Titanium Powder

Titanium, like magnesium, is classified as a combustible metal, but again the size and shape of the metal determine to a great extent whether or not it will ignite. Castings and other massive pieces of titanium are not combustible under ordinary conditions. Small chips, fine turnings, and dust ignite readily and, once ignited, burn with the release of large quantities of heat. Tests have shown that very thin chips and fine turnings could be ignited by a match and heavier chips and turnings by a Bunsen burner.

Coarse chips and turnings 0.79 by 2.7 mm (1/32 by 3/28 in.) or larger may be considered as difficult to ignite, but unless it is known that smaller particles are not mixed with the coarser material in significant amounts, it is wise to assume easy ignition is possible. Finely divided titanium in the form of dust clouds or layers does not ignite spontaneously (differing in this respect from zirconium, plutonium, and certain other metals). Ignition temperatures of titanium dust clouds in air range from 332 to 588 degrees C (630 to 1,090 degrees F), and of titanium dust layers from 382 to 510 degrees C (720 to 950 degrees F).

Titanium dust can be ignited in atmospheres of carbon dioxide or nitrogen. Titanium surfaces that have been treated with nitric acid, particularly with red fuming nitric acid containing 10 to 20% nitrogen tetroxide, become pyrophoric and may be explosive. The unusual conditions under which massive titanium shapes will ignite spontaneously include contact with liquid oxygen, in which case it may explode on contact. It has been found that under static conditions spontaneous ignition will take place in pure oxygen at pressures of at least 2,413 kPa (350 psi). If the oxygen was diluted, the required pressure increased, but in no instance did spontaneous heating occur in oxygen concentrations less than 35%. Another requirement for spontaneous heating is a fresh surface which oxidizes rapidly and exothermically in an oxygen atmosphere.

Titanium powder is a beautiful material, both lightweight and strong, but the dust can be dangerous. Titanium is used in applications where both high strength and low weight are required, such as, aerospace components, high performance auto racing parts, surgical equipment, and joint replacements.

But, titanium powder is pyrophoric: A cloud of titanium dust can ignite with explosive force sometimes spontaneously when exposed to air or when ignited by a spark. It doesn't take much. The lower explosive limit for titanium dust is 20 to 30 micrograms per cubic centimeter. This is about the equivalent of 25 to 40 specks of household dust in a little box 0.4 inches on each side.

In some cases, a static discharge of only 25 millijoules may cause ignition. This is about the same amount of energy you get from the static electricity created after you shuffle across a carpet and touch a light switch.