Although their chemistry is almost identical, the density of zirconium is about half of hafnium. Very pure hafnium has been produced, with zirconium being the major impurity. Hafnium has been successfully alloyed with iron , titanium , niobium , tantalum , and other metals. At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfH1.86. Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon , boron , sulfur , and silicon .
Halogens react directly to form tetrahalides.
Such rods are used in nuclear submarines. Hafnium is used in gas-filled and incandescent lamps, and is an efficient getter for scavenging oxygen and nitrogen . Handling Finely divided hafnium is pyrophoric and can ignite spontaneously in air. Care should be taken when machining the metal or when handling hot sponge hafnium. The yearly demand for hafnium in the U.S. now exceeds 100,000 lb.