Rust free blackening on mild steel plates?

Hi,

Here i am stuck up with blackening on mild steel plate of 12 mm thickness. We are giving mild steel base plate to outside vendors for blackening. Since we are very close to see shore, the plates get rusted very fast after blackening it.

Powder coating also will not work for my application 'cause it will produce scratches and come off when I slide bigger jobs on the surface of plate.

The basic requirement is I needed a black surface on mild steel plate with non corrosive properties. The reason behind to have black surface is, our's is laser machine, and the black surface aids to avoid light scattering.

Thanks!

Ruban J.