Effective inhibition of T95 steel corrosion in 15 wt% HCl solution by aspartame, potassium iodide, and sodium dodecyl sulphate mixture

Sustainable development goal 12 advocates the production and consumption of green and sustainable commodities. As such, pressure is mounting on the oil and gas industries for a paradigm shift. This work explores the potential of aspartame (a derivative of aspartic acid and phenylalanine) based formulation as a green inhibitor. The inhibiting effect of aspartame alone and in combination with potassium iodide (KI) or sodium dodecyl sulphate (SDS) or both on T95 steel in 15 wt% HCl solution at 60–90 °C is investigated using weight loss, electrochemical, and surface analysis techniques. The results show severe metal corrosion especially at 90 °C with a corrosion rate (v) of 186.37 mm/y. Aspartame inhibits corrosion and its inhibition efficiency (η) increases with an increase in temperature. At 6.80 mM, η of 86% is obtained at 90 °C. The addition of SDS to aspartame produces an antagonistic effect. A KI-aspartame mixture produces an antagonistic effect at 60 °C and 70 °C but a synergistic effect at 80 °C and 90 °C. There is a strong synergy when aspartame (6.80 mM), KI (1 mM), and SDS (1 mM) are mixed especially at higher temperatures. The mixture reduces v from 186.37 to 14.35 mm/y, protecting the metal surface by 92% at 90 °C. The mixture can be considered an acidizing corrosion inhibitor.

to boost the corrosion inhibition property of aspartame for T95 steel in a strong acid medium (15 wt% HCl) at high temperatures (60-90 °C).Electrochemical frequency modulation (EFM) and weight loss (WL) techniques are used to revalidate the previous results 12 .The effect of the addition of potassium iodide or sodium dodecyl sulphate (SDS, Fig. 1b) or both on the inhibition performance of aspartame is studied using the WL, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), scanning electron microscope (SEM), and optical profilometer (OP).
The T95-2 steel grade (for convenience, T95 is adopted in the manuscript) is known for its high strength and resistance to sulphide stress corrosion making them the preferred material for oil and gas well tubing 13,14 .Nevertheless, well-stimulation activities such as acidizing whereby a strong acid solution is pumped into a reservoir to dissolve flow channel barriers 15 affect the corrosion resistance property of the tubing negatively.The normal practice has been the fortification of the acidizing solution with an effective corrosion inhibitor 15 which is the basis for corrosion inhibitor research in acid concentrations of 15-28 wt%.Subramania et al. 16 investigated the inhibitive effect of 1-cinnamylidine-3-thiocarbohydrazide (CTCH) and 1,1′-dicinnamylidine-3-thiocarbohydrazide (DCTCH) on carbon steel dissolution in 15 wt% HCl solution.At 110 °C, 1500 ppm of CTCH was found to reduce the corrosion rate of the metal from 14,417.0 to 255.96 mm/y and protected the metal surface by 98.2%.At the same temperature and concentration, a corrosion rate and inhibition efficiency of 140.31 mm/y and 99.0%, respectively was recorded for DCTCH from the weight loss technique.A formulation consisting of 1.0 mM N-acetyl cysteine + 10 −5 M glutathione + 10 −5 M KI was reported to afford an inhibition efficiency of 91.4% at 90 °C on X80 steel in 15 wt% HCl solution 17 .Dicinnamylidene acetone, disalicylidene acetone, divanillidene acetone 18 , N-(2-(2-tridecyl-4,5-dihydro-1H-imidazol-1-yl)ethyl) tetradecanamide 19 , (E)-5-amino-3-(4-methoxyphenyl)-N′-(1-(4-methoxyphenyl)ethylidene)-1H-pyrazole-4-carbohydrazide, (E)-5-amino-N′-(4chlorobenzylidene)-3-(4-chlorophenyl)-1H-pyrazole-4-carbohydrazide 20 , among others, have been reported as corrosion inhibitors for carbon steel in 15 wt% HCl solution.As earlier mentioned, environmental concerns and the complicated synthesis procedures for some of these compounds are the limitations.This communication thus showcases aspartame-based formulation as a green and sustainable corrosion inhibitor for the acidizing process.

