Performance of green corrosion inhibitors from biomass in acidic media

3.3.1 Quinones and napthoquinones

Quinones are aromatic rings with two ketone substitutions. These compounds are highly reactive, and switch between diphenol (or hydroquinone) and diketone (or quinone) easily through oxidation and reduction potential, providing a source of stable free radicals (Cowan, 1999). Napthoquinones (C₆-C₄) are a class of quinone pigments widespread in nature (Lattanzio, 2013). For instance, henna contains lawsone, which is a very popular plant extract with dyeing properties. This compound, technically named 2-hydroxy-1,4-napthoquinone, has been studied for corrosion inhibition of steel in acid media (Chetuani & Hammouti, 2003; El-Etre et al., 2005; Ostovari et al., 2009; Abdollahi & Shadizadeh, 2012).

Lawsonia inermis or Lawsonia alba (Al-Sehaibani, 2000), most commonly known as henna, is rich in lawsone, resin, tannin, coumarins, gallic acid, and sterols (El-Etre et al., 2005). This extract was studied by several authors for protection of steel in different media.

El-Etre et al. (2005) tested henna extract for protection of different metals, including steel. Their experiments with steel in different media revealed that the IE is dependent on the concentration of the extract and pH of the environment. IEs increase with increase of pH: %IE (alkaline)<%IE (neutral)<%IE (acid) is shown in Table 17 (rows 2–4). The formation of metal-Lawsonia complex is shown in Figure 3. The authors proposed delocalization of the lone pair of electrons on the hydroxyl group in the presence of metal cations as the mechanism of inhibition for the lawsone molecule. Abdollahi and Shadizadeh (2012) studied the inhibitive effects of henna leaf extract on N80 API steel immersed in regular stimulation mud acid (HCl/HF 12:3 wt%) at 28°C. The inhibition properties of henna extract were also tested in mud acid samples containing standard doses of separated and combined commercial additives currently applied by service companies in the upstream industry. The list of additives was surfactant, multi-function surfactant, suspended agent, iron control agent, anti-sludge, and a mutual solvent. All experiments were performed at 28°C. The WL and EIS experiments (Table 17) in pure mud acid revealed that henna extract indeed reduced the corrosion rate of steel. In addition, the performance of the inhibitor was proportional to its concentration, i.e. higher IE was achieved when the concentration of inhibitor was increased. The authors showed evidence that this extract behaved as a cathodic inhibitor. Furthermore, the performance of henna in mud acid+additives by EIS and PP experiments showed that only iron control additives applied to the mud improved henna inhibition, while the others were detrimental to it. Therefore, the authors concluded that the corrosion rate depends on the chemical nature of the electrolyte and the medium temperature (Abdollahi & Shadizadeh, 2012).

Figure 3:

Formation of metal-Lawsonia complex.

Reproduced with permission from El-Etre et al. (2005).

Ostovari et al. (2009) also studied henna (Table 17) and the components present in its extract in order to evaluate their individual contribution to protection of mild steel in 1 m HCl solution by WL at different temperatures and electrochemical studies at 25±1°C (PP and EIS). These components were identified as lawsone, gallic acid, α-d-glucose, and tannic acid by the use of gas chromatography and mass spectrometry. Due to the fact that they represent only 18% wt of Lawsonia plant, their individual IE was tested using commercially available samples of these compounds.

One the one hand, the values of enthalpy of adsorption (ΔH_ads=−54.76 kJ/mol) led the authors to conclude chemisorption as the adsorption phenomena, which was represented by the Langmuir adsorption isotherm (Ostovari et al., 2009). However, their calculated values for apparent activation energy increases when henna concentration is increased  ( Eainhibitor≥46.21kJmol> Eafree acid=27.06kJmol ), which is an observed behavior of chemisorption (Bothi Raja & Sethuraman, 2008a, 2009b). On the other hand, the PP results from the individual henna constituents revealed that all of them behave as a mixed-type inhibitor and all the EIS results indicated a decrease of the double-layer capacitance values in the presence of these components. However, not all of them equally impact the ability of henna extract to protect steel in the acid solution. The trend of %IE values from both experiments was the same. Therefore, it was concluded that the efficiency was, in decreasing order, %IE lawsone>%IE gallic acid>α-d-glucose>tannic acid. In consequence, it established lawsone as the main responsible component for the good performance of henna in the experiments, due to the fact that this molecule has better IE when compared to the other ingredients present in this plant extract, and it contributes to 1.02% (i.e. major component) of its composition (Ostovari et al., 2009).

