Nitrogen-doped hydrogenated amorphous carbon (a-C:H:N) films were synthesized via plasma-activated chemical vapor deposition (PACVD) using a capacitively coupled RF plasma (RF-CCP) source to address the critical challenge of carbon steel corrosion in acidic environments. This study is motivated by the urgent need to replace toxic corrosion inhibitors and improve upon conventional nitriding methods that typically offer <80 % protection in hydrochloric acid. We systematically investigated the effects of plasma power (400–700 W), treatment time (5–20 min), and N 2 /C 2 H 2 gas ratios (0–100 % N 2 ) on film properties and corrosion resistance in 1.0 M HCl. Electrochemical measurements revealed exceptional corrosion inhibition efficiencies (IE %) of 99.3 % for C 2 H 2 -derived films and 99.1 % for N 2 /C 2 H 2 (75/25 %) mixtures – the highest reported values for plasma-treated steel in acidic media. Tafel analysis showed these films preferentially inhibit anodic reactions, reducing corrosion current density from 400 μA/cm 2 (untreated steel) to just 2.7 μA/cm 2 . Surface characterization demonstrated that optimal films combine a hydrophobic carbon matrix (30.23 at.% C by EDX) with nitrogen functionalities (0.68 at.% N), forming a nanostructured barrier that resists acid penetration (SEM). The 700 W, 20 min N 2 treatment achieved 97.7 % IE %, while shorter 10 min treatments maintained 97.6 % efficiency, suggesting energy-saving potential. These results establish that a-C:H:N films deposited at room temperature can provide near-complete corrosion protection through two mechanisms: (1) C–H network formation at high power and (2) synergistic N-doping that enhances passivation. The study provides a scalable, environmentally friendly alternative to conventional coatings for industrial applications involving acid exposure, with quantified performance benchmarks that surpass existing plasma-based methods.
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