Effect of methanolic plant extract on copper electro-polishing in ortho-phosphoric acid

2.1 Solution preparation

The solution 8 M H₃PO₄ was planned via analytical grade (85% w/w H₃PO₄) dilution with de-ionized water with a measured resistivity >18 MΩ/cm. Methanol was used in the natural products preparation. All chemicals were analytical grade reagents and supplied by BDH Chemicals Ltd. All experiments were operated in acid free solution and acid containing several concentrations (50 to1800 ppm) of the natural compounds.

2.2 Material sample composition

The copper composition in weight percent was as a follow: Cd: 0.001, Ag: 0.011, Pb: 0.003, Sn: 0.005, and, Cu 99.98.

2.3 Methanolic plant extracts preparation

Four different green inhibitors, Allium sativum (garlic extract), Tilia cordata (Tilia leafs), Foeniculum vulgare (fennel seeds), and Lablab purpureus (Dolichos lablab extract) were procured from the local market, washed by purify water then dried out in the laboratory at room temperature. Fresh garlic (Allium sativum) bulbs was chopped into small pieces and other green inhibitors were separately grounded using electrical mill into fine powder then stored in clean glass bottles at room temperature. About 20 g of samples were soaked in 400 ml methanol for a week (for mixing of the samples with the solvent) at room temperature, and whatman filter papers to remove undesirable solid residues and contamination.

This crude extract of each extracts was kept in refrigerator until it was used. The concentrations of crude extract solutions of each plant was determined by the remained weight of dry solid residue in conical flask after the methanol evaporation completely (and this weight equal the equivalent weight dissolved in 10 ml filtrate). By this relation we can prepare stock plant extract solution with desired concentration

2.4 Electrochemical measurements

The cell used consists of rectangular plexiglass container with a base of 15 × 5 and a height of 10 cm with copper electrodes of 10 cm height and 5 cm width. Electrode separation was 15 cm. The electrical circuit consisted of a 6 V D.C. power supply, a variable resistance and a multi range ammeter connected in a series with cell. A high impedance voltammeter was connected in parallel with the cell to measure its potential. The temperature was regulated by placing the cell in thermostatic water bath at different temperatures (20, 30, 40 and 50 °C) ± 0.5 °C

2.5 Surface investigation:

2.6 Fourier Transform-Infrared spectroscopy (FTIR)

For FT-IR studies, A Bruker Tensor 37 Fourier transform infrared spectrophotometer, instrument was used, vacuum dried plant extract liquid was used. The sample was mixed with KBr. The spectra of all investigated specimens were determined in 400–4000 cm⁻¹ range.

2.7 Gas chromatography

GC analysis was carried out using Gas Hewlett Packard HP-5890 series II with split/split less injector and capillary column (30 m, 0.25 mm, 0.25 µm) fused with phenyl polysilphenylene Siloxane. The injector and detector temperatures were set at 280 and 300 °C, and the oven temperature were kept at 80 °C for 1 min, rose to 300 °C at 20 °C/min. Helium was used as carrier gas at a constant flow of 1.0 ml/min. A volume of 2 µl was injected in the split less mode and the purge time was 1 min.

2.8 Gas Chromatography–Mass Spectrometry (GC–MS) analysis

The MS (Hewlett-Packard 5889 BMS Engine) with selected ion monitoring (SIM) was used. The mass spectrometer was operated at 70 eV and scan fragments from 50 to 650 m/z.

2.9 Identification of components

The methodology carried out for identification of individual components of plant extract was based on chromatographic retention indices and via comparing the attained mass spectra with those accessible in NIST library

3.1 Leveling process (Electro-polishing in H₃PO₄ -natural extracts free solution):

The leveling study is based on the traditional cell voltage-anodic current curves of EP as shown in Fig. 1. Atypical polarization curve is obtained for an electrolyte consisting of ortho-phoshoric acid of concentration ranged from (6–14) M as represented in (Table 1). The curve is divided into three parts which are represents electrolytic etching, polishing of copper surface and O₂ gas evolution with pitting occurs in the ascending portion of the curve. The H₃PO₄ concentration effect on the value of I_L can be interpreted via the mass transfer concept according to Eq. (1) (Landolt, 1987).