3.3.2 Flavones, flavonols, isoflavonoids, and flavanols

Flavonoids are a class of hydroxylated phenolic substance occurring with a C₆-C₃-C₆ unit linked to an aromatic ring. This flavonoid group includes mainly flavones, flavanols, and flavanols (Cowan, 1999; Oliveira et al., 2013).

Cardozo et al. (2010, 2014) studied orange peels as source of corrosion inhibitor for protection of steel. They claimed that orange peel (da Cunha Ponciano Gomes et al., 2011) provided good results due to the fact that it contains the highest concentration of flavonoids among citrus fruits (Cardozo et al., 2014) (Table 18). Orange peel extract also behaved as a mixed-type inhibitor that physisorbs ( Eainhibitor=74.9kJmol> Eafree acid=51.5kJmol ) on the steel surface following the Langmuir isotherm (da Cunha Ponciano Gomes et al., 2011; Cardozo et al., 2014).

The flowers of Ixora coccinea contain vitexin and diosmetin flavonols (Nagarajan & Sulochana, 2006) that protect steel from corrosion when this metal was exposed to 1 m HCl solution at 30°C. The results displayed in Table 19 concluded that these components behaved as a mixed-type inhibitor covering the steel surface by following the Langmuir adsorption isotherm.

Ethyl acetate obtained from Uncaria gambir was examined as an inhibitor for steel in different pH media by Hussin and Kassim (2010). This plant, with (+)-catechin and (+)-epicatechin flavan monomers besides alkaloids as major ingredients provided a concentration-independent inhibition behavior. The optimum %IE for different experiments is shown in Table 20. The authors showed evidence of the formation of a catechin complex on the steel surface by using SEM micrographs and energy-dispersive X-ray analysis of the mild steel surface, comparing the surfaces of steel before the experiments, with those without and with inhibitors (Hussin & Kassim, 2010).

3.3.3 Tannins

The name “tannin” is derived from the French “tanin,” i.e. tanning substance (Lattanzio, 2013). Therefore, tannins are capable of tanning leather (Cowan, 1999), with the ability to precipitate alkaloids, gelatin, and other proteins (Lattanzio, 2013). This group is also characterized by having a relatively high molecular weight (between 500 and 3000), and is divided into two broad categories: hydrolyzable tannins and condensed tannins (Cowan, 1999). Tannins are soluble in water, and their ability to adsorb heavy metal ions must be treated chemically to immobilize the phenolic polymers present in them (Rahim & Kassim, 2008).

Rahim and Kassim (2008) provided a review of recent development related to the use of tannins as corrosion inhibitors in acidic and near-neutral solution, and as an ingredient in pre-treatment formulations. They stated that an IE of >80% was achieved at acid pH 0 and pH 0.5 for all tannins, and that condensed mangrove tannins are superior to mimosa and chestnut counterparts at pH 4.0. Additionally, the authors stated that variations of acid pH range influence the inhibition behavior of tannins, e.g. the mimosa tannins chemisorbed on the steel surface when tested in sulfuric acid solution at pH range from 1 to 3, providing mixed-type inhibition. At 0.5 m HCl, the mangrove tannins behaved as a cathodic inhibitor that also chemisorbed on the metal surface via sharing the donor (-OH) electrons or aromatic π-electrons; however, when chestnut tannins were tested in a 2 m HCl solution, the bonding between inhibitor and steel surface was of the physical type. In the presence of 3.5% NaCl and pre-rusted steel alone, the Ecorr of mangrove tannins decreases, becoming more negative. Lastly, tannins in pre-treatment formulation have the reduction capability to convert active rust into more corrosion-resistant compounds by the interaction of its polyphenolic moieties with iron oxides and oxyhydroxides, forming ferric-tannates as the major product (Rahim & Kassim, 2008).

3.3.4 Stilbenes

These are a class of plant phenolics with a unit structure possessing two aromatic rings joined by a methylene bridge (C₆-C₂-C₆). The most famous representative stilbene is named resveratrol, which occurs in Polygonum cuspidatum root and Vitis species (Shen et al., 2013). Combretum bracteosum extract, which was tested for protection of steel, also contains stilbenes and other types of phenolic compounds able to inhibit corrosion of steel (Okafor et al., 2009).