Experimental aspect
Materials and reagents preparation.Aspartame (CAS No: 22839-47-0, molar mass: 294.3 g/mol), KI, SDS, HCl (37%), and ethanol were purchased from Merck company (USA).They were of analytical grade.T95 steel grade with composition given in Uzoma et al. 12 was sourced from Wuhan Corrtest Instruments Corp., Ltd, China.The dimension of the samples used for weight loss studies was 2 × 2 × 1 and the surface area (A) was calculated following Eq.(1) 21 .
where w is the width (cm), d is the thickness (cm) and l is the length (cm).For samples used for the electrochemical studies, the exposed surface area was 1 cm 2 .The surface pre-treatment followed the procedure listed in ASTM G1-90 22 .Mechanical surface abrasion was accomplished on the EcoMet metallographic abrasion instrument (Buehler Co., USA) with sandpapers of grit size ranging from #120 to #2000.At room temperature, aspartame is not readily soluble in water.Hence, the stock solution (339.79 mM) was prepared by dissolving the appropriate amount in a 1:1 ratio mixture (20 mL) of ethanol and distilled water.The experimental concentrations of aspartame (1.70 mM and 6.80 mM) were obtained by serial dilution of the stock solution with 15 wt% of HCl solution.The concentrations of KI and SDS considered in the study are 1, 3, and 5 mM.

Weight loss experiments.
Before the experiment, the weight of the prepared samples was measured and recorded as the initial weight.Sealed reaction bottles (150 cm 3 capacity) with appropriate label (i.e., blank, 1.70 mM aspartame, etc.) was filled with 100 cm 3 of the respective solutions and placed on a digital thermostatic water bath for the solutions to attain the studied temperature.The temperatures studied were 60 °C, 70 °C, 80 °C, and 90 °C.A thermometer was used to confirm that the temperature of the bottles' solutions was as expected.Thereafter, the prepared samples, in triplicate, were freely suspended in each of the bottles with the help of a thread, the bottles were corked, and allowed to stand at the studied temperature for 4 h.Subsequently, the corroded samples were retrieved, immersed in a pickling solution (50 g NaOH + 200 g granulated Zn dust made up to 1000 mL with distilled water 22 for 10 min, gently scrubbed under running water and acetone, dried with warm air, and re-weighed.The weight loss (g), corrosion rate (mm/y), and inhibition efficiency (η WL %) were calculated as 23 : (1) A = 2(wl + dl + dw)

Results and discussion
Validation of the corrosion inhibition by aspartame.The EIS and PDP techniques were previously used to study the inhibition performance of aspartame for T95 steel in 15 wt% HCl solution at 60-90 °C12 .Before proceeding to the synergistic inhibition studies which is the main focus of this work, the previous results were validated using the classical WL and EFM techniques.The EFM technique, in particular, was opted for because of the internal data validation mechanism, the so-called causality factor 23 .As is known 25 , the ideal value of causality 2 (CF2) and 3 (CF3) factors is 2 and 3, respectively.Hence, experimentally obtained values must fall within the 0-2 and 0-3 range for the results to be meaningful.Figure 2 shows EFM spectra for T95 in 15 wt% HCl solution in the absence and presence of 1.70 mM or 6.80 mM of aspartame at 60 °C, 70 °C, 80 °C, and 90 °C.The i corr , anodic and cathodic slopes (β a and β c ), CF2, and CF3 values derived from the analysis of the EFM data are given in Table 1.η EFM as earlier stated was computed using Eq.(7).
In Table 1, the CF2 and CF3 are in the acceptable region, indicating a favourable causality between perturbation and signal 23 and reliable results.The spectra in Fig. 2 all contain strong harmonic and intermodulation signals as well as weak background noise signals.A close inspection of the EFM spectra reveals that the harmonic and intermodulation signals are suppressed with increasing frequency, especially in the aspartame-containing systems.This, according to Liu et al. 23 is due to a decrease in i corr .An examination of Table 1 discloses that the i corr value of the T95 steel specimen in the blank acid solution slightly decreased from 182.9 at 60 °C to 167.7, 166.5, and 165.0 µA cm −2 at 70 °C, 80 °C, and 90 °C, respectively indicating the slight inhibitive effect of the corrosion products deposited on the surface.The i corr value for the aspartame-inhibited systems noticeably reduced and the Vol:.(1234567890) www.nature.com/scientificreports/steel surface 27 .This could be possible through the donation of electron pairs from the heteroatoms (N and O) of aspartame (Fig. 1a) to the low-lying empty d-orbital ([Ar]3d 6 ) of Fe 2+27 .