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Fig. 1

Typical copper EP polarization curve at 20 °C in H₃PO₄ acid different concentrations.

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Increasing H₃PO₄ acid concentration leads to decreases the saturation solubility of Cu₃(PO₄)₂ with a consequential diminish in I_L consistent with Eq. (1).Also, raising H₃PO₄ concentration lead to raise solution viscosity (η), which effects in a diminish of Cu²⁺ ions diffusivity (D) in proportion to Stokes-Einstein equation (D η/T = constant). In addition the augment in ( $\delta$ ) (which signify the resistance to the mass transfer rate for Cu²⁺ ions from anode surface to the solution bulk. Due to these effects I_L decreases with increasing H₃PO₄ concentration.

Mechanistic studies of EP have revealed that during copper EP in H₃PO₄ (Fig. 1), when a current passes crosswise the electrolyte, the anode surface will be coated with a viscous film. This film has superior viscosity and superior electrical resistivity than the solution bulk. The film breadth change from spot to spot: above protrusions the film is thinner than above the valleys. Therefore peaks dissolve more quickly than valleys. Also the diffusion of metal salt out of a scratch or cavity is slow compared with that over a peak. Due to the non -identical current density metal from projections dissolves more rapidly than from crevices which resulting in a surface leveling effect. Thus, the observed behavior of I_L is due to the fact that the dissolution is controlled by mass transport (Landolt et al., 2003).

In which the reaction products diffusion is limited and is in turn determining the overall reaction rate. The salt -film precipitated mechanism engages rate limiting diffusion of cations of the dissolving metal from the anode into the bulk (Datta and Landolt, 2000; Sedahmed et al., 2001). At I_L, a thin salt film with a saturated concentration of metallic cation is present on the anode surface and limits the rate at which metal ions depart the surface. Copper anodic dissolution in H₃PO₄ demonstrates a limiting current plateau. In the potential range below I_L, copper dissolution involves simultaneous formation of Cu²⁺, the Cu²⁺ formation amount becoming significant at elevated potential. In the potential range corresponding to the limiting current plateau, copper dissolution leads to formation of Cu²⁺only. At potentials higher than I_L copper dissolution is accompanied by O₂ progression.

3.2 Electropolishing of copper in H₃PO₄ acid containing different concentrations of plant extracts:

Fig. 2 illustrates polarization curves for anodic dissolution of copper in solution containing several concentrations of Allium sativum (garlic extract), Tilia cordata (Tilia leafs), Foeniculum vulgare (fennel seeds), and Lablab purpureus (Dolichos lablab extract) methanolic extract respectively. The limiting current values (I_L) were calculated by extrapolation of linear parts of the dissolution potential to the intersection of the corresponding dissolution current. Values of I_L are given in (Table 2). Appearance of a characteristic I_L plateau enlarging over a wide potential range in presence of different concentrate of plant extract, proofs that the extracts do not change the diffusion controlled mechanism of copper (mass transport controlled process) dissolution to charge transfer process.

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Fig. 2

Typical polarization curve for vertical copper plate electropolishing in 8 M H₃PO₄ in presence of different concentrations (a) Garlic(Allium Sativum),(b) Fennel seeds (Foeniculum Vulgre), (c) Tilia leaves (Tilia cordata) and (d) Dolichos lablab (Lablab purpureus) at 20 °C.

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If I_L(blank) is the limiting current in natural extract –free solution and I_L(extract) is the limiting current in natural extract containing solution, then percentage inhibition efficiency (%IE) can be calculated from the following Eq. (2):

The anodic dissolution rate I_L values and percentage inhibition efficiency (%IE) for the studied natural extract at concentration range from 100 to 1800 and temperature range of 20–50 °C with an increment of 10 °C are given in (Table 2).

Fig. 6 and Table 2 show that %IE increases as the studied methanolic extract concentrations increases and temperature increases in case of Allium sativum (garlic extract), and Lablab purpureus (Dolichos lablab extract). While in case of Tilia cordata (Tilia leafs) and Foeniculum vulgare (fennel seeds) %IE amplifies via mounting concentration and decreasing temperature. The %IE of different methanolic extracts could be arranged in the order:

• Allium sativum (garlic extract) > Foeniculum vulgare (fennel seeds) > Tilia cordata (Tilia leafs) > Lablab purpureus (Dolichos lablab extract)

The difference in inhibition efficiencies of the studied methanolic extracts may be due to the fact that each extract contains a large number of compounds (identified using GC–MS their relative percentages are reported in Table 4, which are present in different amounts depending on the type of extract.

The net effect of these compounds was a retardation of the dissolution process of copper in H₃PO₄ solution. It can be proposed that the effective compounds in these extracts contains functional groups which can be used as adsorption centers, since they contain sulphur or oxygen and may contain nitrogen atoms. The electron density on the adsorption centers and consequently the strength of the extract adsorption on the metallic surface depends on its chemical structure, its molecular size and the distribution of charge over the whole inhibitor molecule. This reflects the observed differences in the %IE between the various extracts and lead to the order:

• Allium sativum (garlic extract) > Foeniculum vulgare (fennel seeds) > Tilia cordata (Tilia leafs) > Lablab purpureus (Dolichos lablab extract)

This order indicates that the methanolic extract of the cloves of garlic bulb has the highest %IE (83.75% at 1800 ppm) on the dissolution of copper in H₃PO₄ acid. This effect may be attributed, as above mentioned, to high organo-sulphur compound content and also the presence of conjugate double bonds in these compounds as identified from GC–MS (Table 4) and FTIR (Table 3). Additionally the high Garlic retardation performance proposes powerful bonding of the effective compounds and copper surface owing to hetero-atoms (O, N, S) lone pairs electrons and π- orbital blocking the active sites and decreasing the dissolution rate. Therefore, bonding between inhibitor molecules and copper surface occurs through sharing electrons of the sulphur and oxygen atoms (OH groups) present in the identified compounds (Table 4) of the methanolic extract of garlic (Allium sativum) and vacant copper d-orbital. Also Garlic (Allium sativum) molecules can absorb on the metal surface on the basis of donor–acceptor interaction between aromatic rings π-electrons and vacant d-orbital of copper (Mahmoud, 2006; Rodriguez-Clemente et al., 2015).

Further inspection of Fig. 6 and Table 2 demonstrate that retardation effectiveness values enlarge via mounting concentration but decrease with increasing experiment temperatures for Fennel seeds (Foeniculum vulgare)and Tilia leaves (Tilia cordata), the inhibition efficiency attains a maximum of 73.33% and 66.67% at (1800 ppm) for Fennel (Foeniculum vulgare)and Tilia (Tilia cordata) respectively. Decrease in %IE with increasing temperature indicates that the molecules of Fennel (Foeniculum vulgare) and Tilia (Tilia cordata) were physically adsorbed on the copper surface.

