Microstructure engineering of sol-gel SiO2 coatings with biomass derived carbon dots for enhanced corrosion protection of stainless steel.

Abstract

Biomass-derived pomegranate peel carbon dots (PP-CDs) were incorporated into sol-gel silica coatings to develop environmentally sustainable protective films for stainless steel 304 in hydrochloric acid and the effect of PP-CDs loading on coating microstructure and corrosion resistance was investigated. The PP-CDs were prepared hydrothermally and characterized by TEM, FT-IR XPS, UV-vis and photoluminescence spectroscopy. They are spherical, photoluminescent particles (average diameter ∼ 7.4 nm) with hydroxyl, carbonyl and carboxyl surface groups. To examine their fate during processing, the SiO2-0.003PP-CD material was also studied as a calcined powder (500 °C, 2 h) by TEM and XPS, which showed nanoscale features of about 3.0 nm without noticeable aggregation, the presence of C, N, O and Si, and a shift of the O 1s peaks to lower binding energy relative to pristine PP-CDs. SiO2 coatings containing 0, 0.003 and 0.005 mg PP-CDs were deposited on S304 by spin-coating and calcined at 500 °C. Their microstructure, composition and wettability were examined by SEM, EDS and contact angle measurements and their corrosion behavior after 24 h in 1 M HCl at 25 °C by EFM, PDP and EIS. Among the investigated compositions, the SiO2-0.003PP-CDs coating exhibited the most continuous microstructure, maintaining its integrity after immersion in 1.0 M HCl, whereas the undoped coating underwent severe cracking and delamination. Electrochemical measurements (EFM, PDP, and EIS) gave the same ranking, with inhibition efficiencies of 84-86% (EFM and PDP) for the 0.003 mg coating compared with 50-53% for undoped silica. EDS, contact angle, and AFM results were consistent with better retention of the 0.003 mg coating after immersion. The improvement over the undoped coating is primarily attributed to the addition of PP-CDs, which is accompanied by a change in film morphology and microstructure, from a cracked, agglomerated film to a continuous, more homogeneous one at 0.003 mg. These findings indicate that biomass-derived carbon dots can improve the short-term corrosion resistance of sol-gel silica coatings on stainless steel in hydrochloric acid under laboratory conditions.

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