Combretum bracteosum is rich in tannins, saponins, and stilbenes, among other phytochemicals. The extract obtained from the leaves of this plant was tested as an inhibitor for corrosion of mild steel at different concentrations (0.1–4 g/l) in different acidic solutions of H₂SO₄ (2–5 m) at different temperatures (30–60°C) (Okafor et al., 2009). The experiments measured IE through WL and hydrogen evolution methods, and the results are displayed in Table 21. From these experiments, Okafor et al. (2009) deducted that this extract protects steel by its physisorption (53.79kJmol ≥Eainhibitor>45.51kJmol Eafree acid) on this metal surface following the Frumkin isotherm. Also, from Table 21, it can be observed that inhibition was favored by the high concentration of extract and low temperatures, e.g. 4 g/l at 30°C yielded maximum %IE values through both methods. However, the reverse conditions have a negative impact on the protection of steel.

The peels of mango and cashew fruits, obtained as by-products for their use in the food process industry, have been studied as corrosion inhibitors by Cardozo et al. (2010, 2014) and da Cunha Ponciano Gomes (2011). As a result from their study, it was found that mango and cashew peels contain important amounts of polyphenols and heterosides that protected steel from corrosion when these extracts were tested in 1 m HCl media. The efficiency results, summarized in Table 22, concluded that the peel compounds behaved as a mixed-type inhibitor that physisorbed (75.5kJmol Eacashew>65.8kJmol Eamango>51.5kJmol Eafree acid) on the steel surface following the Langmuir isotherm.

Coffee senna is rich in polyphenol-based structures such as anthraquinones, emodin glycosides, chrysophanol, aloe-emodin, quercetine, rhamnosides, and rhein and vitexin alkaloids (Wagstaff, 2008). Thus, Akalezi et al. (2013) studied the ability of the extract from leaves of this plant to protect mild steel in 1 m HCl and 0.5 m H₂SO₄ solutions. The results from the studies are shown in Table 23. All experiments exhibited an increase in %IE with an increment of extract concentration used. The authors mentioned that a slight shift took place in the corrosion potential into the anodic direction in the HCl solution, while an opposite trend occurred in H₂SO₄ media. It is believed that the difference in behavior in the HCl media is because the Cl⁻ ions are strongly adsorbed on the metal surface. Gravimetric experiments realized in the 30–60°C temperature range showed an increase of %IE with an increase in temperature, indicating chemisorption (63.35kJmol Eafree acid>37.59kJmol Eainhibitor) of coffee senna phytochemicals on the steel surface (Akalezi et al., 2013). Commercial coffee varieties (Coffea arabica and Coffea canephora) were studied for protection of steel in 1 m HCl solution at different temperatures and immersion times by Vasconcelos et al. (2011). The major phenolic compounds identified by the authors in the coffee ground extracts were cholorogenic acids (monoesters or diesters of quinic acid with transhydroxycinnamic acid), such as caffeoylquinic, feruloylquinic, dicaffeoylquinic, and feruloylquinic acids. The results shown in Table 24 led the authors to conclude that the extract provided mixed inhibition of steel by chemisorption (41.9kJmol Eafree acid>16.8kJmol Eainhibitor)  on the steel surface modeled by the Langmuir isotherm.

3.3.5 Lignin

Lignin is an amorphous, highly branched polyphenolic macromolecule with a complex structure and high molecular weight, consisting primarily of phenylpropanoic units cross-linked together in three dimensions via a radical coupling process during its biosynthesis (Wardrop, 1971). Lignin contains several important functional groups (Adler, 1957), including para-hydroxyphenyl, aliphatic hydroxyl, and carboxylic acid groups, which has an impact on its overall reactivity. The corrosion inhibition properties of lignin are attributed to the aforementioned functional groups. Several studies have been performed since the 1980s (Srivastava & Srivastava, 1981; Forostyan & Prosper, 1983; Schilling & Brown, 1988), in order to analyze the IE of lignin derivatives, taking into consideration the extraction process (Akbarzadeh et al., 2011; Altwaiq et al., 2011; Abu-Dalo et al., 2013; Hussin et al., 2016), plant origin (Alaneme & Oulesegun, 2012; Zulkafli et al., 2014), chemical composition (Akbarzadeh et al., 2012), and chemical modifications (Schilling & Brown, 1988; Ouyang et al., 2006; Vagin et al., 2006; Ren et al., 2008; Hussin et al., 2015a). The chemical structure of lignin is highly irregular, and the overall reactivity of several lignins depends on their chemical structures and functional groups (Adler, 1957), including para-hydroxyphenyl (H), aliphatic hydroxyl, and carboxylic acid groups. Softwoods contain almost exclusively guaiacyl (G) units, whereas hardwoods contain both syringyl (S) and guaiacyl units.

