Barnacle cement protein as an efficient bioinspired corrosion inhibitor

Corrosion of metallic structures in seawater is a significant problem, causing substantial material loss and economic damage. Conventional corrosion inhibitors, often organic compounds, are effective but frequently exhibit environmental toxicity. The substantial global cost of corrosion necessitates the development of greener and more sustainable alternatives. This research explores the potential of bio-inspired corrosion inhibitors, focusing on proteins derived from marine organisms known for their strong adhesion and corrosion resistance properties. Barnacles, in particular, exhibit remarkable adhesion to submerged substrates, and while previous studies indicated that dead barnacles accelerate corrosion, live barnacles surprisingly reduce corrosion. This observation suggests that components of their adhesive cement may possess anti-corrosion properties. This study centers on the recombinant protein rMrCP20 from the barnacle *Megabalanus rosa*, a cysteine-rich protein previously shown to have strong underwater adhesive properties due to its high cysteine and charged amino acid content. The hypothesis is that rMrCP20's adhesion properties, coupled with its potential interaction with released iron ions, will result in effective corrosion inhibition of steel in a simulated marine environment. Success in this area would contribute significantly to the development of environmentally benign corrosion protection strategies for marine structures, reducing economic costs and mitigating environmental impact.

The literature extensively documents the use of organic compounds as corrosion inhibitors, including azole derivatives, Schiff bases, phenolic compounds, amine derivatives, thio-compounds, and pyrimidine derivatives. While effective, their environmental impact is a growing concern. Research on bio-based inhibitors, specifically proteins and amino acids, has gained traction. Mussel adhesive proteins (MAPs), particularly Mefp-1 from *Mytilus edulis*, have shown promising anti-corrosion effects on carbon steel. However, exploration of barnacle cement proteins as corrosion inhibitors remains limited, despite the known strong adhesion and corrosion-mitigating effects observed in live barnacles attached to steel. This study addresses this gap by investigating the anti-corrosion properties of rMrCP20, a protein previously characterized for its adhesive properties.

The study employed a multi-faceted approach to investigate the anti-corrosion properties of rMrCP20. rMrCP20 was expressed and purified using established protocols. AH36 mild steel coupons were prepared by embedding them in epoxy and polishing them to a mirror finish. Time-resolved corrosion studies were conducted by immersing the coupons in a pH 8.3 buffer containing 150 mM NaCl and 20 mM Tris, with varying concentrations of rMrCP20 (0.1–10 mg mL⁻¹). Visual inspection and ImageJ analysis were used to assess corrosion. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measured the iron weight loss from the coupons and the solution, calculating the corrosion rate and inhibition efficiency. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) techniques further investigated the impedance and corrosion inhibition mechanism. The EIS data were fitted to an equivalent electrical circuit to extract relevant electrochemical parameters. QCM-D measurements on iron sensors monitored the adsorption of rMrCP20 and its interaction with Fe³⁺ ions. Nanoindentation determined the thickness of the adsorbed protein layer. Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), coupled with molecular dynamics (MD) simulations, identified the molecular interactions between rMrCP20 and Fe ions. Fluorescence microscopy, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS) investigated the protein's adsorption behavior on the steel surface, focusing on grain boundaries.

The results showed a concentration-dependent anti-corrosion effect of rMrCP20. At concentrations above 5 mg mL⁻¹, rMrCP20 significantly reduced corrosion, achieving an inhibition efficiency of up to 88.48% (weight loss measurements), 84.3% (EIS), and 86.5% (PDP). QCM-D revealed strong and rapid adsorption of rMrCP20 onto the steel surface, with subsequent Fe³⁺ addition leading to further changes in the adsorbed layer. Nanoindentation showed an 8 nm thick protein layer, slightly compacted after Fe³⁺ interaction. FTIR and MD simulation identified that Fe³⁺ ions form ionic bridges and coordination bonds with negatively charged side chains (Asp, Glu) and histidine (His) residues of rMrCP20. Fluorescence microscopy, AES, and XPS indicated preferential adsorption of rMrCP20 at grain boundaries, suggesting protection against grain boundary-initiated corrosion. SAXS analysis indicated that rMrCP20 exists as a dimer in solution under optimal corrosion inhibition conditions. The Freundlich adsorption isotherm model best fitted the adsorption data, indicating multilayer adsorption on heterogeneous surfaces. The calculated standard free energy of adsorption (-23.52 kJ mol⁻¹) confirmed the spontaneous interaction between the protein and steel.

The findings strongly support the hypothesis that rMrCP20 exhibits effective anti-corrosion properties. The multimodal mechanism involves strong adsorption of rMrCP20 onto the steel surface, preferentially at grain boundaries, forming a protective layer that hinders the access of corrosive ions. The interaction of rMrCP20 with released Fe ions, forming stable complexes, further enhances the protective effect. This mechanism explains the increased impedance, reduced corrosion current density, and improved corrosion inhibition efficiency observed. The preferential adsorption at grain boundaries suggests a targeted approach to mitigating intergranular corrosion. The data are consistent across multiple analytical techniques, solidifying the conclusions. The use of a bio-inspired, naturally derived protein as an effective corrosion inhibitor offers a significant advantage over traditional organic inhibitors, promoting environmental sustainability.

This study demonstrates the potential of rMrCP20, a barnacle cement protein, as a highly effective and environmentally friendly corrosion inhibitor for steel in marine environments. The protein's strong adhesion, combined with its ability to interact with iron ions and form a protective layer, leads to significant corrosion inhibition. The findings offer valuable molecular-level insights for designing future bio-inspired corrosion inhibitors. Future research should focus on optimizing rMrCP20's performance and exploring its potential for large-scale applications in marine corrosion protection.

The study focused on AH36 steel in a simulated marine environment. The results might not be directly transferable to other steel types or real-world marine conditions, which can vary in salinity, temperature, and presence of other marine organisms. Further investigation is needed to assess the long-term stability and effectiveness of rMrCP20 under diverse environmental conditions and on other metal substrates. The current study primarily focuses on the anti-corrosion mechanism, with limited investigation into the biodegradability and potential toxicity of the protein.