Effect of Field Factors on the Evaluation of Corrosion of Storage Tanks Floor ‎by ‎Magnetic Flux Leakage Method

In this paper, the effects of some field factors in the non-destructive evaluation of the corrosion rate of the floor of oil storage tanks have been investigated using the magnetic flux leakage method at the industrial level. For this purpose, the effect of factors such as the cathodic protection system on the tank, the presence of waste, the presence of metal attachments (spot welding or spatter), distortion of the tank and the coating thickness of plate on the accuracy of the assessment of the size of defects in different situations have been studied. The field inspections were carried out in two oil tanks using the MFL3DiM magnetic flux leakage unit on the detected defects. Two standard steel blocks with thicknesses of 6 and 8 mm and 4 specific defects in different sizes (20, 40, 60 and 80% of the plate thickness) was used for setting and calibrating the device. Ultrasonic thickness measurement was also used to verify the results of MFL measurement.  In addition, the MFL results were clarified by an ultrasonic thickness measuring unit. The results of the evaluations showed that when the cathodic protection system is turn on, it caused a decrease in the measurement accuracy and the corrosion defects with a less than 22 mass loss percent are not identified. Also, the presence of a metal attachments (welding spot) with a height of 5 mm caused defects with a corrosion depth of less than 25% not to be detected. On the other hand, it was observed that the presence of attachments and waste on the bottom of the tank caused the error in detecting the depth of corrosion to be greater than the length and width of the defect. The presence of waste on the bottom of the tank caused defects with a corrosion depth of less than 18% not to be detected. Distortion in the bottom sheet of the tank caused false defects to be detected in the sheet and the dimensions of the real defects were reported completely wrongly. The presence of a coating with a thickness of more than 3000 micrometers made the defects of the calibration plates undetectable. Additional investigations showed that if the difference in the thickness of the non-magnetic coating on the sheet is higher than 500 micrometers, the dimensions of the detected defects are unrealistic and unreliable.