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Article

Amino Acids Reduce Mild Steel Corrosion in Used Cooking Oils

by
Nina Bruun
*,
Juho Lehmusto
,
Fiseha Tesfaye
,
Jarl Hemming
and
Leena Hupa
Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Henrikinkatu 2, FI-20500 Turku, Finland
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(7), 3858; https://0-doi-org.brum.beds.ac.uk/10.3390/su14073858
Submission received: 16 February 2022 / Revised: 18 March 2022 / Accepted: 23 March 2022 / Published: 24 March 2022
(This article belongs to the Special Issue Biomass Conversion and Biofuels)
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Versions Notes

Abstract

:
In this study, we tested several amino acids as eco-friendly inhibitors against corrosion of mild steel by used cooking oils (UCOs). The corrosion inhibition was studied by immersing mild steel rods in the UCOs and reference fresh rapeseed and olive oils mixed with amino acids. The immersion tests were conducted at room temperature for three days. The roles of water and bio-oil preservatives (formic and propionic acids) in the corrosion were explored. The mild steel surface morphology changes after exposure to the oils were analyzed with a scanning electron microscope coupled with an energy dispersive spectroscope (SEM-EDS). The concentration of iron dissolved in the oils was determined with a spectrophotometer. A thick layer was analyzed on the surfaces of the mild steel rods immersed in the oils containing formic or propionic acid and water. This layer provided a minor barrier against corrosion. According to the Fourier transform infrared spectrometer (FTIR) analytical results, the layer consisted of an acid and iron salt mixture. All the tested amino acids decreased the concentration of dissolved iron in the UCOs; particularly, cationic amino acids, L-lycine and L-arginine showed adequate corrosion inhibition properties at low concentrations.

