Structured Thin Films Based on Synergistic Effects of MnTa 2 O 6 Oxide and bis -Carboxy-phenyl-substituted Porphyrins, Capable to Inhibit Steel Corrosion

: Covering steel surfaces with suitable materials with the capacity to protect against corrosion represents a challenge for both research and industry, as steel, due to its paramount utility, is the most recycled material. This study presents the realization of new sandwich type materials based on 5,10-(4-carboxy-phenyl)-15,20-(4-phenoxy-phenyl)-porphyrin or 5,15-(4-carboxy-phenyl)-10,20-diphenylporphyrin and MnTa 2 O 6 designed to improve corrosion inhibition of steel in aggressive media. The thin ﬁlms, designed as single- or sandwich-type structures were obtained on carbon steel through the drop-casting technique. Morphological investigations of thin ﬁlms were carried out by ﬁeld emission-scanning electron microscopy (SEM) and atomic force microscopy (AFM). The inhibition of a steel corrosion process was evaluated in an aggressive environment of 0.1 M HCl by performing electrochemical investigations such as open circuit potential (OCP) and the potentiodynamic polarization technique. The inﬂuence of variations in the cathodic Tafel slopes β c and anodic Tafel slopes β a over the corrosion rates was discussed. The best corrosion inhibition efﬁciency of 91.76% was realized by the steel electrode covered with sandwich-type layers of 5,15-(4-carboxy-phenyl)-10,20-diphenylporphyrin on the bottom layer and MnTa 2 O 6 on the top. The effect of location of the COOH groups in the cis or trans position on the tetrapyrrolic ring was also discussed to understand the corrosion inhibition mechanism.


Introduction
Carbon steel and its alloy derivatives play a vital role in many domains of engineering, such as constructions, all kinds of machinery, petroleum refineries equipment, and pipes due to their excellent resistance, and both physical and mechanical properties [1,2]. One of the main drawbacks of using equipment made of carbon steel is its chemical resistance in an acid environment [3][4][5][6][7][8][9][10]. Steel corrosion inhibitors bring significant benefits when used in descaling or in oil-well-acidizing treatments and in cleaning procedures using acids [11][12][13][14][15][16][17][18]. Among commercially available acids, the most frequently used is hydrochloric acid. Carbon steel and its alloys react very easily in acidic environments, producing large economic losses [19]. The application of the corrosion inhibitors [8][9][10][11][12] leads to a significant reduction in the corrosion of steel in an aggressive acid medium, increasing the feasibility and lifetime of the equipment and installations. Several organic and inorganic compounds deposited as protective layers on metallic surfaces have the desired effect of decreasing the corrosion rate [20,21]. The adsorption of inhibitors on metallic surfaces to uniformly create covering and adherent layers depends on certain physical-chemical properties of the inhibitor, as follows: electronic structure of the whole molecule, nature of grafted functional groups and the capacity to self-aggregate and generate supramolecular architectures [22,23]. Regarding their mechanism for inhibiting corrosion [24], the inorganic inhibitors usually act as anodic or passivation inhibitors or by diminishing the cathodic reaction of the steel by restricting oxygen diffusion. Besides these two mechanisms, the organic inhibitors act also through surface adsorption, thus providing a shield against the steel corrosion [25]. Flat compounds that contain nitrogen atoms, as porphyrins do, can block the corrosion sites with extensive π-bonds [26].
The main aim of the present study is to test the steel corrosion inhibition provided by thin layers of two different A 2 B 2 porphyrins, containing two COOH-functional groups, namely: 5,10-(4-carboxy-phenyl)-15,20-(4-phenoxy-phenyl)-porphyrin or 5,15-(4-carboxyphenyl)-10,20-diphenylporphyrin(structures in Figure 1 a and b)in association with MnTa 2 O 6 pseudo-binary oxide. In order to achieve a high impact, an acid environment of 0.1 M HCl was chosen to perform the corrosion tests. The inhibiting single or sandwich type layers were obtained by drop-casting technique on electrodes of carbon steel.

Electrode
The Before each thin film's deposition, the surfaces of (Ols) were finely polished with emery paper (NIKKEN, Tokyo, Japan) to obtain a mirrorlike surface, rinsed with double-distilled water and degreased with ethanol (99.6%, Merck, Darmstadt, Germany).

