Interfacial Corrosion Protection of ST37 Steel by Borassus flabellifer Husk Extract in 1 M HCl: Effect of Inhibitor Concentration

In the industrial world, steel is the skeleton of modern life, forming everything from skyscrapers to ships. However, to make this metal useful, manufacturers often need to strip away rust and scale, a process called pickling, which involves soaking the steel in strong acids like hydrochloric acid. While this acid is excellent at cleaning the surface, it is indiscriminate; once the dirt is gone, the acid begins to eat away at the steel itself, causing it to dissolve and weaken. To stop this, engineers add corrosion inhibitors, substances that coat the metal and act as a shield. For decades, these shields have been made from synthetic chemicals that work well but can be toxic and harmful to the environment. This has led scientists to look toward nature for alternatives, searching for plant extracts that might offer protection without the ecological cost. The question is not just whether a plant can stop rust, but how the amount of that plant material changes its ability to protect the metal, and whether there is a specific point where adding more becomes less effective.

A team of researchers from Indonesia and Japan set out to answer these questions using the husk of the Borassus flabellifer, a type of palm tree fruit commonly found in tropical regions. They focused on ST37 steel, a standard structural steel, and tested how an extract from the palm's fibrous outer shell performed in a one-molar hydrochloric acid solution. The researchers did not simply guess the right amount to use; instead, they systematically tested the extract at five different concentrations, ranging from 50 parts per million up to 250 parts per million. By immersing steel samples in these solutions, they could measure exactly how much metal was lost over time and observe the chemical changes happening at the surface. Their goal was to find the precise concentration where the plant extract offered the strongest defense against the acid, and to understand the physical mechanism behind that protection.

The results revealed a clear and somewhat surprising pattern. The extract did not work better simply because there was more of it. Instead, the protection peaked at a specific concentration of 100 parts per million. At this level, the extract reduced the weight loss of the steel by nearly 88 percent compared to the unprotected samples. When the researchers looked at the speed at which the steel was corroding, they found that the rate dropped from 5.62 millimeters per year down to just 1.43 millimeters per year at this optimal concentration. However, when they increased the amount of extract beyond this point, the protection actually weakened. At 150, 200, and 250 parts per million, the steel suffered more corrosion than it did at the 100 parts per million mark. This suggests that there is a limit to how the molecules in the extract can arrange themselves on the metal surface, and crowding the surface with too many molecules might actually disrupt the protective layer.

To understand why this happened, the team examined the steel under powerful microscopes and analyzed the chemical signals coming from the surface. They found that at the optimal concentration, the steel surface remained remarkably smooth and intact, covered by a uniform layer of the plant extract. In contrast, the unprotected steel became rough and pitted, with deep valleys and jagged peaks where the acid had eaten away the metal. The chemical analysis showed that the extract contains oxygen-rich compounds and aromatic structures, likely derived from natural substances like flavonoids and tannins found in the plant. These molecules appear to stick to the steel, forming a barrier that blocks the acid from reaching the metal. The researchers observed that this barrier works by slowing down both the release of iron ions from the metal and the generation of hydrogen gas, effectively putting a brake on the corrosion process from both sides.

The study also looked at how the extract molecules interact with the steel surface, testing whether they spread out in a single, orderly layer or clump together in a messy pile. The data fit best with a model that describes a single, even layer of molecules covering the surface, much like a thin film of oil spreading across water. However, the researchers noted that while this model described the coverage well, they could not fully calculate the energy holding the molecules in place due to a specific mathematical irregularity in their data. Despite this limitation, the physical evidence was consistent: the extract formed a protective film that significantly reduced surface roughness. The smoothest surfaces were found at the 100 parts per million concentration, where the average height of surface peaks and valleys was reduced by more than half compared to the damaged, unprotected steel.

Ultimately, the work demonstrates that the husk of the Borassus flabellifer palm is a potent source of natural corrosion protection, but its effectiveness is highly sensitive to the amount used. The researchers concluded that the extract works by adsorbing onto the steel, creating a shield that prevents the acid from attacking the metal. While higher concentrations might seem like they would offer better protection, the study showed that there is an optimal point where the shield is most complete. Beyond that point, the protective layer becomes less effective, perhaps because the molecules interfere with one another. This finding highlights the importance of precision in using natural inhibitors; it is not a case of "more is better," but rather a need to find the exact balance where the plant's natural chemistry can best defend the metal.