1. Introduction

Used cooking oils (UCOs) are non-edible residues from restaurants, households, and the food processing industry. The global vegetable oil utilization leading to the generation of UCOs has significantly increased in recent decades due to the global population growth, the change in food habits, and the rise in the utilization of lipids [1]. The generation of UCOs is estimated to be around 20–32% of the total vegetable oil consumption [2,3].
Increasing interest in the circular economy and sustainable use of resources has made UCOs and UCO-based biodiesel attractive as renewable fuels for diesel engines, particularly for marine engine applications [4]. However, the UCOs must be used within a relatively short period of time after their collection and processing to avoid, e.g., the formation of corrosive degradation components [5,6]. In general, the acid number, viscosity, density, and water content are essential criteria for approving the UCOs as fuels [7]. However, the acid number and water do not directly correlate with the bio-oil properties and corrosivity [8]. The roles of different bio-oil components and corrosion inhibitors in the corrosive properties are not thoroughly understood.
The corrosivity of bio-oils can be lowered with corrosion inhibitors, which are added in small concentrations to alter the environment into a less corrosive one by interacting with the corrosive species [9,10]. Alternatively, the inhibitors interact with the metal to form a protective surface film [10,11,12,13]. Corrosion inhibitors with proven effective performance include those containing multiple bonds, N, O, S, and P organic compounds, in addition to some functional groups [13,14]. Organic compounds with -OH, -COOH, -NH2, etc., are also excellent corrosion inhibitors, especially in acidic solutions [14]. The environmental legislation on toxicity, biodegradability, and bioaccumulation gives strict regulations and rules for the usage and disposal of corrosion inhibitors in different countries [15]. Developing green or eco-friendly corrosion inhibitors with minimal side effects has been considered very important [16,17]. These inhibitors are generally categorized as inorganic or organic [17]. Organic green corrosion inhibitors include surfactants, amino acids, etc. [11]. Proteins in all animal and plant species are composed of twenty different amino acids [18]. Amino acids are biomolecules that have vital significance to all organisms. They form the building blocks of proteins and many essential substances such as neurotransmitters, hormones, and nucleic acids [19]. Amino acids are relatively cheap, non-toxic, biodegradable, soluble in aqueous media, and produced at high purity [14,20]. Amino acids possess at least one carboxyl group and one amino group. They can coordinate with metals through nitrogen or oxygen atoms in the carboxyl group [21]. The strength of the inhibitor-metal bond is an essential part of the corrosion inhibition degree of amino acids [21]. Amino acids can control corrosion of various metals such as pure iron, carbon steel, copper, zinc, nickel, tin, and aluminum alloys [20,21,22]. Furthermore, amino acids behave as corrosion inhibitors in acid medium, neutral medium, and de-aerated carbonate solution [21].
Short-chain carboxylic acids, including formic, acetic, propionic, and butyric acids, have been used as food preservatives for a long time [23,24]. Organic acids are commonly used as preservatives for animal feed (such as chicken, pig, and cattle) and human consumption [23,24]. The reactive carboxyl group present in organic acids makes them essential building blocks for many compounds such as drugs, pharmaceuticals, plastics, and fibers [25]. Rust and oxidation scales formed on the steel substrates can be removed by different inorganic or organic acids such as phosphoric, hydrochloric, sulfuric, formic, or acetic acid. The choice of the acid depends on the oxide scale’s solubility in the solution and the content of various components in the steel. Acid solutions, e.g., hydrochloric and sulfuric acids, are used for pickling, de-scaling, cleaning, oil well cleaning, and pipeline cleaning [26]. The corrosion inhibitors added to these acid solutions must be stable and effective in the hot concentrated acid even in severe environments [27]. Rafiquee et al. [25] have studied the corrosion of mild steel in the presence and absence of inhibitors in 20% formic and 20% acetic acid solutions at 30 °C for 24 h. The polished mild steel exposed to the 20% formic acid solution without corrosion inhibitors showed abrasion and corrosion. In contrast, the metal surface exposed to the 20% formic acid solution containing 100 ppm of the inhibitor 2-amino-5-propyl-1,3,4-thiadiazoles (APT) was smoother, and the inhibitor had adsorbed on the surface [25]. Zhu et al. [28] have examined the effects of temperature and acetic acid concentration on the corrosion behavior of N80 carbon steel. An increased temperature was observed to enhance the dissolution of steel substrate and promote the main corrosion product FeCO3 precipitation. The increased acetic acid enhanced the localized corrosion attack on N80 carbon steel [28].
Carbon steel coupons exposed to biodiesel and different antioxidants (e.g., tert-butylhydroquinone, propyl gallate, and curcumin) for 90 days were examined by Serqueira et al. [29]. After the exposure, the surfaces were analyzed with the SEM-EDX technique. The results indicated that antioxidants had adhered to the metallic surface, forming a protective film that may provide corrosion protection [29].
Qiang et al. [30,31] have studied the inhibition effect of 5-(Benzylthio)-1H-tetrazole (BTTA) and 5-Benzyl-1H-tetrazole (BTA) for Q235 steel (Fe 99.7 wt %) in 0.5 M sulfuric acid and copper in 0.5 M H2SO4 by applying weight loss measurement, electrochemical techniques, scanning electron microscopy (SEM), atomic force microscopy (AFM), etc. Their results suggested that corrosion of the metal surfaces could be significantly better inhibited when 2 mM BTTA was added to the acid solution, compared to 2 mM BTA.
The eco-friendly substance Losartan potassium (LP) for corrosion inhibition of mild steel in HCl medium was recently studied by applying the gravimetric, electrochemical, and scanning vibrating electrode techniques [32]. For 5 mM LP, the authors reported the inhibition performance to increase from about 89% at room temperature to 92% at 318 K.
Our previous works [8,33,34,35] discussed the chemical, physical, and thermal properties and the effect of storage time on the quality and corrosion properties of UCOs and fish oils. In addition, the corrosion of different metal rods in oils was tested [8]. The immersion tests of the UCOs showed variations in corrosion properties depending on the oil batch [8,33]. An oleic acid layer on the steel rod after the immersion in one UCO batch was supposed to provide partial corrosion protection [8]. Thus, it was assumed that suitable additives in UCOs could form a protective layer or provide corrosion inhibition through some other mechanism.
This work studied the role of ten different amino acids, water, and formic and propionic acids in different UCO batches in the corrosion of mild steel rods. Fresh edible oils were used as references. The results provide a novel understanding of the efficiency of different amino acids in the corrosion resistance of mild steel rods in contact with oils.

