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