Abstract
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).
Highlights
Corrosion, which is the most common type of defects on metals, affects the operation reliability of the metal structure and component and can cause huge losses [1]–[3]
The errors of the inversion depths were less than 0.002 mm and the error rates of the inversion depths were less than 1.20%, which proves the effectiveness of the depth inversion method in Finite element (FE) models
DISCUSSIONS AND CONCLUSIONS In this paper, a corrosion depth inversion method based on the lift-off effect of the capacitive imaging (CI) technique was introduced
Summary
Corrosion, which is the most common type of defects on metals, affects the operation reliability of the metal structure and component and can cause huge losses [1]–[3]. When a CI sensor is used to inspect the conducting specimen without insulation layer, as shown, if a corrosion defect is presented (shown as a step shape metal loss in Fig. 1(a)), the distribution of the probing field between the driving and sensing electrode will be altered. For the case in which an insulation layer is presented, as shown, the quasi-static fringing electric field will penetrate through the insulation layer and be influenced by the corrosion defects (metal loss) on the upper surface of the conducting specimen This will again cause charge variation on the sensing electrode, which can be measured and used to reflect depth information of the corrosion defects under insulation.
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