2. Materials and Methods

2.1. Materials

The physicochemical properties of the studied bio-oils have been reported earlier in detail [8]. All the UCO samples were provided by VG EcoFuel Oy (Uusikaupunki, Finland). When naming the oil samples, numbers were used to note different oil batches. All the used cooking oils (filtered) were vegetable-based and obtained from fast food companies. In this work, oil samples from different batches were studied in immersion tests. A fresh edible vegetable oil (Keiju from Bunge Finland Oy, Raisio, Finland) and an organic extra virgin olive oil (Pirkka Luomu, from Granada, Spain) were used as reference samples. The UCOs and vegetable oil were stored in a refrigerator, whereas the olive oil was stored at room temperature.
The mild steel rods (98.64 wt % Fe, 1.00 wt % Mn, 0.21 wt % Si, 0.11 wt % C, 0.03 wt % P, and 0.02 wt % S) used in the immersion tests were cut from an H44 all-round welding rod with a diameter of 1.6 mm, obtained from Linde (Solna, Sweden). All chemicals and solvents used for the analyses were of analytical reagent grade. Formic acid (98–100%) was purchased from Riedel-de Haën (Seelze, Germany), and propionic acid (99.5%) and L-Methionine (98%) were acquired from Fluka AG, Sigma-Aldrich Chemie GmbH (Buchs, Switzerland). L-Alanine (99%) was provided by Sigma-Aldrich Chemie GmbH (Steinheim, Germany). L-Arginine (>98%), L-Cysteine (>98%), and L-Serine (>99%) were provided by Sigma Ultra, Sigma Chemical Co. (St. Louis, MO, USA). Glycine (>99.7%), L-Glutamic acid (>99%), L-Leucine (>99%), and L-Tyrosine (>99%) were provided by Merck (Darmstadt, Germany). L-Lysine (>97%) was acquired from SAFC (St. Louis, MO, USA). The concentrations of amino acids in the oil samples slightly varied due to difficulties in weighing the tiny amounts of powder or liquid needed for a particular concentration. Finally, formic and propionic acids and water were added to the oils to better understand the roles of various components in oil in mild steel corrosion.

2.2. Corrosion Test and Analyses

Corrosion tests were carried out at room temperature to investigate the inhibitive role of amino acids in UCO-induced corrosion of mild steel. The experiments were performed in test tubes of 15 mL, mounted on a rotary mixer rotated at a constant speed of 56 rpm. The steel sample was placed in a 7 mL oil sample for three days. After the immersion test, the sample was removed from the oil and cleaned ultrasonically using a mixture (1:1 v/v) of toluene (≥99.9%, Honeywell, Riedel-de-Haën, Seelze, Germany) and 2-propanol (≥99.8%, Sigma-Aldrich Chemie GmbH, Steinheim, Germany). Then, the oils were subjected to liquid–liquid extraction using 1 mL of sulfuric acid (95–97%, Sigma-Aldrich Chemie GmbH, Steinheim, Germany) and 8 mL of deionized water in a test tube, which was vigorously shaken for 1 min. After the extraction, the mixture was filtered using a quantitative ashless filter paper (Grade No. 42, Whatman International Ltd., Maidstone, England), and the filtrate was analyzed spectrophotometrically. Each experiment contained three parallel samples. More details on the experimental procedures were reported in [8,33].
The amount of iron dissolved in oils was determined from the filtrate after a liquid–liquid extraction step using a spectrophotometer (Perkin-Elmer Lambda 25, Waltham, MA, USA). The spectra were measured between 400 and 600 nm with a scan rate of 480 nm min−1 using a quartz cuvette with a 1 cm path length.
The surface morphologies and the corrosion products were studied with a scanning electron microscope (SEM, LEO Gemini 1530 with a Thermo Scientific Ultra Dry Silicon Drift Detector (SDD), Oberkochen, Baden-Württemberg, Germany) coupled to an elemental X-ray detector (EDS, Energy Dispersive X-ray Spectroscopy, Thermo Scientific, Madison, WI, USA). Cross-sectional samples for SEM imaging were prepared by broad ion beam milling (BIB) (Ilion + Advantage Precision, Model 693, Gatan Inc., Pleasanton, CA, USA).
Organometallic species at the sample surfaces were identified with Fourier transform infrared spectroscopy (FTIR) using a Perkin-Elmer Spectrum Two™ spectrometer (Perkin-Elmer, Llantrisant, UK) with a spectrum area of 4000–450 cm−1 and a resolution of 4 cm−1. The FTIR spectra were interpreted using the spectral analysis function (software KnowItAll Informatics System 2020, John Wiley Sons, Inc., Hoboken, NJ, USA).