Effect of the addition of KI and SDS on aspartame inhibition. Studies on individual inhibition per-
formance by additives.The weight loss technique was adopted to study the individual inhibition performance by KI and SDS against T95 corrosion in 15 wt% HCl solution at a temperature range of 60-90 °C.To be guided in the choice of the concentration selection for synergistic studies, three concentrations, namely 1 mM, 3 mM, and 5 mM were studied.The calculated value of WL, v, surface coverage (θ), and η WL for T95 corrosion in the corrodent free of and containing the chosen concentrations of KI or SDS at various temperatures are summarized in Table 2.As should be expected, WL and v in both uninhibited and inhibited systems increase with a rise in temperature because of the corresponding increase in the kinetic energy of the attacking species 28 .Both KI and SDS exhibit corrosion-inhibiting properties.The WL and v values can be seen to reduce in the additives-containing systems at all temperatures.The θ and η WL values indicate that the additives adsorb on the steel surface, cover some portions, and offer a certain degree of protection against the metal surface corrosion.The η WL value of the additives which is in the range of 35-10% is seen to decrease with an increase in concentration and temperature.
The best inhibition performance is obtained for the 1 mM concentration and at 60 °C.
It has been experimentally proven that iodide ions (I − ) exhibit some degree of inhibition against Fe corrosion in acid media due to its specific adsorption [29][30][31] .Farag and Hegazy 29 reported an inhibition efficiency of 82.7% for the protection of carbon steel against corrosion in 0.5 M H 2 SO 4 solution by 0.05 M KI at 25 °C.Haruna et al. 32 reported an inhibition efficiency value of 70.42% for 2.5 w/v% KI against the corrosion of X60 carbon steel in 15 wt% HCl at 25 °C.Feng et al. 33 however, observed a fluctuating inhibition performance by iodide ions with increasing concentrations typical of the observation in this work (Table 2).In an acid solution with dissolved oxygen and a potential of about 0.625 V SHE or 0.381 V SCE , I − ions are oxidized to I − 3 ions (Eqs.8-10) 33 .
Feng et al. 33 noted that the formation of triiodide ion ( I − 3 ) rather lowered the inhibition efficiency of I − .If the I − 3 species do not provide as much inhibition as the I -ions, it means fewer I -ions would be available for adsorption and protection of the steel surface at higher concentrations of KI and temperatures.The oxidation of I -ions to I − 3 ions should increase as the temperature of the system is increased.More I -ions should be available for oxidation to I − 3 ions for higher concentrations.This may be the reason for the decreasing trend in the inhibition efficiency of KI with increasing concentration and temperature.It should also be mentioned that the inhibition of KI significantly depreciates in 15 wt% HCl at 60-90 °C compared to the value reported for the same corrosive medium but at lower temperatures 32 .This further alludes to the possibility of I − 3 ions being less inhibiting than I -ions.Investigating the inhibition property of various species of iodide ions will be interesting.For the SDS, Tan et al. 34 , based on molecular simulation results proposed the mechanism of inhibition of carbon steel dissolution in 1 M HCl at 25 °C by SDS to originate from self-assembled hydrophobic barrier formation on the metal surface due to a connection between the polar sulfonic acid group to the Fe atoms and the spreading of the long alkyl chain in solution.The authors, notwithstanding noticed that there was no significant change in the inhibition efficiency value upon an increase in SDS concentration from 1 mM (68.6%) to 5.0 mM (69.5%).It is rational to attribute the observed decline in inhibition efficiency of SDS with increasing concentration and temperature to system saturation and detachment, respectively.
Studies on collective inhibition performance by additives with aspartame.WL studies.The 1 mM concentration of KI and SDS was chosen for synergistic inhibition studies with aspartame since it exhibited the highest inhibiting effect.The results of the synergistic studies from the weight loss technique are given in Table 3. Formulation denotes a mixture of 6.80 mM aspartame with 1 mM of KI and SDS.An interesting observation is made in Table 3.The addition of KI to aspartame enhances inhibitive performance at all temperatures while the reverse is observed for the aspartame + SDS combination.For instance, the η WL of aspartame at 60 °C, 70 °C, 80 °C, and 90 °C is 50%, 58%, 63%, and 86%, respectively (Table 1).With a combination with 1 mM KI, the performance improves to 59%, 66%, 69%, and 90% at 60 °C, 70 °C, 80 °C, and 90 °C, respectively.However, a combination with 1 mM SDS sees the inhibiting performance depreciates to 41% and 58% at 60 °C and 80 °C, respectively, and remains the same at 70 and 90 °C (Table 3).This can be explained by the fact that I -ions preferentially adsorbed on the anodic area, create a negative substrate surface that electrostatically attracts protonated aspartame molecules.However, on the surface, molecular interactions occasioned by the chemisorption process involving electron donation from the aspartame heteroatoms and the acceptance by the 3d-orbitals of Fe (in steel) ensured.Thus, I -ions are involved in the stabilisation of the adsorbed aspartame molecules 29,35 .In the case of SDS, according to Tan et al. 34 , the adsorption of SDS molecules on a steel surface in an HCl solution occurs through the S and O atoms of the polar sulfonic acid group.The heteroatoms form covalent bonds with Fe 3d orbitals.But first, the negative charge on the polar head of SDS will facilitate columbic attraction to the anodic area.Therefore, there will be interference adsorption between SDS and aspartame on the anode.This may be the reason for the observed depreciating inhibition performance by the aspartame-SDS mixture relative to aspartame alone.
It is beneficial to rather mix aspartame, KI, and SDS.The iodide ions seem to have helped in stabilising both aspartame and SDS adsorptions.As could be seen in the table, the inhibition efficiency of the formulation is higher than that of the aspartame-KI mixture at all temperatures.The η WL value recorded for the formulation at 60 °C, 70 °C, 80 °C, and 90 °C is 70%, 79%, 89%, and 91%, respectively.Besides the high η WL value obtained for the formulation at the various temperature levels, it is worth drawing attention to the near constancy of the corrosion rate of the steel at all temperatures.The v is 8.67 ± 1.73 mm/y, 12.14 ± 1.73 mm/y, 10.41 ± 3.47 mm/y, and 13.88 ± 5.20 mm/y at 60 °C, 70 °C, 80 °C, and 90 °C despite reaching 149.54 ± 8.67 mm/y at 90 °C in the blank solution.This indicates that the formulation is effective and stable at high temperatures.Therefore, KI and SDS can be used to improve the adsorption stability of aspartame on steel at high temperatures.EIS studies.Figure 3 shows the Nyquist electrochemical diagrams for T95 corrosion in 15 wt% HCl solution without and with 6.80 mM aspartame alone and in combination with 1 mM KI or 1 mM SDS or both at 60-90 °C.The Nyquist diagrams drawn for the steel corrosion in the unprotected acid solution exhibit a capacitive loop at the high frequencies (HF) and an inductive loop at low frequencies (LF) at all temperatures except at 90 °C in which there is an additional poorly resolved capacitive loop at the middle frequencies (MF).A similar impedance characteristic was observed for pure iron 36,37 in 1 M HCl solution and cold rolled steel in 0.5 M H 2 SO 4 solution 38 .The HF semi-circular loop is often linked with the charge transfer of the dissolution process and double-layer behaviour [36][37][38] .The LF inductive loop is believed to result from the slow reaction during the adsorption of Cl − ads and H + ads on the working electrode surface 37 .It is also suggested that the re-dissolution of the passivated surface layer at LF could also give rise to an inductive loop [36][37][38] .The unresolved semi-circular Table 3. Weight loss (WL), corrosion rate (v), surface coverage (θ), and inhibition efficiency (η WL ) for T95 corrosion in 15 wt% HCl solution in the absence and presence of 6.80 mM aspartame in combination with 1 mM KI or 1 mM SDS or both at different temperatures from weight loss measurements..It thus supports the assertion of aggravated corrosion of the steel at 90 °C earlier noted from the WL technique (Tables 1, 2).The corrosion products on the T95 steel surface, at 90 °C may have become too porous such that corrosive species were transported from the bulk solution to the steel/solution interface 39 .In the Nyquist diagrams drawn for the inhibited systems, the inductive loop is still seen but the MF capacitive loop at 90 °C is absent.This observation implies that the T95 steel electrode still corrodes by the direct charge-transfer process even in the presence of the additives but at 90 °C, the diffusion contribution to corrosion was abated.Overall, the inhibiting effect of the additives is demonstrated by the increase in the diameter of the capacitive loop relative to that of the blank.Similar to the observation made from the WL results (Table 2), the diameter of the HF capacitive loop increases in the order: formulation > aspartame + KI > aspartame > aspartame + SDS > blank at 60 °C and 80 °C.The diameter of the aspartame and aspartame + SDS graphs are almost the same at 90 °C.This indicates improved inhibition performance by the formulation followed by the aspartame-KI mixture and the antagonistic behaviour of SDS in the aspartame-SDS mixture.It should be mentioned that in all cases, the HF loops are not perfect semicircles.This phenomenon has always been attributed to the frequency dispersion resulting from the roughness and non-homogenous characteristics of the working electrode surface 38 .