The dissolution inhibiting effect of Fennel (Foeniculum vulgare) and Tilia (Tilia cordata) extracts can be attributed to the adsorption of effective constituents including D-limonene, β-pinene, myrecene and L-Fenchone in Fennel seeds and kaempherol, Quercetin, Tilirosid and p-coumaric acid in Tilia leaves (Tilia cordata). The effective constituents of these extracts are shown in Table 4. FTIR results also shows that Fennel (Foeniculum vulgare) and Tilia extracts (Tilia cordata) contains oxygen and nitrogen atoms in functional groups (OH, C⚌O, C—O, C⚌N, C—N, N—H, C⚌C) and aromatic rings, which meets the general consideration of typical corrosion inhibitors. In aqueous acidic solutions Fennel Foeniculum vulgare) and Tilia (Tilia cordata) may be protonated and exists in the form of cations. However, copper in H₃PO₄ is positively charged, so it is complicated for the protonated Fennel (Foeniculum vulgare) and Tilia (Tilia cordata) to approach the positively charged copper surface due to the electrostatic repulsion since phosphate being weakly adsorbed on the surface of copper there by given rise to negatively charged copper surface. The protonated component of Fennel (Foeniculum vulgare) and Tilia (Tilia cordata) may adsorb through electrostatic interaction between the positively charged molecules and negative charged metal surface (physisorption). When cations (protonated components) are adsorbed on metal surface a coordinate bond may be formed by partial transference of electrons from polar atoms to copper surface (Poornima et al., 2011). In addition owing to lone-pair electrons of polar atoms Fennel (Foeniculum vulgare) and Tilia (Tilia cordata) components may combine with freshly generated Cu²⁺ ions on copper surface forming metal inhibitor complexes. These complexes might get adsorbed onto copper surface by vander waals force to form a protective film to keep copper from dissolution. Results in Table 2 show increasing trend in inhibition efficiency values with increasing experimental temperature for Garlic (Allium sativum) and Dolichos lablab seeds (Lablab purpureus). Inhibition of copper dissolution in 8 M H₃PO₄ by Garlic (Allium sativum) and Dolichos lablab (Lablab purpureus) may be attributed to the adsorption of the effective compounds in these extracts onto the copper surface leading to dissolution inhibition phenomenon. Dissolution inhibition is initiated by the displacement of adsorbed H₂O molecules by the inhibitor leading to specific adsorption of the inhibitor on copper surface increase in %IE with temperature can be attributed to the chemisorptions of the Garlic (Allium sativum) and Dolichos lablab (Lablab purpureus) on the copper surface (Abd El-Maksoud, 2008; Xiang-Hong et al., 2010).

Finally it could be said that the synergistic interaction of the major components in the four studied methanolic extract with other chemical constituents adds for the appreciable inhibitive property of the extracts.

3.3 Characterization of methanolic natural extracts

1. Fourier Transform-Infrared spectroscopic analysis (FTIR):

The existence of the hetero-atoms such as (N, S, O), aromatic ring, beside varied functional groups (O—H, C⚌C, C⚌O) in plants methanolic extract may assist the inhibitor molecules adsorption on copper surface to reduce the copper dissolution. In this work the FTIR spectroscopy was employed to identify whether the plants extract contains some of these functional groups or not. Table 3 show some important functional groups that are present in the methanolic extracts.

The comparison between the absorbance bands observed by FTIR spectra of Allium sativum (garlic extract), Tilia cordata (Tilia leafs), Foeniculum vulgare (fennel seeds), and Lablab purpureus (Dolichos lablab extract) before and after EP is shown in Fig. 3. This absorbance bands are sometimes appear, disappear or shifted after EP to indicate the formation of copper -plant extract complex. Also it is found that almost all the peaks observed for dried plants extract before EP are also noticed for plants extract after EP.

• Obviously:

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Fig. 3

FTIR spectra of methanolic plant extract (a) before EP and (b) after EP.