Method for Electrode Preparations
The electrochemical investigations of the capacity of the realized thin films to protect against corrosion were performed on a Voltalab Model PGZ 402 potentiostat, equipped with an electrochemical cell, made of a working electrode (consisting in bare (OL) for control or drop-casted modified steel disks), a counter-electrode of platinum wire (Metrohm Analytics Romania SRL, Bucharest/Romania) and a saturated calomel reference electrode (SCE) (Metrohm Analytics Romania SRL, Bucharest/Romania). All potentials were referenced to the standard hydrogen electrode (SHE) (Metrohm Analytics Romania SRL, Bucharest/Romania). The potentiodynamic polarization measurements were recorded at 23 • C ± 1 • C, by sweeping the potential in the -1.3 V ÷ -0.6 V domains at a scan rate of 1 mV/s. The corrosive environment was a solution 0.1 M HCl (0.1 N, Merck, Darmstadt, Germany). The open circuit potential (OCP) of the protected electrodes was monitored around 30 min before polarization. The electrodes were fixed into a Teflon (SC NEAGOE FLUOROPOLIMERI SRL, Brasov, Romania) body to obtain a precise active surface of 0.28 cm 2 . The measured parameters, such as corrosion potential (E corr ), corrosion current density (i corr ), polarization resistance (R p ), corrosion rate (ν corr ), the anodic (β a ) and the ca- thodic Tafel slope (β c ), were calculated using VoltaMaster 4 v. 7.09 software. The inhibition efficiency (IE%) was calculated based on the previously reported equation [35].

Apparatus
The morphologic characteristics of the single-and sandwich-deposited layers were analyzed by field emission-scanning electron microscopy-(SEM)/EDAX (Model INSPECT S) (FEI Company, Hillsboro, OH, USA) with two types of signal-detecting backscattered electrons and secondary electrons (EDS detector), using a low vacuum at a magnification = 100×, high voltage = 25.00 KV and weight distance = 10.6 mm and atomic force microscopy (AFM) apparatus (Nanosurf ® EasyScan2, Liestal, Switzerland) using the non-contact mode cantilever (image scan size of 1.1 µm × 1.1 µm, time/line = 1 s, points/line = 1024).
Using the previously reported equations [36] and the Nanosurf ® EasyScan 2 soft, the values of average roughness S a and the mean square root roughness S q were calculated for each sample. The thickness of the layers (S y ) was also calculated for each deposited layer before and after the corrosion tests. The electrode surface had an area of 1.326 pm 2 . Particle size analysis was performed with the AFM software.

Results and Discussion
2D AFM images of the surfaces for each of the covering layers deposited on OL electrodes, taken before and after corrosion tests, using a non-contact mode cantilever, are presented in Figure 2.
Darmstadt, Germany). The open circuit potential (OCP) of the protected electrodes was monitored around 30 min before polarization. The electrodes were fixed into a Teflon (SC NEAGOE FLUOROPOLIMERI SRL, Brasov, Romania) body to obtain a precise active surface of 0.28 cm 2 . The measured parameters, such as corrosion potential (Ecorr), corrosion current density (icorr), polarization resistance (Rp), corrosion rate (νcorr), the anodic (βa) and the cathodic Tafel slope (βc), were calculated using VoltaMaster 4 v. 7.09 software. The inhibition efficiency (IE%) was calculated based on the previously reported equation [35].

Apparatus
The morphologic characteristics of the single-and sandwich-deposited layers were analyzed by field emission-scanning electron microscopy-(SEM)/EDAX (Model IN-SPECT S) (FEI Company, Hillsboro, OH, USA) with two types of signal-detecting backscattered electrons and secondary electrons (EDS detector), using a low vacuum at a magnification = 100×, high voltage = 25.00 KV and weight distance = 10.6 mm and atomic force microscopy (AFM) apparatus (Nanosurf ® EasyScan2, Liestal, Switzerland) using the non-contact mode cantilever (image scan size of 1.1 µm × 1.1 µm, time/line = 1 s, points/line = 1024).
Using the previously reported equations [36] and the Nanosurf ® EasyScan 2 soft, the values of average roughness Sa and the mean square root roughness Sq were calculated for each sample. The thickness of the layers (Sy) was also calculated for each deposited layer before and after the corrosion tests. The electrode surface had an area of 1.326 pm 2 . Particle size analysis was performed with the AFM software.

