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Synergistic Corrosion Inhibition of Carbon Steel in Borewell Water by Beetroot Extract and Zn²⁺: Gravimetric, Electrochemical and Surface-Morphological Investigations

This study demonstrates that an aqueous beetroot extract, which alone shows limited or negative inhibition, synergistically enhances corrosion protection for carbon steel in borewell water when combined with Zn²⁺, achieving an 84% inhibition efficiency through the formation of a protective anodic film confirmed by gravimetric, electrochemical, and surface-morphological analyses.

Original authors: C. S.A. Raj, Susai Rajendran

Published 2026-10-06

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CC BY 4.0

Original authors: C. S.A. Raj, Susai Rajendran

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Metal is everywhere, from the pipes carrying our water to the girders holding up our bridges. Yet, when metal meets water, a slow, invisible battle begins. This is corrosion, a natural process where the metal gradually breaks down as it reacts with the water around it. For the steel used in construction and industry, this reaction can weaken the structure, leading to leaks, failures, and costly repairs. For decades, engineers have fought this battle by adding chemicals to the water to stop the reaction. However, many of these traditional chemicals are toxic and harmful to the environment, prompting scientists to look for safer, "green" alternatives found in nature. The question becomes whether something as simple as a vegetable extract can protect heavy metal as effectively as industrial chemicals.

Researchers set out to test this idea using carbon steel, a common and affordable type of metal, and a liquid made from beetroot. They placed steel samples into borewell water, which is water drawn from deep underground, to see how quickly it would rust. First, they tried using only the beetroot liquid. The results were surprising and complicated. At very low amounts, the beetroot extract slowed down the rusting slightly. But as they added more of the extract, the protection vanished, and in some cases, the steel actually rusted faster than it did in plain water. The beetroot alone was not a reliable shield; in fact, too much of it seemed to make the problem worse.

The story changed when the scientists introduced a second ingredient: zinc ions. Zinc is a metal often used in coatings to prevent rust, but here it was added in tiny amounts to the water. When the researchers mixed the beetroot extract with zinc, the behavior of the steel transformed completely. Instead of fighting each other, the beetroot and the zinc worked together in a powerful partnership. In the best combination, using eight milliliters of beetroot extract and fifty parts per million of zinc, the rusting slowed down dramatically. The steel lost very little weight compared to the unprotected samples, showing that the metal surface was being held in place. This mixture achieved an eighty-four percent reduction in corrosion, a level of protection that neither ingredient could reach on its own.

To understand how this protection worked, the team looked at the steel under powerful microscopes and measured the electrical activity on the metal's surface. They found that without any treatment, the steel surface became rough and pitted, covered in the debris of corrosion. In the water with the beetroot and zinc, the surface remained smooth and intact, as if a thin, invisible film had formed over it. Electrical measurements confirmed that this film acted as a barrier, making it much harder for the water to steal electrons from the metal, which is the fundamental step in the rusting process. The data showed that the beetroot and zinc combined to create a protective layer that stuck to the steel, blocking the water from causing damage.

This study demonstrates that nature can offer solutions to industrial problems, but it also highlights that the solution is not always straightforward. A plant extract alone might fail or even backfire, but when paired with a simple mineral like zinc, it becomes a highly effective guardian for metal. The researchers found that this specific combination of beetroot and zinc creates a synergistic effect, where the whole is far greater than the sum of its parts. While more work is needed to understand the exact chemical makeup of the beetroot and to test how this holds up over long periods, the findings offer a promising path toward safer, environmentally friendly ways to protect the steel that supports our modern world.

Technical Summary: Synergistic Corrosion Inhibition of Carbon Steel in Borewell Water by Beetroot Extract and Zn²⁺

Problem Statement
Carbon steel is widely utilized in structural and industrial applications due to its favorable mechanical properties and low cost; however, it is susceptible to electrochemical degradation when exposed to aqueous environments such as borewell water. Conventional corrosion inhibitors, including chromates and phosphates, are effective but raise significant concerns regarding toxicity and environmental persistence. Consequently, there is a need for "green" alternatives derived from renewable biological sources. While plant extracts have shown promise, their efficacy can be concentration-dependent, and they may sometimes fail to provide adequate protection or even accelerate corrosion at higher concentrations. This study addresses the challenge of developing an effective, environmentally benign corrosion inhibition system for carbon steel in borewell water by investigating the synergistic potential of aqueous beetroot extract combined with zinc ions (Zn²⁺).