2.3. Properties of UCO Batches without Additives

The chemical and physical properties (chemical composition, acid number, water content, density, viscosity, and kinematic measurements) of the UCOs have been reported in [8]. In short, no significant differences between the properties of the UCO batches were found. The mean value for total free fatty acids (FFAs) was about 30.1 ± 3.6 mg/g and the measured acid number (AN) was 7.2 ± 0.8 mg KOH/g oil (Table 1). The mean water content was 2483 ± 672 ppm [8].
The immersion tests showed the highest iron concentrations, >390 ppm, after three days in UCO2, UCO6, and UCO7 [8]. After three days, the dissolved iron concentration in UCO8 was 60 ppm, much lower than in UCO6 (450 ppm) and UCO7 (570 ppm). Although UCO8 had a lower water content (2660 ppm) than the two other oils (3750 and 3090 ppm for UCO6 and UCO7, respectively), the differences in the dissolved iron cannot be explained by the water concentrations. UCO8 had the highest total FFAs, 37.8 mg/g oil, and AN (8.8 mg KOH/g oil) (Table 1). Furthermore, the unsaturated free fatty acid oleic acid content was 21.7 mg/g in UCO8, higher than in UCO6 (15.2 mg/g) and UCO7 (15.9 mg/g). According to our previous study [33], a layer formed of oleic acid on the mild steel rod surface. This layer might provide some corrosion protection and thus partly explain the differences in the observed iron contents.
Table 1 also shows the physicochemical properties and the immersion test results of the reference samples, fresh edible vegetable oil and virgin olive oil. The water contents of these oils were about 600 ppm, and the measured ANs were 0.1 and 0.5 mg KOH/g oil for the vegetable and olive oil, respectively. After the immersion tests, these fresh oils showed low iron content, 8 ppm, the same level as before.

3. Results and Discussion

3.1. Corrosion Inhibition

3.1.1. UCO2 with Amino Acids

Figure 1 shows the dissolved iron concentration after immersion of the mild steel rods in UCO2 with various concentrations of the amino acids tested as corrosion inhibitors. Although significant differences were measured between the different amino acids’ corrosion-inhibiting effects, all amino acids decreased the amount of iron dissolved to UCO2 during the three-day immersion. These results are in accordance with the published results of utilizing amino acids and their derivatives to prevent corrosion of metals and alloys in aqueous media [11,21]. Interestingly, a particular amino acid concentration did not have a marked impact on the iron concentration analyzed in the oil. The best corrosion inhibition was achieved with the cationic acids L-lycine and L-arginine. Low concentrations of these two amino acids almost totally inhibited the corrosion.
Figure 2 shows SEM secondary electron images of polished steel rod surfaces after immersion in UCO2 and the different amino acids. Figure 2a shows the polishing scratches on the rod surface before the exposure. After the immersions, the surface morphologies were smoother (Figure 2b–e). The immersion affected the surface morphology of the rods for all other UCO2 additives but L-lycine (Figure 2f) and L-arginine (Figure 2g). Thus, the images suggest only minor corrosion, further supported by low iron concentrations in UCO2, around 10 ppm Fe with L-lycine and 12 ppm Fe with L-arginine.

3.1.2. Impact of Water in UCO2 on Corrosion Inhibition with L-Lycine and L-Arginine

UCO batches with more than 3000 ppm water (UCO6 and UCO7 in Table 1) dissolved more iron than UCO2 with 1800 ppm water. Whether L-lycine and L-arginine were effective also in the presence of higher water content was tested by adding water to UCO2 and UCO2 mixtures with the two amino acids. Table 2 gives the water and amino acid contents in UCO2 and the measured average Fe concentration of three parallel samples with each UCO2 composition. The iron concentration difference in UCO2 shown in Table 1 and Table 2 was assumed to depend on the difference in the experimental time points. The oil in Table 1 was fresh but, most likely, had aged during the five months of storage before the experiments shown in Table 2 were carried out. Nevertheless, the corrosivity of UCO2 increased with the water content. As indicated by the small amounts of dissolved iron in Table 2, both L-lycine and L-arginine effectively inhibited corrosion for the mixtures with higher water contents.
The surface morphologies in Figure 3 verify the negligible iron dissolution into UCO2 with higher water contents in the presence of the amino acids. However, the SEM images in Figure 2 and Figure 3 do not explain the corrosion inhibition mechanism induced by the amino acids. According to Rafiquee et al. [25], the inhibitive effect occurs when the inhibitors adsorb on the mild steel surface, thereby decreasing corrosion.

