Glyceral Trinitrate: As Potential Corrosion Protector for Mild Steel in Acid Medium Along with Paint-Coated Steel in a Saline Environment

Introduction

Mild steel has excellent mechanical properties ¹ and due to this property, it is used as the material of construction engineering structural material. In petroleum industries, mild steel is extensively used transmission pipelines, dubler, flow lines, etc., where it is susceptible for Oxidative degradation for which different descaling process such as acid well acidification ^2-5, acid cleaning, petrochemical processes are widely used for cleaning process. Mild steel undergoes corrosion attack, due to aggressive nature [6-8] of acidic medium. This corrosive attack can be controlled by using chemical inhibitors. Organic compounds containing N,S  ,O ^9-10 having π electron cloud are used to protect the metals and alloys ¹¹ extensively and these are found to be an effective corrosion inhibitors ^12-13. The corrosion inhibitors forms productive coating, which avoids the chemical species from diffusing in mild steel ionization process. Drug  play an important role as corrosion inhibitors due to i) low cost ii) easy solubility iii) biodegradability iv) less toxicity ^14-18.

The present study deals with the determination of effectiveness of GTN (Fig.1). It is an anti-anginal drug, whose trade name is Monit-gtn 2.6mg. The main constituent of the drug is glyceryl trinitrate (GTN) with the molecular formula of C₃H₅N₃O₉. The IUPAC name of the drug GTN is 1, 3-dinitrooxypropan-2-yl nitrate.

Figure 1: Structure of corrosion inhibitor GTN Click here to View figure

Experimental Work

Preparation of Materials

Size of mild steel specimens were cut into dimensions 5cm² exposed area for gravimetric experiments, Mild steel (Weight %: 0.098 % C, 99.653% Fe, 0.02 % P, 0.012% Ni, 0.201 % Mn, 0.016% S and mild steel rod exposed surface area 1cm² where used for electrochemical analysis. For all the experiments the surface of the mild steel samples were polished by using a separate grade (400-1000 grit) of emery papers, then cleaned by using distilled water and dried. To prepare the corrosive medium analytical grade HCl was used with proper dilution.

Preparation of inhibitor Solution

The testing stock solution of 100 mg/L of GTN was prepared by calculated amount  of powdered drug was dissolved  in 1M HCl solution and the desired concentrations (5 – 60 mg/L) of the GTN solution are prepared by dilution the required aliquot with double distilled water.

Methods

Weight loss method

Standard ASTM G1-03 procedure ¹⁹ was applied for weight loss measurements. Properly polished and preweighed MS ccoupons were suspended in 100ml of HCl solution using glass hooks without and with descired concentrations of GTN for 3h to 24 h. The exposed coupons were then taken out, cleaned, reweighed and the various corrosion parameters were calculated as follows.

where W_o and W_i specimen non-appearance and appearance the inhibitor.

Electrochemical measurements

Mild steel rod  (1cm² area) is used working electrode, SCE  as  refernce and  Pt electrode as counter.  All potentials were measured wth respect to SCE. Before each measurement, OCP was calculated ^20-21. Frequency range of EIS measurements100 KHz to 0.01 Hz ^22-24 using  25 mV amplitude. R_ct, C_dl determined from the plot  Z’ Vs Z . From the electrochemical parameters,the I.E (%) was calculated as in the formula,

where R_ct(y) , with  inhibitor and R_ct(n) , without  inhibitor.The corrosion current densities I_corr(B) and I_corr(I) for without and with different concentration in 1M HCl.

Epoxy Coating

Materials required for preparing 5ml of paint

Volume of binder-1.25ml                                          Zinc -2.675g

 Volume of solvent-3 ml                                           Inhibitor-0.1g

Zinc phosphate – 1g                                                  TiO₂-0.4287g

Mixed well the fine powder of TiO_2, zinc, inhibitor, zinc phosphate. A binder of epoxy resin is mixed with a hardener of  polyamide.3ml of Xylene is used as a solvent.The mixture of epoxy resin-polyamide was added to the mixture of fine powder grained well and solvent was  added   drop by drop until a paint form is  obtained and painted on  mild steel which are was labelled as A, B and C. A-bare mild steel specimen without epoxy coating, B-Mild steel with epoxy coating (2mm thickness), C- Mild steel with epoxy coating containing GTN(2mm thickness).The coated coupons dried at 78hrs and studied in 3.5% NaCl.

Electrochemical Measurements of the paint coating mild steel

Electrochemical measurements were carried out in a similar setup of three  electrode system using the epoxy coated coupons as working electrode in saline environment.

