PURPOSE: To improve the resistance of the surface of a heat exchanger tube to intergranular damage in an environment contg. water at high temp. and pressure when a corrosion resistant coating film is formed on the surface of the tube by heat treatment, by using an atmosphere of an inert gas contg. a very small amt. of O2 under reduced pressure and by specifying the heating temp. and time.

CONSTITUTION: A heat exchanger tube made of an Ni alloy contg. 15W35wt.% Cr and 30W75wt.% Ni is heat treated in an atmosphere of an inert gas contg. 10-2W10-4vol.% O2 under 10-1W10-3atm. In the heat treatment, the heating temp. and time are regulated to the region defined by straight lines connecting seven points AWG in the diagram. By this method, a Cr oxide-based oxide coating film having superior resistance to intergranular damage can be formed on the surface of the heat exchanger tube for generating steam, e.g., for a pressurized water reactor.