Method of surface treatment for stainless steel product for fuel cell

A method of surface treatment for an austenitic stainless steel sheet product for fuel cells, the austenitic stainless steel sheet product being formed by a process comprising forming an ingot, hot rolling, cold rolling and annealing to yield the stainless steel sheet products, whereafter the stainless steel sheet products is to a four step process,

• the austenitic stainless steel sheet product comprising, by weight, C:
• 0.15% or less, Si:
• 0.01 to 1.5%, Mn:
• 0.01 to 2.5%, P:
• 0.035% or less, S:
• 0.01or less, Al:
• 0.001 to 0.2%, N:
• 0.3% or less, Cu:
• 0 to 3%, Ni:
• 7 to 50%, Cr:
• 17 to 30%, Mo:
• 0 to 7%, B:
• 0.005 to 3.5%, and balance:
• Fe and inevitable impurities, with the contents of Cr, Mo, and B satisfying the following formula,
• Cr (%)+3×Mo (%)−2.5×B (%)≧17
• wherein element symbols denote contents (wt.%),
• the four step process comprising a first step of forming a passive film preliminarily on the surface of the austenitic stainless steel sheet product of the specified composition by an oxidizing acid,
• a second step of protruding one or more types of conductive inclusions M23C6 type carbide, M23(C, B)6 type boron carbide, and M2B type boride on the surface by etching the passive film in an acidic aqueous solution,
• a third step of forming a passive film on the surface other than the inclusions protruding on the etched surface by an oxidizing acid, and
• a fourth step of washing in water and drying.

A method of surface treatment for a ferritic stainless steel sheet product for fuel cells, the ferritic stainless steel sheet product being formed by a process comprising forming an ingot, hot rolling, cold rolling and annealing to yield the stainless steel sheet product, whereafter the stainless steel sheet product is subjected to a four step process,

• the ferritic stainless steel product comprising, by weight, C:
• 0.15% or less, Si:
• 0.01 to 1.5%, Mn:
• 0.01 to 1.5%, P:
• 0.035% or less, S:
• 0.01% or less, Al:
• 0.001 to 0.2%, N:
• 0.35% or less, Cu:
• 0 to 1%, Ni:
• 0 to 5%, Cr:
• 17 to 36%, Mo:
• 0 to 7%, B:
• 0.005 to 3.5%, and balance:
• Fe and inevitable impurities, with the contents of Cr, Mo, and B satisfying the following formula,
• Cr (%)+3×Mo (%)−2.5×B (%)≧17
• wherein element symbols denote contents (wt.%).
• the four step process comprising a first step of forming a passive film preliminarily on the surface of the ferritic stainless steel sheet product of the specified composition by an oxidizing acid,
• a second step of protruding one or more types of conductive inclusions M23C6 type carbide, M23(C, B)6 type boron carbide, and M2B type boride on the surface by etching the passive film in an acidic aqueous solution,
• a third step of forming a passive film on the surface other than the inclusions protruding on the etched surface by an oxidizing acid, and
• a fourth step of washing in water and drying.

A method of surface treatment for a bright annealed stainless steel sheet product for fuel cells, the bright annealed stainless steel sheet product being formed by a process comprising forming an ingot, hot rolling, cold rolling and annealing to yield the stainless steel sheet product, whereafter the stainless steel sheet product is subjected to a four step process,

• the bright annealed stainless steel sheet product comprising, by weight, C:
• 0.15% or less, Cr:
• 17 to 36%, and B:
• 0.005 to 3.5%,
• the four step process comprising a first step of forming a passive film preliminarily on the surface of the bright annealed stainless steel sheet product by an oxidizing acid,
• a second step of protruding one or more types of conductive inclusions M23C6 type carbide, M23(C, B)6 type boron carbide, and M2B type boride on the surface by etching the passive film in an acidic aqueous solution,
• a third step of forming a passive film on the surface other than the inclusion protruding on the etched surface by an oxidizing acid, and
• a fourth step of washing in water and drying.

A method of surface treatment for a bright annealed austenitic stainless steel sheet product for fuel cells, the bright annealed stainless steel sheet product being formed by a process comprising forming an ingot, hot rolling, cold rolling and annealing to yield the stainless steel sheet product, whereafter the stainless steel sheet product is subjected to a four step process,

• the bright annealed austenitic stainless steel sheet product comprising, by weight, C:
• 0.15% or less, Si:
• 0.01 to 1.5%, Mn:
• 0.01 to 2.5%, P:
• 0.35% or less, S:
• 0.01% or less, Al:
• 0.001 to 0.2%, N:
• 0.3% or less, Cu:
• 0 to 3%, Ni:
• 7 to 50%, Cr:
• 17 to 30%, Mo:
• 0 to 7%, B:
• 0.005 to 3.5%, and balance:
• Fe and inevitable impurities, with the contents of Cr, Mo and B satisfying the following formula,
• Cr (%)+3×Mo (%)−2.5×B (%)≧17
• wherein element symbols denote contents (wt. %).
• the four step process comprising a first step of forming a passive film preliminarily on the surface of the austenitic stainless steel sheet product of the specified composition by an oxidizing acid,
• a second step of protruding one or more types of conductive inclusions M23C6 type carbide, M23(C, B)6 type boron carbide, and M2B type boride on the surface by etching the passive film in an acidic aqueous solution,
• a third step of forming a passive film on the surface other than the inclusions protruding on the etched surface by an oxidizing acid, and
• a fourth step of washing in water and drying.

A method of surface treatment for a bright annealed ferritic stainless steel sheet product for fuel cells, the bright annealed ferritic stainless steel product being formed by a process comprising forming an ingot, hot rolling, cold rolling and annealing to yield the stainless steel sheet product, whereafter the stainless steel sheet product is subjected to a four step process,

• the bright annealed ferritic stainless steel product for cells comprising, by weight, C:
• 0.15% or less, Si:
• 0.01 to 1.5%, Mn:
• 0.01 to 1.5%, P:
• 0.035% or less, S:
• 0.01% or less, Al:
• 0.001 to 0.2%, N:
• 0.35% or less, Cu:
• 0 to 1%, Ni:
• 0 to 5%, Cr:
• 17 to 36%, Mo:
• 0 to 7%, B:
• 0.005 to 3.5%, and balance:
• Fe and inevitable impurities, with the contents of Cr, Mo, and B satisfying the following formula,
• Cr (%)+3×Mo (%)−2.5×B (%)≧17
• wherein element symbols denote contents (wt.%).
• the four step process comprising a first step of forming a passive film preliminarily on the surface of the ferritic stainless steel product of the specified composition by an oxidizing acid,
• a second step of protruding one or more types of conductive inclusions M23C6 type carbide, M23(C, B)6 type boron carbide, and M2B type boride on the surface by etching the passive film in an acidic aqueous solution,
• a third step of forming a passive film on the surface other than the inclusions protruding on the etched surface by an oxidizing acid, and
• a fourth step of washing in water and drying.