Sesamum indicum as Green Surfactant Corrosion Inhibitor for 316L Steel in Hydrochloric Acid Solution

In the oil and gas industry, the metal equipment that separates and refines crude oil faces a constant, invisible battle. The environment inside these machines is often filled with harsh chemicals, particularly chloride ions, which act like tiny, aggressive agents that strip away the protective layers of steel. When this happens, the metal corrodes, leading to leaks, equipment failure, and massive financial losses for nations that rely on these resources. For decades, engineers have fought back using synthetic chemical inhibitors, but these solutions often come with their own problems: they can be toxic to the environment and expensive to produce. This has led scientists to look toward nature for a gentler, more sustainable alternative. The goal is to find a substance derived from plants that can stick to metal surfaces and form a shield, stopping the corrosive attack without poisoning the water or the air.

A team of researchers from Nigeria set out to test one such candidate: the seed of the sesame plant, known scientifically as Sesamum indicum. They focused on a specific type of steel called 316L stainless steel, a material prized for its strength and resistance to rust, which is commonly used in the critical overhead sections of oil refineries. To simulate the extreme conditions these metals face, the researchers created a laboratory environment using a highly concentrated hydrochloric acid solution, measuring nine moles per liter, a level of acidity that would rapidly destroy unprotected steel. They introduced an extract made from sesame seeds into this acid and watched to see if the plant material could slow down the corrosion. The study was not just about finding a chemical that works; it was about proving that a common agricultural product could serve as a "green" surfactant—a substance that spreads out to cover a surface and repel harmful ions—while aligning with global goals for clean water and responsible production.

The researchers began by preparing the steel samples, polishing them to a mirror finish to ensure a clean starting point. They then submerged these samples in the acid bath, some with the sesame extract added and some without. Over a period of more than 500 hours, they weighed the steel pieces daily to measure exactly how much metal was lost to corrosion. The results were clear: the steel left in the acid without any protection lost a significant amount of weight, showing severe damage. However, as the researchers increased the amount of sesame extract in the solution, the weight loss dropped dramatically. At the highest concentration they tested, the extract reduced the corrosion rate so effectively that the steel retained most of its mass. The extract acted as a barrier, preventing the aggressive acid from eating away at the metal.

To understand how this protection worked, the team looked at the chemical behavior of the sesame extract using a technique that identifies the specific bonds and atoms within a substance. They found that the extract is rich in organic compounds like alkaloids, saponins, and glycosides, which contain nitrogen, oxygen, and sulfur atoms. These atoms have a natural tendency to donate electrons. When the extract was added to the acid, these molecules moved toward the steel surface and attached themselves, forming a thin, protective film. The researchers observed that the functional groups responsible for this attachment were used up on the steel surface, confirming that the molecules had bonded tightly to the metal rather than just floating in the liquid. This film acted as a shield, blocking the chloride ions from reaching the steel.

Further tests using electrical measurements revealed that the sesame extract did not just block one side of the corrosion process; it slowed down both the reaction where the metal dissolves and the reaction where hydrogen gas is produced. This means the extract functions as a mixed-type inhibitor, protecting the steel from multiple angles. The way the molecules settled onto the metal surface followed a specific pattern known as the Frumkin adsorption isotherm, which describes how particles interact with each other as they pack onto a surface. This interaction suggested that the molecules were not just randomly scattered but were organizing themselves into a cohesive layer that covered the steel effectively.

Visual evidence supported these chemical findings. When the researchers examined the steel under a microscope, the unprotected samples showed clear signs of pitting and uniform corrosion, with the surface pitted and damaged by the acid. In contrast, the samples treated with the sesame extract appeared much smoother, with a visible protective layer that had prevented the acid from attacking the metal. The study concluded that Sesamum indicum is a highly effective, non-toxic, and biodegradable inhibitor that can protect 316L stainless steel in extremely acidic environments. By using a common plant seed, the research offers a path toward reducing the economic losses caused by corrosion while simultaneously supporting environmental sustainability and reducing the reliance on toxic synthetic chemicals.