General properties and comparison of the corrosion inhibition efficiencies of the triazole derivatives for mild steel

1.1 Mild steel (MS) corrosion inhibitors (CIs) and their classifications

MS (also known as plain-carbon steel, low-carbon steel, carbon steel, and A36 carbon steel) is one of the most preferred materials for the industry due to its availability, physical properties, and cost. Acidic solutions are extensively utilized in industries; the most vital fields of utilization are acid de-scaling, oil well acidizing, acid pickling, petrochemical processes, and industrial cleaning (Bentiss et al., 1999a,b,c; Soror et al., 1999; Fan et al., 2002). However, acidic conditions are usually detrimental to MS and give rise to significant financial losses (Oguzie et al., 2011). To overcome this issue, CIs are generally employed. The application of CIs in industries and synthesization of new CIs are increasing day by day, owing to the fact that the corrosion inhibition methodology is one of the operative and commercial approaches to shield MS from corrosion in acidic conditions (Lagrenee et al., 2002; Abdallah, 2003). CIs can be classified as either inorganic or organic compounds. Generally, inorganic CIs have anodic or cathodic actions (Fayomi et al., 2018). Organic CIs have both actions, anodic and cathodic, as well as protective action by film adsorption (Figure 1). It is well documented in the literature that organic CIs are more effective and cheap as compared to inorganic CIs (Vishnudevan & Thangavel, 2007).

Figure 1:

Classification of CIs.

1.2 Application of organic compounds as CIs

Organic CIs have achieved outstanding importance in recent years attributed to their many achievements as anti-corrosion agents under a broad spectrum of corrosive environments (Bouayed et al., 1998). It is indicated from a literature survey that, in an acidic environment, there are numerous organic compounds that are highly effective CIs, e.g. aromatic α,β-unsaturated aldehydes, acetylenic alcohols, α-alkenyl phenones, quaternary ammonium compounds, amines, carbonyl containing inhibitors obtained by condensation reaction, and nitrogen- or sulfur-containing compounds (Brindsi et al., 1981; Schmitt, 1984; Cizek, 1994). Among these organic compounds, acetylenic alcohols are broadly utilized as anti-corrosives in industries due to their large inhibition efficiency (IE) and commercial availability. Nevertheless, there are some drawbacks in these inhibiting systems: (i) the effectiveness of acetylenic alcohols is impressive only at large concentrations, and (ii) under acidizing environments, the acetylenic alcohols give rise to toxic vapors (Stupnišek-Lisac et al., 1994; Quraishi & Sardar, 2003).

The toxicity of organic anti-corrosion compounds is a burning issue that encourages synthetic chemists to search for eco-friendly anti-corrosion compounds that contain no heavy metals or toxic units, and are biodegradable (Quraishi & Jamal, 2001). Recently, much exercise has been devoted to the construction of the so-called green CIs that are non-toxic as compared to the majority of the currently practical organic CIs (Sinko, 2001). For example, it has been investigated that a diverse range of eco-friendly plant extracts (Hasson et al., 2011; Ji et al., 2011; Lebrini et al., 2011; Deng et al., 2012a,b) and commercial drugs (Gece, 2011; Farsak et al., 2017) have the property of anti-corrosion for metals in various acidic baths. Meanwhile, investigations of the inhibitive efficiency of eco-friendly natural amino acids for steels (Ashassi-Sorkhabi et al., 2004; Olivares-Xometl et al., 2008; Ghareba and Omanovic, 2010; Amin and Ibrahim, 2011; Hamani et al., 2017), iron (Amin et al., 2010; Espinoza-Vázquez et al., 2014), aluminum (Bereket & Yurt, 2001; Ashassi-Sorkhabi et al., 2005), copper (Barouni et al., 2008; Zhang et al., 2008a,b; Spah et al., 2009; Khaled, 2010; Levin et al., 2012), nickel (Deng et al., 2012a,b), and some alloys (Ghasemi & Tizpar, 2006) in various corrosive conditions have been carried out. More recently, investigations by Prasai et al. (2012) and Kirkland et al. (2012) have revealed that eco-friendly graphene-based CIs have excellent anti-corrosion property for copper and nickel (Novoselov et al., 2004). However, most of these tested non-toxic CIs have certain limitations. The first limitation is that the extraction of the CIs from natural products is a difficult job, i.e. laborious work-up is required; usually, little efficiencies are obtained and the product that is isolated usually contains impurities. Second, the application of drugs as CIs is profligate, attributed to long synthetic steps that are required for their total synthesis, resulting in low total yields and making these drugs expensive.

In the last decade, numerous five-membered heterocyclic compounds have been studied, and the investigation revealed that various five-membered heterocyclic compounds display excellent anti-corrosion efficiency for shielding of numerous alloys and metals (specifically MS), which mainly featured 1,2,4-triazole and 1,2,3-triazole derivatives (Musa et al., 2010a,b; Zhang et al., 2010; Khiati et al., 2011; Mert et al., 2011). However, despite the well-studied 1,2,3-benzotriazole derivatives (Harvey et al., 2011), rare 1,4-disubstituted-[1,2,3]-triazolyl analogues have been investigated as CIs (Zhang et al., 2008a,b; Deng et al., 2012a,b). The triazole-based compounds are almost free from all the above-mentioned limitations; for example, they are eco-friendly, cost-effective, easily synthesizable, and operative at low concentrations. Furthermore, triazoles are highly thermally stable compounds and have excellent coordination ability.

1.3 Triazole derivatives and their applications

The triazole ring is a valuable organic heterocyclic compound consisting of a diunsaturated five-membered ring structure of two carbon atoms and three nitrogen atoms. There are two sets of isomers that differ in the relative positions of the three nitrogen atoms. Each of these has two tautomers that differ by which nitrogen has a hydrogen bonded to it (Figure 2). Among nitrogen-containing five-membered heterocyclic compounds, triazole derivatives have proved to be the most promising scaffolds for biological potencies (Sathish & Kavitha, 2013). Different kinds of triazole derivatives demonstrate a variety of pharmacological activities such as anti-hypertensive, anti-neoplastic, anti-epileptic, anti-histaminic, anti-anxiety, anti-malarial, analgesic, anti-Parkinson’s, anti-diabetic, anti-obesity, local anesthetic, anti-oxidant, anti-tubercular, anti-depressant, anti-inflammatory, anti-fungal, anti-cancer, anti-viral, and anti-microbial. The triazole ring can also be used as a linker of other biological pharmacophore components to afford new drug molecules. These benefits have established triazole derivatives as significant pharmacological frameworks for construction of drugs with a wide spectrum of biological profile, many of which are already on the market (Maddila et al., 2013).

Figure 2:

Structure of 1,2,3-triazole and 1,2,4-triazole heterocyclic compounds.

1.4 Triazole derivatives as CIs

Beside biological activities, triazole derivatives are also known for their corrosion inhibitory ability on MS. Chemically, the triazoles are well suited for CI applications because they are stable against base and acid hydrolysis, as well as in oxidative and reductive conditions. The framework of triazoles is also relatively resistant to decomposition. As aromatic, electron-rich systems, triazole derivatives are able to bind to MS via weak interactions such as cation-π, π-π stacking, ion-dipole, coordination bonds, hydrogen bonds, van der Waals forces, or hydrophobic effect, consequently displaying a wide range of corrosion inhibitory activities (Finšgar & Milošev, 2010; Sathish & Kavitha, 2013). In the past decade, triazoles have received much attention, as their intriguing physical and chemical properties, as well as their outstanding stability, render them promising corrosion control agents. Numerous triazole derivatives have been investigated for their corrosion inhibitory ability, and investigations revealed that triazole derivatives are impressive CIs for MS in acidic conditions (El Din et al., 1997; Bentiss et al., 2002, 2003; Quraishi & Sharma, 2003; Tamilselvi & Rajeswari, 2003; Wang et al., 2003, 2004; Chikh et al., 2005; Wang, 2006; Bentiss et al., 2007). Moreover, among the different nitrogen-containing species investigated as anti-corrosion agents, triazole derivatives have been considered as environmentally friendly compounds (Wang et al., 2004; Wang, 2006; Musa et al., 2010a,b).

