Corrosion inhibition of mild steel in hydrochloric acid solution by the expired Ampicillin drug

Abstract

This study examines the utilization of the expired drug, namely ampicillin, as a mild steel corrosion inhibitor in an acidic environment. The inhibitor was evaluated using weight loss and electrochemical measurement accompanied with surface analytical techniques. The drug showed a potential inhibitory efficiency of > 95% at 55 °C. The inclusion of the inhibitor increased the charge transfer resistance at the steel-solution interface, according to impedance analyses. According to potentiodynamic polarisation measurements, expired ampicillin drug significantly decreased the corrosion current density and worked as a mixed-type corrosion inhibitor. The Langmuir adsorption isotherm was followed by the adsorption of ampicillin drug on the steel substrate, exhibiting an association of physical and chemical adsorption mechanisms. The surface study performed using contact angle and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) measurements supported the inhibitor adsorption on the steel substrate.

Introduction

Mild Steel (MS) is usually employed in petroleum production, chemical processing, construction engineering, marine, automobile, aerospace, etc.^1,2,3. When mild steel comes into contact with corrosive media during some industrial processes, such as pickling, acid descaling, boiler cleaning, oil well acidification, and acid fracturing, the mild steel becomes severely corroded^4,5. Corrosion is a process that occurs when a metal's surface gets damaged due to its interaction with environment. It can be caused by the exposure of the metal to a liquid or gas. It can lead to a financial loss due to its dangerous and hazardous effects. Various methods have been presented to prevent this type of damage. Some of these include the use of corrosion inhibitor^{6,7,8,9,10,11}, coatings^12,13,14, modifying the damaging climate, and the use of anodic protection¹⁵ etc. One of the most crucial methods that can be used during the pickling process is the use of a corrosion inhibitor. Effective steel corrosion inhibitors include chromate and organic molecules. However, due to chromate’s high toxicity, Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) regulations prohibit its practical application^16,17. Organic molecules are limited by poor solubility, poor volatility, and poor biodegradability¹⁸. Since most corrosion inhibitors exhibit toxic behaviour, it is possible to adhere to environmental sustainability and safety principles by using natural inhibitors, such as plant extracts, bio-polymers, amino acids and drugs etc.

Numerous reports have shown that the employment of pharmaceuticals in treating corrosion is friendly approach and does not have a negative effect on the environment¹⁰. Researchers claim that drugs can contest with green corrosion inhibitors. Majority of them can be produced using natural resources. Carbocyclic or heterocyclic systems are very common in drug structures. Various factors affect the selection of the appropriate drug for treating corrosion. First, the molecules used to treat corrosion must contain nitrogen, oxygen, and sulphur. Second, the drug must be environmentally friendly since it can catalyze biological reactions. Large molecular size and solubility are essential factors that can help determine the ideal drug for corrosion treatment. As per the literature survey, many types of drugs such as Tramadol, Cephapirin, Tenormin¹⁹, Phenobarbital²⁰, Ethambutol²¹, Cephapirin², spironolactone²², Atenolol²³, Cephalothin²⁴, Telmisartan²⁵, Modiaquine²⁶, Ciprofloxacin²⁷, Glimepiride²⁸, Ibuprofen and Diclofenac²⁹, Salazopyrin³⁰, Tramadol³¹, Cefixime and Cefpirome³², Moxifloxacin and Betamethasone³³, Amoxicillin, Ciprofloxacin, Doxycycline, and Streptomycin³⁴, Tetracycline and Streptomycin³⁵ etc. have been effectively employed as sustainable inhibitors for the control of corrosion of different metals and alloys. However, most of these are expensive compared to the organic inhibitors in use³⁶. Although it's believed that most drugs can retain their inventive efficacy after expiration, however, their practice for other purposes is restricted because of their liability and professional limitations³⁷. The excessive use of medicine can lead to the accumulation of toxic substances in the environment and homes. It can also affect wildlife by causing toxic effects³⁸. According to the estimates, the cost of drug wastage in Saudi Arabia is about $150 million annually³⁹. The average drug wastage was 25.8% in Saudi Arabia, and in other Gulf regions, the wastage rate is around 41.3%⁴⁰. Therefore, this is not only a critical issue in Saudi Arabia but also has a global impact in both developed and developing countries³⁶.

