Ferritic stainless steel used for bipolar plates of fuel cells, controlling method of surface roughness, method of forming passivation films, and use

A ferritic stainless steel used for bipolar plates of fuel cells, wherein based on that a mass of the ferritic stainless steel is 100%, the ferritic stainless steel comprises:

• C less than or equal to 0.03 wt. %;
• N less than or equal to 0.02 wt. %;
• Si less than or equal to 0.4 wt. %;
• Mn less than or equal to 0.5 wt. %;
• Cr 16-23 wt. %;
• Cu 0-2.0 wt. %;
• Mo 1.8-2.5 wt. %;
• Ni 0.2-2.0 wt. %;
• Ti 0.1-0.5 wt. %;
• Nb 0.005-0.5 wt. %;
• P less than or equal to 0.02 wt. %;
• S less than or equal to 0.02 wt. %;
• and
• a remainder composed of Fe and other unavoidable accompanying elements;
• the ferritic stainless steel has a grain size number of 4-9;
• wherein the ferritic stainless steel is provided with a passivation film on the surface, and the passivation film comprises a p-type passivation film and an n-type passivation film;
• the p-type passivation film corresponds to a p-type semiconductor region, and the n-type passivation film corresponds to an n-type semiconductor region;
• the p-type passivation film refers to a p-type semiconductor passivation film, and the n-type passivation film refers to an n-type semiconductor passivation film;
• the p-type semiconductor passivation film has a higher content of hydroxides of Fe and Cr than a content of oxides of Fe and Cr, and the n-type semiconductor passivation film has a lower content of hydroxides of Fe and Cr than a content of oxides of Fe and Cr;
• a molar ratio of chromium hydroxide to chromium oxide in the p-type passivation film is represented as Ip[Cr(OH)3/Cr2O3], and Ip[Cr(OH)3/Cr2O3] is at least 10;
• a molar ratio of chromium hydroxide to chromium oxide in the n-type passivation film is represented as In[Cr(OH)3/Cr2O3], and In[Cr(OH)3/Cr2O3] is at most 10;
• and
• Ip[Cr(OH)3/Cr2O3]/In[Cr(OH)3/Cr2O3] is more than 3.