Improved corrosion inhibition by heterocyclic compounds on mild steel in acid medium

2.1 Preparation of test solutions

The test solutions of 1 N H₂SO₄, 1 N HCl, 1 mM KI, 1 mM KCl, 1 mM NiCl₂·6H₂O and 1 mM ZnCl₂ were prepared by using distilled water. All chemicals and reagents used were of analytical grade and used without further purification.

(S1) 2,6-Diphenyl-3-methy lazinan-4-one	(S2) 2,6-Diphenyl-3-methyl azinan-4-one semicarbazone	(S3) 2,6-Diphenyl azinan -4-one (R1 = H)	(S4) 2,6-diphenylazinan-4-one semicarbazone (R1 = H)

2.2 Synthesis of S1 and S3

The compounds 2,6-diphenyl-3-methyl-azinan-4-one (S1) and 2,6-diphenyl azinan-4-one (S3) were synthesised as per the literature procedure (Balasubramanian and Padma 1963).

2.3 Synthesis of S2 and S4

Semicarbazide hydrochloride (0.5 g) and sodium acetate (0.5 g) in hot ethanol (20 mL) were added to an alcoholic solution of inhibitor (1.5 g of S1 or S3). This mixture was shaken well for 20 min and kept at room temperature. The product that formed was filtered, slowly washed with cold water and recrystallised from ethanol. The yield was 60% of S2 (m.pt 188–190 °C) and 65% of S4 (m. pt. 176–178 °C) (Govindasamy and Ayappa 2015).

2.4 Weight loss method

The initial weight of the polished plate was taken. The solutions were taken in a 100 mL beaker and the specimens were suspended in duplicate in the solution using glass hooks. After a period of an hour, the specimens were removed, washed with water, dried and weighed to an accuracy of four decimals. From the initial and final masses of the specimen (before and after immersion in the solution), the loss of weight was calculated. The experiment was repeated for various inhibitor concentrations of 1 N H₂SO₄. The procedure was repeated for various concentrations of the inhibitors S1, S2, S3 and S4.

To know the effect of temperature the above procedure was carried out at different temperature ranges i.e. 313–323 K using a thermostat, with best inhibitor efficient concentration.

From the data obtained, the determination of inhibitor efficiency IE (%), corrosion rate, surface coverage (θ), activation energy (E_a) and free energy of adsorption (ΔG°_ads) were calculated by applying the following equations, as shown by Quraishi et al. (2010).

where w_o is the weight loss without inhibitor and w_i is the weight loss with inhibitor.

where w = weight loss in mg, D = density in g/cm³, A = area of exposure in square inches, T = time in hours.

where w_b and w_i are the weight losses per unit time without and with inhibitor, respectively. From this a graph was drawn between C/θ versus °C and C/θ versus log C to know whether the adsorption of inhibitor follows Temkin’s/Langmuir’s isotherm to obtain a linear relationship (Luis et al. 2009; Morad 2008; Tang et al. 2009).

Free energy of adsorption was calculated using the formula

where

where θ is the degree of coverage on the metal surface, C is the concentration of the inhibitor in mol/L, K is the equilibrium constant, R is gas constant and T is temperature.

2.5 Electrochemical study

2.6 Synergistic and salt effect

The synergistic and salt effect were carried out by adding 1 mM KI, 1 mM KCl, 1 mM NiCl₂·6H₂O and 1 mM ZnCl₂ to the steel specimen immersed in 1 N H₂SO₄ containing various concentrations of the inhibitors S1, S2, S3 and S4 for the duration of an hour. From the weight loss and electro chemical studies the corrosion rate and inhibition efficiency were calculated (Sathiyapriya and Rathika 2019; Umoren and Ebenso 2007).