In this review, the application of substituted triazoles as CIs for MS is reviewed by comparing different triazole derivatives with each other with respect to their IEs, which is evaluated from electrochemical measurements, in various acidic media and various concentrations of triazole inhibitors. The approach of the collection, compilation, and summarization of the significant experimental results of triazole derivatives as CIs in one platform is advantageous, helpful, and crucial for synthetic chemists to expand the diversity and complexity of a class of triazole derivatives by synthesizing new triazole derivatives as CIs. Moreover, the reviewing approach also provides guidance to identify the best triazole inhibitor for MS in industry. Moreover, through understanding the mechanism of different inhibitors, this review provides informative opinions to formulate new triazole derivatives having very large IEs.

4.1 Inhibition action of 2[5-(2-pyridyl)-1,2,4-triazol-3-yl] phenol (PPT) on MS in HCl medium

The consequence of the application of PPT on MS dissolution in 1 m HCl has been investigated by Bentiss et al. through gravimetric measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization curve analysis (PPC). From these measurements, it was found that PPT is an excellent anti-corrosion agent (Table 2). Electrochemical measurements, i.e. PPC measurements, demonstrated that PPT is a mixed-type inhibitor (Table 1). It was observed that the IE of PPT does not depend on temperature, and an increase in the concentration of PPT results in decreasing the E_a of corrosion. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) of the MS revealed that PPT is absorbed on the surface of MS through electrostatic force attractions. PPT follows the general adsorption mechanism of the Langmuir adsorption isotherm model (Bentiss et al., 1999a,b,c).

Table 1:

Investigation of 1,2,4-triazole derivatives as CIs.

Sr. no.	CI name	Abbreviation	Molecular structure	Type of inhibitor	References
1	2[5-(2-Pyridyl)-1,2,4-triazol-3-yl phenol	PPT		Mixed-type inhibitor	Bentiss et al., 1999a,b,c
2	3,5-Bis(4-methyltiophenyl)-4H-1,2,4-triazole	4-MTHT		Mixed-type inhibitor	Bentiss et al., 2007
3	3,5-Bis(4-pyridyl)-4H-1,2,4-triazole	4-PHT		Mixed-type inhibitor	Bentiss et al., 2007
4	3,5-Diphenyl-4H-1,2,4-triazole	DHT		Mixed-type inhibitor	Bentiss et al., 2000, 2003, 2007; Arshad et al., 2017
5	3,5-Di(m-tolyl)-4-amino-1,2,4-triazole	m-DTAT		Cathodic-type inhibitor	El Mehdi et al., 2003
6	5-Amino-1,2,4-triazole	5-ATA		Mixed-type inhibitor	Hassan et al., 2007
7	5-Amino-3-mercapto-1,2,4-triazole	5-AMT		Mixed-type inhibitor	Hassan et al., 2007
8	5-Amino-3-methyl thio-1,2,4-triazole	5-AMeTT		Mixed-type inhibitor	Hassan et al., 2007
9	1-Amino-3-methyl thio-1,2,4-triazole	1-AMeTT		Mixed-type inhibitor	Hassan et al., 2007
10	3-Benzylidene amino-1,2,4-triazole phosphonate	BATP		Mixed-type inhibitor	Ramesh & Rajeswari, 2004
11	3-p-Nitro-benzylidene amino-1,2,4-triazole phosphonate	PBAT		Mixed-type inhibitor	Ramesh & Rajeswari, 2004
12	3-Salicylialidene amino-1,2,4-triazole phosphonate	SATP		Mixed-type inhibitor	Ramesh & Rajeswari, 2004
13	3,5-Bis(methylene octadecyldimethylammonium chloride)-1,2,4-triazole	18-Triazole-18		Cationic gemini surfactant	Qiu et al., 2005
14	3-Amino-1,2,4-triazole-5-thiol	3ATA5T		Mixed-type inhibitor	Mert et al., 2011; Tourabi et al., 2014

4.2 Understanding the adsorption of symmetrical pyridine-, benzene-, and thioanisole-containing CIs on the MS surface

Bentiss and co-workers have observed through experimental investigation that 3,5-bis(4-methyltiophenyl)-4H-1,2,4-triazole (4-MTHT) is an excellent CI, and among 4-MTHT, 3,5-bis(4-pyridyl)-4H-1,2,4-triazole (4-PHT), and 3,5-diphenyl-4H-1,2,4-triazole (DHT), the IE order is 4-MTHT>4-PHT>DHT (Table 2). The IE depends mainly upon the nature and type of CI units and the groups attached to CI units. In 1 m HCl solution, PPC displayed that 4-MTHT, 4-PHT, and DHT are mixed-type inhibitors (Table 1). The IE increases with increase in the concentration of CI, namely 4-MTHT, 4-PHT, and DHT, and when the concentration of 4-MTHT is 5×10⁻⁴m, then the maximum IE is achieved, i.e. 99.6%. 4-MTHT, 4-PHT, and DHT adsorption on the surface of MS follows the Langmuir isotherm model. The thermodynamic parameters also show that the anti-corrosion properties of 4-MTHT, 4-PHT, and DHT are attributed to the development of a chemisorbed film on the surface of MS (Bentiss et al., 2007).

4.2.2 DHT as CI for MS in acidic bath

The CI effect of DHT on the corrosion of MS in HCl was examined at a temperature 30°C using electrochemical parameters and weight loss (WL) experiments. PPCs exposed that DHT is a mixed-type inhibitor (Table 1). The maximum IE obtained is 98%, and the value of IE increases with increasing concentrations of DHT CI, and the highest IE is observed when the concentration of CI is 4×10⁻⁴m (Table 2). The surface analysis technique (XPS) displayed that inhibition of corrosion by DHT is attributed to the development of a chemisorbed film on the surface of MS. In 1 m HCl solution, the adsorption of DHT CI on the surface of MS follows the Langmuir adsorption isotherm (Bentiss et al., 2000, 2003; Arshad et al., 2017). The high IE of the CI can also be justified by the localization and the distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) densities along the molecules. Effectively, Figure 4 clearly shows that, for the DHT molecules, the HOMO density is supported by the N3 and N4 nitrogen atoms of the penta-atomic heterocyclic compound. The LUMO density of the molecules (displayed in Figure 4) is preferentially localized on the C-N bond for the DHT molecule (Bentiss et al., 2003).

Figure 4:

HOMO and LUMO of DHT (Bentiss et al., 2003).

4.3 Inhibition performance of 3,5-di(m-tolyl)- 4-amino-1,2,4-triazole (m-DTAT) toward corrosion of MS

The anti-corrosion effect for MS in 1 m HCl by m-DTAT has been investigated by El Mehdi et al. at 30°C using electrochemical parameters and WL techniques. PPC displayed that m-DTAT is a cathodic-type inhibitor (Table 1). IEs up to 95% can be obtained for m-DTAT (Table 2). The inhibitive effect of m-DTAT is attributed to the development of a chemisorbed film on the surface of MS and appears that the substitution of the hydrogen of the triazole, by an amino group (NH₂), increases the IE. The IE is probably linked to the availability of free electron pairs of the nitrogen atom, to form a donor-acceptor-type link necessary for the formation of a protective chemisorbed film. In 1 m HCl medium, the adsorption of m-DTAT CI on the surface of MS follows the Langmuir adsorption isotherm. The temperature effect on the CI behavior with the addition of 4×10⁻⁴m of m-DTAT was also investigated under the temperature range of 30–60°C (El Mehdi et al., 2003).

4.4 Inhibition of MS corrosion by amino group-mediated CIs

The investigation of 5-amino-1,2,4-triazole (5-ATA), 5-amino-3-mercapto-1,2,4-triazole (5-AMT), 5-amino-3-methyl thio-1,2,4-triazole (5-AMeTT), and 1-amino-3-methyl thio-1,2,4-triazole (1-AMeTT) as anti-corrosion agents for MS has been carried out in 0.1 m HCl at 20°C by Hassan and co-workers. The results of the investigation revealed that 5-AMT is the best CI among 5-ATA, 5-AMT, 5-AMeTT, and 1-AMeTT, and displays almost 96% IE at a concentration of 10⁻³m (Table 2). PPC displayed that 5-ATA, 5-AMT, 5-AMeTT, and 1-AMeTT are mixed-type inhibitors (Table 1). The order of IE among four investigated CIs is 5-AMT>5-AMeTT>1-AMeTT>5-ATA (Hassan et al., 2007).