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In Garlic (Allium sativum): A strong and broad peak of —OH stretching is observed at 3434.9 cm⁻¹ before EP and is also observed at 3433.7 cm⁻¹ after EP. Stretching peak —C—H is shifted from 2935.7 to 23916 cm⁻¹. Stretching peak of —N—O disappeared after EP. Furthermore, the vibration peak of —N—O stretching is located in 1513.6 cm⁻¹, which may overlap with other characteristic absorption peaks. The new absorption peaks at 997.8 and 882.1 cm⁻¹ indicate the presence of C⚌C bending which appear after EP.The absorption peak of S⚌O stretching appeared at 1131.9 and shifted to1155.9 cm⁻¹ and S—S stretching has shifted from 552.6 to 503.8 cm⁻¹ (Lu et al., 2011, Barreto et al., 2017). In Fennel seeds (Foeniculum vulgare): The absorption peak of —OH stretching located at 3373.3 shifted to 3175.5 cm⁻¹. Additionally —C—H stretching peak is shifted from 2931.6 to 2393. The stretching frequency of C⚌O and C—O bands are shifted from 1608.5 to 1639.5 cm⁻¹ and 1055.5 to 1069.5 cm⁻¹ respectively, due to electron cloud density shift from O atoms to coordinate with Cu²⁺ to form copper plant extract complex (Hussein et al., 2016). In Tilia leaves (Tilia cordata): A broad band at 3426.8 cm⁻¹ is attributed to —OH and/or N—H stretching vibrations which shifted to 3398.1 cm⁻¹. The stretching peak C—H is shifted from 2935.5 to 2396.1 cm⁻¹, O—H bending peak is located at 1371.1 cm⁻¹ which disappeared after EP. The absorption bands at 1208.2 and 774.3 cm⁻¹ are shifted to1125.5 and 714.5 cm⁻¹ which could be assigned to C—N stretching and C—H bending vibration, respectively (Fouda et al., 2017). In Dolichos lablab (Dolichos lablab extract): A strong —OH band at 3383.6 has shifted to 3429.4 cm⁻¹, C—H stretching band at 2926.2 is shifted to 2393.5 cm⁻¹. The absorption band 1742.1 cm⁻¹ indicates the presence of C⚌O stretching group and this band disappeared after EP. C⚌C bending is shifted from 927.8 to 887.1 cm⁻¹ and C—Cl stretching band are shifted from 552.6 to 501.1 cm⁻¹ (Kahsay et al., 2018).

Finally, the surface analysis using FTIR reveals that function groups like —OH, —COOH, —N—O, C⚌N, C⚌O, S⚌O and S—S are present in the organic constituent of plants extract which indicate that the extracts contains several bioactive molecules (good for corrosion inhibition). The presence and shifted stretching peaks of the functional groups confirm the formation of active compound —Cu²⁺ complex on the copper surface. Hence, the FTIR spectra show that there is an interaction between the extract and the copper substrate which resulted in the inhibition effect (Alaneme et al., 2016).

• UV–Visible spectroscopy:

Fig. 4 shows the UV–Visible spectrum of Garlic Allium sativum (garlic extract), Tilia cordata (Tilia leafs), Foeniculum vulgare (fennel seeds), and Lablab purpureus (Dolichos lablab extract) extract. As seen in Fig. 3a–d, the solution having Cu²⁺ ions display three absorption bands at 209, 252 and 816 nm. Otherwise, the garlic (Allium sativum) extract solution reveals two bands only at 229 and 279 nm. Cu²⁺ ions adding to garlic (Allium sativum) results in substitute the 252 and 816 nm absorption bands of Cu²⁺ ions by powerful absorption band at 323 and 813 nm investigative of Cu-plant extract complex formation. The solution containing fennel (Foeniculum vulgare) extract demonstrates only two bands at 287 and 421 nm. Cu²⁺ ions addition to fennel (Foeniculum vulgare) extract lead to replace the 252 and 816 nm Cu²⁺ ions absorption bands by forceful absorption band at 260 and 813 nm pinpointing to Cu-plant extract complex formation. Conversely, tilia (Tilia cordata) extract containing solution illustrates only two bands at 274 and 379 nm. Cu²⁺ ions addition to tilia (Tilia cordata) extract lead to alternate the 252 and 816 nm Cu²⁺ ions absorption bands by concentrated absorption band at 284 and 804 nm investigative of Cu-plant extract complex formation. But, dolichos lablab (Lablab purpureus) extract containing solution demonstrates only two bands at 218 and 268 nm. Cu²⁺ ions addition to dolichos lablab (Lablab purpureus)extract lead to change the 209, 252 and 816 nm Cu²⁺ ions absorption bands by powerful absorption band at 257 and 807 nm which indicate that Cu-plant extract complex formation. Therefore, Cu-plant extract may be as a stable complex adsorbed over the metal surface resulting in retardation influence or it may be as a soluble complex leading to catalytic effect. In systems that contain several chemical species, there is a opportunity to two types of complexes formation. Therefore, plant extract influence on the copper corrosion behavior in 8 M phosphoric acid could be interpreted on the basis of two issues: stable complex retardation effect and other soluble metal complex catalytic effect. It clear that the adsorbed metal complex retardation effect is primarily principal till a certain concentration where a definite leveling off value of their retardation efficiency is attained. Increasing the plant extract amount lead to appear of other soluble species accelerating effect which lead to diminish of the greatest retardation effectiveness. (Karthiga et al., 2018; Abdel-Gaber et al., 2006)