3.1 IR spectral studies

Analysis of the IR spectra of the compounds S1, S2, S3 and S4 indicates that all the compounds show two strong bands in the regions 3525–3398 cm⁻¹ and 3400–3367 cm⁻¹ assignable to ν_asym(N–H) and ν_sym (N–H), respectively of the –NH₂ group of the compounds (Chandra and Gupta 2005). Another medium intensity broad band around 3319–3306 cm⁻¹ found in the IR spectra of these compounds is due to –NH stretching vibration of ring nitrogen. Bands appearing in the region 3100–3030 cm⁻¹, 2975–2950 cm⁻¹ and 2898–2850 cm⁻¹ are due to aromatic ν(C–H), aliphatic and alicyclic ν_asym(C–H) and aliphatic and alicyclic ν_sym(C–H) respectively. A medium to sharp band observed in the region 1568–1492 cm⁻¹ in the case of S2 and S4 were attributed to ν(C=N) stretching which indicates the presence of azomethine group in the compounds (Afrasiabi et al. 2005). A strong band in the region 1720–1680 cm⁻¹ is mainly due to C=O stretching mode (Hamdy and El-Gendy 2013). In addition to several other bands in the region 1450–900 cm⁻¹ can be attributed to vibrations involving interactions between C=O stretching and –C=N stretching (Beraldo et al. 2001; Shutalev et al. 2014).

3.2 ¹H and ¹³C NMR spectral studies

The ¹H and ¹³C NMR Spectra of all compounds were recorded in DMSOd6 as a solvent. The N–H proton of azinan-4-one ring system of S1–S4 exhibit singlet in the region 2.00–2.07 ppm (Afrasiabi et al. 2005; Chandra and Gupta 2005; Surendra Kumar et al. 2016). All the compounds show multiplet between 7.24 and 7.61 ppm which is due to aromatic protons. The compounds S1 and S3 show a chemical shift value in the region 0.9–1.1 is assignable to C–CH₃ group. In addition to these chemical shift values the compound S2 and S4 show additional peaks at 8.6–8.8 ppm due to =N–NH group. The proton of the –CONH₂ show a characteristic signal δ(NH₂) in the region 5.3 and 6.3 ppm The presence of these broad peaks indicate that these two protons are non-equivalent and this may be due to restricted rotation of –CONH₂ bond as a result of orientation of semicarbazone group in space.

The ¹³C NMR spectra of the compounds S1–S4 show signals in the range 126–142.67 and 36–69 is due to aromatic carbons of the phenyl rings and due to carbons of heterocyclic ring, respectively (Pandeya and Raja 2002; Quraishi et al. 2010; Surendra Kumar et al. 2016). The compounds S1 and S3 show a strong peak at 205–209 due to –C=O of heterocyclic ring. The signals in the range 150–152.5 ppm and at 157.20 ppm appeared in the case of S2 and S4 are due to azomethine carbon(–C=N) and NH–CO–NH₂, respectively (Abdel-Aai and Morad 2001; Raghav and Kaur 2014; Rajak et al. 2010). The compound S1 and S3 show a peak around 10.10 is due to –CH₃ carbon (Surendra Kumar et al. 2016). Thus, IR, ¹H NMR and ¹³C NMR Spectral studies confirm the formation of S2 and S4 from S1 and S3, respectively.

3.3 Weight loss studies

3.3.1 Effect of concentration of inhibitor on inhibition efficiency

It was observed that all the four inhibitors inhibit the corrosion of mild steel to certain percentage at all concentration used in this study. In all the four cases (S1, S2, S3 and S4) the inhibition efficiency was found to increase with increase in inhibitor concentration (Table 1).

Table 1:

Inhibition efficiencies of various concentrations of inhibitor (S1, S2, S3 and S4) for the corrosion of mild steel in 1 N H₂SO₄ obtained by weight loss measurements at room temperature.

