Introduction

Corrosion creates different serious problems in various industries due to its precarious and damaging effect on metals. It is one of the main causes of structural deterioration in offshore and marine structures (Anwar et al. 2019, 2021). Cleaning of pipework steel in order to remove undesirable rust and scale is usually done with acid solutions like hydrochloric acid which subjects the steel to corrosion (Abd-Elaal et al. 2013). In acid solutions, apart from metal dissolution (anodic process) corrosion is usually accompanied by cathodic processes which predominantly involve hydrogen gas evolution. The study of steel corrosion in acid media has been of interest to researchers due to increased applications of acid solution. Pipework steel protection with corrosion inhibitors has greatly been investigated and found to be one of the most desirable corrosion prevention methods (Njoku et al. 2021; Iroha and Nnanna 2021; Kousar et al. 2021). These inhibitors are chemicals usually added in small quantity to the corrosive environments to slowdown the corrosion rate (Iroha and James 2019). Several organic compounds (Arrousse et al. 2021; Kacimi et al. 2017; Poojary et al. 2021; Iroha et al. 2005) have been applied as inhibitors for steel corrosion in acidic media and found to be effective in reducing the corrosion rate. These compounds exert inhibitive action by adsorption of their molecules onto the steel surface, thereby creating a protective barrier to attack of the corrodant. Based on extensive study, the organic compounds are able to adsorb effectively on the metal surface most likely because of the presence of multiple bonds, aromatic rings, heterocyclic and, N, O, S and P atoms present in their molecules. These atoms contain electron lone pairs which are available to the iron empty d-orbital to form a dative bond.

Schiff bases with general formula \(\text{R}\text{C}=\text{N}{\text{R}}^{{\prime }}\) have in their structure, features that make them potent as corrosion inhibitors. Schiff bases are compounds produced by the condensation of amines and a carbonyl compound. The ease of synthesis of these compounds from affordable materials is one the great advantages of Schiff bases in their use as corrosion inhibitors. Recent reports have shown the effectiveness of Schiff bases as inhibitors of steel corrosion in acid media (Yurt et al. 2014; Okey et al. 2020). Literature survey has also revealed the use of Schiff bases derived from pyridine as corrosion inhibitors (Ansari et al. 2015; Iroha et al. 2021). This synthesized pyridine derivative is a non-toxic organic compound which is also useful and important intermediate in various medicinal compounds preparations (Cocco et al. 2003; Hughes et al. 2008). This type of Schiff base contains heterocyclic compounds with nitrogen atoms which can be protonated easily in acidic environment to display effective inhibition of metals corrosion in acid solution.

In an attempt to contribute to this research area which is growing and solving industry problems, we have in our laboratory, synthesized and used some Schiff bases (Dueke Eze et al. 2022; Iroha and Dueke-Eze 2021) which have performed well as corrosion inhibitors. This present work is aimed at studying the inhibitory performance of Schiff base N5MHP on the corrosion of N80 steel in 1 M HCl environment using electrochemical and weight loss techniques. This compound was chosen on the basis of molecular structure, while considering the active centers and the kind of substituents. To correlate the inhibition performance of N5MHP with its structure, quantum chemical calculations were performed.

Experimental

Preparation of metal specimen and corrosion medium

The metal specimens (N80 steel) used for the study contained; 0.92% Mn, 0.01% P, 0.31% C, 0.008% S, 0.19% Si, 0.2% Cr and the remainder Fe. The steel specimens were first cut into 2 × 4 cm2 dimensions for both weight loss and electrochemical measurements and then abraded with emery papers (grades 400–1200). The specimens were thereafter rinsed with distilled water, dried in an oven after degreasing with acetone and then utilize for the experiment. Exposed area of 1 cm2 for the samples was utilized in electrochemical studies. The blank corrosion medium consists of 1 M HCl solution prepared from analytical grade 37% HCl (Merck) by diluting with distilled water.

Synthesis of N5MHP

5-methoxy-2-hydroxybenzaldehyde solution (24.5 mg, 0.20 mmol) in 10 mL of ethanol and formic acid (two drops) were mixed with 2-aminopyridine solution (18.8 mg, 0.20 mmol) in 10 mL of ethanol and stirred. For 6 h the mixture was refluxed and the precipitate formed was filtered and then recrystallized to yield N-(5-methoxy-2-hydroxybenzylidene)pyridine-2-amine (N5MHP). Detailed characterization by NMR and IR has been earlier reported (Dueke-Eze et al. 2013). The molecular structure of N5MHP is shown in Fig. 1.

