  We demonstrate a simple, robust and contactless method for non-destructive
testing of magnetic materials such as steel. This uses a fiber-coupled magnetic
sensor based on nitrogen vacancy centers (NVC) in diamond without magnetic
shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic
field, but in our design we deliberately applied an inhomogeneous magnetic
field. As a consequence of our experimental set-up we achieve a high spatial
resolution: 1~mm in the plane parallel and 0.1~mm in the plane perpendicular to
the surface of the steel. Structural damage in the steel distorts this
inhomogeneous magnetic field and by detecting this distortion we reconstruct
the damage profile through quantifying the shifts in the NVC Zeeman splitting.
This works even when the steel is covered by a non-magnetic material. The
lift-off distance of our sensor head from the surface of 316 stainless steel is
up to 3~mm.
