{HEAD ~ A rod} 5, wherein {TAIL ~ the protective layer} is obtained by controlled oxidation of the hafnium bar, for 3 to 12 hours, at a temperature in the range 820° C. to 950° C., in oxygen diluted to a concentration of 1% to 50% in an inert gas, under low pressure 
wherein
{ HEAD ~ A rod } 5 , wherein { TAIL ~ the protective layer } is obtained by controlled oxidation of the hafnium bar , for 3 to 12 hours , at a temperature in the range 820 ° C. to 950 ° C. , in oxygen diluted to a concentration of 1 % to 50 % in an inert gas , under low pressure
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH