Method for maintenance of used permanent cathode plates

1. A method for maintenance of used permanent cathode plates made of stainless steel, said used cathode plate having scratches, crud formations and oversize grain boundaries on a surface of the cathode plate, the method comprising:

removing of scratches and accumulated crud from the surface of the cathode plate,

removing substantially completely the oversize grain boundaries from the surface, and thereafter

regenerating the grain boundaries of the surface of the cathode plate to an average grain boundary width of 1 to 3 μm and an average grain boundary depth less than 1 μm.

2. The method according to claim 1 , further comprising alkaline treatment of the surface of the cathode plate for removing the accumulated crud before removing the oversize grain boundaries from the surface of the cathode plate.

3. The method according to claim 1 , further comprising mechanical grinding of the surface of the cathode plate for removing the accumulated crud.

4. The method according to claim 1 , further comprising mechanical grinding of the surface of the cathode plate for removing the oversize grain boundaries.

5. The method according to claim 1 , further comprising mechanical grinding of the surface of the cathode plate wherein the mechanical grinding is performed in two phases comprising a first phase grinding to surface roughness Ra of about 0.9-1.1 μm and thereafter a second phase grinding to surface roughness Ra of about 0.2-0.4 μm.

6. The method according to claim 5 , wherein the mechanical grinding is implemented by at least one of belt grinding and circular grinding.

7. The method according to claim 2 , wherein, the alkaline treatment of the surface comprises subjecting the surface to liquid caustic soda (NaOH) having a pH greater than at least one of 10 and a pH of potassium hydroxide (KOH).

8. The method according to claim 7 , wherein the alkaline treatment of the surface comprises subjecting the surface to 10M liquid caustic soda (NaOH) at a temperature of 50° C.

9. The method according to claim 1 , wherein regenerating of the grain boundaries of the surface of the cathode plate is performed by at least one of the chemically and electrochemically.

10. The method according to claim 1 , wherein the electrochemical regenerating of the grain boundaries comprises etching the plate surface with nitric acid 60% solution (HNO 3 ) using a current of 15-40 As/cm 2 , and preferably 20 As/cm 2 .

11. The method according to claim 1 , wherein the chemical regenerating of the grain boundaries comprises subjecting the plate surface to at least one of oxalic acid (H 2 C 2 O 4 ) and sulphuric acid (H 2 SO 4 ) and sulphuric acid-based copper electrolyte.

12. The method according to claim 1 , wherein the electrochemical regenerating of the grain boundaries comprises subjecting the plate surface to sulphuric acid-based electrolyte obtained from electrolysis.

13. The method according to claim 1 , wherein the electrochemical regenerating of the grain boundaries comprises etching the plate surface with sulphuric acid-based electrolyte using current 10-40 As/cm 2 , preferably 20 As/cm 2 .

14. The method according to claim 1 , further comprising passivation of the surface after regeneration of the grain boundaries.

15. The method according to claim 14 , wherein the passivation of the surface comprises dipping the cathode plate into nitric acid (HNO 3 ) or citric acid (C 6 H 8 O 7 ).

16. The method according to claim 15 , further comprising neutralizing and washing of the surface to neutralize and wash out the nitric acid or citric acid after passivation.