treatment be conducted in an inert gas, such as N2, Ar, He or the like, and by means of conducting the heat treatment at a temperature within a range of 100?? to 400?? C., and preferably within a range of 200?? to 370?? C., and for a period of 1 to 5 hours, a fluorocarbon layer is formed which is durable and minute, has good adherence with the metal, and which exhibits sufficient corrosion resist