Theoretical methods

The inhibitor geometrical structure was optimized completely  with the program package  GAUSSIAN 09 followed by B3LYP procedure ^25-27 within the basic set  of 6-31G ²⁸. All the quantum chemical parameters of  electronegativity (χ), dipole moment(µ), global hardness (η), E_LUMO E_HOMO, Global  softness (σ), ∆E, ∆N were discussed for the studied compound.

Results and Discussion

Gravimetric measurements

Based on gravimetric investigation on mild steel specimen, the stability of the inhibitor molecules was evaluated. The measurements obtained after 3h, 6h,12h and 24hrs immersion  time at 298K with different concentartion of GTN (5-60 mg/L) was clearly predicted in Table 1. Thus as  the inhibitor GTN concentration increases there is a increase in the I.E(%) ^29-33 and corrosion rates decreases. The result shows that the metal surface are blocked by the GTN inhibitor molecules from the corrosion solution. After 6 h of immersion, I.E (91.01%) was found to decrease which may be attrributed ^34-38. Metal due to prologed exposure of metal to the medium. With increased immersion time upto 24 hours there is found to be an increase in the corrosion rate ^39-42. This result can be explained by the theory of adsorption.GTN molecules were desorbed, on prolonged immersion on the mild steel specimen surface, thereby increases the surface corrosive environment interaction. Moreover, the corrosion rate increases with increasing contact time, which was assigned to a small amount of GTN molecules being adsorbed in the surface to control the metal dissolution process. Hence it is clear that after 6 hours of immersion time, GTN molecules were desorbed, they turned to be ineffective as they are not involve in the inhibitive process.

Table 1: The inhibition of GTN from Gravimetric measurements.

Conc mg/L	3 h		6 h		12 h		24h
	CR mm/y	IE %	CR mm/y	IE %	CR mm/y	IE %	CR mm/y	IE %
Blank	1168	–	1351.	–	3279.	–	6491.	–
5	601	48.53	422.	68.73	1073.	67.27	3378.	47.95
10	548	53.06	296.	78.04	1038.	68.32	3316.	48.91
15	492	57.82	186.	86.18	714.	78.22	2783.	57.11
20	447	61.68	168.	87.55	671.	79.52	2526.	61.07
25	408	65.08	151.	88.82	592.	81.94	2043.	68.52
30	374	67.91	147.	89.12	597.	81.78	1910.	70.57
35	307	73.70	143.	89.41	551.	83.19	1737.	73.24
40	282	75.85	137.	89.80	536.	83.64	1722.	73.46
45	231	80.16	131.	90.29	477.	85.45	1710.	73.65
50	201	82.77	128.	93.16	378.	88.44	1657.	74.46
55	169	85.49	124.	93.78	314.	90.42	1603.	75.30
60	139	88.10	94.	94.04	291.	91.11	1462.	77.46

 

Effect of the inhibitor concentration on protection ability over time is presented in Figure 2. It was clear from figure that at lower immersion time (3 hrs) there was a steady increase in the inhibition effeciency with concentration but for higher immersion time (6 & 12 hrs) the effeciency of the inhibitor GTN increases upto 15 mg/L concentration after which thereis not much pronouned effect inits corrosion inhibiting performace. Thus optimal concentration of GTN attaining the maximum efficiency at 15 mg/L in 1M HCl.

Figure 2: Influence of various concentrations on the I.E of GTN for mildsteel in 1M HCl Click here to View figure

Potentiodynamic polarization measurements

At different concentration, the electrochemical reaction potential and current were determined from the point intersection of cathodic and anodic curves by extrapolation of linear parts of the Tafel plots. As evidenced from the Tafel plot (Figure 3) GTN affects both bc and ba. It is also proved from the change in both the tafel slopes values which proves that GTN inhibitor behaves with mixed nature. I.E(%), it reaches maximum efficiency 85.7% at 60mg/L (Table 2). This results indicates that the GTN inhibitor, surface of the metal active sites are blocked and formed by a barrier between the charge transfer and mass which are the evidences fort he decrease in the interaction oft he metal surface with the corrosive environment.

Figure 3: Polarization  curves observed for mild steel electrode  in 1M HCl including addition concentrations of GTN. Click here to View figure

Table 2: Polarization parameters for the corrosion of mild steel without and with various concentrations of GTN in 1M HCl.

