A metal object is treated to form an integral surface layer by: (a) immersing the metal object in an anodising electrolyte, and passivating the metal to form an anodised layer on the metal object (b) continuing the application of a potential to modify the surface layer (c) then treating the metal object with a chemical reducing agent so a hydrous metal oxide is formed and (d) then contacting the metal object with a solution containing a biocidal material so as to incorporate biocidal material into the surface layer.