For corrosion applications, three major C6-C3 phenylpropanoid units of lignin have been identified with anti-corrosion properties. These are p-coumaryl, coniferyl, and sinapyl alcohol (Hussin et al., 2015b). The study carried out by Forostyan and Prosper (1983) is perhaps the first work to document the properties of lignin as a corrosion inhibitor for steel in acid media. Their experiments tested different lignins obtained from sunflower husks and fir sawdust in a 5% H₂SO₄ aqueous solution with 5 g of different lignin types (native lignin, ammoniated native lignin, chlorinated hydrolysis lignin, and ammoniated hydrolyzed lignin) with constant stirring at 24°C during 24 h. Their results suggested a direct relationship between the presence of carboxyl groups (or COOH) in lignin structure or modifications, and the IE. The higher the COOH content, the higher the corrosion protection, which in their case occurred for the ammoniated hydrolyzed lignin. Also, they proposed the hydroxyl (OH) group as the second functional group affecting the inhibitory properties in the lignin molecule, and that the presence of nitrogen does not have a great effect on their protection efficacy.

Later, Vagin et al. (2006) compared the protection of steel caused by the electropolymerization of water-soluble lignosulfonates to COREXIT SXT-1003 inhibitor for carbon steel electrodes immersed in test brine, with air-saturated environment at room conditions. The voltammetry analysis of solutions containing lignosulfonate showed that an increase of concentration of KCl from 0.1 to 1 m in brine caused a decrease in oxidation potential to a cathodic shift of values around 1 V. This potential shift indicated that hydrophobic interactions resulted in an increase of interfacial lignosulfonates concentration with the increase of ionic strength. Additionally, when the steel electrode was tested with COREXIT, the corrosion rate was decreased only by 1.5 times, with respect to steel electrode soaked in brine solution alone. Meanwhile, the sulfonate lignin derivative showed a 3.7 times decrease of corrosion rate under the same conditions.

The properties of different types of lignin were investigated by different authors. Altwaiq et al. (2011) tested alkali lignin extracted from sawdust of a maple wood tree as a corrosion inhibitor for mild steel and other metals in 1 m HCl solution. Their experiments consisted in measuring the WL experienced in a period of time by steel immersed in the acidic solution, with different concentrations of alkali lignin dissolved in 10 ml of ethylene glycol, compared to the same solution without lignin extract. They concluded that alkali lignin does not stop the corrosion completely but inhibits the corrosion process by decreasing the corrosion rate. Additionally, they found that the higher the concentration of alkali lignin in solution, the higher the efficiency, and increasing time of exposure weakened the inhibition effect (Table 25).

Hussin et al. (2014) also compared the IE of lignins extracted through different methods: kraft, soda, and organosolv from oil palm fronds (Elaeis guineensis Jacq.). After analyzing the performance of electrochemical studies for all lignin dissolved in methanol and 0.5 m HCl solution in the presence of mild steel, they found better IE when soda lignin was employed, followed by organosolv and kraft. The results of EIS and PP tests, as shown in Table 26, suggest that lignin behaved as a mixed-type inhibitor protecting steel by forming a layer over the metal surface.

They proposed that the difference between the lignin samples is attributed to their composition, which is affected by the pulping process. The authors believed that during soda pulping, scission of β-O-4 bonds occurs, leading to the production of phenolic OH groups, which, combined with low molecular weight, provides better lignin quality for inhibition properties. The authors also examined the surface of the steel samples after the electrochemical experiment finished by SEM analysis and also performed XRD surface analysis of steel immersed for 24 h in 1 g/l lignin in methanol solution as per ASTM B117. The surface analysis revealed the presence of ferric lignates on a smooth surface with contents of lepidocrocite (L: γ-FeOOH), magnetite (M: Fe₃O₄), and traces of geothite (G: α-FeOOH). These results confirm, along with the electrochemical studies, the formation of a protective layer of lignin on the steel. The authors also suggested that inhibition takes place through the interaction of oxygen donation of electrons and sulfur with the metal ion surface (Hussin et al., 2014).

