In such an environment, there is greater interference of OH– ions in the complexat ion leading to the formation of [Zn(II)-BPM G- tartrate -OH] co mplex, which may not contribute to the formation of p rotective film on the metal surface. In acidic mediu m at pH 5, the ligands will be in the protonated form and do not coordinate with Zn(II) as effectively as the deprotonated ligands. Secondly, enough amount of Zn(OH)2 will not be formed on the cathodic sites. Hence, decrease in inhib ition efficiency is observed at pH 5. Thus, BPM G, Zn2+ and tartrate play a very important ro le in the synergistic effect in controlling corrosion through the formation of protective film on the metal surface. It is inferred that the film may consist of various oxides/hydroxides like Fe2O3, Fe3O4.H2O, FeOOH, Zn(OH)2 and a polynuclear multiligand complex, [Fe(III), Zn(II)-BPM G-tart rate]. Each of these constituents contributes itself to make the film highly protective. 4. Conclusions Tartaric acid, a non-toxic organic co mpound is proved to be an excellent synergist in combination with BPM G and Zn2+ for corrosion control o f carbon steel in nearly neutral aqueous environment. The ternary formu lation containing 20 ppm o f BPM G and 30 pp m of Zn2+ along with 25 pp m of tartrate is an effective corrosion inhibitor for carbon steel. Once the protective film is formed, a mixture of only 10 ppm each of BPM G and Zn2+ and 20 pp m of tartrate will serve as the maintenance dosage. The ternary inhibitor system is thus relatively more environ mentally friendly. The inhib itor system is effective in the pH range 6-8. The inhib itor formulat ion acts as a mixed type inhibitor . Electrochemical impedance studies indicated the significant modificat ion of the metal/solution interface by the format ion of dense protective film in presence of the inhibitor formu lation. The protective film consists of mainly [Zn(II)– BPM G–tart rate] complex, Zn(OH)2 and small amounts of oxides/hydroxides of Fe(III). Presence of optimu m amounts of all these compounds is required at a given pH value to make the surface film p rotective. inhibition of steel by 1-hydroxyethane-1,1-diphosphonic acid”, The Electrochemical Society, Journal of The Electrochemical Society, vol.146, no.3, pp.961-969, 1999.  Y. Gonzalez, M . C. Lafont, N. Pebere, F. 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