RETRACTED ARTICLE: Chemical, electrochemical and surface studies of new metal–organic frameworks (MOF) as corrosion inhibitors for carbon steel in sulfuric acid environment

The effects of [Co2 (SCN) 4(hmt)2(H2O)6. H2O] (SC1) and [Co (CN)6 (Me3Sn)3(H2O). (qox)] (SCP2) MOF as corrosion inhibitors on C-steel in 0.5 M sulfuric acid solutions are illustrated utilizing mass reduction (MR), electrochemical [potentiodynamic polarization (PP), and AC electrochemical impedance (EIS)]. The experiments revealed that as the dose of these compounds rose, the inhibition efficacy (IE percent) of C-steel corrosion improved, reaching 80.7–93.1% at dose 25 × 10−6 M for SC1 and SCP2, respectively. IE percent, on the other hand, dropped as the temperature range grew. SC1was adsorbed physically and chemically (mixed adsorption) but SCP2 was adsorbed physically on the surface of C-steel and conformed to the Langmuir adsorption isotherm equation. The PP studies revealed that these compounds act as mixed kind inhibitors. To establish the morphology of the inhibited C-steel surface, scanning electron microscopy (SEM), energy transmitted X-ray (EDX), and atomic force microscopy (AFM) studies were used. All tested experiments were in good agreement.

(SC1). In the presence of ultrasonic radiation, red crystals of [Co 2 (SCN) 4 (hmt) 2 (H 2 O) 6 . H 2 O], SC1, were produced. In an ultrasonic bath, a 0.118 g (0.5 mmol) solution of CoCl 2 .6H 2 O dissolved in 10-mL bidistilled H 2 O was gradually added to a stirred mixture of 0.099 g (0.5 mmol) trimethyl tin chloride and 0.097 g (1 mmol) KSCN in 10 mL of CH 3 CN/deionized H 2 O. Following a few minutes of magnetic stirring, a solution of 0.07 g (0.5 mmol) hexamethylenetetramine (hmt) in 10 mL H 2 O was added to the mixture drop by drop. The resulting mixture was ultrasonically treated for 6 h at 30 °C with a power of 60 W. Filtering, precipitation, washing with 10 mL H 2 O, and drying in the open air were then used to separate the precipitate SC1 had been obtained in the amount of 367 mg (94.2 percent). SC1, C 16     to + 700 mV against (Eocp). At a scan rate of 1 mVs -1 , the power was measured.

Measurement of EIS.
All open-circuit testing with EIS were carried out with AC signals ranging from 100 kHz to 0.1 Hz and peak amplitudes of 10 mV at open circuit potential (OCP). The equipment used in electrochemical experiments was a "Gamry Potentiostat/Galvanostat/ZRA" (PCI4-G750). Gamry comprises the DC105 DC Corrosion Program, the EIS300 EIS Program, and a data gathering computer. To plot and compute data, Echem Analyst version 5.5 was used".

Morphology of the surface. Attenuated Total Reflection Infra-Red (ATR-IR) analysis. ATR-IR spectra
were recorded in the spectral region "4000 to 500 cm −1 " using the Attenuated Total Reflectance (ATR ) technique on an FTIR-Spectrometer iS 10. (Thermo Fisher Scientific, USA). The FT-IR spectrum is a useful tool for comparing inhibitor and corrosion products following inhibitor adsorption. After immersion for a period, the FT-IR peak values for metal-organic and C-steel were obtained. After 24 h of immersion in the acid corrosive solution with 25 × 10 −6 M of metal-organic, the peak values of the FT-IR were recorded for metal-organic and C-steel 32 .
