Cobalt in oxidized state for use as anti-artifact layer (4) covering a metallic substrate (1) of a medical device for reducing the production of artifacts in MRI caused by the magnetic property of the metallic substrate (1), wherein the anti-artifact layer (4) is present at outermost surface of the metallic substrate (1) and has at least 30% of cobalt ratio (at% Co) to the total amount of transition metallic atoms present therein, and at least 90% of cobalt atoms present within the anti-artifact layer (4) are converted into at least one of Co(II) oxidized state and Co(III) oxidized state.
