EFFECTS ON ELEMENTS UNDER REDUCING ATMOSPHERES

If it is intendec to fire an electric kiln under reducing
conditions, either by the introduction of gas, charcoal,
wood chippings or similar, w2 strongly advise adherence to
the comments and recommendations mace in the KHNTHAL hand-
book, from which we detail an extract below. This deals with
harmful effects on Kanthal elements.

“The durability of resistance alloys in air at high temper-
atures is greatly increased by an oxide surface layer form-
ed bya reaction with the oxygen of the air. The protective
nature of this oxide layer is proportioned to its area and
depth. Foreign matter usually interferes with the formation
of the oxide layer, and this causes a reduced life. Kanthal
alloys in this respect are no exception. At high tenperatures
the protective layer of Kanthal materials consists almnst
entirely of aluminum oxide. This has a light grey colour and
good chemical resistance. At temperatures below 1000 C.
(1832 F.) the oxide layer has a dark colour since the alum-
inum oxide is impure.

It is therefore recommendid that Kanthal elements which
are used in reducing gases ere given a protective oxide
layer by first putting the elements into operation in
open air. This process is called pre-oxidation. The elem-
ents should be pre-oxidized at 1050 C. (1922 F.) for 7 to
10 hours. It is advisable to re-oxidize the elements
occasionally for short periods, if service conditions are
Favourable."

This may sound complicated, but, generally speaking, if
you keep a keen observation on the elements and re-oxidize
by means of firing the kiln empty with the spy-holes open
and the damper partially open up to a temperature of appr.
100 C. below the maximum for a period of 7 to & hours,
immediately a discolouration is noticed, the element life
should be quite reasoneble., We hove many kilns being used
under reducing conditions.

Definite Figures for the life of Kanthal elements working
in such atmospheres cannot be given.

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