PURPOSE: To prevent and reduce cavitation effectively by forming a hardened layer consisting of a hardened film by Ni and a thin film of chromium oxide on the inside surface of a thick walled pipe of a small diameter.

CONSTITUTION: A plating film of Ni is first formed by a chemical plating methof on the inside surface 2 of a thick walled small diameter pipe 1 for supplying a high pressure fluid, such as a part of an injection pipe for high pressure fuel or a high pressure injection nozzle and then heated for 2W60min at 250W450°C to form a hardened film 3. The pipe is thereafter dipped in a chromic acid or a chromic acid/chromate soln. prepd. by dissolving the chromic acid and chromate to impregnate the pipe with said soln., and the pipe is then heated for ≥20min at 200W700°C to form a dense hardened layer 4 consisting of a thin layer of chromium oxide resulted from the calcination and conversion of the chromic acid soln. on the film 3. The generation of cavitation in the inside surface of the supply pipe for high pressure fluid is prevented.