4.4.1 Reason for the high relative IE of 5-AMT

The high polarizability of CS double bond and excellent capability to form hydrogen bonding are the main reasons for the high IE of 5-AMT. The reason for the relatively low IE of 5-ATA is poor adsorption via pi-electrons present in ring and N-species. The tautomeric forms of 5-AMT permit classification of 5-AMT not only as aromatic (isomer 1) but also as distorted triazole (isomers 2 and 3) compounds. In acidic condition, isomers 2 and 3 have domination over isomer 1 (Figure 5). Moreover, the replacement of the -SH group with the -SCH₃ group also contributes to the lower IE of 5-AMeTT; that is, the replacement restricts the production of isomers (caused by tautomerization) with C-S bonds like 2 and 3. The greater IE of 5-AMeTT and 1-AMeTT over 5-ATA is specified to increments in the number of adsorption centers in the CI species and the greater density of electrons in them is attributed to the electron-donating effect of methyl groups (Hassan et al., 2007).

Figure 5:

Tautomeric forms of 5-ATA.

4.5 Inhibitive effect of phosphonate-containing CIs toward corrosion in aqueous bath

Ramesh et al. have synthesized an environmentally acceptable multi-component inhibitor (triazole based) containing phenyl, azomethine, and phosphonate groups in the same molecule acting as a good-mixed inhibitor system, with the objective of evaluating its corrosion inhibition properties on MS in natural aqueous atmosphere (lake water atmosphere) by WL, PPC, and alternating current (AC) impedance techniques. Biocide, cetyltrimethylammonium bromide, and molybdate at various concentrations have also been used with the inhibitors to study their interference effect on the corrosion process of MS in lake water. It was observed that the IE increases with the increase in the concentration of SATP. BATP, PBAT, and SATP are mixed-type inhibitors (Table 1). The values of the degree of corrosion inhibition by the inhibitors are in the order SATP>BATP>PBATP (Table 2). Corrosion inhibition may arise from the formation of a protective film consisting of the Fe-phosphonate complex. The higher IE shown by SATP compared to BATP and PBATP may be owing to the increased electron density resulting in the mechanism of transferring of electrons from functional groups to the surface of MS, creating a superior coordinate type of bonding with a greater adsorption and IE. The low double-layer capacitance (C_dl) value and increase in polarization resistance (R_p) obtained in the existence of both inhibitor and biocide showed the development of a thicker inhibitor layer on the surface of MS (Ramesh & Rajeswari, 2004).

4.6 Inhibition performance of 18-triazole-18 for MS corrosion

The anti-corrosion effect in 1 m HCl by 18-triazole-18 has been investigated by Qiu et al. at 30°C using electrochemical parameters and WL techniques. In 1 m HCl, it was observed by Qiu et al. that 18-triazole-18 is a cationic gemini surfactant and an excellent inhibitor (Table 1). The IE of 18-triazole-18 increases with rise of surfactant concentration, and 18-triazole-18 displays maximum IE, i.e. 89% at 1×10⁻³m (Table 2). In 1 m HCl bath, the adsorption of 18-triazole-18 CI on the steel surface obeys the Langmuir adsorption isotherm (Qiu et al., 2005).

4.6.1 Inhibition mechanism in HCl by 18-triazole-18

The adsorption mechanism of the gemini surfactant 3,5-bis(methylene octadecyldimethylammonium chloride)- 1,2,4-triazole (18-triazole-18) on the surface of MS is discussed above, and the plot of the IE versus the logarithmic of concentration of 18-triazole-18 is shown in Figure 6. As illustrated in the figure, the studied concentrations of 18-triazole-18 could be divided into four regions, which correspond to four adsorption mechanisms as shown in Figure 7A–D, respectively. When the concentration of 18-triazole-18 is extremely low, IE increases gradually with rise in surfactant concentration in the first region, in which 18-triazole-18 molecules are adsorbed according to the adsorption mechanism shown in Figure 7A. Then, IE increases rapidly in region II and reaches an unobvious plateau. In region III, 18-triazole-18 may be adsorbed according to the adsorption mechanism as shown in Figure 7B. With further increases in 18-triazole-18 concentrations near the critical micelle concentration (CMC), IE increases rapidly again (region III) and then reaches the second plateau at and above the CMC (region IV). In these two regions, 18-triazole-18 molecules are adsorbed on the MS surface by the adsorption mechanism, as shown in Figure 7C and D, respectively (Qiu et al., 2005).

Figure 6:

Variation of IE with the logarithmic concentration of the gemini surfactant 18-triazole-18 in 1 m HCl at 25°C (Qiu et al., 2005).

Figure 7:

Adsorption model of 18-triazole-18 onto steel surface in HCl bath at various concentrations (Qiu et al., 2005).

4.7 Examination of 3-amino-1,2,4-triazole-5-thiol (3ATA5T) as a CI for steel in HCl bath

The anti-corrosion capacity of 3ATA5T has been examined by Mert and co-workers in 0.5 m HCl bath on steel via EIS and PPC calculations at different quantities of 3ATA5T and temperature conditions. A high IE was observed, which is justified on the basis of protonated CI species, which are strongly adsorbed on steel surface and constructed a shielding layer. In 0.5 m HCl bath, the IE is up to 99% after 168 h at 10 mm CI concentration (Table 2). E_a and ΔG_ads indicated that the mechanism is more like physiosorption instead of chemosorption (Table 1) (Mert et al., 2011; Tourabi et al., 2014).

5.1 Corrosion-inhibiting power of 3,5-bis(2-pyridyl)-4-amino-1,2,4-triazole (2-PAT) on MS

The inhibiting action of 2-PAT on corrosion of MS in HClO₄, HCl, and H₃PO₄ has been tested by Mernari et al. via the application of electrochemical parameters and WL measurements. The inhibition of corrosion by CI is owing to the development of a chemisorbed film (protective inhibitor layer) on the surface of MS (Table 2). The adsorption also results in the development of a protective layer, which increases (layer thickness) with the rise in exposure time. Moreover, the investigation of electrochemical parameters exposed the fact that 2-PAT is anodic inhibitor and follows the Langmuir adsorption isotherm (Table 3) (Mernari et al., 1998; Bentiss et al., 1999a,b,c; Wang et al., 2012; Chaitra et al., 2015; Belghiti et al., 2016).

Table 3:

Investigation of 1,2,4-triazol-4-amine derivatives as CIs.

Sr. no.	CI name	Abbreviation	Molecular structure	Type of inhibitor	References
1	3,5-Bis(2-pyridyl)-4-amino-1,2,4-triazoles	2-PAT		Anodic inhibitor	Mernari et al., 1998; Bentiss et al., 1999a,b,c; Wang et al., 2012; Chaitra et al., 2015; Belghiti et al., 2016
2	3,5-Bis(2-thienyl)-4-amino-1,2,4-triazoles	2-TAT		Mixed-type inhibitor	Bentiss et al., 1999a,b,c
3	3,5-Bis(3-pyridyl)-4-amino-1,2,4-triazoles	3-PAT		Mixed-type inhibitor	Mernari et al., 1998; Wang et al., 2012
4	3,5-Bis(3-pyridyl)-4-amino-1,2,4-triazoles	4-PAT		Mixed-type inhibitor	Mernari et al., 1998; Wang et al., 2012
5	3,5-Diphenyl-1,2,4-triazoles	DAT		Mixed-type inhibitor	Bentiss et al., 2000, 2003; Arshad et al., 2017
6	4-Amino-3-butyl-5-mercapto-1,2,4-triazole	ABMT		Adsorption via Temkin’s isotherm	Quraishi & Sharma, 2003
7	3,5-Di(m-tolyl)-4H-1,2,4-triazole	m-DTHT		Adsorption via Langmuir isotherm	El Mehdi et al., 2003
8	2,5-Bis(4-methoxyphenyl)-4-amino-1,2,4-triazole	4-MAT		Chemisorption	El Mehdi et al., 2003
9	4-Amino-5-phenyl-4H-1,2,4-trizole-3-thiol	APTT		Mixed-type inhibitor	Musa et al., 2010a,b
10	4-Amino-5-methyl-4H-1,2,4-triazole-3-thiol	AMTT		Chemisorption	Musa et al., 2010a,b
11	4-Amino-5-hydrazine-1,2,4-triazole-3-thiol	ATTT		Chemisorption	Musa et al., 2010a,b
12	3,5-Bis(2-thienylmethyl)-4-amino-1,2,4-triazole	2-TMAT		Mixed-type inhibitor	Tourabi et al., 2013
13	4-Amino-4H-1,2,4-triazole-3,5-dimethanol	ATD		Mixed-type inhibitor	John and Joseph, 2012