Parameters	Inhibitor	Inhibitor concentration (mM)
		Blank	0.2	0.4	0.6	1	2
Inhibition efficiency (IE%)	S1	–	50.48	54.29	57.14	66.67	77.14
	S2	–	51.53	55.10	57.65	67.86	81.63
	S3	–	39.15	51.42	59.91	68.40	73.11
	S4	–	51.19	60.32	68.25	76.59	80.95
Degree of coverage (θ)	S1	–	0.5048	0.5429	0.5714	0.6667	0.7714
	S2	–	0.5153	0.5510	0.5765	0.6786	0.8163
	S3	–	0.3915	0.5142	0.5991	0.6840	0.7311
	S4	–	0.5119	0.6032	0.6825	0.7659	0.8095
Corrosion rate (mpy)	S1	1767.87	875.52	808.17	757.66	0.589.29	404.09
	S2	3300.03	1599.50	1481.65	1397.46	1060.72	606.13
	S3	3569.42	2171.96	1734.20	1431.14	1128.07	959.70
	S4	4242.89	2070.94	1683.68	1346.95	993.38	808.17

It is evident from the data that, all the inhibitors are efficient inhibitors even at the concentration as low as 0.2 mM. By increasing the concentration of the inhibitor from 0.2 to 2 mM, the percentage IE increases gradually and linearly. It is noted that the inhibition efficiency of semicarbazones (S2, S4) were higher than their corresponding parent keto-amines. The metal loss was progressively decreased with increasing inhibitor concentration. The tested azinan-4-one semicarbazones (S2, S4) inhibit the corrosion even at low concentration. The maximum efficiency of about 60–81% was obtained at 0.4–2 mM of inhibitors (Table 1).

The corrosion rate in 1 N H₂SO₄ for various concentrations of the inhibitors (S1, S2, S3 and S4) was determined after an hour of immersion. The corrosion rates expressed in mpy decreased with increasing inhibitor concentration as evident from Table 1. The surface coverage (θ) for different inhibitor concentrations were calculated. The observations of the plot of C/θ versus °C (Figure 1) gives a straight line confirming that all the four inhibitors obeyed Langmuir’s adsorption isotherm and the formation of an adsorbed layer of the inhibitors on the active sites of the metal surface.

Figure 1:

Langmuir’s plot of inhibitors (S1, S2, S3 and S4) in 1 N H₂SO₄.

The effect of temperature on corrosion rate and inhibition efficiency (IE%) was carried out at various temperatures ranging from 303 to 333 K in the absence and in the presence of inhibitors at selected concentration (Table 2).

Table 2:

Inhibition efficiencies of various concentrations of inhibitor (S1, S2, S3 and S4) for the corrosion of mild steel in 1 N H₂SO₄ obtained by weight loss measurements at different temperatures.

Temperature (K)	Inhibitor	IE (%)		Corrosion rate (mpy)
		Inhibitor concentration		Inhibitor concentration
		Blank	0.4 mM	Blank	0.4 mM
313	S1	–	29.07	9091.92	6448.53
	S2	–	31.30	9091.92	6246.48
	S3	–	33.89	9091.92	6010.77
	S4	–	44.07	9091.92	5084.74
323	S1	–	20.57	11,297.55	8974.06
	S2	–	22.65	11,297.55	8738.34
	S3	–	27.42	11,297.55	8199.56
	S4	–	37.26	11,297.55	7088.33
333	S1	–	15.56	13,637.87	11,516.43
	S2	–	16.05	13,637.87	11,449.08
	S3	–	21.73	13,637.87	10,674.58
	S4	–	27.31	13,637.87	9900.89

It is evident from Table 2 that in 1 N H₂SO₄ the efficiency of inhibitor decreases with increase in temperature indicating weak adsorption. The decrease in IE with temperature indicates the fact that the inhibitor film formed on the metal surface is less protective in nature at high temperature (Hamdy and El-Gendy 2013).