Atomic force microscopy (AFM) analysis. AFM is a modified test that provides data on the surface of a C-steel sample with metric linear purity. Persecution is used to apply and appraise measured knowledge 33 . Adapted from the SPM management computer code 34 .  Table 3 contains the lattice constants and refinement parameters of SC1, whereas Table 4 has the bond lengths and angles. SC1's structure displays two unique complexes composed of two crystallographic and chemically distinct Co II atoms, one hmt molecule, two thiocyanate ligands, three coordinated water ligands, and one H 2 O molecule of crystallization (Fig. 1a). SC1's unit cell structure, on the other hand, consists of two neutral complexes with two different Co II atoms, four thiocyanate ligands, and one thiocyanate Two hmt ligands, six coordinated water molecules, and one uncoordinated water molecule (Fig. 1b). The Co1 atom exhibits octahedral shape based on bond lengths and angles (Table 4, Fig. 1b). The Co-N-C angle shows bent structure (163.93°). To create the OC-6 structure, the Co2 atom coordinates with two thiocyanate groups in apical positions and four water molecules in an equatorial plane geometry, which is maintained by bond lengths and angles (  Table S4 shows that the C-Co-C angles imply an octahedral shape of the Co III core. The Co (CN) 6 building blocks are the primary components that make up the host network that is bridged by the Me 3 Sn + cations (in the suplimintary file). Tin atoms are coupled to the nitrogen ends of the cyanide groups, resulting in a trigonal bipyramidal structure. Surprisingly, the Sn3 atom has a distinct crystallographic structure than the Snl and Sn2 atoms. The Sn3 atom coordinates with three methyl ligands to create the Tp-3 configuration, whereas the N3 atom and one water molecule are located at axial locations, as shown in Fig. 2. As a result, the Me3Sn1 and Me3Sn2 cations act as connectors between the Co (CN) 6 building blocks, resulting in 1D-coordinated chains (Fig. S3). while the Me3Sn3 cation structure ends with an H 2 O ligand, which helps in the formation of H-bonds (1.988-3.085) and-stacking (qox-O = 2.771). Surprisingly, five of the cyanide ligands behave as 2-ligands, while the C5N5 ligand has a free uncoordinated nitrogen end that may make H-bonds with the guest qox molecules (2.703-2.735) and water molecules (2.739). SCP2's structure propagates three-dimensionally based on infinite, but nonlinearly coordinated -[-Co-CN-Sn-NC-Co]-chains that cross each other at quasi-octahedral Co sites, as seen in Fig. S4 9 ] ring, as illustrated in Fig. S5. The network space comprises methyl groups and guest qox molecules in addition to the coordinated water group, resulting in a stunning structure.

Mass reduction (MR) tests.
The mass loss which calculated from MR is given by Eq. (1): W 1 , W 2 are the weights of the C-steel specimens before and after reaction with solution. Equation 2 was used to calculate the IE percentage: Table 3. Crystal data and structure refinement parameters of SC 1.  www.nature.com/scientificreports/  www.nature.com/scientificreports/ where ΔW and ΔWi represent the MR per unit area in the absence and presence of prepared samples, respectively. This measurement was performed in accordance with ASTM standard G 31-72 35 . The MR-time curves for C-steel in the presence and absence of changed dosages ranging from 5 × 10 -6 to 25 × 10 -6 M for SC1 and SCP2 are shown in Fig. 3. The k corr grew as the temperature increased, therefore the k corr increased while the IE percent decreased. The curves in the presence of inhibitors are lower than those in the absence of inhibitors. The higher IE percent with increased dosage of metal-organic compounds can be attributed to the formation of an inhibitor layer on the C-steel surface via adsorption. This layer is formed by the free electron pairs on the oxygen and nitrogen atoms of metal-organic compound molecules, as well as the π-electrons of aromatic rings. The reduction in IE percent with rising temperature is most likely due to a higher rate of desorption, which is physical adsorption; the IE percent order was: SC1 > SCP2 Table 5 for example, shows the IE percent and k corr at various doses of metal-organic SC1 of C-steel at temperatures ranging from 298 to 318 K for 120 min immersion. As seen in the Table, raising the temperature lowers the % IE while raising the inhibitor doses raises it.
Temperature influence on corrosion procedure. The activation energy E a * , which can be derived from Eq. (3), is an essential component that influences the speed of reaction and the kind of adsorption.