In order to find the impact of different improved lignin samples obtained from oil palm (Elais guineensis) fronds, the lignins extracted by soda, kraft, and organosolv pulping were ultra-filtrated in a 5-kDa interval membrane and tested for corrosion inhibition of mild steel in acid media by Hussin et al. (2016). They found that IE was better when soda lignin was employed, followed by organosolv and kraft types. After the ultra-filtrated lignin samples were dissolved in a small volume of methanol (1% v/v) to increase solubility, the solutions for different concentrations of lignin were diluted with 0.5 m HCl in order to carry out the experiments. All electrochemical studies were performed at 28±2°C, while the gravimetric tests took place at different temperatures with the objective to identify the existence of physical or chemical adsorption. The electrochemical results, included in Table 27, indicated that this ultra-filtrated lignin behaved as a mixed-type inhibitor. This finding suggested 500 ppm as the optimal concentration. The alkali lignin behaved predominantly as a cathodic inhibitor, while the organosolv showed mainly anodic behavior. For both EIS and PP experiments, the efficiency obtained by soda lignin was superior to that of organosolv, followed by kraft lignin. In addition, the thermodynamic analysis (ΔGoads≥−25.51 kJ/mol) indicated that all lignin samples are physically adsorbed on the steel following the Langmuir isotherm.

Therefore, in order to evaluate the most convenient method for pulping lignin for production of corrosion inhibitors, kraft and soda lignins extracted from oil palm empty fruit bunch were compared by Akbarzadeh et al. (2011) through gravimetric and electrochemical studies. The steel samples were immersed in solutions containing kraft (KL) and soda (SL) lignin at different concentrations (50, 100, 200, and 400 ppm) with 3.5% NaCl, with pH adjusted to 6 and 8 by addition of 0.1 m NaOH at 25°C under naturally aerated conditions during 240 h.

On the one hand, the results from WL experiments in Table 28 showed that IE increased with concentration, until a value from which the IE did not improve any further but instead remained constant if the inhibitor concentration increased. It is suspected that this behavior is explained by the adsorption of the lignin molecules on the steel surface through the polyphenolic monomers with electron-donating groups. These results were successfully fitted to the Langmuir isotherm.

In contrast, the electrochemical studies, also displayed in Table 28, exposed the mixed inhibition behavior of both types of lignin, and while the PP results concluded that the SL achieved higher protection, the EIS data reported better results for the KL. Additionally, the EIS tests suggested that in both SL and KL, the increase of inhibitor concentration caused an increase in the electrical resistance and diameter of capacitive loops, and these were due to the formation of a protective film onto the metal surface. This film was later confirmed by the X-ray microanalysis along with the FTIR spectra for inhibited steel samples that evidenced the formation and deposition ferric-lignin components. In addition, Akbarzadeh et al. (2011) also reported that SL has better adsorption than KL, with ΔGoads values between −23.40 and −22.01 kJ/mol, establishing that both lignin types are first adsorbed physically and then chemically.

During the separate studies, the same authors investigated key ingredients naturally present in lignin that directly impact its inhibition properties. As a result, the individual and synergistic effects of three monomers of SL, p-coumaric acid (CA), ferulic acid (FA), and hydroxybenzoic acid (HB), were tested by Akbarzadeh et al. (2012) for the corrosion inhibition of mild steel in 3.5% w/v NaCl media by PP and linear polarization resistance (LPR) duplicate measurements.

In order to isolate the individual effect of these monomers, air stagnant solutions of 100, 200, 400, and 800 ppm of each individual monomer were prepared with neutral (7) pH adjusted at 25°C. The results are displayed in Table 29 for the electrochemical studies of individual components. Their analysis established that lignin monomers behaved as a mixed-type inhibitor, creating a protective layer on its metal surface. This phenomenon was explained by the displacement of protons (H⁺) in solution, caused by the presence of carbonyl and carboxyl groups in the molecule. As a result, the IE of individual monomers is in the following decreasing order: CA>FA>HB (Akbarzadeh et al., 2012).

