NOVELTY - A bipolar plate as fuel cell comprises a stainless sheet bipolar parent metal (1) and modified films (2), (3) on the two connected side surfaces. The modified films (2), (3) on the surface are the chromium nitride film with thickness being 0.1-5 mu m. The connection between the parent metal (1) and the modified films (2), (3) is a continuous matching connection on the atom size of the material.    USE - The process is useful for preparing bipolar plate for fuel cell and chromium nitride membrane on surface (all claimed).    ADVANTAGE - The parent metal is a stainless sheet whose surface layer is chromium nitride film. The two are connected by matching on the atom size as the ion metallic-membrane deposition so that the bipolar plate has low cost based on that it has multiplex function as tarnish resistance, conductance, strengthening and hydrophobic nature.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for preparing the chromium nitride membrane on surface of the dipolar plate for fuel cell deposited on the surface of the dipolar parent metal with the voltaic arc ion plating technology comprises: (a) stainless dipolar plate sheet parent metal is ultrasonically cleaned and treated by parching, it is put into the vacuum chamber of the voltaic arc ion plating. The vacuum in the vacuum chamber is pumped to be 5 three quarters 10 to the power of negative 3 Pa with the vacuum system; (b) pumping working air; the cathode target arc is started with bias voltage to remove the base plate passive film; the bias voltage is reduced to be negative 300-negative 500 V and operating air pressure is adjusted to be 0.2-0.8Pa; (c) adjusting the arc current and the reacting air nitride is pumped in to start the film deposition process with the time being 20-120 minutes; and (d) coating dipolar plate is taken out by releasing the vacuum after the furnace is fully cooled. The removal treatment of the parent plate positive film executes ion sputtering method. The film deposition process adopts the matching method of the pumping reacting air nitride amount and the adjusting arc current with ratio between the nitride pumping amount and the total arc current being 1 sccm/24A to 1 sccm/2A.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of bipolar plate for fuel cell and chromium nitride membrane on surface.    Stainless sheet bipolar parent metal; and (1)    Modified films (2)