The difference in the shape of the obtained impedance plot at 90 °C from others prompted the fitting of the impedance data with different equivalent circuits involving constant phase element (CPE), charge-transfer resistance (R ct ), inductance (L), inductive resistance (R f ), Warburg impedance (W), and Warburg resistance (R W ). The equivalent circuits are shown in Fig. 4.
The double-layer capacitance (C dl ) was calculated using the following equation 38 .
(11) where Q dl and n are the components of CPE and f max represents the frequency at which imaginary value reaches a maximum on the Nyquist plot 38 .The values for the fitted parameters are listed in Table 4.The inhibition efficiency (η EIS ) was computed using Eq. ( 12) 24 .
where R p(0) and R p(1) are the polarization resistances in the absence and presence of additives, respectively 24 .As expected, the R ct , R f , and R p values increase in the presence of additives while the C dl value decreases indicating inhibitor adsorption on the steel surface and the mitigation of corrosion 40,41 .For all the studied inhibitors, η EIS increases with increasing temperature due to the chemical adsorption of the inhibitor molecules 42 and thus agrees with the WL results.The formulation produced the highest η EIS of 92% at 90 °C.Contrary to the observation from WL studies (Table 3), the η EIS for ASP + SDS marginally increase at 70 and 90 °C.Such a marginal increase is typical of competitive co-adsorption (antagonistic effect).As explained in Section "Studies on collective inhibition performance by additives with aspartame", the polar head of SDS, chloride ions, and aspartame would compete for adsorption on the anodic region of the substrate.The competitive co-adsorption could at first result in a marginal increase in inhibition efficiency as suggested by the η EIS at 70 and 90 °C.However, at prolonged immersion time due to the intensified molecules interactions, inhibition efficiency could slightly depreciate as noted in Table 3.
PDP studies.To further evaluate the influence of the addition of SDS or KI or both on the inhibition performance of aspartame, polarization measurements were performed.Figure 5 displays the PDP plots for T95 corrosion in 15 wt% HCl solution without and with 6.80 mM aspartame alone and in combination with 1 mM SDS or 1 mM KI or both (formulation) at 60 °C, 70 °C, 80 °C, and 90 °C.The values of the polarization parameters including the corrosion potential (E corr ), i corr , and anodic and cathodic slopes (β a and β c ) are given in Table 5.The corrosion rate ( v ) and η PDP were computed using Eqs.( 14) and ( 15) 43 , respectively.
(12) where K is the conversion constant (3.27 × 10 −3 mm g/μA cm y when v is in mm/y), ρ is density in g/cm 3 , EW is the equivalent weight, i 0 corr and i inh corr are the corrosion current densities in the absence and presence of inhibitor.The presence of the additives is seen to cause both the anodic and cathodic curves to shift in the direction of low current density at all temperatures.The trend is as noted in other techniques, that is, the formulation produced the most effect.The results in Table 5 reveal that the i 0 corr increases with the rise in temperature reaching a value of 16,100 μA cm −2 at 90 °C.Consequently, a startling corrosion rate of 186.37 mm/y is obtained at 90 °C confirming the aggravated corrosion at 90 °C that the results from other techniques (Tables 1, 2, Fig. 3) suggested.The remarkable reduction of v from 186.37 mm/y at 90 °C to 14.35 mm/y corresponding to an inhibition efficiency of 92% by the formulation is worth noting.One of the key requirements for an acidizing oil well inhibitor is thermal stability and effectiveness retention at high temperatures.The results imply that the formulation could satisfactorily restrain the corrosion of oil well tubing during acidizing operation.It is obvious from the polarization curves that the aspartame mixture exhibits a mixed-type inhibiting behaviour.The insignificant displacement of the E corr in the presence of the additives and the simultaneous reduction of the anodic and cathodic current densities attest to the mixed-type behaviour 44 .However, it could be seen that there is suppression dominance of the cathodic half-reduction reaction especially at 60 °C and 70 °C (Fig. 3).In Table 5, the β c values are seen to be much bigger than the β a values.This is reasonable since the protonated form of aspartame is expected to adsorb at the cathodic region to suppress the adsorption of H + ads and the subsequent evolution of hydrogen gas.The inhibition of the anodic oxidation reaction will very much depend on the recharging power of the additives.The PDP and the EIS results are in good agreement.
