Environmental Considerations for Electric Resistance Heating Element Materials
Assuming that you have properly designed the heating element and
based on the guidelines in the watt-loading tables the element you
have designed should operate properly in air (or the normal environment
for that specific type of element). What now happens when you place
that oxidation resistant material into an atmosphere containing
As is described in the section Electric Resistance Heating Materials,
there are two types of heating element materials, Oxidation Resistant
and Non-Oxidation Resistant. Each behaves slightly differently in
atmospheres containing other gasses.
The Oxidation Resistant Materials rely on a layer of oxide to develop
and protect the element from not only further oxidation, but also
from other types of attack. Because of this it is recommended that
if possible these elements should be run in air at the maximum temperatures
expected for several hours. By forming this initial oxide layer,
the element will not only be better prepared to resist most atmospheres,
but it will also proved an extra level of protection against contaminants
that may be present in the pieces that support the element.
Materials that would prove to be hazardous to the heating element
include those that would disrupt, inhibit, or reduce the oxide that
has been formed. Specifically, most fluxes contain halogens (Chlorine,
or Fluorine) which can very quickly reduce the oxide exposing the
heater to further attack.
For the Chrome-Oxide forming alloys, the temperatures in
For Aluminum-Oxide forming alloys, the temperatures in different
Molybdenum Disilicide, forms a Silicon-Oxide. Chlorine and fluorine
rapidly attack this oxide.
In vacuum the silica is actually evaporated directly from the element.
Dry Hydrogen being very reducing also pulls the silica from the
base material, leaving nearly pure molybdenum.
Silicon Carbide, while being resistant to may chemicals, will also
not tolerate high Chlorine or Fluorine. In Nitrogen the element
is limited due to interaction with the element, forming Silicon
Nitride that will appear as a fuzzy white mass surrounding the heater.
For the non-oxidation resistant materials, they cannot form any
protective coating and are therefore directly exposed to the atmospheres
they are placed.