Fig. 1
figure 1

Molecular structure of N5MHP

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Electrochemical measurements

Electrochemical measurements were perform by utilizing the conventional three-electrode electrolytic cell with N80 steel as working electrode, platinum foil as counter electrode and saturated calomel electrode (SCE) [Hg/Hg2Cl2/KClsat] as reference electrode. All measurements were performed using Gamry framework at 30 °C. Samples were mounted in epoxy resin with an exposed area of 1 cm2 to the test solution. The experiment proceeded by first letting the working electrode attain a steady state via immersion in the test solution for 30 min at open circuit potential (OCP). The electrochemical impedance spectroscopy (EIS) measurement performed on the N80 steel electrode at OCP proceeded by applying a signal amplitude of 10 mV within the range of frequency 100 kHz to 0.01 Hz. Fitting and analyses of the EIS data were done with Echem Analyst software version 5.50. The polarization measurement were performed from potential (cathodic) of -0.25 V vs. OCP to potential (anodic) of + 0.25 V vs. OCP at a scan rate 1 mVs− 1. Results reproducibility was tested by repeating the measurements three times.

Weight loss measurement

The weight loss measurements were conducted using ASTM G31-72 standard (ASTM 2004). In this measurement, the N80 steel specimens were weighed and suspended in solutions containing 100 mL bare 1 M HCl and different concentrations of N5MHP for 6 h immersion time at 30 °C, 40 and 50 °C temperatures unstirred. The N80 steel specimens were retrieved washed, dried and accurately re-weighed. The average of three weight loss measurements was recorded. The corrosion rate (CR), surface coverage (\(\theta\)) and inhibition efficiency (IEWL %) for N5MHP were respectively computed using Eqs. 1, 2 and 3:

$${C}_{R}=\frac{\varDelta W}{S.t}$$
(1)

where ∆W represents mean weight loss (mg), t and S stand for immersion time (s) and surface area (cm2) respectively.

$$\theta = \frac{{C}_{R}-{C}_{R\left(i\right)}}{{C}_{R}}$$
(2)
$$I{E}_{WL} \%= \frac{{C}_{R}-{C}_{R\left(i\right)}}{{C}_{R}} \times 100$$
(3)

where CR and CR(i) are corrosion rate values of X60 steel without and with the inhibitors respectively.

SEM analysis

SEM micrographs of the N80 steel specimen immersed in blank 1 M HCl and inhibited with 1 × 10− 2 M N5MHP were examined utilizing Ziess Evo 50 XVP model of scanning electron microscope. The SEM operating at 20 kV recorded the topography of the surface with resolution of electron microscope of 1.5 nm. The SEM images were taken at 2000× magnification. After 6 h immersion at 30 °C, the specimens were retrieved from the test solutions, washed properly with distilled water and dried in air before SEM analysis.

Quantum Chemical Calculations

The quantum chemical calculations were performed using Spartan 14.0 software. The calculations and geometrical optimization of N5MHP inhibitor were carried out utilizing the B3LYP model of density functional theory (DFT) in combination with the 6-311G (d, p) basis sets. The HOMO energy (EHOMO), LUMO energy (ELUMO) and the energy gap (\(\varDelta \text{E}= {{\text{E}}_{\text{L}\text{U}\text{M}\text{O}}-\text{E}}_{\text{H}\text{O}\text{M}\text{O}}\)) were calculated to determine the inhibition behavior of N5MHP. The calculated EHOMO and ELUMO were used to calculate some important parameters like electron affinity (\(\text{E}\text{A}= {-\text{E}}_{\text{L}\text{U}\text{M}\text{O}}\)), ionization potential (\(\text{I}\text{P}= {-\text{E}}_{\text{H}\text{O}\text{M}\text{O}}\)) and other such as electronegativity (χ), global hardness (η) and global softness (σ) as shown in Eqs. (4)–(6) (Zhang et al. 2015; Iroha et al. 2022a):

$${\upchi }= \frac{IE+EA}{2}$$
(4)
$${\upeta }= \frac{IP-EA}{2}$$
(5)
$${\upsigma }= \frac{1}{{\upeta }}$$
(6)

The change in the number of transported electrons (ΔN) from the inhibitors to the Fe surface was calculated by Eq. (7) (Xia et al. 2015; Maduelosi and Iroha 2020):

$$\varDelta N= \frac{{{\upchi }}_{Fe}- {{\upchi }}_{inh}}{2({\eta}_{Fe}+ {\eta}_{inh})}$$
(7)

where χFe and χinh are the electronegativity of Fe and inhibitor molecules, while ηFe and ηinh are the global hardness of Fe and inhibitor molecules. From Pearson’s electronegativity scale, the theoretical values of ηFe and χFe are 0 eV and 7 eV (Zhang et al. 2019).