Currently research works were conducted using expired drugs as corrosion inhibitors. Expired drugs or unused drugs in the environment results in negative impact on human, aquatics as well as terrestrial eco-system thereby the use of expired drugs as corrosion inhibitors leads to the effective waste management of expired drugs and also reduces the economic losses³⁶. Many expired drugs such as carbamazepine and paracetamol³⁶, Atenolol and Nifedipine³⁷, declophen(2-(2,6-dichloranilino) phenylacetic acid) ampoules⁴¹, Voltaren⁴², Asthalin⁴³, Atorvastatin⁴⁴, Carvedilol⁴⁵ for carbon steel, ampicillin (AMP) and ceftriaxone (CRO) for Sabic Fe corrosion⁴⁶, Meloxicam⁴⁷, Levothyroxine for SS⁴⁸, Oseltamivir for Aluminium⁴⁹, etc. were reported as sustainable inhibitors.

As a result, in the current study, we chose an expired antibiotic drug namely ampicillin. Ampicillin is a semi-synthetic penicillin derivative that acts as an orally active broad-spectrum antibiotic. It treats and prevents various bacterial infections, including urinary tract infections, respiratory tract infections, meningitis, endocarditis and salmonellosis. It may also be used to protect new-borns from group B streptococcal infection. It's application to inhibit mild steel corrosion in 5% HCl at different temperatures was studied. The corrosion studies include weight loss measurement, potentiodynamic polarization measurement, electrochemical impedance, and contact angle measurement and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) studies.

Experimental details

Preparation of studied surface and tested media

Expired drug collected from pharmaceutical drug store. The drug was obtained in pure form and used for this study without any further purification. Figure 1 shows the structural formula of the drug employed. The composition (wt%) of mild steel used for experiments is (wt%) C, 0.061; Mn, 0.181; P, 0.017; Cr, 0.035; Al, 0.017 and remain Fe. For the weight loss measurement measurements, the MS specimens were sized into rectangular pieces that were 2.5 × 2 × 0.1 cm³ in size. They are then cleaned and polished using acetone and SiC paper of grades 180–1200. The specimens are then subjected to 5% HCl solution that is made by the dilution of 37% HCl in bi-distilled water. The solution was standardized using already prepared standard 1 M Na₂CO₃ solution.

Figure 1

Chemical structures of the tested inhibitor.

Weight loss measurements

The corrosion protection effect of the drug on mild steel in 5% HCl solution at varying temperatures was analysed through the weight loss method⁵⁰.

The testing samples were suspended in 5% HCl solution in the absence and presence of various inhibitor concentrations for 6 h at 25, 35, 45 and 55 °C, respectively. Specimens were washed with water immediately after immersion and then rubbed with a soft brush; finally, the samples were washed with acetone and anhydrous ethanol and weighed accurately again after drying. In this study, the average corrosion rate (ν in mpy) and inhibition efficiency (%η_w) of mild steel were calculated according to the following formulas⁵¹:

$$\nu =\frac{KW}{At\rho }$$

(1)

where ν—corrosion rate, K—constant (3.46 × 10⁶), t—immersion time in h, W—weight loss in grams, A—coupon’s exposed area in cm², ρ—density of MS, whose value is 7.86 g cm⁻³.

Surface coverage (θ) and corrosion inhibition efficiencies (%η_w) were determined through corrosion rate data by Eqs. (2) and (3), respectively.

$$\theta =\frac{{\nu }_{0}-{\nu }_{i}}{{\nu }_{0}}$$

(2)

$$\%{\eta }_{w}=\frac{{\nu }_{0}-{\nu }_{i}}{{\nu }_{0}}\times 100$$

(3)

where corrosion rates are indicated by ν₀ and ν_i in the absence and presence of inhibitor, respectively.

Electrochemical analyses

Potentiodynamic polarization (PDP) and Electrochemical impedance (EIS) measurements investigations were done using the three-electrode cell system comprising of a circular MS specimen with exposed area of 1 cm² as a working electrode, Ag/AgCl saturated by 3 M KCl and platinum rod (The surface area is approximatively 1 cm²) as a reference and counter electrodes, respectively. Before the measurement, the MS samples were immersed in a corrosive environment for 30 min to achieve a steady-state open circuit potential (OCP) to check that the surface of working electrode attained a steady-state potential. The test was performed at a frequency range of 10^–2 to 10⁵ Hz with a potential perturbation of 10-mV. All the experiments were performed in this investigation at 25, 35, 45 and 55 °C.