The values of activation energy (E_a) were calculated from the plot of log(corrosion rate) versus 1000/T. The free energy of adsorption (ΔG°_ads) at various temperatures was calculated. The values of E_a and ΔG°_ads are given in Table 3. The less negative values of ΔG°_ads with increase in temperature indicate the physical adsorption of the inhibitors on the metal surface (Abdel-Aai and Morad 2001). The values of E_a in the inhibited acid solution are appreciably greater than those obtained in the uninhibited acid solution. This suggested that the presence of reactive centres on the inhibitors, block the active sites for corrosion resulting in an increasing in activation energy. This also indicates that these types of indicators are more effective at room temperature than at higher temperature.

Table 3:

Activation energies (E_a) and free energies of adsorption (ΔG°_ads) for the corrosion of mild steel for selected concentration (0.4 mM) of the inhibitors in 1 N H₂SO₄.

Inhibitor	E a (kJ)	−ΔG°ads at various temperatures (kJ)
		313 K	323 K	333 K
Blank	33.70	–	–	–
S1	48.25	10.52	9.47	8.71
S2	50.36	10.79	9.80	8.81
S3	47.87	11.09	10.46	9.80
S4	55.34	12.21	11.66	10.63

3.4 Electrochemical studies

A.C. impedance measurements were carried out at room temperature for corrosion of mild steel in 1 N H₂SO₄ after immersion for about 10 min. The Nyquist plots for mild steel uninhibited acid and for the three concentrations of the inhibitors (S1, S2, S3 and S4) are shown in Figure 2 and the data are tabulated (Table 4).

Figure 2:

Nyquist diagrams for mild steel in 1 N H₂SO₄ for selected concentrations of S1, S2, S3 and S4.

The charge transfer resistance (R_t) value for mild steel in uninhibited H₂SO₄ significantly changes after the addition of inhibitor. The value of charge transfer resistance increase with increase in concentration. The fact is advocated by the increase in inhibitor efficiency. The semicircular nature of Nyquist plots obtained for all experiments indicate that the corrosion of mild steel is controlled by charge transfer process (Ahamad et al. 2010; Ashish Kumar and Quarishi 2010). The double layer capacitance values of the systems were also examined and calculated using the expression:

The suggested equivalent circuit model for the studied system is given below .

The various corrosion kinetic parameters such as corrosion current (I_corr), corrosion potential (E_corr), anodic and cathodic Tafel slopes (β_a and β_c) were derived from potentiodynamic polarisation studies on mild steel in 1 N H₂SO₄, both in the presence and in the absence of inhibitors at different concentrations (Table 4) and the polarisation curves are depicted in (Figure 3). The E_corr values were shifted slightly in the presence of the inhibitors. The I_corr value decreased with the addition of inhibitor. Tafel slopes β_a and β_c were affected to the same extent by the addition of inhibitors (S1, S2, S3 and S4) (Ferreira et al. 2004; Sathiyapriya et al. 2019).

Figure 3:

Polarisation curves of mild steel recorded in 1 N H₂SO₄ for selected concentrations of inhibitors S1, S2, S3 and S4.

3.5 Synergistic effect

It has been reported that halide ions and metal ions have the most adsorbable character on steel surface. Hence, a comparative study of synergistic effect of corrosion on mild steel in 1 N H₂SO₄ by a combination of inhibitors with and without addition of I⁻,Cl⁻, Zn²⁺ and Ni²⁺ ions were studied by weight loss and electro chemical method. The results are given in Table 5 and the corrosion inhibition was found to increase with addition of halide ions and metal salts in addition to inhibitors (Pavithra et al. 2010; Umoren and Solomon 2015).

The addition of I⁻ ions to the inhibiting solution is found to have better inhibition efficiency than Cl⁻ ions. It is evident from Table 5 that both Zn²⁺ and Ni²⁺ ions have the potential to enhance the inhibition action Zn²⁺ ions have better enhancing power than Ni²⁺ ions. Potentiodynamic polarisation and AC impedance data follow the same trend and it is evident from Table 5, Figures 4 and 5.