Results and discussion

Potentiodynamic polarization measurements (PDP)

A potentiodynamic polarization test was carried out to gain insight into the kinetics of the anodic and cathodic reactions. The anodic reaction involves N80 steel dissolution through oxidation process as earlier described (Hamani et al. 2014; Iroha et al. 2022b):

$$Fe+C{l}^{-}\rightleftharpoons (FeC{l}^{-}{)}_{ads}$$
(8)
$$(FeC{l}^{-}{)}_{ads}\rightleftharpoons (FeCl{)}_{ads}+{e}^{-}$$
(9)
$$(FeCl{)}_{ads}\to (FeCl{)}^{+}+{e}^{-}$$
(10)
$$(FeCl{)}^{+}\rightleftharpoons F{e}^{2+}+C{l}^{-}$$
(11)

While on the contrary, the cathodic reaction has to do with the hydrogen evolution through reduction process as follows described (Hamani et al. 2014; Iroha et al. 2022b):

$$Fe+{H}^{+}\rightleftharpoons (Fe{H}^{+}{)}_{ads}$$
(12)
$$(Fe{H}^{+}{)}_{ads}+{e}^{-}\to (FeH{)}_{ads}$$
(13)
$$(FeH{)}_{ads}+{H}^{+}+{e}^{-}\to Fe+{H}_{2}$$
(14)

The polarization curves without and with various concentrations of N5MHP in 1 M HCl media are depicted in Fig. 2. Figure 2 shows that both the anodic and branch cathodic reactions were influenced by the addition of N5MHP, which reduced both the cathodic hydrogen evolution and the anodic N80 steel dissolution (Mobin and Rizvi 2017; James and Iroha 2021). This indicates that N5MHP behaved as a mixed-type inhibitor. The electrochemical polarization parameters like corrosion current density (icorr), corrosion potential (Ecorr), inhibition efficiency (IEPDP, %), anodic branch slope (βa) and cathodic branch slope (βc) were deduced by means of Tafel extrapolation and listed in Table 1. The IEPDP was calculated as follows:

$${IE}_{PDP}= \frac{{I}_{corr}^{0}-{I}_{corr}^{i}}{{I}_{corr}^{0}}\times 100$$
(15)

where \({I}_{corr}^{0}\) and \({I}_{corr}^{i}\) are the corrosion current densities without and with N5MHP inhibitor respectively. The results as shown in Table 1 indicate that the presence of various concentrations of N5MHP cause a significant decrease in the icorr values. The observed decrease of the icorr values was due to the inhibitor adsorption on the N80 steel/HCl interface (Obot et al. 2015; Iroha and Akaranta 2020). Maximum inhibition efficiency was obtained at the concentration of 1.00 mM of N5MHP. Generally, a shift in Ecorr above 85 mV, categorizes the inhibitor as cathodic or anodic and a shift lower than 85 mV (as seen in this study) suggest that the inhibitor is mixed-type (Olasunkanmi et al. 2017; Abeng et al. 2023; Iroha and Ukpe 2020). The Tafel slopes (βa and βc) decrease with an increase in N5MHP concentration, which further suggests the suppression of both cathodic and anodic partial reactions.