The inhibition efficacies of the inhibitor using EIS parameter i.e., polarization resistance (R_p) was denoted as %η_E and calculated using Eq. (4):

$${\%\eta }_{E}=\frac{{R}_{p}^{(i)}-{R}_{p}^{(o)}}{{R}_{p}^{(i)}}\times 100$$

(4)

R⁽ⁱ⁾_p and R^o_p are the polarization resistances in the presence and absence of inhibitor, respectively.

The PDP measurements evaluates the corrosion current potential and inhibition efficacy of MS that has been exposed to acidic environments both in the occurrence and absence of inhibitor within a range of + 250 to − 250 mV vs. open circuit potential (OCP) and at scan rate of 0.1 mV s⁻¹. The %η_I (percent inhibition efficiency calculated by corrosion current density values) can be determined using Eq. (5).

$$\mathrm{\%}{\eta }_{I}=\frac{{I}_{corr}^{(o)}-{I}_{corr}^{(i)}}{{I}_{corr}^{(o)}}\times 100$$

(5)

I^(°)_corr and I⁽ⁱ⁾_corr are the corrosion current densities in the absence and presence of inhibitor, respectively.

Contact angle measurement

To determine the effective wetting characteristics of mild steel, the KRUSS Germany model FM41Mk2 Simple drop wettability analyser was used. The samples were prepared according to the weight loss measurements.

Surface characterization methods

In order to better understand the surface film morphology and composition, the samples were investigated through SEM–EDS analysis. The specimens were analysed using the JEOL JSM-6510LV model, equipped with an INCA, Oxford energy dispersive X-ray spectrometer. Morphologies of all the coupons were recorded with a magnification of  250× to present the constant view.

Results and discussion

Weight loss test

The Table 1 shows the outcomes of MS corrosion behaviour in HCl with varying concentrations of expired ampicillin drug for 6 h in the temperature range of 25–55 °C. The results of this Table shows that the rate of MS corrosion and inhibition efficiencies increased with increasing temperature^52,53. This suggests that at high temperatures, the ampicillin drug's improved performance could result from the alteration in adsorption behaviour from physical to chemical adsorption. In the case of chemical adsorption, the chemical interactions among active sites of metal and the adsorption centres of inhibitor molecules occur, making the desorption process difficult. Besides, increasing temperature results in the desorption of water molecules from the surface and increasing accessibility of the surface sites for the adsorption of inhibitor molecules. The results confirmed that the drug molecules were chemically adsorbed on the MS⁵⁴ and are a suitable temperature-resistant corrosion inhibitor.

Table 1 Weight loss test results of MS following 6 h immersion in 5% HCl solution in the absence and occurrence of diverse concentrations of expired Ampicillin drug at varying temperatures.

Assessment of Adsorption Isotherms and thermodynamic parameters

The results of the gravimetric measurement were used to select the most appropriate isotherm for the adsorption of drug molecules on the metal surface (Fig. 2a–c). The linear regression coefficient (R²), meticulously related to the unity, authenticated the Langmuir adsorption model as shown in Eq. 6 where θ is the fraction of the surface covered by the adsorbed inhibitor molecules and C is the inhibitor’s concentration⁵⁵:

$$\frac{C}{\theta }=\frac{1}{{K}_{ads}}+C$$

(6)

where the equilibrium adsorption constant is denoted by K_ads, which can be attained as the intercept value from the plot of C/θ against C (Fig. 2a)_. The conquered unit slope of Eq. (6) given in Table 2 confirmed the fitted Langmuir adsorption model. Moreover, high K_ads value at 55 °C given in Table 2 suggests the more pronounced adsorption process (Table 2).

Figure 2

(a) Langmuir, (b) Temkin, (c) Freundlich adsorption isotherm.

Table 2 Thermodynamic characteristics of adsorption for MS in 5% HCl at different temperatures.

