Investigation and Optimization of Patchouli Leaf Waste Extract (PLWE) as a Green Corrosion Inhibitor for Aluminum in HCl via Box–Behnken Design

Patchouli leaf waste extract (PLWE) was investigated as a waste derived green corrosion inhibitor for AA5052 aluminum in hydrochloric acid (HCl) media and optimized using response surface methodology (RSM). PLWE was prepared from patchouli leaf waste through ethanol maceration followed by concentration using microwave-assisted distillation. Corrosion performance was evaluated by gravimetric immersion (weight loss) testing conducted under ASTM G31 conditions, and inhibition efficiency (IE, %) was modeled with a three factor Box–Behnken design (BBD) using HCl concentration, PLWE concentration, and immersion time as process variables. Across the BBD experimental matrix, IE (%) varied substantially, with the highest experimental IE (%) of 77.9528% obtained at 0.5 M HCl, 7.5 g L -1 PLWE, and 6 h immersion time. The fitted quadratic model was statistically significant and demonstrated good predictive capability. Numerical optimization identified 0.5 M HCl, 7.29 g L -1 PLWE, and 5.83 h as the primary optimum condition, giving an experimental IE (%) of 80.94% compared with a predicted value of 80.10%. Corrosion rate calculations further supported the inhibitory performance, where Vcorr decreased to 17.2054 mm/y under effective inhibited conditions, while markedly higher Vcorr values were observed in uninhibited systems. Overall, the results demonstrate that patchouli extraction residue can be valorized into a functional corrosion inhibitor for aluminum in acidic media, supporting a waste-to-resource pathway and providing an RSM based framework for selecting robust operating windows. • Patchouli leaf waste extract (PLWE) is a green inhibitor for AA5052 Al in HCl • •Ethanol maceration + microwave-assisted distillation produced PLWE inhibitor • •ASTM G31 weight-loss + 3-factor Box–Behnken RSM modeled IE(%) vs HCl, PLWE, and immersion time • •Optimization gave 80.94% IE at 0.5 M HCl, 7.29 g L -1 PLWE, and 5.83 h • •Quadratic model fit R 2 = 0.9293; Vcorr decreased to 17.2054 mm/y with PLWE