Stainless separator for fuel cell and method of manufacturing the same
A stainless steel separator for fuel cells and a method of manufacturing the same are disclosed. The method includes preparing a stainless steel sheet as a matrix, performing surface modification on a surface of the stainless steel sheet to form a Cr-rich passive film having a comparatively increased amount of Cr in a superficial layer of the stainless steel sheet by decreasing an amount of Fe in the superficial layer of the stainless steel sheet, and forming a coating layer on the surface of the surface-modified stainless steel sheet. The coating layer is one selected from a metal nitride layer (MN x ), a metal/metal nitride layer (M/MN x ), a metal carbide layer (MC y ), and a metal boride layer (MB z ) (where 0.5≦x≦1, 0.42≦y≦1, 0.5≦z≦2).
1. A method of manufacturing a stainless steel separator for fuel cells, the method comprising:
preparing a stainless steel sheet as a matrix;
performing surface modification directly on a surface of the stainless steel sheet to form a Cr-rich passive film having a comparatively greater amount of Cr in a superficial layer of the stainless steel sheet by dissolving an amount of Fe directly from the superficial layer of the stainless steel sheet; and
forming a coating layer directly on the Cr-rich passive film, wherein the coating layer is a metal boride layer (MBz),
wherein 0.5≦z≦2.
2. The method according to claim 1 , wherein the metal (M) used in forming the coating layer is at least one of chromium (Cr), titanium (Ti), zirconium (Zr), and tungsten (W).
3. The method according to claim 1 , wherein the metal (M) used in forming the coating layer includes a transition metal.
4. The method according to claim 1 , wherein the Cr-rich passive film of the superficial layer of the stainless steel sheet has a (Cr+Ni)/Fe ratio of 1 or more in terms of an atomic weight ratio.
5. The method according to claim 1 , wherein the coating layer is formed as a film having a thickness of 30˜300 nm.
6. The method according to claim 1 , wherein the surface modification comprises immersing the stainless steel sheet in a solution comprising sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ), or spraying the solution onto the surface of the stainless steel sheet.
7. The method according to claim 6 , wherein the surface modification solution further comprises one or more additives including any of hydrogen peroxide (H 2 O 2 ) and oxalic acid (C 2 H 2 O 4 ).
8. The method according to claim 1 , wherein the surface modification comprises immersing the stainless steel sheet in a surface modification solution comprising sulfuric acid (H 2 SO 4 ), and applying a potential or current in an SHE region of greater than 0 to 1.0V.
9. The method according to claim 1 , wherein the coating layer is formed by sputtering or arc ion plating.
10. The method according to claim 9 , wherein the sputtering is reactive sputtering.
11. The method according to claim 1 , wherein the stainless steel sheet contains 16˜28 wt % chromium.
12. The method according to claim 1 , wherein the surface modification solution comprises 5-20 wt % nitric acid (HNO 3 ), 2-15 wt % sulfuric acid (H 2 SO 4 ), and a balance of water, and wherein the surface modification is performed at 50-80° C. for an immersion duration of 30 seconds to 30 minutes or less.
13. The method according to claim 12 , wherein the surface modification is performed while adjusting the concentrations of the nitric acid and the sulfuric acid with respect to treatment duration.
14. The method according to claim 12 , further comprising accelerating a metal dissolution rate on the surface of the stainless steel sheet by adding one or both of oxalic acid (C 2 H 2 O 4 ) and hydrogen peroxide H 2 O 2 to the surface modification solution.