However, the thermodynamic nature of inhibition of steel by lignin is not defined yet. Thus, Yahya et al. (2015) intended to clarify it by studying the temperature effect in the efficiency and adsorption phenomena of lignin in low carbon steel coupons through WL methods. They found that lignin fitted properly to the Langmuir isotherm and also its optimum concentration to maximize metal inhibition. In order to accomplish this analysis, the steel samples were soaked in a 1 m HCl solutions during 3 h at the presence of lignin concentrations between 500 and 2500 ppm and at a temperature range of 30–70°C. After the calculation of IEs, the authors proceeded with thermodynamic calculations using the Arrhenius plot and transition equations. The %IE values disclosed that at >500 ppm, the %IE reached a plateau, and its values are not consistent until 1000 ppm of lignin is employed. They suggested that this was probably because lower concentrations than this value are unfavorable for lignin molecules competing with the Cl⁻ ions in solution. Additionally, 50°C is the optimum temperature to attain the higher efficiency value, and at temperatures higher than this value, the vibration of lignin molecules might impede the formation of coordinated bond with atoms in steel interface, and desorption of lignin will occur. Moreover, the authors suggested chemical adsorption based on the proximity of E_a values at 1500–2000 ppm to the chemisorption threshold (78.95 kJ/mol≤E_a≤79.85 kJ/mol). Additionally, E_a≤80 kJ/mol for other concentrations, along with −40 kJ​/​mol≤ΔGoads≤−20 kJ​/​mol as a possible explanation for the comprehensive (chemical and physical) adsorption behavior of lignin.

Later, another thermodynamic analysis of inhibition by lignin was realized (Alaneme & Oulesegun, 2012). The authors studied the efficiency of protection of medium-carbon low-alloy steel for soda lignin extracted from Tithonia diversifolia (sunflower) stems in 1 m H₂SO₄ solution in a temperature range of 30–60°C with an exposure time of 12 days. The authors found 55.5% ≤IE≤78.8% for different temperatures, with the thermodynamic parameters indicating physisorption (Ea≤80 kJ​/​mol and ΔGoads≥−20 kJ​/​mol). The experimental data were fitted to the Langmuir isotherm.

In previous analyses, several authors reported some of the lignin samples as insoluble materials (Ren et al., 2008; Abu-Dalo et al., 2013). Therefore, different attempts to modify their structures have been done in several studies in order to improve their practical use and effectivity as corrosion inhibitors. Some reports found in the literature show, for example, the formation of a water-soluble polyamine-aldehyde lignin, as described by the patent from Schilling and Brown (1988), the graft copolymerization of soda lignin (Ren et al., 2008), the incorporation of scavengers in lignin molecule (Hussin et al., 2015a), and the variation of the sulfonation degree on kraft lignin (Ouyang et al., 2006; Abu-Dalo et al., 2013). The improvement of lignin by graft copolymerization of soda lignin with acrylamide and dimethyl diallyl ammonium chloride (DMDAAC) was accomplished by Ren et al. (2008). This modification was done with the aim of adding hydrophilic groups containing nitrogen, which provides better electrostatic adsorption of lignin to the steel surface. These molecular changes resulted in IEs >85% for experiments run at 25°C and 80°C in a 10% HCl solution, as can be observed in Table 30. Also, the electrochemical analysis showed that grafted lignin behaved as a mixed-type inhibitor preferentially anodic at low temperatures but predominantly cathodic at high temperatures. After applying the Arrhenius equation and fitting the data to the Temkin isotherm, the authors concluded that this modified lignin adsorbs chemically to the steel surface through the positively charged nitrogen atoms (N⁺) and negatively charged metal surface. The authors also revealed the inhibition enhancement of the inhibition effect as a consequence of the amphiphilic nature of the new molecule. The hydrophilic portion of the amide and the DMDAAC group would form links with the lonely sp² orbital electrons from Fe atoms. Meanwhile, the hydrophobic part of the compound (lignin) would isolate the metal from the solution, creating a barrier to chloride ions (Ren et al., 2008).

