Development of a renewable corrosion-inhibiting agent from Acacia auriculiformis seed oil Free

Corrosion is a natural phenomenon where refined metals undergo electrochemical or chemical reactions with their surroundings, resulting in the transformation of the metals into more stable forms such as its oxide and sulfide. Attention is focused on the corrosion of the most commonly used plain carbon (mild) steel because it is the most widely used in industries, due to its low cost.1 Corrosion inhibitors are materials that are employed in low concentrations in a harsh environment to minimise, inhibit or prevent pitting or localised corrosion.2,3 The use of a corrosion inhibitor is not limited only to incorporation into the paint, but it may also be added during the manufacturing process.3 Organic corrosion inhibitors derived from natural sources are receiving a lot of attention these days due to their ready availability, ecological acceptability, cheapness and renewability.4–6 Because of their hydrophobic nature, organic corrosion inhibitors have restricted solubility, particularly in polar solutions, which adversely affects their protection efficiency.7,8 However, raising the temperature has been shown to boost their protection effectiveness, where interaction between the inhibitor molecules and metal surface involves chemical bonding.2,4,9,10 

Amide-based organic compounds have extensive utilisation as corrosion inhibitors in different industries.11 The literature reveals that the corrosion inhibition properties of fatty amide are due to the hydrophobic nature of the fatty acid chain,12 which can shield the metal surface from the aqueous corrosion media13,14 and the available lone pair of nitrogen for bonding with the metal surface through coordination bonds,14 as well as by creating an alkaline environment.8 It has been reported that amide-based organic molecules with long carbon chains can inhibit the corrosion of metals and alloys under acidic conditions.11 Rice bran oil with aminoethylethanolamine,14 hydroxylated Khaya senegalensis seed oil with ethylenediamine,15 oleic acid with diethylenetriamine (DETA)13 and ethoxylated fatty amide16 also serve as good corrosion inhibitors. The inhibiting effect of amide-based cationic surfactants has also been reported.17 

The Acacia auriculiformis tree is indigenous to Australia and Papua New Guinea and is typically found in tropical climates worldwide. It has the ability to grow fast in a wide range of soil, which makes it abundant in this region. Its seed has an excellent fatty acid composition, containing linoleic acid (C18:2) and vernolic acid (C18:1, 12-epoxy), suitable for surface-coating applications.18,19 In this study, the aim was to synthesise a fatty amide with a novel raw material source – namely, A. auriculiformis seed oil (ASO), which to date has not been utilised. Aminolysis was done with diethyltriamine, a very common raw material for epoxy coating. In this investigation, an ASO-based fatty amide (AFAM) was synthesised and characterised in terms of physico-chemical properties. The corresponding fatty amide was blended in different ratios with a synthetic lacquer formulated with soya-based alkyd resin. The films formed in these trials were then characterised for surface morphology and corrosion-inhibiting properties. The optimum concentration of fatty amide in the blend was also determined. Such optimisation was not reported earlier.

Toluene (assay 99%) and other analytical-reagent (AR)-grade solvents were purchased from Merck India Ltd and used as received. The DETA (Dow Chemical Company), metallic driers (Patcham FZC) and mineral turpentine oil used in this study were obtained from M/s Apex Chemicals India Pvt. Ltd. Xylene (extra pure, AR grade, 99.8%) was procured from SRL Chemicals.

The acid value, saponification value, iodine value, colour, specific gravity and refractive index of ASO fatty acid (AFA) and the amine value, colour, specific gravity and refractive index of DETA were measured according to standard laboratory methods.

AFA (0.3 mol) and toluene (∼7.5 g) were taken in a three-necked round-bottomed flask with a magnetic stirring bead in it. The three necks of the flask were fitted with Dean–Stark apparatus, an addition funnel and a thermometer pocket. The Dean–Stark apparatus was fitted with a condenser provided with continuous water outflow to allow effective distillation. A thermometer was inserted into the thermometer pocket. DETA (0.15 mol) was taken through the addition funnel fitted into the flask. The temperature was increased gradually accompanied by stirring until the temperature of the mixture reached 140°C. After attaining the required temperature, the knob of the addition funnel was adjusted to allow dropwise addition of DETA to the hot mixture for 30 min. The reaction was carried out for 4 h until the theoretical quantity of water was liberated and the acid value attained was below 10.

The acid value, amine value, non-volatile matter (NVM) and refractive index of the synthesised AFAM were determined using standard laboratory methods.

