Biodiesel products show corrosive properties. Biodiesel contains components of saturated and unsaturated esters which tend to be unstable, sensitive to light, temperature, and metal ions. Thus, the study aims to synthesize biodiesel from various vegetable oils (palm oil, sunflower seed oil, and candlenut oil), and to analyze its corrosiveness to ferrous nails and characterization of biodiesel. The research stages were: synthesis of methyl ester and its characterization, and corrosion test. The results showed that the methyl ester characteristics of the samples meet requirements with SNI7182 : 2015. In GC-MS results, the largest components of methyl esters from candlenut oil and sunflower seed oil were 35.04% methyl oleate and 46.79% methyl oleate respectively, while in palm oil, the largest components were 41.60% methyl oleate and 41.16%. methyl palmitate. Corrosion test showed that the corrosion rate of ferrous nail in biodiesel at room temperature was lower than 70 ℃. Based on GC-MS and SEM results, biodiesel contained high unsaturated fatty acids and had a corrosion rate, i.e., at room temperature, the methyl ester of palm oil, candlenut oil, and sunflower seed oil were 0.006 mpy, 0.011 mpy, and 0.011 mpy respectively, while at 70 ℃, they were 0.011 mpy, 0.016 mpy, and 0.017 mpy, respectively. The results corresponded to SEM results at high temperature and significantly high content of unsaturated fatty acids. It was indicated by the formation of pits.

The synthesis of biodiesel via transesterification needs to be improved by the heterogeneous catalysts. So, the study aimed to determine the best conditions for the synthesis of biodiesel produced through a transesterification reaction using waste cooking oil with a Na₂O/γ-Al₂O₃ catalyst assisted by ultrasonic wave. The steps were: catalyst preparation, oil preparation, esterification, trans-esterification reactions using methanol and various Na₂O/γ-Al₂O₃ catalysts, and characterization. The results showed that the refined waste cooking oil using FTIR was known that there was still a carbonyl group indicating the presence of triglycerides. Free Fatty Acid content of waste cooking oil after esterification was 0.40%, so that the transesterification reaction could be carried out. The XRD results of the catalyst showed a conformity with the reference and it was determined by the Scherrer formula that the crystal size of the catalyst was 30.59 nm. The best condition for biodiesel synthesis was obtained at a catalyst ratio of 1:1 as much as concentration 3% w/w for 15 minutes of ultrasonication, 65 ℃, and molar ratio of methanol: oil (12:1), for the yield of 83.51%. After identification through GC-MS, it was known that the main components in the transesterified biodiesel from waste cooking oil were methyl elaidate (38.54%), methyl palmitate (30.90%) and methyl linoleate (16.61%).