Corrosion is the surface decay of metals and non-metallic materials, including ceramics, plastics, rubber, and wood, due to exposure to certain combinations of liquids and/or gases. The mechanism and the rate of corrosion depend on the exact nature of the atmosphere in which the corrosion takes place. Metal corrosion includes the rusting of iron, the tarnishing of silver, the dissolution of metals in acid solutions, and the growth of patina on copper. Corrosion occurs in public infrastructures such as bridges, pipelines, vehicles, utilities (electrical, water, telecommunications, and nuclear power plant), engineering and manufacturing, chemical industry, and the oil and gas industry. Acid solutions are widely used in process industries as corrosion inhibitors through the prevention of metallic scales deposition. The application of corrosion inhibitors has been widely used as an acceptable engineering practice owing to their protective nature against acid attack [
1]. Hydrochloric acid (HCl) is widely used for the pickling, cleaning, descaling, and etching of metals [
2]. However, HCl is known to be responsible for metal surface corrosion through acid attack mechanisms, resulting in millions of dollars being spent annually on maintenance. It is reported that, with proper corrosion prevention techniques, 25–30% of maintenance costs can be avoided [
3]. Applying corrosion inhibitors, even in the smallest amounts, is one of the most economical and environmentally friendly solutions to the metallic corrosion problem [
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9,
10]. Several factors are considered when making a selection of a suitable corrosion inhibitor. This includes cost, effectiveness and the amount to be used, availability and stability, toxicity, and environmental impact. It was found that a number of organic corrosion inhibitors could be used during acid pickling with appreciable inhibition capabilities [
11,
12,
13]. For example, a mixture of benzotriazole, chitosan, polyacrylic acid, and zinc salt was used as a corrosion and scale inhibitor of carbon steel in cooling towers [
14]. As this organic corrosion and scale inhibitor is phosphate-free, it tends not to cause eutrophication. Considering the product performance and environmental influence, the phosphate-free corrosion and scale inhibitor are superior to the traditional one. Additionally, the application of natural
Jatropha curcas oil as a steel anti-corrosion agent was recently investigated in a hydrochloric acid medium using weight loss technique, an electrochemical and electrochemical impedance spectroscopy (EIS) polarization technique [
15]. It was observed that the corrosion inhibition efficiency increased with inhibitor concentration. Additionally, the potentiodynamic polarization technique revealed that the presence of the natural
Jatropha curcas oil did not alter the mechanism of hydrogen evolution reaction and instead acted as a surface protection inhibitor. The corrosion inhibition data was fitted to the Frumkin adsorption isotherm model. Hydrazine carbodithioic acid derivatives were also studied as a corrosion inhibitor for mild steel in 0.5 M hydrochloric acid solutions [
16]. Langmuir isotherms were used to fit these inhibition adsorption data with adequate accuracy. However, the common denominator for the most widely used corrosion inhibitors is their toxicity and negative environmental impact. Replacing toxic inhibitors with more eco-friendly inhibitors for metal protection in acidic media has been the focus of many studies in recent years. Molasses and vegetable oil have been used as corrosion inhibitors for steel sheets in acid pickling processes [
17]. More recently, several studies have focused on the inhibition effect of different environmental friendly materials as corrosion inhibitors for stainless steel and mild steel corrosion in HCl media [
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24]. While organic corrosion inhibitors are toxic in nature, green inhibitors are biodegradable, and contain no heavy metals or other toxic compounds [
25]. Spent organic and inorganic inhibitors are released into the environment as aqueous wastes which causes harmful effects to living organisms. In offshore operations, corrosion inhibitors are used to form a barrier layer between the oil and the water phase. The water–inhibitor system is eventually released into the sea, which has toxic effects on marine life [
26].
The increasing emphasis on renewable energy is due to the rapid increase in demand for clean transportation fuels, environmental concerns, and shrinking oil reserves. Biodiesel is a biodegradable and renewable fuel that has recently been viewed with increasing interest. Although global biodiesel production was once expected to reach higher levels, the high production cost and the production of high volumes of crude glycerol, a by-product of the biodiesel making, have slowed the rate of biodiesel production [
27]. For every 100 kg of biodiesel produced, approximately 10 kg of crude glycerol is generated [
28,
29]. Crude glycerol is typically transformed to pure glycerol using energy-intensive and expensive processes for applications in the food, pharmaceutical, and cosmetics industries. Various methods for conversion and utilization of crude glycerol have been proposed such as combustion, composting, anaerobic digestion, animal feeds, and thermochemical/biological conversions to value-added products. The chemical composition of crude glycerol depends on the composition of the feedstock, the type of catalyst used in the biodiesel production process, the trans-esterification efficiency, the recovery efficiency of biodiesel, and whether the methanol and catalysts were recovered [
30]. Disposal of crude glycerol, which contains salt, methanol, free fatty acids, residual catalyst, etc., represents an economic and environmental challenge. The high cost of purification and market saturation behooves biodiesel producers to seek cheaper and alternate ways to utilize crude glycerol [
31]. The presence of impurities has a negative effect on the fermentation process by inhibiting product formation. Hence, microbial fermentation is an alternative and competitive approach for biodiesel producers. Canada’s annual biodiesel production in 2018 is estimated at 750 million liters [
32]. Hence, large quantities of crude glycerol (~75 million liters) will need to be either purified for use in secondary industries or use in nonconventional applications. In this study, non-purified crude glycerol from biodiesel production was evaluated as a potential green inhibitor for steel specimens in a hydrochloric acid medium.