Fig. 2
figure 2

Potentiodynamic polarization curves of N80 steel in 1 M HCl solutions without and with various concentrations of N5MHP of N5B2HP at 30 °C

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Table 1 Potentiodynamic polarization data of N80 steel in 1 M HCl without and with various concentrations of N5MHP at 30 °C
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Electrochemical impedance measurement

The impedance study was carried out to elucidate the kinetics and characteristics of the metal/solution interface and how N5MHP obstructed the reaction. The Nyquist plots of N80 steel in 1 M HCl medium without and with various concentrations of N5MHP is depicted in Fig. 3. The figure clearly shows a depressed semicircular shape for all the impedance spectra. Slight deviations of the shape from a perfectly circle suggest interfacial impedance frequency dispersion (Da-Rocha et al. 2010; Pavithra et al. 2010). The equivalent circuit used in modeling the N80 steel/HCl solution interface is depicted as an insert in Fig. 3, where Rs denotes solution resistance, Rct stands for charge transfer resistance and CPE is the constant phase element used to replace double layer capacitance (Cdl). The CPE impedance is expressed as:

$${Z}_{CPE}= \frac{1}{{Y}_{o}}(j\omega {)}^{-n}$$
(16)

where Y0 denotes CPE constant, \(\omega\) is angular frequency, j2 = -1 is imaginary number and n is the CPE exponent which presents information on the degree of surface inhomogeneity resulting from inhibitor adsorption, surface roughness and formation of porous layer (Abdel-Gaber et al. 2009). The Cdl values are computed from CPE utilizing Eq. 17:

$${C}_{dl}= {Y}_{0}({\omega }_{max}{)}^{n-1}$$
(17)

Where, \({\omega }_{max}\) denotes the frequency at which the impedance imaginary part has attain maximum value. The impedance parameters such as Rct, Cdl and inhibition efficiency (IEEIS, %), are listed in Table 2. The IEEIS was computed as follows:

$${IE}_{EIS}= \frac{{R}_{ct}-{R}_{ct}^{0}}{{R}_{ct}}\times 100$$
(15)

where \({R}_{ct}^{0}\) and \({R}_{ct}\) are the charge transfer resistances without and with N5MHP inhibitor respectively. It is obvious that Rct values increase with increasing N5MHP concentrations. This indicates a decrease in corrosion rate in the inhibitor presence and an increase in IEEIS. On the other hand, the Cdl values were found to decrease on adding N5MHP inhibitor, which indicates a decrease in local dielectric constant and/or an increase in the electrical double layer thickness, suggesting that N5MHP function by protective layer formation on the N80 steel surface (Ostovari et al. 2009; Iroha et al. 2023a, b).

Fig. 3
figure 3

Nyquist plot of N80 steel in 1 M HCl without and with various concentrations of N5MHP (insert: equivalent circuit)

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Table 2 Electrochemical impedance parameters for N80 steel in 1 M HCl without and with various concentrations of N5MHP at 30 °C
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Weight loss method

Effect of concentration

The variation of weight loss parameters with the concentrations of N5MHP is depicted in Fig. 4. It is observed that N5MHP protected N80 steel corrosion in the HCl solution at all studied concentrations i.e. 0.50 to 1.00 mM, with CR decreasing with an increase in the inhibitors concentration while the reverse was the case for IEWL %. However, the Schiff base pyridine-2-amine derivative showed maximum protection at 303 K with 1.00 mM concentration. The results in Fig. 1b reveal that N5MHP performed well in protecting the steel surface, achieving an inhibition efficiency of 97.0% at 303 K with 1.0 mM concentration. The corrosion rate on the other hand, decreased from 8.465 mg/cm2/h to 0.452 mg/cm2/h in the presence of 1.00 mM N5MHP at 303 K. This observed result is related to increased adsorption of inhibitor species leading to hydrophobic thin film formation on the acid-substrate interface (Iroha et al. 2015).

Fig. 4
figure 4

Variation of a CR and b IEWL with N5MHP concentration for N80 steel at different temperatures

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Impact of temperature

The impact of temperature on N80 steel corrosion behaviour in inhibited and uninhibited test solutions was studied. The observed variation of CR and IEWL% with temperature ranging from 303 to 323 K are displayed in Table 3. The table reveals that CR of X60 steel in both inhibited and uninhibited solutions increased with increase in temperature while IEWL % for N5MHP at constant concentration, decreased with temperature rise. This observation could be due to desorption of adsorbed inhibitor species caused by elevated temperature (Fragoza-Mar et al. 2012; Mourya et al. 2016).