• Gas Chromatography–Mass spectrometry (GC–MS) analysis:

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Fig. 4

UV–visible spectrum of Garlic Allium sativum (garlic extract), Tilia cordata (Tilia leafs), Foeniculum vulgare (fennel seeds), and Lablab purpureus (Dolichos lablab extract).

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The organic compounds are found in Allium sativum (garlic extract), Tilia cordata (Tilia leafs), Foeniculum vulgare (fennel seeds), and Lablab purpureus (Dolichos lablab extract) were identified through GC–MS analysis Fig. 5. The molecular formula, molecular weight, retention time (RT), % compositions and probability percentage are presented in Table 3.

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Fig. 5

GC–MS spectrum of (a) Allium sativum (garlic extract), (b) Foeniculum vulgare (fennel seeds), (c) Tilia cordata (Tilia leafs), and (d) Lablab purpureus (Dolichos lablab extract).

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Fig. 6

Percentage inhibition efficiency (% IE)- concentration dependence curves for copper in 8 M H₃PO₄ solution containing different concentration of (a) Garlic (Allium sativum), (b) Fennel seeds (Foeniculum vulgare), (c) Tilia leaves (Tilia cordata) and (d) Dolichos lablab (Lablab purpureus).

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3.4 Temperature effect

The temperature effect on metal dissolution reaction is extremely difficult, since several modification take place on the metal surface for instance speedy etching and desorption of additives and the additives themselves many undergo decomposition and/or rescheduling. The effect of temperature on the dissolution rate of copper in 8 M H₃PO₄ and in the presence of different plant extract concentrations was studied in the temperature range of 293–323 K using galvanostatic measurements. We curiosity in discover the dissolution process activation energy. This was consummate by inspecting the temperature as function of dissolution current.

The activation parameters for the copper dissolution process were calculated from Arrhenius-type plot according to Eq. (3):

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Fig. 7

(a) Arrhenius plot and (b) Transition state plot of the dissolution for copper in presence of different concentration of Garlic (Allium sativum) extracts.

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Extract adsorption on copper surface might modify the surface through jamming fraction or the complete active sites; thus they may reduce electrochemical reactions rate on that surface which leading to enlarge Ea values, also the type of adsorption is typical of physical adsorption. In another word, an enlarge of dissolution activation energy in solution containing Fennel seeds (Foeniculum vulgare) and Tilia leaves (Tilia cordata), accompanied with an inhibition efficiency diminishing with mounting temperature, is regularly explained owing to adsorption film formation which have physical nature, i.e. linking electrostatic interactions with the metal surface. On the other hand, a chemisorption mechanism matches to an amplify in inhibition efficiency with temperature and lower activation energy in the presence of Garlic (Allium sativum) and Dolicus lablab (Lablab purpureus) (Fouda and Elabbasy, 2019; Dahmani et al., 2017).

It can be seen in Table 5 that Ea is lower in attendance of plant extract than in the plant extract free solution. This surveillance additional sustain, the proposed chemisorptions mechanism because lower activation energy in inhibited systems compared to blank has been reported to be indicative of chemical adsorption mechanism (Ekanem et al., 2010). Further in section of Table 5 also revealed that activation energy decreases with increases in concentration of Garlic (Allium sativum) and Dolichos lablab (Lablab purpureus) with the lower values 6.88 kJ·mol⁻¹ and 2.86 kJ·mol⁻¹ respectively being obtained at the highest extract concentration 1800 ppm studied. This is in agreement with the previous study of temperature effects on the corrosion inhibition of mild steel in acidic solutions of aqueous extract of Fenugreek leaves. The fall in the value of Ea may probably attribute to increase of metal surface area sheltered by the inhibitor molecules as the temperature rises (Noor, 2007).