Figure 4:

Nyquist diagrams for the optimum concentration of S1, S2, S3 and S4 in 1 N H₂SO₄ in the presence of KI, KCl, ZnCl₂ and NiCl₂·6H₂O.

Figure 5:

Polarisation curves for the optimum concentration of S1, S2, S3 and S4 in 1 N H₂SO₄ in the presence of ZnCl₂ and NiCl₂·6H₂O.

The corrosion current obtained for mild steel plates tested in the inhibited solutions were lower than uninhibited solution (blank). This shows that the presence of inhibitors lowered the i_corrr value. A lowest i_corr value was observed for 1 mM concentration of S4 which showed a maximum inhibition efficiency of 85.14%. Similar trend of decrease in i_corr value and increase in IE% was observed by the addition of salts KCl, KI, ZnCl₂ and NiCl₂ (synergism).

A typical Tafel polarisation curve obtained for different concentration of inhibitors and blank show a change in shape of the anodic and cathodic curves. Corrosion is an electrochemical phenomenon and inhibitors decrease the velocity of electrochemical reactions. The lower i_corr value by the addition of inhibitors and salts implies that the rate of electrochemical reaction were reduced due to the formation of layers over the mild steel surface by the inhibitor molecules. The E_corr values of inhibited and uninhibited systems did not vary significantly. It shows that the addition of inhibitors affected both anodic and cathodic reactions, suggesting that the inhibitors behaved as a mixed type inhibitors.

The polarisation resistance increases noticeably with increase in inhibitor concentration and the IE% values also increase with inhibitor concentration. The IE% calculated from polarisation resistance show the same trend as those obtained from corrosion current I_corr.

Polarisation curves and Tafel line obtained for mild steel with the addition of salts (along with inhibitors are shown in Table 5). In all the cases the polarisation resistance and IE% were found to increase with increase in the concentration of inhibitors. It is found that same trend is obtained for the addition salts (Stanly Jacob and Parameswaran 2010).

3.6 Inhibiting efficiency of the inhibitors in 1 N HCl

Weight loss studies were also carried out with various concentrations of inhibitors of S1, S2, S3 and S4 in 1 N HCl to verify the inhibition efficiency of the inhibitors in another acidic medium. It is found that all the four inhibitors are effective against corrosion and the inhibition efficiency (IE) increased with increase in concentration (Appa Rao et al. 2005, 2011) (Table 6). All the compounds inhibit corrosion by adsorption mechanism and adsorption of these compounds follow Langmuir’s adsorption isotherm. Even in this study the inhibitors S2 and S4 (semicarbazones) show high inhibition efficiency than S1 and S3 (heterocyclic keto-amines) (Table 6 and Figure 6).

Figure 6:

Effect of various concentrations of the inhibitor versus inhibition efficiency in 1 N HCl.

3.7 Mechanism of inhibition

The extent of the adsorption of the different inhibitors at a fixed concentration would depend upon the surface area of the inhibitor molecules, size, shape, orientation, number of active centres such as N and O atoms and the intensities of lone pairs of electrons on these sites along with intensities of π electrons on aromatic rings and the electronic charge of the molecules (Al-Azawi et al. 2016).

In this present study the percentage of inhibition efficiency exhibited by these inhibitors is high due to strong adsorption of the inhibitor molecules on the metal surface. The inhibitors are expected to get adsorbed through the lone pairs of electrons on N and O atoms. The π electron density on the phenyl rings also support the strong coordination of these groups with the metal surface.

From all the above studies it is revealed that the semicarbazones (S2, S4) show higher corrosion inhibition efficiency than S1 and S3 and it may be due to the presence of –NH–CO–NH₂ polar functional groups. The additional O and N of semicarbazone strongly adsorbed on the surface of mild steel through their lone pair of electrons thereby increased the inhibition of corrosion of mild steel in acid solutions.