Stainless steel for polymer fuel cell separator and method for preparing same
There are provided a ferrite stainless steel for a polymer fuel cell separator having excellent corrosion resistance and interfacial contact resistance under an operating environment of a polymer fuel cell, and a preparation method of the stainless steel. A stainless steel includes C: 0.02 wt % or less, N: 0.02 wt % or less, Si: 0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, S: 0.02 wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt % or less, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, waste Fe and inevitably contained elements. A preparation method of the stainless steel having a second passive film formed on a surface thereof includes forming a first passive film on the surface of the stainless steel by bright-annealing or annealing-pickling the stainless steel; removing the first passive film by pickling the stainless steel in a 10 to 20 wt % sulfuric acid solution at a temperature of 50 to 75° C. for a predetermined time; water-washing the stainless steel; and forming the second passive film by performing a passivation treatment on the stainless steel in the mixture of a 10 to 20 wt % nitric acid and a 1 to 10 wt % fluorine acid at a temperature of 40 to 60° C. for the predetermined time. Accordingly, it is possible to prepare a stainless steel having reduced elution resistance and excellent corrosion resistance and to produce a stainless steel for a polymer fuel cell separator, which has low interfacial contact resistance and excellent long-term performance even under a fuel cell operating condition of 60 to 150° C. and various surface roughness conditions.
1. A preparation method of a stainless steel for a polymer fuel cell separator, comprising C: 0.02 wt % or less, N: 0.02 wt % or less, Si: 0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, S: 0.02 wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt % or less, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, waste Fe and inevitably contained elements, in which a second passive film is formed on a surface of the stainless steel, the method comprising:
forming a first passive film on the surface of the stainless steel by bright-annealing or annealing-pickling the stainless steel;
removing the first passive film by pickling the stainless steel in a 10 to 20 wt % sulfuric acid solution at a temperature of 50 to 75° C. for a first processing time controlled based on a surface roughness Ra of the stainless steel, wherein the first processing time is increased as the surface roughness Ra is increased;
water-washing the stainless steel; and
forming the second passive film by performing a passivation treatment on the stainless steel in the mixture of a 10 to 20 wt % nitric acid and a 1 to 10 wt % fluorine acid at a temperature of 40 to 60° C. for a second processing time controlled based on the surface roughness Ra,
wherein, in the forming of the second passive film, the second processing time is controlled according to the following expression:
120+6.73(1/ Ra )≦second processing time(second)≦140+6.73(1/ Ra ).
2. The method of claim 1 , wherein the stainless steel is a ferrite stainless steel further comprising Mo: 5.0 wt % or less.
3. The method of claim 1 , wherein the stainless steel further comprises one or two or more elements selected from the group consisting of V: 0.1 to 1.5 wt %, W: 0.1 to 2.0 wt %, La: 0.0005 to 1.0 wt %, Zr: 0.0005 to 1.0 wt % and B: 0.0005 to 1.0 wt %.
4. The method of claim 1 , wherein, in the forming of the second passive film, the second passive film is formed to have a thickness of 2 to 4.5 nm.
5. The method of claim 1 , wherein, in the forming of the second passive film, the Cr/Fe oxide ratio of the second passive film is 1.5 or more in a region of 1.5 nm or less.
6. The method of claim 1 , wherein, in the forming of the second passive film, the Cr(OH) 3 /Cr oxide distribution of the second passive film has a ratio of 0 to 0.7 in a region of 1 nm.
7. The method of claim 1 , wherein the removing of the first passive film, the water-washing and the forming of the second passive film are performed on a steel plate after the bright-annealing or annealing-pickling of the stainless steel.
8. The method of claim 1 , wherein the removing of the first passive film, the water-washing and the forming of the second passive film are performed just before forming a flow path of the separator after the bright-annealing or annealing-pickling of the stainless steel.
9. The method of claim 1 , wherein the removing of the first passive film, the water-washing and the forming of the second passive film are performed just after forming a flow path of the separator after the bright-annealing or annealing-pickling of the stainless steel.
10. The method of claim 1 , wherein the contact resistance of the stainless steel is 10 mΩcm 2 or less under an operating environment of 60 to 150° C.
11. A preparation method of a stainless steel for a polymer fuel cell separator, comprising C: 0.02 wt % or less, N: 0.02 wt % or less, Si: 0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, S: 0.02 wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt % or less, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, waste Fe and inevitably contained elements, in which a second passive film is formed on a surface of the stainless steel, the method comprising:
forming a first passive film on the surface of the stainless steel by bright-annealing or annealing-pickling the stainless steel;
removing the first passive film by pickling the stainless steel in a 10 to 20 wt % sulfuric acid solution at a temperature of 50 to 75° C. for a first processing time controlled based on a surface roughness Ra of the stainless steel, wherein the first processing time is increased as the surface roughness Ra is increased;
water-washing the stainless steel; and
forming the second passive film by performing a passivation treatment on the stainless steel in the mixture of a 10 to 20 wt % nitric acid and a 1 to 10 wt % fluorine acid at a temperature of 40 to 60° C. for a second processing time controlled based on the surface roughness Ra,
wherein, the first processing time is controlled according to the following expression:
99−3.18(1/ Ra )≦first processing time(second)≦153−3.18(1/ Ra ).
12. The method of claim 11 , wherein the stainless steel is a ferrite stainless steel further comprising Mo: 5.0 wt % or less.
13. The method of claim 11 , wherein the stainless steel further comprises one or two or more elements selected from the group consisting of V: 0.1 to 1.5 wt %, W: 0.1 to 2.0 wt %, La: 0.0005 to 1.0 wt %, Zr: 0.0005 to 1.0 wt % and B: 0.0005 to 1.0 wt %.
14. The method of claim 11 , wherein, in the forming of the second passive film, the second passive film is formed to have a thickness of 2 to 4.5 nm.
15. The method of claim 11 , wherein, in the forming of the second passive film, the Cr/Fe oxide ratio of the second passive film is 1.5 or more in a region of 1.5 nm or less.
16. The method of claim 11 , wherein, in the forming of the second passive film, the Cr(OH) 3 /Cr oxide distribution of the second passive film has a ratio of 0 to 0.7 in a region of 1 nm.
17. The method of claim 11 , wherein the removing of the first passive film, the water-washing and the forming of the second passive film are performed on a steel plate after the bright-annealing or annealing-pickling of the stainless steel.
18. The method of claim 11 , wherein the removing of the first passive film, the water-washing and the forming of the second passive film are performed just before forming a flow path of the separator after the bright-annealing or annealing-pickling of the stainless steel.
19. The method of claim 11 , wherein the removing of the first passive film, the water-washing and the forming of the second passive film are performed just after forming a flow path of the separator after the bright-annealing or annealing-pickling of the stainless steel.
20. The method of claim 11 , wherein the contact resistance of the stainless steel is 10 mΩcm 2 or less under an operating environment of 60 to 150° C.