Metal treatment to form a surface layer

1. A method of treating a metal object so as to form thereon a surface layer which is integral with the metal object, and which includes a biocidal metal, the method comprising:

(a) immersing the metal object, which is to provide a substrate for the surface layer, in an anodising electrolyte containing a solvent, and anodising the metal, by application of a positive voltage, to form an anodised integral surface layer on the metal object, wherein the metal of the metal object comprises titanium, niobium, tantalum, zirconium and/or an alloy thereof;

(b) continuing the application of a positive voltage to produce pits through the integral surface layer and into the substrate;

(c) producing a hydrous metal oxide in the pits by treating the metal object with a chemical reducing agent after steps (a) and (b), by connecting a corrodible metal in solid form to the metal object to act as a reductant, wherein the corrodible metal is in contact with the metal object through an electrical connection and the corrodible metal is in ionic connection with the metal object through an electrolyte, and wherein the corrodible metal corrodes preferentially to the metal object; and

(d) contacting the anodised metal object resulting from step (c) with a solution containing a biocidal metal so as to incorporate said biocidal metal into the surface layer.

2. The method according to claim 1 , wherein the corrodible metal is Fe or Zn or an alloy thereof

3. The method of claim 1 wherein the anodising is performed employing an electrolyte comprising phosphoric acid, sulphuric acid, acetic acid or an alkaline base.

4. The method of claim 3 that uses phosphoric acid at a concentration in a range of from 0.01 to 5 molar.

5. The method of claim 1 wherein the anodised metal object is treated with a solvent to remove electrolyte and soluble cations prior to contacting the anodised metal object with the solution containing a biocidal metal.

6. The method of claim 1 wherein the anodising is performed with a current density in the range from 0.1 to 25 mA/cm 2 .

7. The method of claim 1 wherein during the anodising, movement of the electrolyte relative to the surface of the metal object is inhibited or suppressed, at least during the pit growth phase of step (b).

8. The method of claim 1 wherein the biocidal metal comprises copper, gold, platinum, silver or mixtures thereof.

9. The method of claim 8 wherein the biocidal metal comprises silver.

10. The method of claim 1 comprising multiple anodisation steps of anodising the metal by application of a positive voltage.