3.1.3. Amino Acids, L-Lycine and L-Arginine in Different UCO Batches

The corrosion-inhibiting effects of L-lycine and L-arginine were also tested with UCO6 and UCO7, which had shown significant corrosion of the mild steel rods in earlier experiments (Table 1) [8]. In addition, the batch UCO8 showing less intense corrosion was included in the experiment series.
Adding 300 ppm L-lycine to UCO6, UCO7, and UCO8 decreased the iron dissolution to 15–20 ppm. Similarly, 300 ppm L-arginine in UCO6, UCO7, and UCO8 decreased the dissolved iron concentration to 22–44 ppm.
Figure 4 shows SEM images of the mild steel rod surfaces after immersion in UCO6, UCO7, and UCO8 without and with 300 ppm L-lycine. The rods immersed in oil batches containing L-lycine had similar surface morphology as before the exposure, suggesting that the amino acid effectively suppressed the corrosion.

3.1.4. Formic and Propionic Acids in UCOs with Amino Acid Inhibitors

The three-day immersion test was used to measure the impact of two carboxylic acids, formic and propionic acids, on the corrosive properties of used cooking oils with and without amino acids.
The iron concentration in UCO6 without any additives was 450 ppm. After immersion in UCO6 containing formic acid and added water, the iron content was lower, around 230 ppm. Adding 300 ppm L-lycine to the UCO6 mixture with formic acid and water slightly decreased the iron content to 180 ppm (Figure 5). Similar trends were also seen in UCO7, i.e., the iron content in the oil decreased from 570 to 190 and further to 140 ppm upon adding formic acid and water, followed by L-lycine addition (Figure 5).
The SEM images of the steel surface after immersion in UCO6, UCO7, and UCO8 with additions of formic acid (10,000 ppm), water (10,000 ppm), and L-lycine (300 ppm) show a surface layer of large (>10 µm) particles (Figure 6). The surface layer structure is similar to that reported by Rafiquee et al. [25] for mild steel after being in contact with a 20% formic acid solution.
SEM-EDS analysis suggested that the layer consisted mainly of carbon and oxygen, with some iron and manganese. The rod cross-section was revealed by BIB cutting for the SEM analysis. According to the image analysis, the layer mean thickness was 46 µm (Figure 7).
The layer was removed from the rod surface and then analyzed using FTIR. The spectrum in Figure 8 and the absorption bands and their interpretations in Table 3 suggest that the layer consisted of formic acid and iron salt.

3.2. Corrosion in Fresh Edible Oils

The impact of formic and propionic acid on corrosion was studied by immersing the mild steel rods in fresh olive and vegetable (rapeseed) oils with acid and water additions.

3.2.1. Olive Oil with Formic Acid, Propionic Acid, and Water

The iron content in fresh olive oil and vegetable oil was 7 ppm. After immersing mild steel rods in oils, the iron level was still on the same level (8 ppm). Adding (1) water (10,000 ppm), (2) formic acid (10,000 ppm), or (3) propionic acid (10,000 ppm) gave only a negligible increase in the iron content (8–11 ppm) (Table 4). However, adding 10,000 ppm formic acid and water resulted in 200 ppm iron in olive oil. Similarly, 10,000 ppm propionic acid and 10,000 ppm water increased the iron content in olive oil to 1200 ppm (Table 4).
After the immersion, SEM images of the rod surface morphologies were in line with the dissolved iron concentrations in oil mixtures. Adding only water, formic acid, or propionic acid in olive or rapeseed oils did not induce corrosion noticeably. However, marked corrosion occurred as soon as both an acid and water were present simultaneously. The corrosion was observed with increased concentrations of iron in the oils (Table 4) and changed morphologies of the steel surfaces (Figure 9).
The observed corrosion by an acid–water mixture originates from the dissociation of acid in aqueous solutions. According to Kahyarian et al. [36], acetic acid, often used as a model for short-chain carboxylic acids, does not directly participate in the corrosion reaction. Instead, after acetic acid has dissociated in water, the hydrogen ions participate in the cathode reaction and thus induce steel corrosion; in contrast, undissociated acid does not possess similar corrosivity. Similar behavior has been reported for formic acid, which is readily dissociated in aqueous solutions, increasing the conductivity and the corrosion rate of the solution [37,38,39]. Furthermore, acid concentration, solution temperature, and pH affect the corrosion rate [38,39,40]. Since formic acid is a relatively strong acid, any bio-oil containing formic acid can be considered corrosive towards steels with a Cr concentration below 11 wt % [41]. The steel studied contained no Cr, thus explaining the high measured amount of dissolved iron. It should be mentioned that the possible phase separation of oil and acid–water was not examined. Therefore, it cannot be said with certainty whether only one phase or two separate phases existed in the oil mixture.