Equation (7) can be employed to measure Gibbs free energy of adsorption (∆G°_ads)⁵⁶:

$${\Delta G}_{ads}^{0}=-RT\mathrm{ln}\left(55.5{K}_{ads}\right)$$

(7)

where R − 8.314 J K⁻¹ mol⁻¹ (universal gas constant), T—absolute temperature in K, and 55.5—water’s molar concentration in mol L⁻¹. The negative sign of ∆G°_ads indicates that the adsorption process is spontaneous⁵⁷.

Adsorption enthalpy change (ΔH^o_ads) was obtained by plotting to log K_ads against 1/T (Fig. 3) using Eq. (8)⁵⁷:

Figure 3

Relationship between log K_ads vs. 1/T.

$$\mathrm{log}{K}_{ads}=\left(-\frac{\Delta {H}_{ads}^{0}}{2.303RT}\right)+constant$$

(8)

The standard activation entropy (ΔS°_ads) can be measured utilizing Vant’t Hoff Eq. (9)⁵⁸:

$$\Delta {G}_{ads}^{0}= \Delta {H}_{ads}^{0}-{T\Delta S}_{ads}^{0}$$

(9)

Table 2 shows the values of ΔS^°_ads and ΔH^°_ads measured from Fig. 3. The positive ΔH^°_ads values depict the endothermic nature of the activated step of the corrosion process. Instead, the negative ΔS^°_ads values suggest that the entropy is reduces by the adherence of inhibitor molecules to a metal surface reduces^59,60.

Activation parameters

The increasing temperature can significantly affect a material's various properties, such as its kinetics, corrosion rate, and equilibrium. These parameters were attained through Arrhenius and transition-state Eqs. (10) and (11)⁴¹:

$$\mathrm{log}\nu =logA-\frac{Ea}{2.303RT }$$

(10)

where the pre-exponential factor is denoted by A, absolute temperature is indicated by T in Kelvin, and the Gas Constant denoted by R in J K⁻¹ mol⁻¹.

$$\nu = \frac{RT}{Nh}\mathrm{exp}\frac{\Delta S}{R}\mathrm{exp}\frac{-\Delta H}{RT}$$

(11)

where N is Avogadro’s number, and h is Planck’s constant.

The activation energy of a process is computed from the log CR vs. 1/T plot (Fig. 4a). The lower activation energy of a process in the occurrence of a drug compared to that of a process in its absence is due to its chemical adsorption⁶¹. The increase in the %η with temperature increase also suggests chemical adsorption^62,63.

Figure 4

(a) Arrhenius plot, (b) transition state plot.

The entropy and enthalpy of activation were computed by taking into account the Transition State (Eq. 9). From the intercept of Fig. 4b and the slope of (− ∆H*/2.303R), the values of ∆S* and ∆H* were determined. The ∆H* value with positive sign indicates the endothermic process of the MS, which suggests that the solution containing the synthesized inhibitor has difficulty in preventing the steel from corrosion. The reduction in the ΔS* values (Table 3) when using inhibitor-protected acid solutions suggests that the transition state is formed through an associative mechanism⁶⁴.

Table 3 Activation parameters of MS in 5% HCl.

EIS measurement

The impact of temperature on the ampicillin drug's efficacy in 5% HCl solution containing MS was investigated at a temperature ranging from 25 to 55 °C. The electrochemical parameters namely, the polarization resistance (R_P), double layer capacitance (C_dl), and percent inhibition efficiencies (%η_E), were analysed and given in Table 4. Table 4 shows that the absence and presence of the inhibitor significantly affects the these parameters. The radius of the MS capacitive loops in an inhibited solution is larger when compared to the solution with no inhibitor. As the concentration of inhibitor increases, this effect continues. The increase in the temperature leads to a decrease in the radius of the capacitive loop, but the increase in the presence of an inhibitor increases its diameter. The increase in impedance of mild steel due to the increased surface coverage of inhibitor molecules on the surface of electrode results in an increase in inhibition efficacy⁶⁵. Additionally, Nyquist plots (Fig. 5) show a single semicircle loop for each concentration that was depressed in the centre of the plot. The capacitive loop's shape is imperfect as a result of surface imperfections and defects. Since the semicircle curves were imperfect, the constant phase element (CPE) was utilized instead of an ideal capacitor. The CPE can be calculated using the equation below⁵⁴:

$${\mathrm{Z}}_{\mathrm{CPE}}= {\mathrm{Y}}_{0}^{-1}(\mathrm{j\omega }{)}^{-\mathrm{n}}$$

(12)

where Z_CPE represents the CPE’ impedance, j—an imaginary number associating to the square root of − 1, Y₀—proportionality integral, ω—angular frequency, and n—the CPE exponent which is detailing about the degree of surface inhomogeneity.

