Corrosion Depth Inversion Method Based on the Lift-Off Effect of the Capacitive Imaging (CI) Technique

As a new Non-destructive Evaluation (NDE) technique, capacitive imaging (CI) has been used to detect defects in oil pipelines, composite sucker rods and storage tanks in recent years. When a CI sensor is used to detect corrosion defect on a conducting surface with or without an insulation layer, the depth information of corrosion defect is difficult to acquire due to the non-linearity of the probing field. This paper proposed a corrosion depth inversion method based on the lift-off effect of the CI technique. The proposed depth inversion method, which includes three steps, namely establishing the lift-off curve, obtaining the scan curve and fitting inversion, are introduced and evaluated. In the FE simulations, the error rates of the inversion depths are less than 1.20%. CI experiments were carried out to acquire the depth information of a step shape specimen using the CI experimental system and the proposed corrosion depth inversion method. The results show that the depth inversion method can meet the need of obtaining the actual depth information with the error rates of the inversion depths being less than 10.00%. A CI experiment on a machined and corroded specimen was carried out to further demonstrate the feasibility of the proposed inversion method for corrosion defects with different depths. Both results from FE models and experiments indicate that the proposed method is promising to achieve the quantitative NDE of the Corrosion Under Insulation (CUI).