Design/methodology/approach This paper proposes a method of micro-arc oxidation of valve metal surface assisted by visual defect inspection and using composite hydrogel. The system is mainly composed of a vision inspection system, auxiliary coating preparation and micro-arc oxidation of metal surface. Then the specific environment of the reaction is provided by the composite hydrogel containing the electrolyte solution. Findings The feasibility of this method is proved by experiments, and the surface and cross-sectional morphology of the prepared coating are characterized. In addition, the elemental composition and distribution of the coating were analyzed by energy dispersive spectroscopy (EDS) surface scanning. Finally, the tribological properties and corrosion resistance of the coating were further demonstrated by friction experiments and electrochemical experiments. Originality/value This paper uses machine vision for defect detection on underwater metal surfaces, exploring the possibility of using machine vision in ship body defect repair. It involves preparation of a highly water-containing composite hydrogel to replace the electrolyte solution for micro-arc oxidation, exploring the possibility of local area micro-arc oxidation. The feasibility of this method was experimentally verified and analyzed. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-10-2024-0394/