Table 4 Electrochemical impedance characteristics for MS at different temperatures in 5% HCl solution in the absence and presence of varying concentrations of expired Ampicillin drug.

Figure 5

Nyquist plots for MS in 5% HCl solution in the absence and presence of different concentration of drug at (a) 25 °C, (b) 35 °C, (c) 45 °C, and (d) 55 °C.

The data collected during the experiment were then fitted to an equivalent circuit model as illustrated in Fig. 6, which comprises the R_p, R_s (solution resistance) and C_dl. The values of C_dl were then obtained through the given equation⁶⁶:

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

(13)

where ω_max stands for angular frequency for the mythic proportion of impedance being maximum.

Figure 6

Equivalent circuit model adapted to fit the impedance data.

The data indicated that the R_p values escalated with the increase in the solution temperature and the concentration of the inhibitor^67,68. The reduced C_dl values could be used to support the idea of the metal surface's ability to absorb the inhibitor molecules.

Potentiodynamic polarization (PDP) measurement

Potentiodynamic polarization investigations were achieved in the temperature range of 25–55 °C in the absence and presence of 0.1, and 20 mM of expired ampicillin drug and illustrated in Fig. 7. Table 5 records the parameters extracted from PDP measurements such as I_corr, corrosion potential (E_corr), and %η_I of MS with and without inhibitor. It was discovered that the E_corr values of the used inhibitor differ from those of the blank solution, but the change does not exceed 85 mV, proving that these inhibitors are mixed type and reduces the values of both anodic and cathodic Tafel slope⁶⁹. The polarization curves remain constant even when the diverse concentrations of the inhibitor are used at different temperatures (Fig. 7). The curve's shift for a lower current density indicates that the molecules' adsorption on the metal surface slowed down the corrosion process. In addition, the inclusion of an inhibitor to the blank solution results in a reduction in the values of I_corr. In the absence of the studied inhibitor, I_corr was 286.2 (µA cm⁻²), which increased to reach 71.2 (µA cm⁻²) at 25 °C in the occurrence of 20 mM Ampicillin drug suggesting the adsorption of drug molecules on the metal surface. The results of the polarization curve verified that the increasing temperature caused the increase in the current density and the intensity of inhibition efficiencies. In addition, the time lag amid the desorption and adsorption of the inhibitor molecules on the metal surface increases with the temperature. The increase in the concentration and solution temperature of the inhibitor and its proficiency is consistent with the outcomes of the weight loss and the electrochemical impedance measurements. It could be due to the strong adsorption of drug molecules on the surface of the metal, which could improve the stability of the protective film of the inhibitor⁷⁰.

Figure 7

Polarization curves for MS in 5% HCl solution in the absence and presence of different concentration of drug at (a) 25 °C, (b) 35 °C, (c) 45 °C, (d) 55 °C.

Table 5 Potentiodynamic polarization parameters for MS at diverse temperatures in 5% HCl solution in the absence and presence of varying concentrations of expired Ampicillin drug.

Comparative study

The results of a comparison study of the corrosion inhibition behaviour of previously reported drugs that were tested as corrosion inhibitors for various metals and media are shown in Table 6^{34,71,72,73,74,75,76,77}. It is obvious from comparing the results that the expired drug under study has significantly more potency than other drugs. This approach improves the applicability of using the investigational inhibitor. Additionally, the fact that this inhibitor is used in low concentrations compared to previously reported drugs, can be used to explain the cost effectiveness.

Table 6 Comparative data showing the performance of the other drugs.

