Zirconium and hafnium metallurgy

The elements zirconium (atomic number 40) and hafnium (atomic number 72), together with titanium (atomic number 22), form the only three stable elements in column 4 of Mendeleyev's periodic table with the same number of electrons on the outer layer and therefore similar chemical properties:

Ti : (Ar) 3 d ² 4 s ² 8 electrons are missing on the 3 d sublayer,

Zr : (Kr) 4 d ² 5 s ² 8 electrons are missing on the 4 d sublayer,

Hf: (Xe) 4f ¹⁴ 5 d ² 6 s ² 8 electrons missing on sublayer 5 d.

Note that the fourth element in this column is rutherfordium, atomic number 104, the first transactinide to disintegrate spontaneously by fission:

These three elements, Ti-Zr-Hf, form a remarkable trio of reactive metals with very similar chemical properties, making them very difficult to separate. They are also young metals whose industrial expansion only began in the 1950-1960s with the development of cutting-edge industries such as aeronautics, nuclear power and chemical engineering.

Although their chemical properties are similar, their main applications have proved to be fundamentally different, in direct relation to their physical properties. Titanium is a light metal with a density of 4.5 (g/cm ³ ). Thanks to their very high specific strengths, Ti alloys are mainly used in the aerospace industry [M 557] . Zirconium, a metal with a very low thermal neutron capture cross-section, is the nuclear material par excellence. Its main application is the cladding of nuclear fuel in the core of light water (pressurized or boiling water) or heavy water reactors. Hafnium, on the other hand, has a very high capture cross-section, hence its primary application in nuclear reactor control and shutdown rods. On the other hand, the remarkable properties of the passive layer of each of the three metals Ti, Zr, Hf...