5.2 Controlling corrosion of MS using 3,5-bis(2-thienyl)-4-amino-1,2,4-triazole (2-TAT) in HCl bath

An investigation of CI 2-TAT has been carried out by Bentiss and co-workers in HCl and H₂SO₄ media using the WL technique and EIS measurements, the results of which illustrate that CI 2-TAT is an anodic inhibitor and it follows the Langmuir adsorption isotherm (Table 3). One of the best features of this molecule is that it is found to be non-cytotoxic. It has also been investigated by Bentiss and co-workers that 2-TAT on the surface of MS leads to the development of a protective layer, which develops more with the increment in exposure time (Table 4) (Bentiss et al., 1999a,b,c).

5.3 Studies on symmetrical pyridine-containing CIs for MS in 1 m perchloric acid

An investigation of 3,5-bis(3-pyridyl)-4-amino-1,2,4-triazoles (3-PAT), 2-PAT, and 3,5-bis(3-pyridyl)-4-amino-1,2,4-triazoles (4-PAT) has been done by Mernari et al. in 1 m HClO₄ medium at 30°C using the electrochemical impedance spectroscopy (EIS) technique and gravimetric measurements, the results of which illustrated that the IE of 3-PAT is 95% at a concentration of 12×10⁻⁴m, while 4-PAT and 2-PAT possess IEs of 92% and 65% at concentrations of 12×10⁻⁴m (Table 4). The inhibiting features of 3-PAT, 2-PAT, and 4-PAT are influenced by the concentration of CI applied on the surface of MS, and IE also depends on the position of the nitrogen unit in the pyridinium group of CI. From the application of EIS and WL, it has been tested that among 3-PAT, 2-PAT, and 4-PAT, the order of decreasing IE is 3-PAT>4-PAT>2-PAT, and the IE highly depends on the position of the nitrogen unit in pyridinium groups of 3-PAT, 2-PAT, and 4-PAT. These CIs follow the Langmuir adsorption isotherm, and ΔGoads demonstrates that in 1 m HClO₄ medium, 2-PAT, 3-PAT, and 4-PAT are mixed-type inhibitors (Table 3) (Mernari et al., 1998).

5.3.1 Effect of the position of the nitrogen unit in pyridinium groups of 2-PAT, 3-PAT, and 4-PAT

In the case of 2-PAT and 4-PAT, the direct electron-withdrawing effect of the pyridinium group is present in the 1,2,4-triazole unit. It is noted that in the case of 3-PAT, the electron-withdrawing effect of the pyridinium group is very minor (Figure 8). Hence, in the comparison of 3-PAT with 4- PAT and 2-PAT, 3-PAT displays somewhat better success (Table 4). Moreover, the potentially tetradentate chelating molecule (2-PAT) demonstrates less CI performance in comparison with 3-PAT and 4-PAT, which are non-chelating isomers. It is believed that the reason for the low performance of 2-PAT in 1 m HClO₄ medium is attributed to the development of a non-adherent Fe(II)-2-PAT-ClO₄ complex that facilitates Fe dissolution (Table 3). Anyhow, 2-PAT displays excellent inhibiting performance in HCl, H₂SO₄, and H₃PO₄ media (Mernari et al., 1998; Wang, 2006; Wang et al., 2012). In these cases, the reason for the excellent inhibiting performance is the non-formation of coordination complexes in the inhibition mechanism (Mernari et al., 1998).

Figure 8:

Withdrawing effect of pyridinium group on 3-PAT, 2-PAT, and 4-PAT.

5.4 Inhibition of corrosion of steel in hydrochloric acid by 3,5-diphenyl-1,2,4-triazole (DAT)

The corrosion IE of triazoles and oxadiazoles at the MS electrode in 1 m HCl has been investigated by Bentiss et al. Their IE might be explained in terms of molecular parameters (Table 4), and they used quantum-mechanical calculations to relate the experimental IEs to the molecular structure for some organic inhibitors (Bentiss et al., 2000, 2003; Arshad et al., 2017).

5.4.1 Reason for the high IE of MS in HCl by DAT

The high IE of the molecule can also be justified by the localization and distribution of HOMO and LUMO densities along the molecules (Table 4). Effectively, Figure 9 clearly shows that, for the DAT molecule, the HOMO density is supported by the N3 and N4 nitrogen atoms of the penta-atomic heterocyclic compound. The LUMO density of the molecules displayed in Figure 9 is localized preferentially on the C-N bond in the DAT molecule (Bentiss et al., 2003).

Figure 9:

Density of electrons in HOMO and LUMO of DAT (Bentiss et al., 2003).

5.5 4-Amino-3-butyl-5-mercapto-1,2,4-triazole (ABMT) CI for MS in H₂SO₄

The IE of ABMT has been investigated in 1 n H₂SO₄ by Quraishi and Sharma using WL and PPC measurements in MS. The PPC technique exposed that ABMT is a mixed-type inhibitor (Table 3), and on the surface of MS, ABMT follows Temkin’s adsorption isotherm (Quraishi & Sharma, 2003).

5.6 Inhibitive capacity of 3,5-di(m-tolyl)- 4H-1,2,4-triazole (m-DTHT) toward corrosion of MS

The inhibitive effect of m-DTHT toward corrosion of MS has been investigated by El Mehdi et al. at 30°C using the application of WL and electrochemical parameters. PPC displayed that m-DTHT is a mixed-type inhibitor (Table 3). An IE of up to 91% for m-DTHT can be obtained (Table 4). The inhibitive effect variations at different temperature ranges, i.e. 30–60°C, was also studied using 4×10⁻⁴m of m-DTHT (El Mehdi et al., 2003).

5.7 Controlling corrosion of MS using 2,5-bis(4-methoxyphenyl)-4-amino-1,2,4-triazole (4-MAT) in HCl bath

4-MAT has been found by Bentiss et al. to be an extraordinary CI for controlling the corrosion of MS in 1 m HCl bath at a temperature of 30°C, and the IE increases with rise in the quantity of 4-MAT. The maximum IE, i.e. 98%, is achieved at 4×10⁻⁴m (Table 4). The IEs were investigated by WL, PPC, and EIS techniques. The adsorption of 4-MAT follows the Langmuir adsorption isotherm with negative values of ΔG_ads, which throws light on the fact that the process of adsorption of the 4-MAT molecules is spontaneous (Table 3). The negative value of ΔG_ads also suggests that the process of controlling anti-corrosion by 4-MAT is attributed to the development of a chemisorbed layer on the surface of MS. XPS analysis of the tested steel surface supports the result of thermodynamic measurements and displays indication of the chemisorption of 4-MAT, which explains its high corrosion IE (Bentiss et al., 2009).

5.7.1 Mechanism of action of 4-MAT

In HCl bath, 4-MAT displays cationic units with regard to pK_a of the protonated function (Figure 10).

Figure 10:

Protonation of 4-MAT in HCl bath.

It is believed that, initially, adsorption of Cl⁻ occurs on the surface of MS and the net positive charge, which is present on the surface of MS, supports the specific adsorption of Cl⁻ (Lagrenee et al., 2002). The adsorption of the cationic forms of 4-MAT (form II) is limited by the anion concentration on the surface of MS. The para-methoxyphenyl-substituted heterocyclic compound may also be adsorbed via donor-acceptor electrostatic forces of interactions between electrons in p-orbitals of the aromatic systems and the unshared electron pairs of the heterospecies (viz. nitrogen atom and oxygen atom) to make a strong bond with the empty d-orbitals on surface of MS, which acts as a Lewis acid, resulting in the development of a chemisorbed layer that acts as a protective shield. This effect is enhanced by the strong electron-donating ability of the methoxy group of the anisyl substituent, leading to an excess of negative charges on the nitrogen atoms of the triazole ring (Figure 11). Moreover, 4-MAT molecules adsorb on the surface of MS by electron sharing between nitrogen atom and/or the p-electrons and vacant d-orbitals of iron (Lagrenee et al., 2002).