The structural analysis of the obtained AFAM was analysed using a Cary 630 Fourier transform infrared (FTIR) spectrometer (Agilent) coupled with an attenuated total reflectance accessory. Each analysis was conducted in the 4000–650 cm⁻¹ wavelength range. Nuclear magnetic resonance (NMR) analysis was also conducted to determine the structure of the fatty acid amide. The sample was dissolved in deuterated chloroform (CDCl₃) and analysed using a Bruker AV-III-400-HD spectrometer. Analysis was conducted at a frequency of 400 Hz.

Film properties were evaluated by making a synthetic lacquer with a long oil alkyd resin (60% NVM) and metallic driers (cobalt octoate, lead octoate, calcium octoate) according to the formula reported by Chakraborty and Ghosh.18 Three sets of trial samples were prepared, Tr-1, Tr-2 and Tr-3, by addition of the synthetic lacquer with 0, 2 and 4% fatty amide, respectively. The trial samples were applied on the mild steel (MS) panel and cured under ambient conditions to make a thin film with a dry film thickness (DFT) of 20–22 μm. After a week of drying under ambient conditions, film performance was investigated.18 

The mechanical properties of all the films were determined. The gloss at a 60° angle and DFT of the coatings were measured by using a BYK gloss meter and a DFT meter (ASTM D 6132)20, respectively. The impact resistance (ASTM D 2794)21, flexibility (ASTM D 522)22, pencil hardness (ASTM D 3363)23 and adhesion (ASTM D 3359)24, using a cross-hatch cutter, of the coated films were investigated. Visual appearances such as the flow of lacquer and levelling of the surface were also observed. The corrosion resistance of the films was evaluated in 3% sodium chloride (NaCl) solution according to the ASTM D 130825 method and analysed every 24 h, and the type of surface imperfection was noted.

The hydrophobicity of coating indicates the water repulsion properties of the coated surface; stronger water repulsion means more resistance towards water penetration, which improves the corrosion resistance properties.26 To check the hydrophobicity of the coating, a drop of deionised water was placed on all three trials. The contact angle (CA) of deionised water was observed at room temperature using CA meters (model ACAM HSC 06, Apex Instrument).11 

Scanning electron microscopy (SEM) images of films developed using resins were observed using a Zeiss EVO 18 microscope (Germany). The magnification of the SEM images was ×2000.27 

The properties of AFA, DETA and AFAM are provided in Table 1. The acid value and saponification value, which indicate the purity and average chain length of a fatty acid,18,28 correspond to the specifications of similar products available in the industrial market. The iodine value indicates the average number of double bonds present (1.7) per fatty acid chain of 18 carbon atoms (based on stearic acid).

The observed amine value of DETA also supports the theoretical amine value based on the chemical formula.

The final acid value of the synthesised fatty amide was maintained at 9.73 (<10), which indicated that a nearly 95% fatty acid conversion took place during this reaction to obtain a nearly 95% yield.

The hydrogen-1 (¹H) NMR spectra of AFA and AFAM are shown in Figure 1. The characteristic peak at 2.30–2.35 parts per million (ppm) in AFA suggests a proton of –CH₂ adjacent to the carbonyl (>C=O) group of the fatty acid. This peak is absent in AFAM and shifts to 2.15–2.19 ppm, which verifies amide formation during the reaction.28 The peaks at 3.41–3.42 ppm signify the proton of –CH₂ tied with nitrogen, which is notably absent in AFA.29 The characteristic broad peaks at 3.81 ppm may be for the proton of the neighbouring carbon of the epoxy group, which has been shifted to 4.67 ppm in AFAM.30 This epoxy group comes from vernolic acid. The characteristic peaks at 0.85–0.91 ppm are most likely caused by the protons of the terminal methyl group. The peaks at 1.24–1.37 and 1.55–1.64 ppm correspond to the presence of the protons of internal –CH₂ groups and protons of the second –CH₂ group from the hydrophobic end of the fatty acid chain, respectively. The peaks for allylic protons and vinylic protons are evident at 1.97–2.08 and 5.28–5.40 ppm, respectively. The peaks at 2.74–2.79 ppm indicate the proton attached to the common allylic carbon between two double bonds. The peak at 7.25 ppm arises from the residual chloroform (CHCl₃) present in the deuterated chloroform solvent.27 

The FTIR spectra of the fatty acid and prepared fatty amide are shown in Figure 2. The sharp peak at around 1647 cm⁻¹ denotes the formation of the O=C< linkages of the amide group, and N–H bending and stretching are observed at 1546 and 3276 cm⁻¹, respectively. These peaks are notably absent in the case of the fatty acid. The spectrum of O=C< is represented at 1710 cm⁻¹ for the fatty acid. Symmetrical and asymmetrical C–H stretching are found at 2857 and 2926 cm⁻¹, respectively. The perk at 3011 cm⁻¹ corresponds to the stretching of vinylic hydrogen.