Activation and thermodynamic parameters were considered to help in the understanding of the adsorption and inhibition mechanism. The CR is observed to depend on temperature and this dependency on temperature is expressed utilizing the Arrhenius equation (Verma et al. 2016; Chokor et al. 2022):

$$log \ {C}_{R}=\frac{{-E}_{a}}{2.303RT}+log \ A$$
(16)

Where Ea is denotes activation energ, T is the absolute temperature, R stands for gas constant, and A denotes pre-exponential factor. Plot of log CR against 1/T (Arrhenius plot) for N80 steel dissolution without and with different concentrations of N5MHP is displayed in Fig. 5a. The Ea values deduced from the Arrhenius plot slope are listed in Table 4. The data in Table 4 clearly shows that Ea values in presence of N5MHP were higher when compared with the values without N5MHP. This observation indicates that N5MHP retards the dissolution N80 steel by causing an increase in the energy barrier involved in corrosion (Krishnegowda et al. 2013). An alternative formula for the Arrhenius equation is the Eyring’s transition state equation given as:

$$\text{log}\frac{{C}_{R}}{T}=\text{log}\left(\frac{R}{Nh}\right)+\left(\frac{\varDelta {S}^{*}}{2.303R}\right)-\left(\frac{\varDelta {H}^{*}}{2.303RT}\right)$$
(17)

where 𝑁 is Avogadro’s number, ℎ denotes Planck’s constant, Δ𝑆∗ stands for activation entropy, Δ𝐻∗ denotes activation enthalpy. From the slope (-ΔH*/2.303R) of the plot of log CR/T against 1/T (Transition state plot) depicted in Fig. 5b, ΔH* values were computed. The ΔS* values were deduced from the intercept, [log (R/Nh) + (ΔS*/2.303R)] of the same plot. These values are listed in Table 4. The positive Δ𝐻∗ values obtained shows the endothermic nature of the N80 steel dissolution in the presence of N5MHP. The values ΔS* in the presence of N5MHP and its absence are both negative with less negative values in the presence of N5MHP. This suggests that the activation complex in the rate determining step denotes association instead of dissolution step, which implies decreased disorderliness on going from reactants to activated complex. The negative ΔS* values also shows the non-spontaneous nature of the N80 steel dissolution by adding N5MHP (Ikeuba et al. 2015).

Table 3 Variation of CR and IEWL with temperature without and with 1.00 mM N5MHP in 1 M HCl
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Fig. 5
figure 5

a Arrhenius plots and b Transition state plots for N80 steel in the absence and presence of different concentrations of N5MHP

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Table 4 Activation parameters for the dissolution of N80 steel in the absence and presence of various concentrations of N5MHP.
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Adsorption isotherm

Adsorption isotherm was studied to better understand the interaction between the N80 steel surface and N5MHP in 1 M HCl environment. Among the various adsorption isotherms tested, Langmuir isotherm gave the best fit. The Langmuir isotherm is given as:

$$\frac{{C}_{inh}}{\theta }=\frac{1}{{K}_{ads}} + {C}_{inh}$$
(17)

where \({C}_{inh}\) denotes N5MHP concentration and \({K}_{ads}\) stands for adsorption equilibrium constant. A plot of Cinh/θ against Cinh as depicted in Fig. 6, gives a straight line with the values of regression coefficient (R2) and slope very close to unity suggesting that N5MHP adsorption on the N80 steel surface in test solution obeys the Langmuir adsorption isotherms (Iroha et al. 2012). The Kads is related to the adsorption free energy (\({\varDelta G}_{ads}^{0}\)) as follows:

$$\varDelta {G}_{ads}^{o}= -RT\text{ln}\left(55.5 {K}_{ads}\right)$$
(18)

where R denotes the gas constant and T stands for absolute temperature. The value, 55.5 is the molar concentration of water in solution. The computed values of Kads and \({\varDelta G}_{ads}^{0}\) are presented in Table 5. The values of the Langmuir isotherm slopes show deviations from unity expected for ideal an ideal Langmuir isotherm equation. The deviation from 1 could be as a result of interactions between the adsorbed molecules on the X60 steel surface. The observed negative \({\varDelta G}_{ads}^{0}\) values show that the adsorption process is spontaneous. General, \({\varDelta G}_{ads}^{0}\) values of within − 20 kJmol− 1 are associated with physisorption while values more negative than − 40 kJmol− 1 are compatible with chemisorption (Yadav et al. 2015a). The computed \({\varDelta G}_{ads}^{0}\) values for N5MHP are between − 31.46 to -33.41 kJmol− 1 which are within the threshold values for physisorption and chemisorption, suggesting that the adsorption process of N5MHP on N80 steel surface involves both chemical and physical adsorption (Yadav et al. 2015b).

