Assessing the efficacy of non-destructive testing methods to detect pitting corrosion

Calibration of the non-destructive testing (NDT) methods for pipeline inspections is commonly done via machined defects with simple shapes and known sizes. However, such defects do not represent, e.g. an actual pit morphology observed in the field, which could lead to the unreliable interpretation of the inspection results. This work proposed a systematic approach to create and characterise real pits in stainless steel pipe specimens to assess the sensitivity of selected NDT methods. Pipe samples with three different diameters and thicknesses were used. Pits of different geometries and dimensions were generated by electrochemical methods, and immersion in an acidic ferric chloride solution filled in pre-drilled holes on the external surface of the specimens. NDT methods including radiography, Eddy current and dye penetrant testing were used for detecting and sizing the pits. The results were then compared with true pit depths obtained from cross-section analysis. Radiography and Eddy current methods failed to detect small pits with depths less than 70 µm. The porous metal cover on the pit mouth affected the detectability of radiography, while it had little impact on Eddy current testing. The findings of this investigation could be used to design training and calibration blocks for NDT methods.管道检测用无损检测（Non-Destructive Testing, NDT）方法的校准，通常采用形状简单、尺寸已知的机加工缺陷来完成。然而，此类缺陷无法复现现场实际观测到的真实点蚀形貌，可能导致检测结果出现不可靠的判读。本研究提出了一种系统方法，用于在不锈钢管道试样上制造并表征真实点蚀缺陷，以评估所选无损检测方法的检测灵敏度。本次实验选用了三种不同直径与壁厚的管道试样，通过电化学方法，以及在试样外表面预钻孔内注入酸性氯化铁溶液后浸泡的方式，制备得到了不同几何形态与尺寸的点蚀缺陷。随后采用射线检测（Radiography）、涡流检测（Eddy Current）与渗透检测（Dye Penetrant Testing）三种无损检测方法，对上述点蚀缺陷进行检测与尺寸定量，并将检测结果与通过断面分析得到的真实点蚀深度进行对比。研究结果显示，射线检测与涡流检测均无法检出深度小于70 μm的小型点蚀缺陷。点蚀孔口处的多孔金属覆盖层会对射线检测的检出能力造成不利影响，但对涡流检测几乎无影响。本研究的成果可用于设计无损检测方法的培训与校准试块。