FTIR and hydrogen-1 NMR data reveal that the corresponding fatty amide has been successfully synthesised from AFA.

The coating film was formed using a complex radical polymerisation process in the presence of aerial oxygen and metal drier as a catalyst through the unsaturation present in alkyd resin.31 Based on drying time, it was observed that Tr-1 dried more quickly than other trials; the drying rate of Tr-2 was nearly the same as that of Tr-1, but the drying time of Tr-3 was significantly longer. This might be due to the incorporation of the fatty amide, which had a low molecular weight compared with the corresponding alkyd, reducing the drying rate. The drying rate influences the hardness of the paint film, which occurs because of the chain flexibility and the degree of cross-linked network. Hardness depends on the substrate type and the structural configuration of the monomer involved in the synthesis.32 Trial 3 had poor hardness compared with trials 1 and 2 due to the presence of excess fatty amide.

All trials had comparable adhesion properties on the MS substrate. The effect of the fatty amide on this characteristic was not observed. Gloss is an important physical feature of the coating film that emerges from the interaction of light with the coating film surface. Gloss was found comparable. All trials remained unaffected in the impact and flexibility test (Table 2).

The interfacial CA between the water droplet and the coating surface acts as a determining factor of the hydrophobicity of the coating. A higher angle of contact means higher hydrophobicity of the surface. As evident from Figure 3, the surface of Tr-2 bears the highest CA, followed by Tr-1 and Tr-3. A concentration of 2% fatty amide may result in hydrophobicity due to the presence of a hydrophobic fatty acid chain, but as the percentage of AFAM increases, the amine content also increases and the amine being hydrophilic results in lowering of the CA. Hence, 2% fatty amide could be the optimum concentration of fatty amide to be used in the alkyd coating formulation.

‘X-cut’ lines were made on the coated portion of the panels, and then corrosion resistance was observed according to ASTM D 1308. The status of the panels was checked at intervals of 24 h (Table 3).

By performing the corrosion resistance study, it was observed that after 120 h, the panel bearing 2% corrosion inhibitor (Tr-2) exhibited the maximum corrosion resistance. The test failed for the Tr-1 formulation, whereas blisters were observed for the 4% inhibitor-bearing panel (Tr-3) (Figure 4). Thus, the authors conclude that the maximum allowable inhibitor that can be added to the formulation is 2%. This study also supports the inference of the hydrophobicity test (Section 3.4).

From Figure 5, it may be concluded that the cured films of Tr-1 and Tr-2 produce smooth surfaces, which indicates that the fatty amide is compatible with the alkyd, although they have different true solvents. Few surface defects – namely, cissing and unevenness – were observed in Tr-3, which might be due to the incompatibility of solvents of the long oil alkyd and fatty amide. This might also be attributed to the use of a comparatively high concentration (4%) of fatty amide in the formulation.

From the study, it was observed that AFAM can be successfully synthesised from AFA and DETA. The properties show that AFAM has the potential to act as an effective, green corrosion inhibitor because of its biodegradability, ease of availability and non-toxic nature. It is clear from the investigation that the incorporation of 2% fatty amide into the alkyd coating formulation can remarkably improve corrosion resistance over MS surface, but the gradual increase in the percentage of fatty amide increases the hydrophilicity of the test material, which in turn degrades its corrosion resistance ability. The raw materials used in this investigation are common in the coating industry. Hence, the production unit can easily use the same in-house raw materials to generate such a corrosion-resistant coating formulation, thereby making the system self-reliant. From the observed data, it may be concluded that the Tr-2 film (2% fatty amide) has optimum mechanical and corrosion resistance properties.

The authors express their gratitude to the University of Calcutta, Kolkata, for financial support; Apex Chemicals India Pvt. Ltd, for providing technical assistance; and the Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, for performing the SEM analysis.