Figure 11:

Electron donor ability of the methoxy group.

5.8 Thiol-based inhibitors for MS corrosion

Musa et al. found that thiol-based triazole derivatives are effective inhibitors of corrosion of MS in sulfuric acid bath. Their IEs increase with the concentration of inhibitors in the order of 4-amino-5-hydrazine-1,2,4-triazole-3-thiol (ATTT)<4-amino-5-methyl-4H-1,2,4-triazole-3-thiol (AMTT)<4-amino-5-phenyl-4H-1,2,4-trizole-3-thiol (APTT). The adsorption model of AMTT and ATTT follows the Langmuir adsorption isotherm. The interaction energy of the studied inhibitors increases in the order ATTT<AMTT<APTT (Table 4). These molecules have been chosen because they contain one coordinate covalent bonded group (=N-, -NH₂) and one covalent bonded group (-S, -H), which develop a protective layer on the MS surface. PPC and EIS measurements are used in this study to obtain the IE. The electrochemical measurements results revealed that the IEs increase with rise in the concentration of inhibitors. The molecular dynamic method results displayed that higher forces of interaction between the CI and surface of MS result in higher IE. The quantum chemical calculation results showed that the triazole ring is the active site in these inhibitors, and they can adsorb on the Fe surface by donating electrons to Fe d-orbital. According to the i_corr and IE% data, the inhibitive properties of the studied compounds can be given by the following order: APTT>AMTT>ATTT. Free energies (ΔGoads) are calculated to be −41.18 and −37.05 kJ/mol for AMTT and ATTT, respectively. The adsorption mechanism of AMTT and ATTT is chemisorption (Table 3). The difference in the inhibition actions of triazole compounds can be justified on the basis of the carbon atoms present in the substituent type, which contribute to the chemisorption strength through the donor-acceptor bond between the non-bonding electron pair and the vacant d-orbital of the MS surface. The IE increases with rise in the concentration of CI, and the IE is inversely proportional to temperature. At all the investigated temperature conditions, the adsorption in APPT follows the Langmuir adsorption isotherm. Thermodynamic parameters (ΔG_ads, ΔH_ads, and ΔS_ads) displayed that the process of adsorption is exothermic and spontaneous (Musa et al., 2010a,b).

5.9 Inhibitive capacity of 3,5-bis(2-thienylmethyl)-4-amino-1,2,4-triazole (2-TMAT) toward corrosion of MS in HCl pickling medium

The CI named 2-TMAT has been tested by Tourabi and co-workers for corrosion inhibition of MS in HCl bath at a temperature of 30°C using PPC and EIS measurements. It has been found by Tourabi and co-workers that 2-TMAT is an excellent CI for steel especially for carbon steel and that its IE increases with rise in the concentration of CI (Table 4). PPC displayed that 2-TMAT is a mixed-type inhibitor (Table 3). The ΔG_ads value is highly negative, which also indicates that 2-TMAT follows the Langmuir isotherm. XPS is carried out to find the mechanism of action of 2-TMAT on steel surface (Tourabi et al., 2013).

5.10 Inhibitive behavior of 4-amino-4H-1,2,4-triazole-3,5-dimethanol (ATD) on MS

The interaction and anti-corrosion characteristics of ATD on MS in 1 m HCl have been investigated by John et al. via application of PPC, EIS and WL measurements. PPC exposed the fact that ATD is a mixed-type inhibitor (Table 3). It has been found that when the density of electrons increases around the ATD CI (due to the electron-releasing effect of the electron-donating group), then IE also increases (Table 4). The concept of quantum calculation was used to find out the relation between the structure of ATD molecule and inhibitive properties. Electrochemical (PPC and electrochemical impedance) techniques and theoretical measurements impartially agreed with each other. With rise in the concentration of ATD, the available surface for the attack of acidic species decreases. It is believed that CI ATD adsorbs on the surface of MS and hence blocks the active sites available for the reactions. A higher surface coverage on the surface of MS is attained with larger ATD concentrations (John and Joseph, 2012).

6.1 Anti-corrosion effect of 1,2,3-triazole derivatives in HCl

Substituted 1,2,3-triazole benzyls, viz. BBTMPD, BClBTMPD, BBrBTMPD, BBTMMPD, BClBTMMPD, and BBrBTMMPD, of thymine and uracil have been evaluated by Espinoza-Vázquez and co-workers. The effects of concentration and immersion time on the IE were estimated. It was found that, for all cases, the IE increased with rise in the concentration of CI. IEs >90%, at rather low concentrations of 20 ppm, were found (Table 6). For all cases, these inhibitors followed a physisorption mechanism, behaving as mixed-type inhibitors (Table 5). IEs >88% were measured even after 350 h of exposure. The ΔG_ads values were lower than −16 kJ mol⁻¹; therefore, in this case, the adsorption types of all the inhibitors tested can be classified as physisorption. Six substituted 1,2,3-triazole benzyls (BBTMPD, BClBTMPD, BBrBTMPD, BBTMMPD, BClBTMMPD, and BBrBTMMPD) of thymine and uracil entities were evaluated in the system: pipeline MS in 1 m HCl using EIS measurements. The influence of the CI concentrations on the IE of all these entities was evaluated. The obtained results demonstrated that in all cases, the IE increased with rise in the amount of applied CI, and that, except for one of the di-alkyl analogous of uracil, which is capable of accomplishing solely an IE of 84% at 200 ppm, the remainder demonstrated IEs >90% at rather low concentrations, viz. around 20 ppm. In this sense, the most extraordinary inhibitor is one of the 1,2,3-triazole benzyl analogues of uracil, which showed 96% IE with merely 5 ppm. For all cases, it was found that these inhibitors follow a physisorption mechanism, which can be adequately defined by the Langmuir isotherm. Furthermore, it was exposed that these entities can affect both the anodic and cathodic processes; thus, it can be regarded as a mixed-type inhibitor capable of blocking both anodic and cathodic sites on the API 5L X52 steel surface immersed in HCl bath (Table 5). The influence of the steel immersion time on their IE was also measured in the corroding medium for the two most efficient inhibitors. It was revealed that even when the IE of both inhibitors diminished practically linearly as a function of time, both can deliver adequate protection (IE >88%) to the steel substrate even after 350 h of exposure to this highly corrosive medium (Espinoza-Vázquez et al., 2014).

Table 5:

Investigation of N-substituted-1,2,3-triazole derivatives as CIs.

Sr. no.	CI name	Abbreviation	Molecular structure	Type of inhibitor	References
1	1,3-Bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)pyrimidine-2,4(1H,3H)-dione	BBTMPD		Mixed-type inhibitor	Espinoza-Vázquez et al., 2014
2	1,3-Bis((1-(4-chlorobenzyl)-1H-1,2,3-triazol-4-yl)methyl)pyrimidine-2,4(1H,3H)-dione	BClBTMPD		Mixed-type inhibitor	Espinoza-Vázquez et al., 2014
3	1,3-Bis((1-(4-bromobenzyl)-1H-1,2,3-triazol-4-yl)methyl)pyrimidine-2,4(1H,3H)-dione	BBrBTMPD		Mixed-type inhibitor	Espinoza-Vázquez et al., 2014
4	1,3-Bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)-5-methylpyrimidine-2,4(1H,3H)-dione	BBTMMPD		Mixed-type inhibitor	Espinoza-Vázquez et al., 2014
5	1,3-Bis((1-(4-bromobenzyl)-1H-1,2,3-triazol-4-yl)methyl)-5-methylpyrimidine-2,4(1H,3H)-dione	BClBTMMPD		Mixed-type inhibitor	Espinoza-Vázquez et al., 2014
6	1,3-Bis((1-(4-chlorobenzyl)-1H-1,2,3-triazol-4-yl)methyl)-5 methylpyrimidine-2,4(1H,3H)-dione	BBrBTMMPD		Mixed-type inhibitor	Espinoza-Vázquez et al., 2014
7	I(benzyl)l-H-4,5-dibenzoyl-1,2,3-triazole	BDBT		Mixed-type inhibitor	Abdennabi et al., 1996

6.2 Inhibition action of I(benzyl)l-H-4,5-dibenzoyl-1,2,3-triazole (BDBT) on MS in HCl medium

The inhibition action of BDBT on MS in HCl bath has been investigated by Abdennabi et al. using PPC, MS, and EIS measurements. In the existence of BDBT in 50 ppm concentration, the corrosion rate of MS in HCl was decreased up to 95% (Table 6). BDBT has a mixed mode of corrosion inhibition with a momentous shift in the free corrosion potential to the cathodic direction (Table 5). Film persistency tests displayed that BDBT fabricates a stable layer on the surface of MS electrode (Abdennabi et al., 1996).

