The Kiln Gods on sun 11 oct 98
Hello Vince & fellow Clayarters,
Yes Vince, I am listening. I try to read most of your posts. I applaud your
education & diplomacy.
You are correct....."If you erode that layer of oxidation, they continue to
oxidize deeper into the core metal, and eventually they fail.".....
Oxidizing elements in a standard electric kiln definately increases their
life.
My perspective is to charge at 10-20% more for any work that has been fired
in an electric reduction kiln, buy elements as heavy as possible. KISS
....also oxidize the elements, at least once.......If they are still working
after 3-6 months..oxidize them again.
The following are excerpts from a Thyssen alloy handbook & a Kanthal
handbook, regarding resistance to corrosion/chemical attack;
Resistance to corrosion-Thyssen:
The rate of chemical reactions generally increases rapidly with the
temperature. For particular attacking agents, however, such as sulphur,
carbon-nitrogen, ect..., there are certain ranges of temperature over which
they are especially damaging. This is usually connected with the formation
of certain compounds or crystalline phases.
For heating alloys, clean air (ie: its oxygen) is required for the
formation of the protective oxide layers. Iron-chrome-aluminium heating
alloys (used in most kilns) require a stable protective layer of aluminium
oxide. This is formed at temperatures above 900/1000`C. Longer periods of
time under 1000`C, especially in water vapour, can lead to a reaction
between the heating element and atmospheric nitrogen, forming very brittle
nitrides.
Resistance to chemical attack-Kanthal:
The durability of resistance alloys in air, at high temperature, is due to
the formation of a protective oxide layer formed by reation with the oxygen
in the air. The protective nature of this oxide layer is dependent on
structure & nature of crystals. Foriegn materials usually interfere with
the formation of the oxide layer & this causes reduction in life. Kanthal
alloys, in this respect, are no exception. At high temperatures the
protective layer on Kanthal materials consists almost entirely of aluminium
oxide. This has a gray-white colour & good chemical resistance. At
temperatures under 1000`C (1830`F), the oxide layer has a darker look since
it does not consist of pure aluminium oxide.
In gases with a low oxygen-yeilding content, the oxide layer can become
porous & admit damaging gases such as nitrogen. It is therfore recommended
that Kanthal elements which are to be used in reducing gases be
pre-oxidized by first operating the element in open air. The elements are
best pre-oxidized if the furnace is held at 1050`C (1920`F) for 7-10 hours,
after which the elements can be used in some protective atmospheres. If
possible, it is advisable to pre-oxidize the elements periodically.
There is alot more info. If there is an interest I will continue.
Chris @
Euclid's Kilns and Elements
1-800-296-5456
Web Site: http://www.euclids.com
E-Mail: mail@euclids.com
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