Synergism studies and explanation of inhibition mechanism.Synergism and antagonism are the two terms often used to describe the nature of adsorption in a binary or multiple-component system.By definition, synergistic effect implies the cooperative co-adsorption of the participating species on a substrate surface while antagonistic effect means the separate co-adsorption of the species 45 .The synergism parameter (S) since used by Aramaki and Hackerman 46 has been overwhelmingly applied in classifying co-adsorption as synergistic or antagonistic 34,47,48 .By the S classification, a S > 1 indicates a synergistic effect and S < 1 means an antagonistic effect 45 .Most recently, Kokalj 49 proposed the computation of S from the corrosion activity ( α ) and the threshold corrosion activity ( α threshold ) of an inhibitor following Eq.( 16).The α can be obtained using Eq. ( 17).Equa- tion ( 18) is used for the calculation of α threshold for a binary system or ternary system 49 .In this work, θ is the surface coverage obtained using the inhibition efficiency value from the EIS, i.e., θ = ηEIS 100 .α 1 , α 2 , and α 3 are the corrosion activity of aspartame, KI, and SDS, respectively.The plot of S as a function of temperature is shown in Fig. 6.
For the aspartame-SDS system, S is less than one at all temperatures implying an antagonistic effect between aspartame and SDS.This result is reasonable, agrees with the results from the PDP studies (Fig. 5), and can be explained thus.
1.In the blank acid solution (15 wt% HCl), Fe oxidation takes place at the anode ( Fe → Fe 2+ + 2e − ) and hydrogen ions reduction occurs at the cathode (2H + + 2e − → H 2 ) 50 .Chloride ions ( Cl − ) would be attracted to the positive anodic area and would be adsorbed on the metal surface making the anodic area partially negative.2. In the acid solution containing aspartame, the amino group in aspartame is expected to gain a proton which would make aspartame exist in cationic form and adsorbable at the cathode.Such adsorption would lead to the suppression of the cathodic hydrogen reduction reaction and the PDP results (Fig. 5) attest to this.Some of the protonated aspartame would be attracted to the partially recharged anodic area through coulombic attraction.On the anode, by donation of electron pair on the unprotonated heteroatoms to the d-orbital of Fe, chemical bonding takes place.The variation of the inhibition efficiency with temperature alludes to the existence of chemisorption.3.In the acid solution containing SDS, because of the negative sulphate head, the surfactant would be attracted to the anode and would be in contention of adsorption with chloride ions.As earlier mentioned, the electron pair on the heteroatoms would be donated to the d-orbital of Fe to ensure a covalent type of bonding.Therefore, when aspartame and SDS are mixed, both would compete for adsorption at the anodic area making  the anodic inhibition diminish and the aspartame molecules adsorbed on the cathodic area to be the main contributor to the inhibition process.This can be seen in Fig. 5.It also explains the reason behind the lower inhibition efficiency value of the aspartame-SDS mixture relative to aspartame alone (Table 4).
Figure 6 reveals an antagonistic behaviour between KI and aspartame at 60 °C, 70 °C, and 80 °C but a synergistic effect at 90 °C.Unlike the aspartame-SDS system, the S value for the aspartame-KI mixture is very close to unity meaning better synergy between aspartame and KI than SDS.Aramaki and Hackerman 46 reported a similar antagonistic effect between iodide ions and cyclic imine and attributed it to interference in the adsorption process at the anode.Iodide ions, as explained in section "Studies on individual inhibition performance by additives" are preferentially adsorbed on the anode at the expense of chloride ions 51 .The higher recharging power of iodide ions facilitates the attraction of more aspartame cations onto the anodic region.Aramaki and Hackerman 46 explained that a high concentration of I − ions (in other words, a thick adsorbed iodide ion layer) could interfere with the electron pair donation and acceptance process on the anode.This may have been the case at 60-80 °C.At 90 °C, because a significant amount of I − ions have been oxidized to I − 3 ions, interference of the adsorbed I − ions with the donation and acceptance process between aspartame and Fe on the anode may have been reduced while the stabilization by diminishing the coulombic repulsion between the cationic aspartame and the charged steel surface enhances their synergism.