Methodology
The study employed a multi-faceted approach to evaluate the corrosion behavior of carbon steel specimens (composition: 0.026% S, 0.06% P, 0.4% Mn, 0.1% C, balance Fe) in borewell water. The experimental design included:

Inhibitor Preparation: An aqueous extract was prepared by boiling powdered fresh beetroot in double-distilled water.
Gravimetric Analysis: Weight-loss measurements were conducted to determine corrosion rates and inhibition efficiencies (IE) across varying concentrations of beetroot extract (0–8 mL) and Zn²⁺ (0, 25, and 50 ppm).
Electrochemical Characterization: Potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) were utilized to analyze corrosion potential ( $E_{corr}$ ), corrosion current density ( $I_{corr}$ ), linear polarization resistance (LPR), charge-transfer resistance ( $R_t$ ), and double-layer capacitance ( $C_{dl}$ ).
Surface Morphology: Scanning Electron Microscopy (SEM) was used to visually assess surface damage and the formation of protective films on specimens exposed to uninhibited and inhibited solutions.

Key Contributions and Results
The research demonstrates that the corrosion inhibition behavior of beetroot extract is highly concentration-dependent and that its efficacy is significantly enhanced through synergy with Zn²⁺.

Limitations of Beetroot Extract Alone: When used in isolation, the beetroot extract exhibited a non-linear relationship with concentration. At 2 mL, it provided moderate inhibition (~35%). However, as the concentration increased to 4–8 mL, the inhibition efficiency dropped, eventually becoming negative (e.g., −30% at 8 mL), indicating an acceleration of metal dissolution rather than protection.
Synergistic Effect with Zn²⁺: The addition of Zn²⁺ fundamentally altered the performance of the system.
- At 25 ppm Zn²⁺: The combination improved inhibition compared to the extract alone, reaching a maximum of 48% at 4 mL extract.
- At 50 ppm Zn²⁺: A pronounced synergistic effect was observed. While 50 ppm Zn²⁺ alone provided only ~14% inhibition and 8 mL beetroot extract alone resulted in −30% inhibition, the combination of 8 mL extract and 50 ppm Zn²⁺ achieved a maximum inhibition efficiency of 84%.
Electrochemical Insights:
- Polarization: The presence of the synergistic inhibitor system shifted the corrosion potential ( $E_{corr}$ ) from −756 mV/SCE (blank) to −531 mV/SCE. The study reports this shift as suggesting a predominantly anodic contribution to corrosion inhibition. The corrosion current density decreased, and LPR increased from 3385 to 3980 $\Omega$ cm².
- EIS: The charge-transfer resistance ( $R_t$ ) increased dramatically from 312 to 2189 $\Omega$ cm², while the double-layer capacitance ( $C_{dl}$ ) decreased from $16.36 \times 10^{-9}$ to $2.329 \times 10^{-9}$ F cm². These changes indicate the formation of a protective interfacial layer that hinders charge transfer and displaces water molecules at the metal surface.
Morphological Evidence: SEM images confirmed that carbon steel surfaces exposed to borewell water without inhibitors were rough and covered in corrosion products. In contrast, surfaces treated with the beetroot extract–Zn²⁺ system exhibited substantial smoothing and reduced evidence of corrosion product formation.

Significance and Claims
The paper claims that the combination of beetroot-derived phytochemicals and Zn²⁺ creates an effective, synergistic corrosion inhibition system for carbon steel in aqueous environments. The study highlights that while plant extracts alone may be insufficient or counterproductive at certain concentrations, their integration with specific inorganic ions can yield high-performance "green" inhibitors.

The authors conclude that this beetroot extract–Zn²⁺ system represents a promising approach for corrosion control. However, they maintain a modest stance regarding the mechanistic interpretation, noting that the precise molecular composition of the extract was not determined in this study. Therefore, the proposed mechanism of synergistic film formation is presented as a chemically plausible working model rather than a definitive molecular explanation. The authors suggest that future work should focus on quantifying the specific phytochemicals, establishing adsorption isotherms, evaluating temperature dependence, and determining long-term stability before practical deployment.

Technical Summary: Synergistic Corrosion Inhibition of Carbon Steel in Borewell Water by Beetroot Extract and Zn²⁺

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