7.1 Anti-corrosion properties of condensed 1,2,4-triazoles on MS

Gurudatt et al. have reported that the compounds namely 8-bromo-5-morpholino-3-(4-propylphenyl)-[1,2,4]triazolo[4,3-c]pyrimidine (8-BMPTP), 8-bromo-3-(2-fluoro-3-methoxyphenyl)-5-morpholino-[1,2,4]triazolo[4,3- c]pyrimidine (8-BFMMTP), and 8-bromo-3-(2-fluoro-4,5-dimethoxy-phenyl)-5-morpholin-4-yl-[1,2,4]triazolo [4,3-c]pyrimidine (8-BFDMTP) exhibited excellent inhibitory properties in 0.5 m HCl solution, and the IE is higher with rise in the concentration of CI. The inhibition ability is in the order of 8-BMPTP>8-BFMMTP>8-BFDMTP (Table 8). The adsorption of 8-BMPTP, 8-BFMMTP, and 8-BFDMTP follows the Langmuir adsorption isotherm and ΔG_ads is negative. The negative value throws light on the fact that the process of adsorption of the 8-BMPTP, 8-BFMMTP, and 8-BFDMTP molecules is spontaneous. The PCC study indicated that 8-BMPTP, 8-BFMMTP, and 8-BFDMTP are mixed-type inhibitors (Table 7). The calculated ΔG_ads and ΔH_ads values showed that the process of adsorption mechanism of CI is of physisorption type. SEM investigation showed that CI develops a film layer on the surface of MS, and this film layer is highly stable and has little permeability in HCl bath than the uninhibited surface of MS (Gurudatt et al., 2015).

Table 7:

Investigation of condensed 1,2,4-triazole derivatives as CIs.

Sr. no.	CI name	Abbreviation	Molecular structure	Type of inhibitor	References
1	8-Bromo-5-morpholino-3-(4-propylphenyl)-[1,2,4]triazolo[4,3-c]pyrimidine	8-BMPTP		Mixed-type inhibitor	Gurudatt et al., 2015
2	8-Bromo-3-(2-fluoro-3-methoxyphenyl)-5-morpholino-[1,2,4]triazolo[4,3-c]pyrimidine	8-BFMMTP		Mixed-type inhibitor	Gurudatt et al., 2015
3	8-Bromo-3-(2-fluoro-4,5-dimethoxy-phenyl)-5-morpholin-4-yl-[1,2,4]triazolo[4,3-c]pyrimidine	8-BFDMTP		Mixed-type inhibitor	Gurudatt et al., 2015

8.1 Inhibition action of symmetrical diol-based CIs on MS in HCl medium

The interaction and anti-corrosion characteristics of 4-(benzylideneamino)-4H-1,2,4-triazole-3,5-diyl (BATD) on MS in 1 m HCl bath has been tested by John et al. through PPC, EIS, SEM, WL, and XPS. PPC illustrates that BATD is a mixed-type inhibitor (Table 9). It has been found that when the density of electrons increases around the BATD CI (due to the electron-releasing effect of the electron-donating group), then IE also increases (Table 10). The concept of quantum calculation was used to find out the relation between the structure of BATD molecule and inhibitive properties. Electrochemical (PPC and electrochemical impedance) technique and theoretical measurements impartially agreed with each other. With rise in the concentration of BATD, the available surface for the attack of acidic entities decreases. It is believed that CI BATD adsorbs on the surface of MS and hence blocks the active sites available for the reactions. Higher surface coverage on the surface of MS is attained with larger BATD concentrations (John and Joseph, 2012).

Table 9:

Investigation of N-substituted 1,2,4-triazole derivatives as CIs.

Sr. no.	CI name	Abbreviation	Molecular structure	Type of inhibitor	References
1	4-(Benzylideneamino)-4H-1,2,4-triazole-3,5-diyl	BATD		Mixed-type inhibitor	John and Joseph, 2012
2	(3-Phenylallylidene) amino-5-(pyridine-4-yl)-4H-1,2,4-triazole-3-thiol	SB-1		Mixed-type inhibitor	Ansari et al., 2014
3	3-Mercapto-5(pyridine-4-yl)-4H-1,2,4-triazole-4-yl) imino) methyl)phenol	SB-2		Mixed-type inhibitor	Ansari et al., 2014
4	(4-Nitrobenzylidene) amino)-5-(pyridine-4-yl)-4H-1,2,4-triazole-3-thiol	SB-3		Mixed-type inhibitor	Ansari et al., 2014
5	(3-Bromo-4-fluoro-benzylidene)-[1,2,4]triazol-4-yl-amine	BFBT		Mixed-type inhibitor	Chaitra et al., 2015
6	4-Trifluoromethyl-benzylidene-[1,2,4] triazol-4-yl-amine	TMBT		Mixed-type inhibitor	Chaitra et al., 2015
7	(2-Fluoro-4-nitro-benzylidene)-[1,2,4]triazol-4-yl-amine	FNBT		Mixed-type inhibitor	Chaitra et al., 2015
8	1-[2-(4-Nitro-phenyl)-5-[1,2,4]triazole-1-ylmethyl-[1,3,4]oxadiazol-3-yl]enthanone	NTOE		Mixed-type inhibitor	Zhang et al., 2009
9	1-(4-Methoxy-phenyl)-2-(5-[1,2,4]triazol-1-ylmethyl-4H-[1,2,4]triazol-3-ylsulfanyl)-enthanone	MTTE		Mixed-type inhibitor	Zhang et al., 2009
10	4-Salisylidineamino-3-hydrazino-5-mercapto-1,2,4-triazole	SAHMT		Adsorption; obeys Temkin’s isotherm	Quraishi & Jamal, 2001
11	1-(4-Chlorophenyl)-2,2-dimethylpropan-1-one O-(2-(4H-1,2,4-triazol-4-yl)propan-2-yl) oxime	CATM		Mixed-type inhibitor	Guo et al., 2014
12	1-(4-Fluorophenyl)-2,2-dimethylpropan-1-one O-(2-(4H-1,2,4-triazol-4-yl)propan-2-yl) oxime	FATM		Mixed-type inhibitor	Guo et al., 2014
13	1-(3,4-Dichlorophenyl)-2,2-dimethylpropan-1-one O-(2-(4H-1,2,4-triazol-4-yl)propan-2-yl) oxime	DATM		Mixed-type inhibitor	Guo et al., 2014
14	1-(4-Chlorophenyl)ethanone O-((4H-1,2,4-triazol-4-yl)methyl) oxime	CATM		Mixed-type inhibitor	Li et al., 2007
15	1-(4-Methoxyphenyl)ethanone O-((4H-1,2,4-triazol-4-yl)methyl) oxime	MATM		Mixed-type inhibitor	Li et al., 2007
16	1-(4-Fluorophenyl)ethanone O-((4H-1,2,4-triazol-4-yl)methyl) oxime	FATM		Mixed-type inhibitor	Li et al., 2007
17	1-(4,5-Dihydro-3-phenylpyridine-1-yl)-2-(1H-1,2,4-triazole-1-yl)ethyl ketone	DTE		Mixed-type inhibitor	Xu et al., 2008
18	1-(4,5-Dihydro-3-phenyl pyridine-1-yl)-2-(1H-1,2,4-triazole-1-yl)ethyl ketone	4F-DET		Mixed-type inhibitor	Fengling & Baorong, 2009
19	1-(4,5-Dihydro-3-phenyl pyridine-1-yl)-2-(1H-1,2,4-triazole-1-yl)ethyl ketone	4Cl-DET		Mixed-type inhibitor	Fengling & Baorong, 2009