The enthalpies (ΔH*) positive signs reveal the copper dissolution process is endothermic nature, ΔH* value increases with the addition of Fennel seeds (Foeniculum vulgare) and Tilia leaves (Tilia cordata), which means more energy is needed for the copper dissolution in the presence of Tilia (Tilia cordata), as a result, the dissolution of copper is inhibited. Entropies (ΔS*) value entail that at the rate determining step, the activated complex signifies an organization more than dissociation, significance that a reduce in disordering occur on departing from reactants to the activated complex (Fouda et al., 2016). (ΔH*) and (ΔS*) values designate the dissolution process energy barrier enlarges in the attendance of extract. The (ΔG*) values in solution containing the four plant extracts were more positive than that for free solution enlightening that in the solution containing plant extracts, the activated complex becomes less stable as compared to its free solution.

3.5 Weight loss measurements

Results gained from weight loss measurement were fitted into equation (5) to test for the order of dissolution reaction in the presence of plant extracts.

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Fig. 8

Variation of different concentration for natural extracts with time for copper coupons in 8 M H₃PO₄ acid solution at 25 °C.

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Values of rate constant (k₁) obtained from the slope of the plots of Eq. (5) and the half life calculated from Eq. (6) is presented in Table 6. From the results obtained it is significance to note that half life time increases in the following trend;

• Garlic (Allium Sativum) < Fennel seeds (Foeniculum Vulgre) < Tilia leaves (Tilia cordata) < Dolichos lablab (Lablab purpureus)

Also the rate constant increase by following order;

• Garlic (Allium Sativum)  > Fennel seeds (Foeniculum Vulgre)  > Tilia leaves (Tilia cordata)  > Dolichos lablab (Lablab purpureus)

The results of gravimetric experiments for the copper dissolution in 8 M H₃PO₄ solution free solution and 8 M H₃PO₄ solution containing several concentrations of plant extract for at 298 K are given in Table 7. It is clear that the dissolution rates decrease and the retardation efficiencies increase with the increasing plant extract concentrations, the inhibition efficiency obtained is excellent conformity with the result obtained by electrochemical measurements.

3.6 Surface characterization:

3.6.1

3.6.1 Scanning electron microscope (SEM)

Surface morphology was inspected on sample that electro-polished at 20 °C for 10 min. Fig. 9a demonstrates copper morphology before EP at the magnification of 5000. Etch pits were found with huge size and elevated deepness dispersed above the surface are observed. On the contrary, after EP in 8 M H₃PO_4, A smooth surface without major defects, the grain structures were observed (Fig. 9b).

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Fig. 9

The surface morphology before and after EP at 293 K in the absence and presence of natural extracts.

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Inspection of Fig. 9c (After EP + 1800 ppm Dolichos lablab (Lablab purpureus), it is observed that there is a gradual increase in surface quality to some extent as proved by the absence of pits grooves and cracks on the surface. By examination of Fig. 9d (After EP + 1800 ppm Tilia leaves(Tilia cordata) it was found that, uniformity increased, the surface is well polished and increase in surface appearance to great extent. By further inspection of Fig. 9e (After EP + 1800 ppm Fennel seeds (Foeniculum Vulgre), the surface appear smooth and completely uniform, in the other hand, Fig. 9f show the SEM of copper specimen after EP in presence of 1800 ppm Garlic (Allium Sativum). Leveling and brightening consequences were noticeably improved by the addition of Garlic (Allium Sativum) which is filling up the grooves and grain boundaries, also etching effect are eliminated. This behavior might be attributed to increase in the adsorption power of Garlic (Allium Sativum) methanolic on copper surface which confirms the superior retardation efficiency % recorded for 1800 ppm Garlic.