Another alternative documented in the literature is the modification of insoluble organosolv lignin from oil palm fronds by the incorporation of aromatic scavengers: 2-napthol (AHN EOL) and 1,8-dihydroxyanthraquinone (AHD EOL) (Hussin et al., 2015a). The efficiencies of the obtained samples were tested (after being dissolved in a small volume of methanol) by electrochemical studies (i.e. PP, EIS) at 28±2°C and gravimetric methods at different temperatures in a 0.5 m HCl solution. The authors concluded that both modified EOL lignins (AHN and AHD) were better for inhibiting steel in acid media than the original organosolv lignin. The results, as shown in Table 31, demonstrated that AHD EOL yielded higher inhibition than AHN; this difference was attributable to the presence of more polar hydroxyl functional groups in the 1,8-dihydroxyantraquinone than in naphthol, increasing the hydrophilicity of lignin, promoting dissolution in water. The thermodynamic analysis from the WL methods suggested that the inhibitor adsorbs physically to the steel following the Langmuir isotherm.

Other studies on the partial or total modification of the kraft lignin structure tested previously include the work done by Abu-Dalo et al. (2013). They compared sulfonated kraft lignins to phosphate inhibitor for water distribution systems. They found 0.3–0.4 mg/l as the optimum concentration of lignosulfonates as viable options for preventing corrosion and pit formation in water pipelines. For their experiments, different concentrations of commercially obtained lignin were tested in solutions of water samples obtained from the pre-stabilized water (PRSW) stage from the post-dam water treatment in New York versus the samples obtained from the post-stabilized water at the same plant. The results from the corrosion tests (LPR, EIS, and PP) are shown in Tables 32 and 33, respectively. The authors suggested that the LPR values in Table 32 evidenced the corrosion occurring in the water treatment plant that can be treatable with any of the sulfonated lignin studied if a dosage between 0.4 and 0.5 mg/l is used, as supported by the EIS and PP results from LS_A in Table 33. However, they also found that LS_E and LS_C would be good inhibitors, because at low pH, more sulfonated groups, rather than phenolic sites, would be available for adsorption in anionic form to approach the positively charged metal surface (in acidic media), enhancing the electrostatic attractions between the H₃O⁺/metal interface and lignin molecules.

Lignosulfonates were also studied for inhibition of corrosion and scale deposition in recirculating cooling water systems by Ouyang et al. (2006). Initially, they compared the performance of sulfate lignin (SL) obtained from mason pine with 1-hydroxyethane-1,1-diphosphonic acid (or HEDP, a commercial scale and corrosion inhibitor) for carbon steel immersed in a naturally aerated stagnant solution at 60°C during 14 days. The solution consisted of 0.005 m calcium chloride (CaCl₂), 0.002 m sodium bicarbonate (NaHCO₃), 0.002 m magnesium sulfate (MgSO₄), and 0.01125 m sodium chloride (NaCl), with different concentrations of SL. Due to the poor performance of SL when compared to HEDP for this set of experiments (results in Table 34), it was decided to modify the sulfate lignins by grafting it with acrylic acid onto SL. This modification added more negative charge than the original SL owing to the presence of carboxyl in the new molecule. Therefore, the structural changes in the SL structure reflected an improved efficiency (about 96%) for mixed-type inhibition at the same dosage of HEDP. However, this new SL does not overcome the scale inhibition performance of HEDP or the turbidity of the solution. In this regard, the authors suggested further research in order to surpass these disadvantages, as the grafted SL promises to be a viable alternative for corrosion inhibition in cooling water systems (Ouyang et al., 2006).

Different samples of commercial lignosulfonates were tested for protection of steel in acid media by Abu-Dalo et al. (2016). WL experiments were performed at 25°C in a closed test tube for an extended period of time along with gravimetric tests carried out during 24 h at different temperatures (25°C, 40°C, and 60°C). The results showed that IE depends on the lignin concentration and the ionic strength between the solution and the pH of the lignin sample, as shown in Table 35. The performance of lignin in decreasing order was Reax 88A>Reax 100M>Reax 88B. Additional analysis concluded that lignin adsorbed to the steel following the Langmuir isotherm. Furthermore, FTIR analysis and energy-dispersive spectroscopy evidenced the formation of a protective layer of lignin on the steel surface by the development of an Fe⁺²-S-lignin complex (Abu-Dalo et al., 2016).