3.2.2. L-Lycine and L-Arginine in Rapeseed Oils Containing Carboxylic Acids and Water

L-lycine and L-arginine were tested as corrosion inhibitors in fresh oil mixtures containing carboxylic acid and water.
The immersion tests in rapeseed oil with propionic acid–water additions led to heavy corrosion of iron (Figure 10), and 300 ppm L-lycine or L-arginine in the oil mixture slightly decreased the corrosion. At the higher addition (3000 ppm), the amino acids did not decrease the corrosion to an acceptable level.
After immersion of a polished mild steel rod (3 d) in rapeseed oil containing 10,000 ppm formic acid and water, 210 ppm iron was analyzed in the oil. When L-lycine 300 ppm was added to the oil mixture, the iron content was 180 ppm.
An increase in the L-lycine concentration to 3000 ppm in rapeseed oil containing 10,000 ppm formic acid and water somewhat decreased the corrosion. However, the iron concentration, 100 ppm, was still high. Similar results, i.e., a decrease in the concentration of metal ion, were measured in the oil–water–formic acid mixture with 300 ppm L-arginine (Figure 11).
Although amino acids decrease corrosion, the concentration needed for corrosion inhibition is likely high. The aging of bio-oils might lead to increased water content and the formation of carboxylic acids, which could induce significant corrosion. However, amino acids L-lycine and L-arginine effectively suppress room temperature corrosion if the oil does not simultaneously include water and carboxylic acid.

4. Conclusions

The corrosivity of used cooking oils was addressed in this study with three-day immersion tests of polished mild steel rods. Furthermore, the roles of contaminants, bio-oil preservatives, and corrosion inhibitors in bio-oil-induced corrosion were examined with oil samples containing added water, short-chain carboxylic acids, and amino acids. Among the ten studied amino acids, L-lycine and L-arginine showed noticeable corrosion inhibition effects, even when low concentration were added to an used cooking oil. More importantly, the level of corrosion inhibition was maintained even after the addition of water to the oil.
A minor increase in corrosion resistance was observed when a short-chained carboxylic acid (formic or propionic acid) was added to the oil. The improved corrosion resistance could have originated from a thin layer, which was formed on the surface of the sample. However, the simultaneous presence of water and one of the carboxylic acids led to corrosion of the sample rods. Furthermore, neither L-lycine nor L-arginine could provide notable corrosion protection when both water and a carboxylic acid were present in the oil. This suggests that used cooking oils contaminated with water and containing short-chained carboxylic acids increase the corrosion of mild steels. This should be taken into account when selecting the materials for oil storage vessels.

Author Contributions

Conceptualization, N.B., L.H. and J.L.; methodology, N.B., L.H. and J.H.; formal analyses, N.B., J.H., L.H., J.L. and F.T.; investigation, N.B., L.H., J.L., J.H. and F.T.; writing—original draft, N.B.; writing—review and editing, L.H., F.T., J.L. and J.H.; supervision, L.H., J.L. and F.T.; project administration, N.B. and L.H.; funding acquisition, N.B. and L.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Swedish Cultural Foundation in Finland. The authors are also grateful to the CircVol 6Aika project for financial support as part of the activities of the Johan Gadolin Process Chemistry Centre at Åbo Akademi University. The APC was funded by Åbo Akademi University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