8.2 Schiff bases of pyridyl-containing triazoles as CIs for MS in HCl bath

(3-Phenylallylidene)amino-5-(pyridine-4-yl)-4H-1,2,4-triazole-3-thiol (SB-1), 3-mercapto-5(pyridine-4-yl)-4H-1,2,4-triazole-4-yl) imino) methyl)phenol (SB-2), and (4-nitrobenzylidene) amino)-5-(pyridine-4-yl)-4H-1,2,4-triazole-3-thiol (SB-3) are three well-known Schiff bases of pyridyl-containing triazoles that have been investigated for anti-corrosion in 1 m HCl bath for MS using electrochemical parameters and WL techniques. It has been found by Ansari et al. that the IE order is SB-1>SB-2>SB-3 (Table 10); hence, among all, SB-1 has the best performance and at 150 mg/l its IE is 96.6%. PPC studies indicated that the adsorption process follows the Langmuir adsorption isotherm and SB-1, SB-2, and SB-3 have a mixed mode of corrosion inhibition (Table 9). The correlation of molecular structure with IE was also studied by application of density function theory (DFT) and theoretical measurements. It has been observed that SB-1, SB-2, and SB-3 develop a shielded layer on surface of MS, which blocks the active side that is available for the reaction. The impedance showed that the amount of polarization resistance enlarged and the double-layer capacitance diminished. The increase in the density of the electrical double layer is the main reason behind this fact (Ansari et al., 2014). DFT was used to estimate the effect on IE due to the structure of the molecule. Surface analysis supported the development of a protective inhibitor layer on the surface of MS (Ansari et al., 2014).

8.2.1 Plausible mechanism of MS in boiling HCl by pyridyl-containing triazoles

It is believed that numerous active sites are available on the surface of MS for the adsorption of CI. Hence, on the basis of adsorption phenomena, the mechanism of inhibition action is proposed, as follows (Figure 12):

1. Protonation occurs in neutral N units of inhibitor entities through attaining protons from acidic media:

   [ S B ] + x H + ↔ [ S B x ] x + .

   There is also the occurrence of favorable phenomena that promote adsorption, i.e. the already adsorbed inhibitor entities promote more adsorption of positively charged inhibitor ions (protonated CIs) on the surface of MS through electrostatic forces of interactions (physical adsorption) (Figure 12).

2. On the surface of MS, the proton in positively charged inhibitor ions gains electrons, which results in the formation and releasing of hydrogen gas (Figure 12). The positively charged inhibitor ions return to the neutral state and the lone pairs present on the heteroatom of CI favor chemical adsorption (Ahamad et al., 2010).

3. In order to make adsorption stronger on the surface of MS, the electrons present in the d-orbital of iron atom transfer to the vacant π* (anti-bonding) orbital of CI, which is referred to as retro-donation (Figure 12), and hence the inhibitor gains extra negative charges and strengthens the adsorption (Ansari et al., 2014).

Figure 12:

Adsorption and inhibition mechanism of SB CI on MS (Ahamad et al., 2010).

8.3 Corrosion control of MS using triazol-4-yl-amine-mediated CIs in HCl medium

Triazole Schiff bases, namely 4-trifluoromethyl-benzylidene-[1,2,4] triazol-4-yl-amine (TMBT), (3-bromo-4-fluoro-benzylidene)-[1,2,4]triazol-4-yl-amine (BFBT), and (2-fluoro-4-nitro-benzylidene)-[1,2,4]triazol-4-yl-amine (FNBT), have been studied by Chaitra and co-workers as CIs in 0.5 m HCl by WL, PPC, and EIS measurements. The TMBT, BFBT, and FNBT Schiff bases CI show excellent IE in 0.5 n HCl, and the order of anti-corrosion efficiency is BFBT>TMBT>FNBT; hence, among all, BFBT is found to be efficient (Table 10). It was also observed that with rise in the concentration of CI, the IE increases and with rise in the temperature, the IE decreases. Different measurement parameters showed that the process of adsorption follows the Langmuir adsorption isotherm and the process of adsorption is physisorption. The AC impedance technique illustrated that with rise in the concentration of TMBT, BFBT, and FNBT CI, increase in charge transfer resistance is observed. PPC proves strongly that TMBT, BFBT, and FNBT are mixed-type inhibitors (Table 9). SEM and Fourier transform infrared were used for the investigation of the morphology of the surface of MS. The studies confirmed that the inhibitor develops a protective layer on the surface of MS and provides shielding to protect MS from attack. In the comparison of BFBT with TMBT and FNBT, BFBT is superior, attributed to large values of EHOMO, minor values of ELUMO, lesser gaps between bands (ΔE), and larger dipole moment. These features make BEBT a CI of maximum IE (Chaitra et al., 2015).

8.4 Effect of di-triazol-type compounds as anti-corrosion agents

Investigation has been done by Zhang et al. on 1-[2-(4-nitro-phenyl)-5-[1,2,4]triazole-1-ylmethyl-[1,3,4]oxadiazol-3-yl]enthanone (NTOE) and 1-(4-methoxy-phenyl)-2-(5-[1,2,4]triazol-1-ylmethyl-4H-[1,2,4]triazol-3-ylsulfanyl)-enthanone (MTTE) as CIs for MS in HCl bath by WL, PPC, XPS, SEM, and EIS. It was found that both NTOE and MTTE are non-toxic CIs. NTOE and MTTE act as mixed-type inhibitors, and no change in mechanism is observed either in the reaction of hydrogen evolution or MS dissolution (Table 9). The quantum calculations, ΔG_ads and ΔH_ads values, displayed that the process of adsorption of NTOE and MTTE has two kinds of forces of interaction that are involved, viz. physisorption and chemisorption, and the process of adsorption of 1,2,4-triazole is spontaneous. The order of the IE of NTOE and MTTE is NTOE>MTTE; hence, NTOE is a better CI than NTOE (Table 10) (Zhang et al., 2009).

8.5 Corrosion inhibition of MS in 15% HCl by 4-salisylidineamino-3-hydrazino-5-mercapto-1,2,4-triazole (SAHMT)

SAHMT has been investigated by Quraishi and Jamal as CI of MS in 15% HCl bath under boiling condition of HCl, i.e. high-temperature investigation using WL and electrochemical parameters. PPC displayed that SAHMT acts as mixed-type inhibitor and performs its anti-corrosion activity by blocking the active side of MS available for the reactions. In 15% HCl, SAHMT follows Temkin’s adsorption isotherm on the surface of MS. The best feature of SAHMT is that at high temperature, this CI does not produce any toxic vapors; hence, it is a green CI that does not produce harmful effects (Quraishi & Jamal, 2001).

8.6 Oxime-containing dichloro-, chloro-, and fluoro-derivatized CIs for MS in acidic bath

The corrosion inhibitive performance of 1-(4-chlorophenyl)-2,2-dimethylpropan-1-one O-(2-(4H-1,2,4-triazol-4-yl)propan- 2-yl) oxime (CATM), 1-(4-fluorophenyl)-2,2-dimethylpropan-1-one O-(2-(4H-1,2,4-triazol-4-yl)propan-2-yl) oxime (FATM), and 1-(3,4-dichlorophenyl)-2,2-dimethylpropan- 1-one O-(2-(4H-1,2,4-triazol-4-yl)propan-2-yl) oxime (DATM) in acidic bath for the MS surface has been investigated by Guo et al. through the application of DFT. Calculation by application of quantum parameters revealed that the CATM, FATM, and DATM have well-proportioned active sites by which the inhibitor molecules can directly adsorb onto the MS surface via sharing of electrons with iron atoms. The results revealed that the electron-donating abilities of CATM, FATM, and DATM are with small differences, and the order from high to low is FATM>CATM>DATM (Table 10). The best IE attained by DATM cannot be interpreted in terms of the number of transferred electrons values but by the location of the LUMO density, the geometry of these molecules, as well as other environmental factors. The results of QSAR may also be utilized to suggest possible routes to modify the studied triazole inhibitors and to obtain more effective CIs (Table 9). The Monte Carlo simulation suggested that triazole derivative species adsorb on the surface of iron in a nearly flat way in gas phase, but show some distortions or dihedral angles with higher adsorption energies in aqueous phase. The solvent effect produces small changes in the molecular and electronic structure of the inhibitor molecules. Nevertheless, it can enhance the stability of the anti-corrosion system (Guo et al., 2014).

