Study on the nanoscale pyrogallol film to improve the corrosion resistance of carbon steel by atmospheric pressure chemical vapor deposition

A nanoscale pyrogallol film was deposited on carbon steel via atmospheric pressure chemical vapor deposition (APCVD) of tannic acid (TA). The resulting film exhibited high coverage (>80 %) and uniform thickness (1–2 nm), as confirmed by TG, SEM, FTIR, XPS and AFM. A polyurethane (PU) topcoat was then applied by spray coating onto the modified surfaces. For comparison, additional PU coatings were deposited over steel substrates pre-treated with TA-based self-assembled monolayers (SAMs) prepared at two different pH conditions. The resulting composite coatings are denoted as APCVD-PU, SAM-PU (pH = 5), and SAM-PU (pH = 8). Electrochemical impedance spectroscopy (EIS) revealed that after 50 days of immersion in a 3.5 wt% NaCl solution, the APCVD-PU coating exhibited the highest low-frequency impedance modulus (|Z| 0.01Hz ), which was 2.8, 1.7, and 6.9 times greater than those of SAM-PU (pH = 5), SAM-PU (pH = 8), and the unmodified PU coating, respectively. Crosshatch adhesion testing, pull-off testing and salt spray testing indicated that the APCVD-PU coating achieves the highest adhesion rating and the significant improvement of corrosion resistance. This work proposes a method for reinforcing the coating-metal interface, providing a feasible solution for the development of long-term corrosion-resistant coatings in harsh environments.