Concentration mg/L	Ecorr mV	Icorr μA/cm2	IE %	bc mV/dec	ba mV/dec	Rp Ω cm2	IE %
Blank	-543	217	–	101	117	108	–
5	-525	130	40.09	76	146	166	34.94
15	-525	75	65.44	91	136	314	65.61
30	-529	54	75.12	86	139	430	74.88
45	-529	45	79.26	81	135	469	76.97
60	-529	31	85.71	82	137	492	78.05

 

Electrochemical impedance spectroscopic measurements

From Figure 4 and Table 3, the experimental curves are best fit and the electrochemical parameters are obtained. The results shows that as inhibitor concentration increases there is an increase in the R_p and decreases in C_dl ⁴³. R_p values are increases with inhibitor concentration designates growth in the θ, causing the inhibition efficiency increases ^44-46

Figure 4: Nyquist plots for mild steel corrosion in 1M HCl without and with GTN Concentrations Click here to View figure

Table 3: EIS paprameters for mild steel in HCl with GTN

Concentration mg/L	RS Ω cm2	Rct Ω cm2	IE %	Cdl μF/cm2	θ
Blank	6.81	19	–	12.9	–
5	9.23	45	58.04	26.4	-1.0465
15	16.6	68	72.42	23	-1.0465
30	16.1	107	82.40	19.1	-0.7829
45	19.2	114	83.39	18.2	-0.4806
60	20.3	135	86.03	17.5	-0.4109

 

Because the adsorption of the inhibitor molecule increases with increased concentration, the C_dl value lowers, indicating that the contact of the metal surface area with the medium reduces. The interface layer is affected by the increased substitution of water at the solution-metal border, which decreases the dielectric constant value ^47-51. The shift in the C_dl value, which corresponds to the decrease in the amount of the metal deterioration, is a proof for the displacement of water at the interface double layer. The superficial heterogeneity is represented by constant n values. The n rate increases, indicating that the surface of mild steel becomes more and more homogeneous as the inhibitor concentration rises due to the increased surface coverage.

Epoxy coating behavior of GTN in 3.5% of NaCl

Figure 5 depicts an equivalent circuit model of the cell system. It was used to fit curves and determine the resistance and coating capacitance of epoxy coating systems. R_t – charge transfer resistance, CPE_coat – coating capacitance, R_s – solution resistance, R_coat – coating resistance, CPE_dl – dobl elayer capacitance.

Figure 5: Combining the EIS data with the Euivalent circuit model Click here to View figure

From the figure 6, Nyquist plot for the coating in 3.5 percent NaCl  was investigated. The coatings investigated show a larger capacitive loop in the Nyquist plots at first, with the dielectric response matching that of the coating ^52-53. The coating acts as a barrier layer and has a high coating resistance (R_coat)⁵⁴. The impedance spectroscopy shows only one gradually diminished capacitive arc after a few hours of immersion, indicating ongoing penetration of the electrolyte solution and little electrochemical processes. In addition, the coating’s macroscopic appearance remains unaffected. Because the mass transfer pcorrosion mechnaism is impeded in presence of GTN, a diffusion tail forms in the impedance spectroscopy as the immersion time increases.

Figure 6: Nyquist plot of epoxy coated mild steel coupons in NaCl  (a) 0 hours, (b) 48 hours, and (c) 168 hours Click here to View figure

Table 4: Electrochemical impedance characteristics for paint coated mild steel corrosion in NaCl

Sample Name	Ageing period (hours)	Rs (Ω cm2)	Cdl (mF/cm2)	Rcoat (Ω cm2)	Rt (Ω cm2)	Rtot (Ω cm2)
Bare Mild Steel	0	8.1	9 x 10-7	3922	-3849	73
	48	7.3	9  x 10-7	-8.54	7.4	-1.1
	168	9.6	1.13  x 10-5	-31.4	29	-60.4
Epoxy coated mild steel	0	22.2	132	939	151	1090
	48	26.4	9  x 10-7	804	-7535	499
	168	33.8	9  x 10-7	-176	168	-8
Epoxy coated mild steel with GTN	0	58	118	1830	285	2115
	48	48	571	1217	98	1315
	168	59	9  x 10-7	-5665	6012	347

 

Corrosion in NaCl

Potentiodynamic polarization studiesin 3.5% of NaCl

As illustrated in Figure 7, the potentiodynamic polarisation plots reveal the favourable effect of paint coating when compared to bare MS (A), epoxy coated MS (B), and epoxy coated MS including the inhibitor (C) in a saline environment. The electrochemical characteristics of the corrosion process, E_corr, I_corr, ba & b care given in Table. 5 shown. (ba,bc). The corrosion potentials on painted mild steel surfaces moved to lower negative values, and the anodic current densities on these surfaces were lower than on polished mild steel surfaces ⁵⁵. These findings can be taken as demonstrating that an epoxy-coated steel surface is more corrosion resistant than an untreated steel surface. This study demonstrates a significant improvement in corrosion resistance coating. The resistance was determined to be low on the seventh day.