Results and Discussion
2D AFM images of the surfaces for each of the covering layers deposited on OL electrodes, taken before and after corrosion tests, using a non-contact mode cantilever, are presented in Figure 2.

Before corrosion test
After corrosion test (g) 5,15-(4-carboxy-phenyl)-10,20-bis-phenylporphyrin/MnTa2O6. The particle dimensions and nano-roughness of the surfaces are presented in Table  2. The particle dimensions and nano-roughness of the surfaces are presented in Table 2. As in Table 2, MnTa 2 O 6 single layer had the smoothest surface, characterized by the smallest roughness value and, in the case of sandwich layers, higher values were obtained (the highest values are for sandwich layers composed of trans 5,15-(4-carboxy-phenyl)-10,20-bis-phenylporphyrin/MnTa 2 O 6 ). AFM measurements data provided particle sizes ( Table 2) that vary between 23 and 45 nm, with the last value being obtained for the sample with the highest roughness value. As can be observed from Table 2, the differences obtained before and after the corrosion tests for Sa, Sq and Sy are considerably smaller in the case of steel covered with a sandwich layer consisting of 5,15-(4-carboxy-phenyl)-10,20-bis-phenylporphyrin/MnTa 2 O 6 , meaning that the layer was sufficiently stable in the aggressive medium [37]. Figure 3 shows the most significant morphological aspects of the deposited thin films f and g after performing the corrosion tests. These features (Figure 3a,b) were examined by SEM, and recorded at a magnification of 100×. The sandwich structures present a larger density of particles at the surface. There is a strong relationship between the SEM images aspect and the values for rugosity in the two samples. The non-uniform-type architectures of sample a have a higher value of rugosity [38] than that of sample b, which uniformly displays crystals organized in multiple layers. The evolution of the OCP versus time measurements (Figure 4) were performed for the bare and modified carbon-steel disk electrodes in 0.1 M HCl solution for 30 min. As can be seen in Figure 4, an initial analysis of these curves shows that, in all cases, including bare electrode, a 30 min exposure time leads to a shift in the free potential toward more negative values. The OCP measurements (Figure 4) showed that the covered electrodes stabilize at around 400-500 s, while the uncoated electrode stabilizes at around 1700 s. This result provides preliminary information on the nature of the processes occurring at the interfaces between metal/thin film and the electrolyte. The OCP is a qualitative indicator of the state of corrosion of the steel substrate in acid medium and helps to determine the immersion time required to reach the steady state [39]. Figure 5 are shows the Tafel plots of the investigated OL electrodes recorded after 30 min OCP in 0.1 M HCl solution. The slopes were determined in the Tafel region of the anodic and cathodic curves before and after reaching the corrosion potential (U). The Tafel parameters, which were calculated using VoltaMaster 4 v. 7.09 software, are presented in Table 3. The degree of inhibition efficiency IE (%) was calculated using the following equation [34].
In Table 3, where the calculated parameters from the Tafel plots are summarized, the corrosion potential (E corr ) of the OL electrode is −406.3 mV and the corresponding corrosion current density (i corr ) is 0.9548 mA/cm 2 . All the thin films deposited in single or sandwich structures on electrodes display significantly lower corrosion current densities in comparison with the OL disk.   The polarization resistance (R p ) increases from 101.35 Ω × cm 2 , which is characteristic of bare OL, to 131.64 Ω × cm 2 in the case of 5,15-(4-carboxy-phenyl)-10,20-bisphenylporphyrin/MnTa 2 O 6 . Table 3 presents the recorded Tafel slopes β a and β c . The cathodic Tafel slopes β c are found to increase more significantly than the decreases in the anodic Tafel slopes β a . The anodic reaction can be presumed to be dependent on activation polarization. Nevertheless, the cathodic reaction cannot be neglected, resulting in further polarization. As β a is decreasing at a slower rate than the β c is increasing, the difference in potential between the anodic and cathodic regions is also decreasing and, thus, a lower corrosion rate occurs, as is visible in our case (see Table 3) [36]. The values of the corrosion inhibition efficiencies are influenced by the thickness of the protective layers. The highest inhibition efficiency of 91.76% was found in the case of the OL protected with the 5,15-(4-carboxy-phenyl)-10,20-bisphenylporphyrin/MnTa 2 O 6 successive layers; this also presented the smallest difference before and after the corrosion tests for Sa, Sq and Sy, meaning that the layer was sufficiently stable in the aggressive medium (see Table 2). This result can also be correlated with SEM image (Figure 3b) that reveals the uniform coverage of the particles over the steel surface.
Analyzing the variation in corrosion potential and corrosion current as the inhibition efficiency is increasing (Table 3), it can be noted, as expected, that the corrosion current is decreasing proportionally to the corrosion rate, and the corrosion potential is shifted to more cathodic values. If a cathodic reaction is involved in the corrosion process, the corrosion potential becomes more negative, as noted in our case. The deposited couple of porphyrin/oxide inhibitors generate a barrier over the metal, thus limiting the contact between the steel and the corrosive acid medium, and finally decreasing the corrosion rate [40].
Another significant but expected result is that the porphyrin 5,15-(4-carboxy-phenyl)-10,20-bisphenylporphyrin, with a smaller steric hindrance and the possibility of creating more continuous supramolecular aggregates, due to the presence in trans of two COOH groups, offered the best corrosion protection of the steel surface [31]. Therefore, the trans-COOH porphyrin proved to be better suited to our purpose than the porphyrin with the COOH groups in cis vicinal position.
The mechanism by which the inhibitors hinder the corrosion of the carbon steel is presumed to be simply based on the physical protection effect caused by the compact and adherent layers of the two bis-carboxyl-substituted porphyrins. These form large supramolecular architectures that are chemically resistant to acids (see Figure 6), thus avoiding direct contact between the acid and the steel [41]. This mechanical barrier effect is because the compact layers of triangular building blocks aggregated both in J and H-type processes is more pronounced in the case of trans-porphyrin (Figure 6a) that in the cisderivative case (Figure 6b). The cis-porphyrin is helicoidally aggregated in small rings that join together but leave space for little pores and large voids (Figure 6b), which can form places of contact between acid and metal and points at which pitting corrosion begins [42]. As can be seen from the SEM images of the cross-sectional sandwich f sample, MnTa 2 O 6 /5,15-(4-carboxy-phenyl)-10,20-bis-phenylporphyrin (Figure 7), there is no cleavage between the steel surface and the covering layers, so it can be expected to have very good mechanical protection against the aggressive HCl medium.

