Abstract: Corrosion under the insulation layer of carbon steel (CUI) is a common damage mode for the panels of atmospheric storage tanks for hazardous chemicals. When conducting regular inspection of storage tanks, the methods such as macroscopic inspection after removing the insulation layer and ultrasonic thickness measurement are often used. Using traditional techniques will result in high cost of dismantling the insulation layer, difficulty in coupling ultrasonic thickness measurement, low inspection efficiency, low reliability of inspection data, and low accuracy of local corrosion evaluation. In response to the above issues, this paper proposed a detection method for local corrosion evaluation based on using ultrasonic phased array C-scan for CUI detection inside the tank, and conducted experimental research on the detection and evaluation of a storage tank with a large area of CUI in a certain refining enterprise. The results showed that using ultrasonic phased array C-scan technology to detect large-area CUI had a good effect, which can significantly improve the quantitative accuracy of the minimum thickness of the tank wall and improve the reliability of local corrosion evaluation results.