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Conversion coating in metal material was a method to control the corrosion. It applied in parts of car, aircraft, factory installation, and other appliances. There were three types of conversion coating: phosphate conversion coating, chromate conversion coating, and oxalate conversion coating. There were several aspects to consider in controlling the corrosion fully. This research used phosphate because phosphate conversion coating had a low corrosion rate, affordable production cost, and environmentally friendly. This research aimed to find out the corrosion resistance of ST37 carbon steel using phosphate conversion coating in various immersion durations. Therefore, the result was a breakthrough in using phosphate conversion coating for the industries. This research used the weight loss method to calculate the corrosion rate and macro photos to obtain the corrosion form during the test. This research used ST37 carbon steel with 100 mm x 30 mm x 10 mm as the specimen and phosphate with various coating durations (10, 20, and 30 minutes). Each variation had three specimens, so this research had nine specimens in total. This research calculated the daily corrosion rate for seven days using 5% NaCl as the corrosion solution. The average corrosion rate in specimens with 10 minutes duration was 1.9599 mpy, specimens with 20 minutes immersion was 1.7647 mpy, whereas specimens with 30 minutes duration were 1.3287 mpy. Thus, the longer immersion duration created a smaller corrosion rate. Also, the corrosion formed during the test was pitting and uniform corrosion.