8.6.1 Mechanism of adsorption and inhibition

Due to the activation effect of chloride ions, the pitting corrosion of the blank specimen under HCl proceeded according to the following steps (Burstein et al., 2004; Solmaz et al., 2008; Chen et al., 2012; Moretti et al., 2013):

Fe+Cl − ↔ ( FeCl ) ads + e − ( FeCl ) ads ↔ ( FeCl + ) + e − F e C l + ↔ Fe 2 + + Cl − .

For specimens in the inhibited HCl solutions, the [Fe(0)Inh] complex is formed at the flat area via the following reaction:

Inh+Fe(0) → [ Fe ( 0 ) Inh ] .

In the low-lying area, Cl⁻ anions are first adsorbed onto the surface of MS, which contains positive charges. Because substituted triazoles are organic bases, then FAMT, CAMT, and DATM entities can survive as protonated species (InhH⁺) in HCl medium, which is in equilibrium with its molecular species (Inh). Figure 13 presents the adsorption model of the inhibitor on the iron surface. The inhibitor quickly reacts with Fe(0) and forms a strong protective layer in the non-corroded area. The layer is very thin and is presumably a single monolayer. However, InhH⁺ reacts with Fe(II) and forms a thick and protective [FeCl⁻InhH⁺] complex. Ultimately, caustic ions are obstructed by the protective film, and the steel is effectively protected from corrosion (Guo et al., 2014).

Figure 13:

Adsorption model of CI on the MS surface (Guo et al., 2014).

8.7 Oxime-containing methoxy-, chloro-, and fluoro-derivatized CIs for MS in acidic bath

Three triazole derivatives, namely 1-(4-chlorophenyl)ethanone O-((4H-1,2,4-triazol-4-yl)methyl) oxime (CATM), 1-(4-methoxyphenyl)ethanone O-((4H-1,2,4-triazol-4-yl)methyl) oxime (MATM), and 1-(4-fluorophenyl)ethanone O-((4H-1,2,4-triazol-4-yl)methyl) oxime (FATM), have been investigated by Li et al. as CIs for MS in 1 m HCl bath by means of WL, ESI, and PPC. The investigation illustrated that the CATM, MATM, and FATM are excellent CIs and their anti-corrosion power increased with rise in the concentration of CIs. Among CATM, MATM, and FATM, the order of IE is CATM>MATM≈FATM; hence, among these three CIs, CATM is the best (Table 10). The quantum calculations and ΔG_ads throw light on the fact that in the process of inhibition, two types of forces of interaction are involved, i.e. chemisorption and physisorption, and a mixed mode of corrosion inhibition (Table 9) (Li et al., 2007).

8.8 Inhibitive effect of 1-(4,5-dihydro-3-phenylpyridine-1-yl)-2-(1H-1,2,4-triazole-1-yl)ethyl ketone (DTE) toward corrosion of MS

In 1 m HCl bath, the inhibiting action of DTE has been examined by Xu and co-workers via WL, PPC, EIS, and SEM. The investigation illustrated that DTE is an excellent CI in 1 m HCl and the maximum IE of DTE is 90.9% at 1.0×10⁻³m (Table 10). PPC showed that DTE is a mixed-type inhibitor (Table 9). EIS displayed that in the inhibited and uninhibited solution condition, the charge transfer is the key that controls the corrosion course. The inhibiting action of DTE follows the Langmuir adsorption isotherm, and the ΔG_ads value indicates that the process of adsorption is spontaneous and by chemisorption (Xu et al., 2008).

8.9 Ketone-type CIs for MS in HCl medium

Investigation of 1-(4,5-dihydro-3-phenyl pyridine-1-yl)-2-(1H-1,2,4-triazole-1-yl)ethyl ketone (4F-DET) and 1-(4,5-dihydro-3-phenyl pyridine-1-yl)-2-(1H-1,2,4-triazole-1-yl)ethyl ketone (4Cl-DET) as CI for MS in HCl bath has been done by Fengling and Baorong via WL, PPC, and EIS, and this investigation suggested that 4F-DET and 4Cl-DET are excellent inhibitors (Table 10). PPC revealed that 4F-DET and 4Cl-DET are mixed-type inhibitors (Table 9), and the process of adsorption follows the Langmuir adsorption isotherm and the ΔGoads value demonstrates that the adsorption process is spontaneous and chemisorption (Fengling & Baorong, 2009).

9.1 Effect of temperature on corrosion of MS in acidic media

Temperature has a great influence on the phenomenon of MS corrosion. It can modify the interaction between the acidic medium and MS electrode in the presence of triazole-based inhibitors. The results of the literature review showed that, generally, the rate of corrosion increases with the rise in the temperature. Hence, the triazole-based inhibitors are more effective at lower temperatures than at the higher temperatures. The effect of temperature on the inhibited metal-acid reaction is very complex, because various changes take place on the MS surface, for example desorption of inhibitor, rapid etching, and the CI itself may undergo decomposition. Triazole derivatives are usually stable toward decomposition. Indeed, this kind of behavior can be justified on the basis that at a higher temperature, hot movements of the organic molecules become accelerated and, hence, leads to a shift of the equilibrium position of the desorption-adsorption phenomenon toward desorption of the CI molecules at the MS surface. Owing to a higher rate of desorption, the larger surface area of MS becomes influenced by the corrosive environment, resulting in increased rates of corrosion with an increase in temperature.

9.2 Effect of inhibitor concentration on corrosion of MS in acidic media

In general, the percentage inhibition increases with the increase of the concentration of the triazole-based inhibitor. This may be on account of the fact that the CI ionizes more immediately under higher strength of the acid and is absorbed more easily on the surface of MS. The results obtained from electrochemical studies and WL method were in good agreement in all the studies. The results also suggested that most triazole-mediated CIs behave as mixed-type inhibitors in the reported conditions. The adsorption of the majority of the triazole-based CIs on the surface of MS follows the Langmuir isotherm model.

9.3 Effect of immersion time on corrosion of MS in acidic media

In order to assess the stability of the triazole-based inhibitor behavior on a time scale, the effect of immersion time on the corrosion behavior of MS in an acidic medium with addition of a triazole-mediated inhibitor has also been studied in various reports. In the majority of cases, the IE of the triazole derivatives is increased with increasing immersion time up to 10–12 h; thereafter, IE decreases slightly. The increase in IE up to 10–12 h implies a slow rearrangement and slow surface fixation process of the triazole derivative on the surface of MS, resulting in the development of a more protective layer at the acid-metal interface. Beyond 10–12 h, the inhibiting effect decreases, and this may be possibly attributed to some defects existing on the film, leading to the approach of aggressive ions to the acid-metal interface.

9.4 Effect of type of acid

Generally, the investigated IE of the triazole derivatives is more effective in HCl than in H₂SO₄, which is justified on the basis of the adsorption capability of both SO₄⁻² and Cl⁻ on the surface of MS. Cl⁻ anions are probably more pronounced compared to SO₄⁻² anions and, hence, Cl⁻ anions are specifically adsorbed on the surface of MS leading to charge reversal. The resultant negatively charged MS surface attracts the positive (cationic) species of the inhibitor, forming a barrier to the aggressive acid solution. On the contrary, the surface of MS in H₂SO₄, where little specific adsorption of the SO₄⁻² anion takes place, is positively charged. Thus, the positively charged cationic species of the inhibitor are hardly adsorbed on the MS surface.

9.5 Summary of reported properties of triazole-based inhibitors

The variations in the inhibitive performance of triazole derivatives are mainly influenced by the nature and type of the substituents present in the CI molecule. Table 11 displays a quick review of the properties and features of triazole-based CIs.