Ozone cracking

However, it does occur in many other safety-critical items such as fuel lines and rubber seals, such as gaskets and O-rings, where ozone attack is considered unlikely.

The corners represent stress concentrations, so the tension is at a maximum when the diaphragm of the seal is bent under air pressure.

The seal shown at left failed from traces of ozone at circa 1 ppm, and once cracking had started, it continued as long as the gas was present.

The problem was solved by adding effective filters in the air line and by modifying the design to eliminate the very sharp corners.

By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms.

The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected using energy-dispersive X-ray spectroscopy in the environmental SEM, or ESEM.

A common and low cost antiozonant is a wax which bleeds to the surface and forms a protective layer, but other specialist chemicals are also widely used.

On the other hand, the problem does recur in unprotected products such as rubber tubing and seals, where ozone attack is thought to be impossible.

For high value equipment where loss of function can cause serious problems, low cost seals may be replaced at frequent intervals so as to preclude failure.

Ozone is also produced by the action of sunlight on volatile organic compounds or VOCs, such as gasoline vapour present in the air of towns and cities, in a problem known as photochemical smog.