Conclusions
The drop-casting deposition of single and sandwich coatings consisting of MnTa 2 O 6 pseudo-binary oxide nanomaterials in association with 5,10-(4-carboxy-phenyl)-15,20-(4phenoxy-phenyl)-porphyrin or 5,15-(4-carboxy-phenyl)-10,20-bis-phenylporphyrin on carbon steel electrodes were realized. The AFM analysis reveals that the main morphological features present in the single layers are preserved in the sandwich layers. Correlating the results obtained from corrosion tests with the features revealed by AFM analysis, it can be concluded that the corrosion inhibition degree increases with the decreasing difference obtained for Sa, Sq and Sy roughness, before and after performing the corrosion tests. The best value for corrosion inhibition was exhibited by the OL covered with a mixed layer of 5,15-(4-carboxy-phenyl)-10,20-bis-phenylporphyrin and MnTa 2 O 6 on top, i.e., 91.76% inhibition efficiency. Another significant but expected result is that the porphyrin 5,15-(4-carboxy-phenyl)-10,20-bisphenylporphyrin with a smaller steric hindrance and the possibility to create-due to the presence in trans of two COOH groups-more continuous supramolecular aggregates, offered the best corrosion protection on the steel surface. Therefore, the trans-COOH porphyrin proved to be better suited to our purpose than the porphyrin with the COOH groups in cis vicinal position. Funding: This research was funded by UEFISCDI, grant number PN III nr. 528PED/2020 "Hybrid ceramics/porphyrins, deposited by pulsed laser deposition as single and sandwich layers for corrosion inhibition of steels in acid environment" and partially by Romanian Academy through Programme 3/2021 from the Institute of Chemistry "Coriolan Dragulescu".
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.

Conflicts of Interest:
All authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.