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 no oxygen?

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 environments are:

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.