VG EcoFuel Oy is gratefully acknowledged for providing the oil samples.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Iron dissolved (ppm) from the mild steel rods during 3 d immersion in UCO2 without and with different additions of amino acids.
Figure 1. Iron dissolved (ppm) from the mild steel rods during 3 d immersion in UCO2 without and with different additions of amino acids.
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Figure 2. SEM images of polished mild steel surface exposed to UCO2 and amino acid additives: (a) before exposure, (b) after exposure without any additive, (c) with 300 ppm L-methionine, (d) 720 ppm L-glutamic acid, (e) 220 ppm L-leucine, (f) 160 ppm L-lycine, and (g) 160 ppm L-arginine. All images have the same magnification.
Figure 2. SEM images of polished mild steel surface exposed to UCO2 and amino acid additives: (a) before exposure, (b) after exposure without any additive, (c) with 300 ppm L-methionine, (d) 720 ppm L-glutamic acid, (e) 220 ppm L-leucine, (f) 160 ppm L-lycine, and (g) 160 ppm L-arginine. All images have the same magnification.
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Figure 3. SEM secondary electron images of mild steel rod surfaces after immersion in UCO2 with (a) no additives, 1800 ppm water, (b) 5500 ppm water, (c) L-lycine 430 ppm + 4200 ppm water, and (d) L-arginine 380 ppm + 3400 ppm water. The scale is the same for all images.
Figure 3. SEM secondary electron images of mild steel rod surfaces after immersion in UCO2 with (a) no additives, 1800 ppm water, (b) 5500 ppm water, (c) L-lycine 430 ppm + 4200 ppm water, and (d) L-arginine 380 ppm + 3400 ppm water. The scale is the same for all images.
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Figure 4. SEM secondary electron images of mild steel rod surfaces: (a) before exposure and after three days in (b) UCO6, (c) UCO7, (d) UCO8, (e) UCO6+ L-lycine 300 ppm, (f) UCO7+ L-lycine 300 ppm, and (g) UCO8+ L-lycine 300 ppm. The magnification is the same for all images.
Figure 4. SEM secondary electron images of mild steel rod surfaces: (a) before exposure and after three days in (b) UCO6, (c) UCO7, (d) UCO8, (e) UCO6+ L-lycine 300 ppm, (f) UCO7+ L-lycine 300 ppm, and (g) UCO8+ L-lycine 300 ppm. The magnification is the same for all images.
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Figure 5. Mean iron concentrations in UCO6 and UCO7 with different additives after three days of immersion. All values are the means of three parallel experiments.
Figure 5. Mean iron concentrations in UCO6 and UCO7 with different additives after three days of immersion. All values are the means of three parallel experiments.
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Figure 6. SEM images of rod surfaces after immersion in oils with formic acid (10,000 ppm), water (10,000 ppm), and L-lycine (300 ppm): (a) UCO6, (b) UCO7, and (c) UCO8. The magnification is the same for all images.
Figure 6. SEM images of rod surfaces after immersion in oils with formic acid (10,000 ppm), water (10,000 ppm), and L-lycine (300 ppm): (a) UCO6, (b) UCO7, and (c) UCO8. The magnification is the same for all images.
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Figure 7. SEM image of rod surface layer after 3 days in UCO6 + formic acid, water, and L-lycine.
Figure 7. SEM image of rod surface layer after 3 days in UCO6 + formic acid, water, and L-lycine.
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Figure 8. FTIR spectrum of the removed scale from mild steel surface after immersion in UCO6 with formic acid, water, and L-lycine.
Figure 8. FTIR spectrum of the removed scale from mild steel surface after immersion in UCO6 with formic acid, water, and L-lycine.
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Figure 9. SEM images of steel surfaces: (a) before immersion, and after three days in (b) rapeseed oil, (c) rapeseed oil + 10,000 ppm water, (d) rapeseed oil + 10,000 propionic acid, and (e) rapeseed oil + 10,000 propionic acid + 10,000 ppm water. The magnification is the same in all images.
