Method for improving surface properties of the stainless steels for bipolar plate of polymer electrolyte membrane fuel cell
Disclosed is a method for improving surface properties of the stainless steels for bipolar plate of polymer electrolyte membrane fuel cell having low interfacial contact resistance and excellent corrosion resistance. The method for improving surface properties of the stainless steels according to one exemplary embodiment of the present invention includes: pickling a stainless steel with an aqueous sulfuric acid solution, the stainless steel comprising, by weight: C: 0.02% or less, N: 0.02% or less, Si: 0.4% or less, Mn: 0.2% or less, P: 0.04% or less, S: 0.02% or less, Cr: 25-32%, Mo: 0.1 to 5%, Cu: 0.1 to 2%, Ti: 0.5% or less, Nb: 0.5% or less, and the balance of Fe and other inevitable elements, and then washing the stainless steel with water, and then immersing the stainless steel in a mixture solution of nitric acid and hydrofluoric acid to form a passivation layer.
1. A method for improving surface properties of stainless steel for bipolar plate of polymer electrolyte membrane fuel cell, the method comprising:
press forming and hydro forming cold-rolled annealing stainless steel for bipolar plate;
degreasing the press formed and hydro formed cold-rolled annealing stainless steel;
pickling the degreased stainless steel with an aqueous sulfuric acid solution, the stainless steel comprising, by weight: C: 0.02% or less, N: 0.02% or less, Si: 0.4% or less, Mn: 0.2% or less, P: 0.04% or less, S: 0.02% or less, Cr: 25˜32%, Mo: 0.1 to 5%, Cu: 0.1 to 2%, Ti: 0.5% or less, Nb: 0.5% or less, and the balance of Fe and other inevitable elements;
washing the pickled stainless steel with water;
passivating the washed stainless steel by immersing the washed stainless steel in a mixture solution of nitric acid and hydrofluoric acid to form a passivation layer; and
plasma-nitriding the passivated stainless steel to form a nitride layer on a surface of the stainless steel,
wherein the nitride layer includes CrN and/or Cr 2 N and has a thickness of 0.01 to 100 nm.
2. The method according to claim 1 , wherein the stainless steel further comprises one or two or more elements selected from the group consisting of V: 0 to 1%, W: 0 to 4%, La: 0 to 1%, Zr: 0 to 1% and B: 0 to 0.1%.
3. The method according to claim 1 , wherein a concentration of the sulfuric acid in the aqueous sulfuric acid solution is 5 to 20% by weight, and the pickling with the aqueous sulfuric acid solution is carried out at a temperature of 50 to 75° C. for 20 seconds to 5 minutes.
4. The method according to claim 1 , wherein a concentration of the nitric acid in the mixture solution is 10 to 20% by weight, a concentration of the hydrofluoric acid in the mixture solution is 1 to 10% by weight, and the immersing in the mixture solution is carried out at a temperature of 40 to 60° C. for 30 seconds to 10 minutes.
5. The method according to claim 1 , wherein the plasma nitriding is carried out at a temperature of 200 to 500° C. for 1 minute to 5 hours.