In the formulation system, a strong synergistic effect is observed at 80 °C and 90 °C.Individual dominance may have been minimal in the ternary system which ensured their cooperative co-adsorption resulting in the observed high inhibition performance.
Surface observation.SEM and EDX studies.The surface of the T95 steel specimen before and after immersion in 15 wt% HCl solution without and with 6.80 mM aspartame alone and in combination with 1 mM KI or 1 mM KI + 1 mM SDS at 60 °C and 90 °C for 4 h was analyzed with SEM and EDX.The SEM micrographs of the T95 sample before and after corrosion at 60 °C and 90 °C are shown in Fig. 7.The composition of the steel surface products at 90 °C from EDX analysis is displayed in Fig. 8. Figure 7a exemplifies the smooth surface morphology of the T95 steel sample after mechanical abrasion which based on Fig. 8a has 85.9 wt% Fe, 8.6 wt% Cr, 3.6 wt% C, 1.1 wt% Mo, 0.4 wt% Si, 0.3 wt% Mn, and 0.1 wt% Ni in conformity with the chemical composition of T95-2 steel grade 12 .The steel underwent severe corrosion in the acid solution that resulted in surface deformation (Fig. 7b,c).Heaps of porous and cracked corrosion product is seen on the surfaces shown in Fig. 7b,c.Relative to the EDX spectrum in Fig. 8a,b, reveals the presence of a significant amount of Cl (20.5 wt%), an increase in the wt% of O and C to 11.0 and 10.5, respectively and a decrease in the wt% of Fe to 47.0.This observation implies that the corrosion product is a mixture of chlorides, oxides, hydroxides, and carbonates (Fig. 8b) which aligns with previous reports 45 .The presence of N and the increase in the wt% of C (12.7) in Fig. 8c compare to Fig. 8b provide experimental evidence that the product in Fig. 7d,c is adsorbed aspartame molecules.The significant decrease in the wt% of Cl from 20.5 (Fig. 8b) to 16.3 (Fig. 8c) indicates corrosion inhibition by the adsorbed aspartame molecules.However, an inspection of Fig. 7d, c reveals that aspartame alone could not satisfactorily protect the steel surface, especially at 90 °C.A worn-out and cracking surface morphology is seen in Fig. 7c.In agreement with the other experimental results (Tables 3, 4, 5), the addition of KI enhanced the inhibiting ability of aspartame but adding KI and SDS to aspartame benefitted the inhibiting property of aspartame the most, especially at 90 °C.As could be seen in Fig. 7f-i, the adsorbed surface product is compact and more evenly distributed at 90 °C. Figure 8d,e reveal a substantial decline in the Cl concentration on the aspartame-KI (decreased to 4.8 wt%) and the formulation (decreased to 4.4 wt%) protected surfaces which again confirm the effectiveness of the formulation as an acidizing corrosion inhibitor.OP studies.Figure 9 shows the OP images of abraded T95 steel (a) before and after immersion in 15 wt% HCl solution (b,c) without inhibitor, (d,e) containing 6.80 mM aspartame, (f,g) containing 6.80 mM aspartame + 1 mM KI, (h,i) containing 6.80 mM aspartame + 1 mM KI + 1 mM SDS (formulation) at 60 °C (b,d,f,h) and 90 °C (c,e,g,i) for 4 h.Generally, the surface property can be quantified using roughness parameters.Commonly used roughness parameters are the average roughness (R a ), root mean square roughness (R q ), and the total height of profile (R t ) and give information on the irregularity of deviation from an ideal smooth surface 52 .The value of R a , R q , and R t for the studied steel surface is given in Table 6.R a , R q , and R t value of 0.154 µm, 0.186 µm, and 2.119 µm, respectively was recorded for the smooth abraded surface in Fig. 9a.The rougher surface after corrosion in the blank acid solution can be picturized in Fig. 9b,c.The R a , R q , and R t values increase by 93% and 96% at 60 °C and 90 °C, respectively.For instance, the R a value increases from 0.154 to 2.269 and 4.350 µm at 60 °C and 90 °C, respectively.The surfaces in Fig. 9f-i are smoother compared to the surfaces in Fig. 9b,c due to the adsorption of the inhibitors and the protection of the surface against corrosion.It is again worth emphasizing the outstanding inhibitive performance of the formulation at 90 °C.As could be seen in Fig. 9i, the surface is very smooth when compared to the aspartame-KI surface in Fig. 9g.In Table 6, the R a value is seen to decrease from 4.350 µm at 90 °C to 0.550 µm for the formulation-inhibited surface corresponding to a 79% reduction.It is therefore concluded that the formulation is highly effective against the corrosion of T95 steel in 15 wt% HCl solution at high temperatures.

