Hafnium can react with air to form a protective layer of oxide film. When the temperature is 500 ~ 750℃, the oxide film will lose its protective effect, and combine with oxygen, nitrogen and other gases when heating to form oxides and nitrous compounds. At temperatures above 800 ° C, hafnium rapidly oxidizes to HfO2.
Hafnium has good corrosion resistance and does not react with dilute hydrochloric acid, dilute sulfuric acid and strong alkali solutions, but is soluble in hydrofluoric acid and aqua regia.
Hafnium in compounds often have a +4 valence, the main compound is hafnium oxide HfO2, there are three different variants: monoclinic variant, tetragonal variant, and cubic variant. The +4 valence hafnium is also easy to form complexes with fluorides consisting of K2HFF6, K3HFF7, (NH4) 2HFF6, (NH4) 3HFF7.
