Solid hafnium does not burn. However, particles of 10 microns
or less can spontaneously combust
at room temperature. This can happen even though the particles are wet. Spraying burning hafnium with water is not a good idea
. A CO2
fire extiguisher is also not effective on Hafnium fires. If a fire starts in a mass of wet metal fines, such as a barrel of damp machining chips, the initial fire may be followed by an explosion and a very high temperature flash radiation due to the evolution of hydrogen
Hafnium metal is rapidly dissolved by hydrofluoric acid or
hydrofluoric-nitric acid mixtures. Above 200°C, hafnium reacts exothermically with fluorine,
chlorine, bromine, iodine and with halocarbons, including carbon tetrachloride, carbon
tetrafluoride and Freons. Nitryl fluoride, FNO, will initiate a reaction with hafnium metal at room
temperature to produce a glowing or white incandescence.
Pure hafnium is not known to be toxic or carcinogenic, and is not soluble in water, saline, or other fluids found in the body.
Info paraphrased from http://www.thermadyne.com/tdc/literature/pdfs/msds/TR702.pdf
thanks to C-Dawg, eliserth and montecarlo