Probing microscale defects on the metal surface with a diamond quantum sensor

The detection of defects in metals is critically important for industries utilizing hydrogen energy, as hydrogen can induce embrittlement in metals whose initial defect sizes are often on the micrometer scale. However, existing techniques for detecting such microscale defects are predominantly destructive, making it difficult to monitor their temporal evolution. Here, we propose a nondestructive method that probes defects via the magnetic field generated by an electric current applied to the metal. To address the difficulty of reconstructing the three-dimensional defect shape from the current distribution, we model the defect as a current dipole—a localized region inducing a counter-current to the background flow. As a proof of concept, we detected defects as small as 38 μm on a copper surface, and the reproducibility of the measurements, together with numerical simulations, supports the experimental demonstration. Furthermore, defects were detected from the opposite side of the metal sample, highlighting the potential for subsurface sensing. This approach provides a promising route toward nondestructive, real-time inspection of metallic infrastructure, particularly for hydrogen-related applications.