Barnacle cement protein as an efficient bioinspired corrosion inhibitor

The study addresses the need for environmentally benign corrosion inhibitors for metals exposed to marine environments, where high salinity accelerates corrosion and incurs significant economic costs. Conventional organic inhibitors are effective but raise toxicity concerns. Proteins and amino acids, with functional groups capable of interacting with metal surfaces, are promising green alternatives. Inspired by barnacle adhesion and previous observations that barnacle attachment can mitigate certain corrosion modes on metals, the authors hypothesize that the barnacle cement protein MrCP20—characterized by high cysteine and charged residue content and strong underwater adhesion—could inhibit corrosion of steel by forming an adsorbed layer and interacting with iron ions to block corrosion pathways.

The paper reviews common classes of synthetic corrosion inhibitors (azoles, Schiff bases, phenolics, amines, thio-compounds, pyrimidines) and their mechanisms via adsorption through electronegative atoms and polar functional groups. Environmental concerns over such inhibitors motivate bio-based alternatives. Prior work has shown amino acids can inhibit corrosion of metals in acids and that mussel adhesive protein (Mefp-1) can protect carbon steel. Reports note barnacle adhesion to metals and differential effects on corrosion under live versus dead barnacles, suggesting adhesive proteins may influence corrosion processes. The authors’ previous structural studies on MrCP20 revealed adhesive properties linked to cysteine-rich and charged sequences, motivating its evaluation as a green inhibitor.

• Protein production: Recombinant MrCP20 (rMrCP20) expressed in E. coli BL21(DE3) using pET-22b(+), purified by Ni-NTA affinity and SEC in 20 mM Tris, 150 mM NaCl, pH 8.3.
• Substrate preparation: AH36 mild steel coupons (10×10×2 mm) embedded in epoxy, polished to mirror finish (to 3 µm diamond), cleaned and dried; for electrochemistry, wired prior to embedding, exposing 1 cm² area.
• Corrosion tests: Coupons immersed 24 h in pH 8.3 buffer (20 mM Tris, 150 mM NaCl) with rMrCP20 at 0–10 mg mL−1. Visual time-lapse imaging (0–24 h) and post-cleaning inspections; ImageJ quantified corroded area.
• Weight loss by ICP-OES: Total Fe loss from dissolved species and corrosion products removed by 1% HCl/hexamethylenetetramine; Fe quantified (ICP-OES at 238.86 nm). Corrosion rate (CR) and inhibition efficiency η computed from standard equations.
• Electrochemical measurements: EIS and potentiodynamic polarization (PDP) in three-electrode cell (Ag/AgCl reference, Pt counter). EIS at OCP with 10 mV perturbation, 0.01–10 kHz; Nyquist/Bode/phase analyses and equivalent electrical circuit fitting to extract Rct, film resistance, and CPE parameters; ηEIS computed from Rct. PDP from −0.25 V vs OCP to −0.4 V vs Ag/AgCl at 10 mV min−1; Tafel extrapolation for Ecorr, icorr, βa, βc; CR and ηPDP computed. Cyclic voltammetry assessed redox changes.
• Adsorption and Fe3+ interaction: QCM-D on iron-coated quartz (4.95 MHz) to track adsorption of rMrCP20 (23 µM), rinsing, FeCl3 (100 µM) addition, and final rinse; Δf and ΔD analyzed across overtones.
• Film thickness: Nanoindentation (cube-corner tip) on rMrCP20-coated Fe sensors hydrated in water to determine layer thickness from load–displacement slope changes, with/without FeCl3 exposure.
• Spectroscopy: ATR-FTIR on lyophilized samples (rMrCP20 1 or 5 mg mL−1 with varying FeCl3 ratios), processed to deconvolute amide I and identify functional group interactions and secondary structure changes.
• Adsorption isotherms: Degree of surface coverage from weight loss fitted to multiple models; Freundlich model selected; Kads and n determined; ΔGads calculated.
• Structure and simulations: SAXS (1.3, 4.1, 6 mg mL−1) to assess oligomeric state and Rg; model fitting (CRYSOL). MD simulations (AMBER99SB-ILDN, TIP3P water) of an AlphaFold2-based dimer in high Fe3+ conditions to map Fe–residue interactions and binding preferences.
• Surface mapping: Fluorescence microscopy with FITC-labeled rMrCP20 on etched AH36 to visualize adsorption preference; AES elemental mapping with topography correction and XPS survey and high-resolution scans to confirm protein adsorption and functional groups.