3.6.2

3.6.2 Degree of brightness

The Vis-IR spectra of unpolished copper specimen that electro-polished in 8 M H₃PO₄ (blank) and copper that electro-polished in 8 M H₃PO₄ electrolyte containing 1800 ppm of different methanolic plant extracts is shown in Fig. 10.

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Fig. 10

Reflectance diagram in the absences and presence of methanolic natural extract different concentrations (I) Garlic (Allium sativum), (II) Fennel seeds (Foeniculum vulgare), (III) Tilia leaves (Tilia cordata) and (IV) Dolichos lablab (Lablab purpureus).

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It is seen that the electro-polished specimens reflectance is a function of the type of methanolic plant extracts. It is clear that the reflectance of the specimen that electro-polished at 8 M H₃PO₄ electrolyte increases compared with untreated specimen. From Table 8, the specular reflectance value before EP is 35.8. After an EP in 8 M H₃PO₄ electrolyte is 40.11. By addition of Dolichos lablab (Lablab purpureus), Tilia leaves (Tilia cordata), Fennel seeds (Foeniculum vulgare) and Garlic (Allium sativum), the degree of surface brightness and reflectance increased to 45.66, 49.77, 52.43 and 56.26 respectively.

Table 8 Measured roughness (Ra), Rq (RMS), (PV) Peak – valley ratio, Reflectance value and brightness of copper samples in absent and present natural extracts.

1800 ppm methanolic extract	Raw sample	Blank	Dolichos lablab (Lablab purpureus)	Tilia leaves (Tilia cordata)	Fennel seeds (Foeniculum vulgare)	Garlic (Allium sativum)
Ra (μm)	141.92	112.3	69.09	62.87	58.99	28.88
Rq (RMS) (μm)	179.50	141.3	82.82	80.90	78.11	35.24
PV (μm)	133.00	101	94.09	59.69	49.14	46.11
Reflectance value	35.81	40.11	45.66	49.77	52.43	56.26
Gloss value	42.67	48.98	53.44	57.83	60.34	67.98

It is clear that the development of the insulating layer on the anode is very significant. The anode layer is a slim film of solid material or adsorbed atoms and molecules. The polishing accomplishment is considered to be due to a diffusion-controlled anodic dissolution procedure being set up through the anode layer and film. The depth and confrontation of insulating layer increase by addition of the four studied methanolic plant extracts. It is credible that the adsorption of the four studied methanolic plant extracts, the behavior of the normal anodic film in improving the microstructure of the surface, i.e. the four studied methanolic plant extract will increase the insulating layer thickness to great extent.

3.6.3

3.6.3 Atomic force microscope (AFM)

The surface roughness of the electropolished copper surface sample, various roughness values have been defined which are used to quantitatively describe the roughness of a material surface, one of them are shown in Figs. 11 and 12 as an example, including average roughness Ra, root mean square roughness Rq, and maximum peak-to-valley height PV.

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Fig. 11

Two dimensional AFM images before and after EP at 298 K in the absence and presence of natural extracts.

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Fig. 12

Three dimensional AFM images before and after EP at 298 K in the absence and presence of natural extracts.

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From Table 8, the surface roughness will be reduced by the addition of different types of natural extracts. Since the trends of Ra and RMS are almost the same, only Ra values are provided for the following discussion.

From an examination of Table 8, it is observed that, raw copper sample records highest surface roughness (141.92 μm). Oppositely, the electropolished copper with a low Ra value of (112.3 μm) obtained from the bath containing 8 M H₃PO₄ solution without any addition (Blank). EP in the solution containing 8 M H₃PO₄ and 1800 ppm of plant extract should be very different from the case containing phosphoric acid only. Hence, their corresponding surface roughness parameters are mainly decreased to a great extent.Ra value decreases depending on the type of plant extract. Where Ra values are 69.09, 62.87, 58.99 and 28.88 μm in the presence of Dolichos lablab (Lablab purpureus), Tilia leaves (Tilia cordata), Fennel seeds (Foeniculum vulgare) and Garlic (Allium sativum) respectively.