Figure 9. SEM images of steel surfaces: (a) before immersion, and after three days in (b) rapeseed oil, (c) rapeseed oil + 10,000 ppm water, (d) rapeseed oil + 10,000 propionic acid, and (e) rapeseed oil + 10,000 propionic acid + 10,000 ppm water. The magnification is the same in all images.
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Figure 10. The concentration of iron dissolved from the mild steel immersed in rapeseed oil containing 10,000 ppm propionic acid and water with different amounts of either L-lycine or L-arginine.
Figure 10. The concentration of iron dissolved from the mild steel immersed in rapeseed oil containing 10,000 ppm propionic acid and water with different amounts of either L-lycine or L-arginine.
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Figure 11. The concentration of iron dissolved from the mild steel in rapeseed oil containing formic acid and water with either L-lycine or L-arginine.
Figure 11. The concentration of iron dissolved from the mild steel in rapeseed oil containing formic acid and water with either L-lycine or L-arginine.
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Table
Table 1. The physicochemical properties and immersion test results of UCOs [8] and the reference vegetable and olive oils. The UCO batches marked with bold were used for the corrosion tests in this work.
Table 1. The physicochemical properties and immersion test results of UCOs [8] and the reference vegetable and olive oils. The UCO batches marked with bold were used for the corrosion tests in this work.
SampleMeasured ANWater ContentTotal FFAOleic AcidImmersion Test (3 d)
(mg KOH/g Oil)(ppm)(mg/g)mg/gFe (ppm)
UCO16.9185028.414.912
UCO26.5177628.414.8390
UCO36.9218028.316.0102
UCO47.0206730.016.914
UCO58.0249332.918.774
UCO66.7374827.015.2449
UCO76.9308928.015.9571
UCO88.8266437.821.759
Mean Value
UCO1-UCO87.2 ± 0.82483.0 ± 67230.1 ± 3.616.8 ± 2.4209 ± 223
Vegetable oil0.16040.50.28
Virgin Olive oil0.55811.00.48
Table
Table 2. Impact of water, L-lycine, and L-arginine content (ppm) in UCO2 on iron released from the mild steel rod during the three-day immersion test.
Table 2. Impact of water, L-lycine, and L-arginine content (ppm) in UCO2 on iron released from the mild steel rod during the three-day immersion test.
Chemicals in UCO2 (ppm)Dissolved Fe (ppm)
H2OL-LycineL-Arginine
1800--490
5500--530
4200430-10
3400-38010
Table
Table 3. Peaks in the FTIR spectrum (Figure 8) and their interpretation.
Table 3. Peaks in the FTIR spectrum (Figure 8) and their interpretation.
Peak (cm−1)StrengthStructureInterpretation
3259Weak-OH (hydroxyl)Iron oxide or water
2877Weak-CH stretchingOrganic compound
1564StrongAsymmetric C=O stretching
of carboxylic acid salt		Iron and carboxylic acid salt, formic acid
1330WeakSymmetric C=O stretching
of carboxylic acid salt		Iron and carboxylic acid salt, formic acid
755WeakRocking of primary amine salt
667WeakUnknown
Table
Table 4. Iron content in olive and rapeseed oils after immersing mild steel rods in oils with different concentrations of formic acid, propionic acid, and water.
Table 4. Iron content in olive and rapeseed oils after immersing mild steel rods in oils with different concentrations of formic acid, propionic acid, and water.
Additive (ppm)Iron Content (ppm)
Formic AcidPropionic AcidWaterOlive OilRapeseed Oil
---88
--10,000109
10,000--1111
10,000-10,000200170
-10,000-89
-10,00010,00012001550
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Bruun, N.; Lehmusto, J.; Tesfaye, F.; Hemming, J.; Hupa, L. Amino Acids Reduce Mild Steel Corrosion in Used Cooking Oils. Sustainability 2022, 14, 3858. https://0-doi-org.brum.beds.ac.uk/10.3390/su14073858

AMA Style

Bruun N, Lehmusto J, Tesfaye F, Hemming J, Hupa L. Amino Acids Reduce Mild Steel Corrosion in Used Cooking Oils. Sustainability. 2022; 14(7):3858. https://0-doi-org.brum.beds.ac.uk/10.3390/su14073858

Chicago/Turabian Style

Bruun, Nina, Juho Lehmusto, Fiseha Tesfaye, Jarl Hemming, and Leena Hupa. 2022. "Amino Acids Reduce Mild Steel Corrosion in Used Cooking Oils" Sustainability 14, no. 7: 3858. https://0-doi-org.brum.beds.ac.uk/10.3390/su14073858

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