where k corr is the corrosion rate. Figure 4 depicts Arrhenius diagrams for SC1 and SCP2 [log (k corr ) versus 1/T], where the E * a energy of the activation of the results was obtained in Table 6, It suggests that the surface reaction dominates the overall activity since the activation corrosion process (E a * ) is more than (20 kJ mol −1 ) and the activation energy increases as the dosage of metal-organic compound increases. Energy rises as the dose of metal-organic compound increases, it appears that the surface reaction dominates the overall activity. The adsorption nature of metal-organic compounds on C-steel causes this rise, which correlates to the physical adsorption of metal-organic compounds [36][37][38][39] . The transitional state equation was used to calculate the changes in entropy and enthalpy. The activation enthalpy (ΔH * ) and entropy (ΔS * ) increases for C-steel corrosion in 0.5 M H 2 SO 4 are calculated using the equation below: where symbol "h" is the Planck's constant and N is the Avogadro's number. Graph of log (k corr /T) versus (1/T) for unprotected C-steel at 0.5 M H 2 SO 4 and in the existence of metal-organic compounds is shown in Fig. 5, which gave straight lines with slope equal (− ΔH * /2.303R) and an intercept equal (log R/Nh − ΔS * /2.303R) from which ΔH * and ΔS * data were calculated and depicted in Table 6. Negative results for (ΔH * ) on the C-steel surface, indicating that the reaction that occurs during the dissolving process is exothermic, and it is known that they may be used to chemical and physical adsorption [40][41][42] . The mean values (ΔS * )" are both high and negative, indicating that the activated complex is associated rather than dissociated during the rate-determining stage.
Adsorption isotherm behavior. Studding of adsorption isotherms help us to explain the reaction occurred among the C-steel surface and metal-organic additives. It is deduced that θ increased with raising the inhibitor dose; this is because of the adsorption of metal-organic additive molecules on the C-steel surface. It is also supposed that the adsorption of the studied metal-organic additives is proceeding with the monolayer adsorption so that the adsorption process may obeys Langmuir isotherm. The Cinh/relationship dependence for  www.nature.com/scientificreports/ SC1 and SCP2 is shown in Fig. 6, because of the dosage of metal-organic compounds (C inh ) obeying Langmuir isotherm adsorption.
where K ads is the equilibrium adsorption constant intricate in chemical reaction In which the free adsorbent energy is stimulated by a 55.5 dosage of molar water in solution. The data pattern revealed a negative sign of G • ads due to the spontaneous and stable adsorbed layer on the metal surface 43 . The adsorption characteristics for the metal-organic compounds found are shown in Table 7. The free energy findings show that the kind of adsorption for SC1 is physical and chemical adsorption but physical in case of SCP2, since it is known that negative values are greater than 20 kJ mol −1 and less than 40 kJ mol −1 for SC1. The G • ads values ranged between − 22.7 and − 23.1 kJ mol −1 , suggesting physical and chemical adsorption (mixed adsorption), but between 21 anf 21.5 kJ mol −1 for SCP2 which showed that it adsorbed on C-steel surface physically. The enthalpy of adsorption, H • ads , was determined using the Vant Hoff equation: Figure 7 shows plotting of log K ads with 1/T for C-steel in 0.5 M H 2 SO 4 with SC1. The negative sign of the H • ads value indicates that the adsorption process is exothermic. Adsorption can be physical or chemical in an exothermic process, while it can only be chemical in an endothermic process. Finally, the following equation may be used to calculate S • ads .
(5) C/θ = 1/ K ads + C    Electrochemical measurements. PP measurements. PP diagrams of C-steel in 0.5 M sulfuric acid in the existence and absence of altered doses of metal-organic compounds at 298 K are shown in Fig. 8. From this Figure we see that Tafel extrapolation obtained the electrochemical parameters at E corr and were depicted in Table 8. The current density reduced as the accumulation of inhibitors increased. According to the results of the tests, β c is somewhat greater than β a , suggesting that the inhibitors favor cathodic rather than nodic action. As a result, these inhibitors function like a combination of inhibitors. Also, E corr change slightly (less than ± 85 mV) which confirm that these compounds exert on both cathodic (hydrogen reduction) and anodic (metal dissolution) processes. The efficacy of inhibition (IE%) was determined from the curves of polarization as in Eq. (9):   44,45 . The parallel Tafel lines with and without inhibitors indicate that there is no change in corrosion mechanism.
