Anti-corrosion effect of ethanol extract from Piper guineense leaves on mild steel in water-in-diesel emulsion

Abstract The growing demand for cleaner fuel alternatives has driven significant interest in water-in-diesel emulsions to reduce nitrogen oxides and particulate matter emissions, particularly in diesel-dependent regions. However, corrosion of fuel system components remains a critical barrier to water-in-diesel emulsions adoption. This study presents the novel application of ethanol extract of Piper guineense leaves as a green corrosion inhibitor for mild steel in water-in-diesel emulsion environments while also addressing the stability of water-in-diesel emulsion with varying water content, hydrophilic-lipophilic balance, surfactant dosage and mixing time. The anti-corrosion performance was evaluated by immersing mild steel specimens in a water-in-diesel emulsion containing Piper guineense extract at concentrations of 0.5, 1.0, and 1.5 g/L, with weight loss measurements conducted over 672 h. The Piper guineense extract demonstrated a maximum inhibition efficiency of 91.47% at 1.0 g/L, significantly reducing the corrosion rate from 0.0140 to 0.0008 mm/year. Stability evaluation revealed that the maximum emulsion performance was achieved with water-in-diesel emulsion formulated with 20% water content and a 5% surfactant dosage at a hydrophilic-lipophilic balance value of 9, maintaining stability for 8 h without visible phase separation. Surface characterization using Atomic Force Microscopy revealed a 23% reduction in apparent surface roughness and a 28% decrease in overall surface roughness for treated specimens. Fourier-Transform Infrared Spectroscopy analysis identified the adsorption of Piper guineense extract bioactive compounds on the steel surface, forming a protective barrier. These findings highlight the potential of Piper guineense extract as a sustainable additive that enhances material protection while supporting the operational viability of water-in-diesel emulsion fuels.