Metal treatment
Metal objects are treated by anodising (P, SE) the metal object in contact with an acidic solution, and then subjecting the anodised metal object to a reversed voltage (VR). The anodising is performed in two stages, firstly to passivate (P) with the formation of a surface layer, and secondly to form pits in this surface layer (SE). The second stage (SE) of anodising is performed at a lower voltage than the first stage (P). After the reversed voltage step (VR) the metal object is then contacted with a biocidal metal-containing solution. Biocidal metal is absorbed into the surface of the metal object, resulting in improved biocidal properties. The lower voltage of the second stage anodising (SE) results in reduced processing time.
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) contacting the metal object, which is to provide a substrate for the surface layer, with an anodising electrolyte comprising phosphoric acid, and applying an anodising voltage of between 30 V and 150 V to the metal object to passivate the metal by forming an anodised integral surface layer on the metal object, wherein the metal object comprises titanium, niobium, tantalum or zirconium, or an alloy comprising such a metal;
(b) continuing, in the same electrolyte, the application of an anodising voltage to produce pits through the integral surface layer and into the substrate;
(c) producing a hydrous metal oxide in the surface layer by electrochemical reduction in contact with an electrolyte or a solution;
(d) removing or separating the anodised metal object resulting from the electrochemical reduction of step (c) from the electrolyte or the solution of step (c); and
(e) contacting the anodised metal object with a solution containing a biocidal metal so as to incorporate said biocidal metal by ion exchange into the surface layer;
wherein the voltage in the pit-forming step (b) is less than the maximum voltage applied during the passivating step (a) and is between 20 V and 60 V; and wherein the anodising steps (a) and (b) are performed in a total time of no more than 60 minutes.
2. The method of claim 1 , wherein the voltage applied in the passivating step (a) is increased at no more than 5 V/s.
3. The method of claim 1 , wherein the voltage applied in the passivating step (a) is increased stepwise.
4. The method of claim 1 , wherein in passivating step (a) the voltage is maintained at a value below 50 V for a period between 1 and 20 minutes, prior to raising to the maximum voltage.
5. The method of claim 4 , wherein the voltage is maintained at a value between 3 and 35 V for a period between 2 and 5 minutes, prior to raising to the maximum voltage.
6. The method of claim 1 , wherein in passivating step (a) the voltage is raised to a value below 50 V, and then dropped to zero, before raising to the maximum voltage.
7. The method of claim 1 , wherein the amount of charge employed for anodising in steps (a) and (b) is in the range from 0.5 to 10 coulomb/cm 2 .
8. The method of claim 1 , comprising multiple passivation steps (a).
9. The method of claim 1 , wherein the hydrous metal oxide is produced in step (c) by application of a negative voltage to the metal object, and the negative voltage is applied for less than 300 seconds.
10. The method of claim 1 , wherein the anodizing electrolyte is agitated during the passivating step (a), and/or movement of the electrolyte relative to the surface of the metal object is inhibited during the pit-producing step (b).
11. The method of claim 1 , wherein the voltage in the pit-forming step (b) is 30 V.
12. The method of claim 1 , wherein the voltage in the pit-forming step (b) is between 25 V and 50 V.