Summary and conclusions
The inhibiting ability of aspartame, a natural sweetener in combination with potassium iodide and sodium dodecyl sulphate was explored against the corrosion of a typical oil well tubing material (T95-2 grade) in 15 wt% HCl solution at 60-90 °C.The weight loss, electrochemical (EFM, EIS, PDP) and surface analysis (SEM, EDX, OP) techniques were used in the investigation.The findings from the investigation are summarized as follows.

Figure 3 .
Figure 3. Nyquist electrochemical plots for T95 corrosion in 15 wt% HCl solution without and with 6.80 mM aspartame alone and in combination with 1 mM SDS or 1 mM KI or both (formulation) at different temperatures.

Figure 4 .
Figure 4.The equivalent circuit was used in the analysis of (a) all the electrochemical impedance data and (b) the data for blank at 90 °C.

Figure 5 .
Figure 5. Potentiodynamic polarization plots for T95 corrosion in 15 wt% HCl solution without and with 6.80 mM aspartame alone and in combination with 1 mM SDS or 1 mM KI or both (formulation) at different temperatures.

Figure 6 .
Figure 6.Variation of synergism parameter with temperature.

Table 2 .
Weight loss (WL), corrosion rate (v), surface coverage (θ), and inhibition efficiency (η WL ) for T95 corrosion in 15 wt% HCl solution in the absence and presence of various concentrations of potassium iodide (KI) and sodium dodecyl sulphate (SDS) at different temperatures from weight loss technique.KI, potassium iodide; SDS, Sodium dodecyl sulphate.ConcWL ± S.

Table 4 .
EIS parameters were obtained for T95 corrosion in 15 wt% HCl at different temperatures without and with 6.80 mM aspartame alone and in combination with additives.a R w value; ASP, Aspartame; KI, potassium iodide; SDS, sodium dodecyl sulphate; Formulation = ASP + KI + SDS.

Table 5 .
Potentiodynamic polarization parameters derived for T95 corrosion in 15 wt% HCl at different temperatures without and with 6.80 mM aspartame alone and in combination with 1 mM KI or 1 mM SDS or both.ASP, aspartame; KI, potassium iodide; SDS, sodium dodecyl sulphate; Formulation = ASP + KI + SDS.

Table 6 .
Surface parameters derived from profilometer surface analysis of the corroded T95 immersed in 15 wt% HCl without and with additives at 60 °C and 90 °C after 4 h of immersion.ASP, aspartame; KI, potassium iodide; Formulation = ASP + KI + SDS.