Fuel cell stack and method of producing its separator plates

A fuel cell stack comprising:

• a membrane electrode assembly including electrode catalyst layers and an electrolyte membrane between the electrode catalyst layers;
• a metal separator on each side of first and second surfaces of the membrane electrode assembly, each metal separator having a base material comprising a stainless steel containing at least one of Fe, Ni, and Cr as a main component and having a corrosion-resistant coating layer provided only on a reaction-side surface facing the membrane electrode assembly, each metal separator constituting gas flow paths,
• the membrane electrode assembly and the metal separators constituting a cell of a fuel cell;
• and
• a temperature-control medium flow path provided by forming joined portions where back surfaces of the metal separators of adjacent stacked cells are in contact with each other;
• wherein the joined portions are formed by welding portions of the metal separators that constitute groove bottoms of the gas flow paths;
• and
• wherein an oxide film is provided on a surface of respective welded portions after welding by at least one of:
• irradiating a laser beam on the surfaces of the welded portions to form an oxide film;
• immersing the surfaces of the welded portions in an acidic solution to form an oxide film;
• and
• performing electrolysis while the surfaces of the welded portions arc immersed in an acidic solution to form an oxide film.