• Corrosion inhibition efficacy is concentration-dependent. Visual inspection showed extensive rusting without protein and markedly reduced corrosion with ≥5 mg mL−1 rMrCP20; corroded area decreased from ~91% (0 mg mL−1) to ~2% (10 mg mL−1) after 24 h.
• Weight loss/ICP-OES: Corrosion rate decreased with increasing rMrCP20; lowest CR ≈ 0.12 mm yr−1 with η ≈ 88.48% at optimal concentrations.
• EIS: Nyquist semicircle diameter increased with protein concentration, indicating higher charge transfer resistance. Fitted Rct increased from ~926 Ω cm² (1 mg mL−1) to ~4721 Ω cm² (5 mg mL−1); ηEIS rose from 19.8% to 84.3%. Double layer capacitance decreased (control Cdl ≈ 757 µF cm−2), and CPE n increased, indicating more homogeneous films.
• PDP: icorr dropped from 29.5 µA cm−2 (no inhibitor) to 4.0 µA cm−2 (5 mg mL−1); CR reduced from 0.34 to 0.05 mm yr−1; ηPDP ≈ 86.5%. Ecorr shifted slightly cathodically; βc relatively unchanged versus concentration, while βa decreased, suggesting modified Fe dissolution mechanism via metal–protein complexation. CV peak currents decreased with protein, consistent with inhibited redox activity.
• QCM-D: Rapid, strong adsorption of rMrCP20 on Fe (Δf ≈ −40 Hz; ΔD ≈ 1.6×10−6 at 5th overtone), forming a compact, largely rigid adlayer that further tightened over time. Addition of Fe3+ induced additional Δf and ΔD shifts, evidencing strong Fe3+ binding to the pre-adsorbed protein layer and adlayer restructuring.
• Nanoindentation: Hydrated oxide layer ≈ 74 nm. Protein adsorption added a ~8 nm film (total ≈ 82 nm). After FeCl3 interaction, the protein layer compacted to ~5 nm (total ≈ 79 nm).
• ATR-FTIR: At higher FeCl3 (protein:FeCl3 ~1:60), amide I shifted 1641→1631 cm−1 with secondary structure bias toward antiparallel β-sheets; new peaks indicated interactions: His imidazole chelation (1043 cm−1 split to 1038/1053 cm−1), carboxylate–Fe3+ ionic bridges/coordination (emergent ~1437 cm−1 with reduced 1398 cm−1), cysteine oxidation (S=O ~1403 cm−1), and additional side-chain interactions (1138, 1262, 1296 cm−1).
• Adsorption thermodynamics: Freundlich isotherm best fit (R²=0.96), Kads=1.33×10−2 L mg−1, n=2.18 (>1, favorable), ΔGads=−23.52 kJ mol−1, consistent with mixed physisorption and chemisorption.
• SAXS: rMrCP20 exists predominantly as a dimer in solution at ~4–6 mg mL−1 (MW ≈ 42–45 kDa; monomer ~21 kDa).
• MD: Fe ions preferentially interact with acidic residues (Asp/Glu), with multiple Fe ions bound per residue at interfaces; ~54 close-contact Fe ions around the dimer in the largest cluster; supports Fe-mediated protein–protein interactions and aggregation observed experimentally.
• Surface mapping: FITC–rMrCP20 fluorescence concentrated at grain boundaries (GBs). AES/XPS confirmed protein adsorption and presence of protein functional groups; stronger signals at GBs suggest preferential adsorption that helps mitigate GB-initiated corrosion.
• Overall mechanism: Strong adsorption to steel, preferentially at GBs, combined with Fe ion coordination/bridging forms a compact adlayer that increases impedance and blocks charge transfer, delaying corrosion processes.

The findings demonstrate that rMrCP20 effectively inhibits corrosion of AH36 steel in a simulated seawater buffer by adsorbing quickly to form a homogeneous, protective protein layer that blocks electrochemical reactions at the metal–electrolyte interface. Electrochemical data (EIS and PDP) consistently show increased charge transfer resistance and reduced corrosion current with rising protein concentration, aligning with weight loss reductions. Spectroscopy, QCM-D, and nanoindentation confirm that the protein strongly adheres and reorganizes into a compact film that incorporates Fe ions through interactions with deprotonated acidic and cysteine residues, and His imidazole groups, reinforcing the adlayer and impeding charge transfer pathways. Preferential adsorption at grain boundaries—common corrosion initiation sites—provides targeted protection that likely suppresses intergranular and pitting corrosion. SAXS and MD support a dimeric solution state and highlight Fe-mediated interactions that enhance film cohesion. Together, these results validate the hypothesis that the barnacle cement protein’s adhesive and metal-binding properties yield synergistic corrosion protection, offering a greener alternative to conventional inhibitors.

rMrCP20, a barnacle cement protein, serves as an effective green corrosion inhibitor for AH36 steel in high-salinity, pH 8.3 conditions. Above ~5 mg mL−1, it rapidly adsorbs to steel, increases coating impedance, reduces corrosion currents and rates, and forms a compact adlayer that incorporates Fe ions via electrostatic and coordination interactions, accompanied by secondary structure adjustments. The protein preferentially adsorbs at grain boundaries, mitigating corrosion initiation. These insights offer molecular-level guidelines to design protein-based or bioinspired corrosion inhibitors and indicate potential for scale-up as environmentally friendly additives for marine corrosion protection. Future work could optimize formulation and delivery methods, assess long-term stability in real seawater conditions, and evaluate compatibility with industrial coatings.