Contact angle analysis

A contact angle test was executed to determine if a surface has a hydrophilic or hydrophobic characteristic. This is useful in identifying a solid substrate's ability to repel liquids. A contact angle measurement was conducted on the mild steel surface that was immersed in 5% HCl in the absence and presence of drug. The contact angle of a steel surface without an inhibitor was measured at 29.8°. The results indicated that the surface's wettability could give it favourable water-related hydrophilicity⁷⁸. The increase in the contact angle from 29.8° to 37.3°, 58.2°, 73.8° to 89.4° with the addition of inhibitor, as shown in Fig. 8, proposes that the steel surface has a hydrophobic layer due to the adsorbed drug molecules.

Figure 8

Illustrates the surface contact angles of the steel surface uninhibited with various concentrations of expired Ampicillin drug at 25 °C.

Surface morphology

The SEM micrographs were used to analyse the morphological variations in MS exhibited by 5% HCl solution. The polished MS surface exhibited a defect-free appearance Fig. 9a, while the rough MS surface was seen in the presence of 5% HCl solution because of the intense dissolution of the metal surface Fig. 9b. Figure 9c showed the changes in the surface appearance which were caused by the existence of a protective layer on the surface. This layer prevented the surface from getting rougher. The occurrence of the drug on the surface of the metal can act as a barrier against corrosion Fig. 9c.

Figure 9

SEM micrographs of MS surface immersed in 5% HCl with (a) absence of inhibitor (b) presence of inhibitor.

EDS can determine the presence of chemical composition in a sample. The surface configuration for the polished MS and MS in uninhibited and inhibited drug solutions are illustrated in Fig. 10a–c. Figure 10a displays the EDS peak of the polished MS surface before exposing it to any aggressive media, which shows some of the constituent peaks of MS. When the MS surface was exposed to 5% HCl without inhibitor (Fig. 10b), the peaks of oxygen and chlorine appeared, suggesting the formation of corrosion products in the form of oxide/chlorides due to the continuous degradation of metal in acidic phase. The absence of the oxygen peak and the increase in the peaks of iron and carbon were also observed in the 5% HCl solution. These effects led to the establishment of the surface's safety from corrosive solution because of the adsorption of inhibitor molecules (Fig. 10c).

Figure 10

EDS photographs of MS surface immersed in 5% HCl with (a) absence of inhibitor (b) existence of inhibitor.

Mechanism of corrosion protection

In the presence of HCl solution, the MS surface is positively charged. It is well known that chloride ions have a low degree of hydration and that, through specific adsorption, they provide an excessive amount of negative charge to the metal's solution side, attracting the majority of cations⁶⁹. In an acidic media, inhibitor molecules can remain as neutral or as protonated molecules. As a result, the adsorption modes of drug can be considered by neutral molecules or protonated forms⁷⁹. The structure of ampicillin drug includes a benzene ring, beta-lactam ring and thiazolidine ring and heteroatoms like S, N, and O. Neutral drug molecules can be adsorbed on the mild steel’ surface via the chemisorption mechanism, which involves the water molecules’ displacement from the steel surface and the sharing of electrons between Fe and heteroatoms.

Moreover, the drug is adsorbed on the MS surface via donor–acceptor interactions among the ampicillin drug and the vacant d-orbital of iron atoms. Ampicillin heteroatoms can establish a coordinate bond by donating a lone pair of electrons to the metal's vacant d orbital. Besides that, π-electron in benzene ring may form the similar bond with the metal atom. The inhibitor molecules in the protonated form can bind to the metal surface through chloride ions, which form interconnecting bridges between the protonated drug cations and the positively charged metal surface. As a result, a densely packed layer will form on the metal surface, reducing charge and mass transfers.

Conclusion

To develop an effective and eco-friendly corrosion inhibitor, the effect of expired ampicillin drug on the corrosion of MS in 5% HCl was studied. The study's results revealed that the drug exhibited a 96.9% inhibition efficiency at 55 °C when 10 mM of concentration of drug was used. The potentiodynamic polarization results validated that the drug exhibited mixed-type inhibition behaviour. The impedance spectra also disclosed that the inhibitor efficacy increases with the increased concentration of inhibitor, and the diameter of the semicircle increased with increased inhibitor concentration. SEM–EDS analysis confirmed the surface film formation. It can be utilized as an eco-friendly alternative to toxic corrosion inhibitors in industrial processes such as descaling and acid pickling.