Electrochemical impedance spectroscopy (EIS) measurements. Figures 9 and 10 show the C-steel Nyquist and Bode diagrams at OCP in the presence and absence of different dosages of metal-organic SC1 and SCP2 at 298 K. The circuit that represents metal organic compounds and electrolyte is presented in Fig. 11, with R s as the solution resistance. The impedance spectra show that the diameter decreases as the dose of studied inhibitors rises. The interfacial capacitance C dl values can be estimated from CPE parameters (Y 0 and n) and is defined in Eq. (10) 46-50 : where Y 0 is the CPE magnitude, and n is the variance CPE data of the: − 1 < n < 1. Using Eq. (10). Table 9 shows the impedance data that established the data of R ct increasing with increasing the dosage of the metal-organic (10) C dl = Y 0 (ω max ) n−1  www.nature.com/scientificreports/   www.nature.com/scientificreports/ compounds, pointing to an increase in IE percent. This might be due to an increase in the thickness of the adsorbed layer caused by increasing the metal-organic compound dosages. The Table also shows that (n) value varies directly with SC1 and SCP2 dosages. (n) value is a measure of surface roughness 51 , and its rise might indicate a reduction in the heterogeneity of the metal surface caused by SC1 and SCP2 adsorption. The inclusion of SC1 and SCP2 results in lower C dl values, which the Helmholtz model ascribed to an increase in the thickness of the electric double layer or/and a drop in the local dielectric constant 52 : where ε is the dielectric constant of the medium, ε° is vacuum permittivity, A is the electrode area and δ is the thickness of the protective layer. Bode graphs (Fig. 11) in the presence of inhibitors revealed that the Bode amplitude value increases over the whole frequency range with the addition of SC1 and SCP2. Equation 12 was used to get the percent IE and θ from the impedance testing: where R • p and R p are the resistances unprotected and protected metal-organic compounds, individually. Table 10 shows the values of parameters such as R s and R ct obtained from EIS fitting, as well as the derived parameters C dl and IE percent. The usual criteria for evaluating the best fit of these compounds were followed: the chi-square errors were low (χ 2 ≈ 10 -4 ) and the allowable errors of elements in fitting mode were low (5%). As a result, the utilised circuit is acceptable in this situation.     www.nature.com/scientificreports/ Surface analysis. AFM analysis. AFM in Table 10 Table 10. The values displayed that the roughness rises with adding H 2 SO 4 due to the corrosion occurs on the C-steel surface but decreased with adding the prepared 53 .
FT-IR analysis. Fourier transform infrared spectroscopy (FT-IR) identifies chemical bonds in a molecule by producing an infrared absorption spectrum. "FT-IR spectrum of the corrosion product at C-steel surface in 0.5 M H 2 SO 4 does not show any useful adsorption peaks 54 . FT-IR fingerprint spectra of the stock metalorganic SC1and the C-steel surface after dipping in 0.5 M H 2 SO 4 + 25 × 10 -6 M of metal-organic SC1 for 24 h was obtained and compared to each other it was obviously clear that the same fingerprint of metal-organic SC1solution present on C-steel surface except the absence of some functional group and it suggested to be due to reaction with H 2 SO 4 . From Fig. 13 there are small shift in the peaks at C-steel surface from the original peak of the stock inhibitor solution", these shifts indicate that there is interaction between C-steel and metal-organic (SC1&SCP2).
Corrosion inhibition mechanism analysis. Metal-organic compound inhibitors prevent C-steel corrosion primarily by adsorption on the C-steel surface, where it moves H 2 O molecules, forming a tight barrier layer 55 . Adsorption is related to inhibitor functional groups such as O, N and S, as well as the potential electronic density and steric effect of active centers, which can donate their lone electron to the d-orbital of Fe, forming a chemical bond that is characteristic of chemical adsorption as in case of SC1 and this confirmed from the values of ΔG o ads which are more than 20 kJ mol -1 . On the other hand, the surface of the C-steel sample is positively charge in aqueous acid solution 56 . The SO 4 2− ions get adsorbed on C-steel sample and turn it as negatively charged surface, the protonated inhibitor metal-organic molecules (cationic) get adsorbed on the negatively charged metal surface by an electrostatic attraction. The protonated molecules may adsorb on C-steel samples, resulting in physi-

Conclusions
The metal-organic compounds investigated have a high inhibition efficiency ranging from 93.1 to 91.2% at 25 × 10 −6 based on measurements of mass reduction as it gives linear variation of mass reduction over time. Electrochemical measurements also provide high inhibition efficiency as Tafel lines moved to higher potential regions and the EIS analysis showed a rise in R ct and a lowered in C dl as the dose of the inhibitors improved. The investigated compounds adsorption obeyed Langmuir isotherm. Thermodynamic and kinetic parameters indicated that the metal-organic compound act as mixed kind as the adsorption is spontaneous and involving physical adsorption.