Prolonged Protection of Copper in Acidic Media Through the Synergistic Effect of Fat-Soluble Vitamins

The long-term corrosion protection of copper surfaces modified with self-assembled hydrophobic layers (SAHLs) based on stearic acid (SA) and two fat-soluble vitamins, vitamin K3 (menadione) and vitamin E (E307), was investigated in simulated acidic urban rain (pH 5) over 7 days. The SAHLs were characterised by SEM, contact angle goniometry, ATR-FTIR, potentiodynamic polarisation, and electrochemical impedance spectroscopy (EIS). Surface modification was achieved by immersing copper samples in ethanolic SA solutions containing 2.0 wt% of fat-soluble vitamins. Variants included individual additives, (SA + 2.0 wt% K3) and (SA + 2.0 wt% E307), as well as mixtures with a constant total additive content of 2.0 wt%: (SA + [1.5 wt% K3 + 0.5 wt% E307]) and (SA + [1.0 wt% K3 + 1.0 wt% E307]). The (SA + 2.0 wt% K3) modification produced needle-like microstructures with strong short-term inhibition but poor long-term stability, while (SA + 2.0 wt% E307) formed smoother, more stable films. The mixture containing equal mass fractions of vitamins, (SA + [1.0 wt% K3 + 1.0 wt% E307]), exhibited a synergistic effect, yielding hierarchically structured, flower-like morphologies with high polarisation resistance and stable impedance over 7 days. These results show that combining K3 and E307 with stearic acid provides robust, environmentally friendly, and durable protection for copper surfaces.