Fig. 6
figure 6

Langmuir isotherm for the adsorption of N5MHP on N80 steel surface using weight loss method

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Table 5 Thermodynamic parameters for the adsorption of N5MHP on N80 steel in 1 M HCl at various temperature
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SEM analysis

The surface morphology of uninhibited and inhibited N80 steel samples after immersion period of 6 h are depicted in Fig. 7. The SEM image of the specimen immersed in uninhibited 1 M HCl environment (Fig. 7a) is porous and rough due to aggressive and fast acid corrosion reaction. Moreover, the SEM images of the N80 steel specimen in presence of N5MHP (Fig. 7b) is less damaged, indicating a retardation of the corrosion attack and protective film formation of N5MHP on the steel surface.

Fig. 7
figure 7

SEM images for N80 steel a exposed to 1 M HCl b exposed to 1 M HCl containing 1.00 mM N5MHP.

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Quantum chemical calculation

The frontier molecular orbitals (FMOs) are very important tool in determining the chemical reactivity of the inhibitor adsorbed onto metal surfaces (Daoud et al. 2015). Figure 8 shows graphical representations of the HOMO and LUMO density distribution of N5MHP. The depiction in Fig. 8 shows that the HOMO and LUMO orbitals’ sites of distribution are mainly the azomethine group, π-electrons, including the heteroatoms of oxygen and nitrogen. The quantum chemical properties of the neutral and protonated forms of N5MHP are listed in Table 6. The EHOMO values are generally related to the ability of the inhibitor molecules to donate electron to the unoccupied metal surface’s d-orbital. Higher values of EHOMO imply that the electron donating ability including the inhibition efficiencies of the inhibitor would be high. However, ELUMO values are related to the ability of the inhibitor molecule to accept electron from the metal. Therefore, the electron accepting properties including the inhibition efficiency increases with reduced ELUMO values (Verma et al. 2016). As observed in Table 6, the high EHOMO of neutral N5MHP is stabilized in N5MHP-H+ (protonated form) which suggests lower electron-donating ability of N5MHP-H+. The energy band gap (∆E) relates the stability and chemical reactivity of the inhibitor molecules. Lower values of ∆E facilitate and increase the adsorption of the inhibitor molecule on the metal surface through donor-acceptor process (Boughoues et al. 2020). In the present study, ΔE of N5MHP-H+ is lower than that of N5MHP, which promotes its adsorption to the N80 steel surface and enhances its efficiency. The hardness (η) and softness (σ) are other properties that are related to molecular reactivity and stability. A hard molecule has higher ∆E between the ELUMO and EHOMO and is related to lower reactivity and inhibition efficiency. On the other hand, a soft molecule has a lower ∆E between the ELUMO and EHOMO and is related to larger reactivity and inhibition efficiency (Mohamed et al. 2021). The fraction of transferred electrons (∆N) from our study are less than 3.6 for N5MHP and N5MHP-H+ molecules, suggesting transfer of electrons from the molecules to N80 steel leading to formation of dative bonds. This promotes protective layer formation against corrosion.

Fig. 8
figure 8

Frontier molecular orbitals HOMO and LUMO of neutral and protonated N5MHP

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Table 6 Calculated quantum chemical parameters for N5MHP and N5MHP-H+ molecules
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Conclusion

The present study has shown that the synthesized N-(5-methoxy-2-hydroxybenzylidene)pyridine-2-amine (N5MHP) can function as an effective corrosion inhibitor for N80 pipeline steel in 1 M HCl solution and the inhibition efficiency increases with increasing concentration of the inhibitor. The corrosion process was inhibited by adsorption of the Schiff base molecules on N80 steel surface leading to the formation of a protective film on the metal/acid solution interface, decreasing the dissolution of the steel. Results from Potentiodynamic polarization showed that the studied inhibitor exhibit mixed-type inhibition activity. The results of the EIS reveal that there is a decrease in the charge transfer resistance in the presence of N5MHP. The impact of temperature on the inhibition performance of N5MHP was investigated using weight loss measurements and the results indicated that the inhibition efficiency of N5MHP decreases with increase in temperature. Adsorption of the studied inhibitor obeys Langmuir adsorption isotherm. The results obtained from EIS, weight loss and polarization techniques are in good agreement. Surface analysis by SEM confirms the adsorption performance of the inhibitor and the development of a protective film on the steel surface. Additionally, the results obtained by the DFT-based quantum chemical calculations supported the experimental findings.