Analysis of the Effect of Belimbing Wuluh Leaf Extract (Averrhoa Bilimbi L.) as a Green Inhibitor on Corrosion Rate and Validation of Optimized Operating Conditions in a Circulating Cooling Water System

Corrosion and fouling in circulating cooling water systems can reduce equipment performance. This study evaluates star fruit (Averrhoa bilimbi L.) leaf extract as a green corrosion inhibitor using the weight loss method and optimizes operating conditions through Response Surface Methodology (RSM) based on Central Composite Design (CCD). Inhibitor concentrations of 0, 6, 8, 10, 12, and 14 g/L were evaluated at temperatures of 25–55°C and stirring speeds of 100–400 rpm. Surfaces were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray (EDX) analysis. The tannin content of the extract was 0.0223% (w/w). Increasing inhibitor concentration reduced the corrosion rate, with 14 g/L achieving 1.697 mpy and 71% inhibition efficiency. The observed corrosion rates ranged from 1.9319 to 5.7682 mpy. The lowest corrosion rate, 1.9319 mpy, was obtained at 55°C and 200 rpm. FTIR identified O–H, C=O, and C–O functional groups, while SEM–EDX indicated reduced surface deterioration and formation of an organic protective layer. RSM–CCD and ANOVA predicted optimum conditions at 54.53°C and 212.86 rpm, with a predicted corrosion rate of 1.9673 mpy. These findings demonstrate the potential of star fruit leaf extract as an environmentally friendly inhibitor for circulating cooling water systems.