Figure 7: Tafel plot of painted coupons in NaCl (3.5%) at (a) 0 hr (b) 48 hrs (c) 168 hrs Click here to View figure

Table 5: Parameters of potentiodynamic epoxy coated coupons in NaCl (3.5%)

Sample	Ageing period (hours)	Ecorr (mV)	Icorr (µA/cm2)	ba (mV/dec)	bc (mV/dec)
A	0	-634	190	481	126
	48	-647	9.92	299	118
	168	-638	16	142	161
B	0	-647	9.92	299	118
	48	-693	66	122	287
	168	-595	21	155	121
C	0	-638	16	142	161
	48	-608	18	9	155
	168	-738	27	84	208

 

Quantum chemical Studies

Frontier molecular orbitals (HOMO, LUMO) is the calculation form of quantum chemical studies. It is used to relate the inhibition efficiency and the molecular structure of the inhibitor ^56-59. Figure 8 represents the charge distribution of the GTN inhibitor. The electron donating and electron accepting ability is related to energies of frontier molecular orbitals in a molecule. Figure 9 a & b represents the E_HOMO and E_LUMO of the inhibitor molecule

Figure 8: Charge distribution structure of  GTN Click here to View figure

Figure 9: (a). The GTN’s HOMO Click here to View figure

Figure 9: (b). The GTN’s LUMO Click here to View figure

From Table 6, quantum chemistry parameters that are arrangement molecules’ well-known minimal energy configuration. High E_HOMO values ⁶⁰ indicate a molecule that has a proclivity for donating electrons to the Fe atom’s vacant d orbital ^61-62. The high value of HOMO, -11.99 eV, and the very low value of LUMO, -1.80 eV, corresponding to the value of HOMO, -11.99 eV, and the very low value of LUMO, -1.80 eV, corresponding to the value of E (10.19 eV), have an insignificant value, indicating that the energy required for electron elimination, resulting in good inhibition efficiencies and a better dipole moment. The dipole value of 4.677 eV indicates that the investigated inhibitor system has good adsorption characteristics on the metal surface.

Global Hardness η are  important pointer of a molecules tendency concerning covalent interaction, softness are accelerate the surface and molecules interaction as well as the metal state and hybridization ⁶³. From Table 6 visibly η have lower values (5.09eV) that indicates the high corrosion inhibition potential of GTN. Global softness (σ) is 0.09 eV. According to Lukovis et al, a N value of less than 3.6 implies that GTN has a stronger potential to donate electrons ⁶⁴. The electron-donating capacity of GTN is confirmed by the same order of N and E_HOMO.

Table 6: Quantum chemical parameters obtained by DFT calculation

Quantum chemical Parameters	GTN
Molecular Formula	C3H5N3O9
Total Energy (a.u)	-947.36
EHOMO (eV)	-11.99
ELUMO (eV)	-1.80
∆E (eV)	10.19
Dipole Moment (µ) (eV)	4.677
Global Hardness (η) (eV)	5.09
Global Softness (σ) (eV)	0.09
Electronegativity (χ) (eV)	6.89
∆N	0.1

 

Conclusion

The inhibitive nature of a pharmaceutically active chemical, glyceryl trinitrate GTN, against acid corrosion and as an epoxy coating protection in a salty environment was investigated using the following data.

In 1M HCl, the pharmaceutically active chemical glyceryl trinitrate GTN inhibited mild steel corrosion, and notable efficiency increase when noted for incremental change in inhibitor concentration.

The inhibitive action was attributed to the inhibitor molecule after becoming physically adsorbed. The Langmuir isotherm governs the adsorption process.

The mixed type nature of GTN in acid media is demonstrated by its polarisation behaviour.

Impedance studies of paint coating in 3.5 percent NaCl demonstrate that including GTN into the paint improves the performance of the coating, resulting in a coating with strong antipermiability resistance. It’s also worth noting that, thanks to these new refined passivation methods, the produced epoxy coatings with GTN might save the a lot money.

Inhibitive impact examined substances was connected with data derived from DFT calculation. Both experimental and theoretical calculations coincide well.

Acknowledgement

The authors express their gratitude to PSGR Krishnammal College for Women, DST-FIST, UGC, for the financial support providing the essential facilities.

Conflict of interest

There are no conflicts of interest declared by any of the authors

Funding Sources

There are no funding source.

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