The use of any heating element material is decided based on how it reacts with oxygen at high temperatures. With certain materials that include tungsten, graphite or molybdenum the oxidation process takes place until the material is totally consumed. In certain other materials a protective oxide layer is formed which hinders further oxidation and provides protection from being affected by other compounds. Formation of oxides is dependent on temperature and time.
Oxide Formation on Heating Elements
The various wire alloys used as heating elements that aid in oxide formation are listed below:
In nickel-chrome alloys, a temperature of 800 to 900°C should be sufficient to ensure the formation of a good oxide layer. The material forms a layer of chromium oxide when exposed to air. The oxide layer is comparatively thick and has a green color can flake off while cycling.
Due to flaking, the base metal is exposed to further oxidation. Consequently chrome gets depleted resulting in element failure. Flaking may also result in contamination of the product unless the elements are either positioned in such a way that falling oxides do not fall on the product or elements placed in protective casings. The oxide offers minimal protection against being infiltrated by carbon and reacts with sulphur.
These alloys form an oxide that consists primarily of alumina (Al2O3). The alloy is highly stable just like chromium oxide but has a distinct characteristic. The alloy is very fine and attaches firmly to the base metal ensuring that flaking is minimized and product contamination is avoided.
Since the element is cycled thermally, tiny cracks may form in the oxide that will result in aluminum depletion in the base metal. Aluminum depletion takes place at a much lesser rate than chromium depletion in nickel-chrome materials having almost the same configuration.
Oxide has a high resistance to carbon infiltration and is especially stable in the presence of sulphur. The oxide also acts as a superior electrical insulator that is very useful in case there is contact between element sections. In these alloys, temperatures more than 1000ºC ensure formation of a good oxide protective layer.
Element life is based on parameters that include atmosphere, temperature, chemical attack and mechanical shock. Due to variations in mounting, operating cycles and other variables it is not easy to provide a precise figure for a given element’s life.
Generally flaking of chromium oxide on nickel-chrome materials has more effect on depletion of the base material when compared to alumina cracking on iron-chrome-aluminum materials. On coupling this with a higher use temperature, it is found that in oxidizing atmospheres, for a specific watt loading, furnace temperature and atmosphere composition, the iron-chrome-aluminum material should be long-lasting.
Figure 1 shows a “life index” for both the alloys in air. Though it is not possible to convert the same directly to hours it can be inferred that elements having a life index of 400 can last twice as long as elements having a life index of 200.
Figure 1. Life Index comparison of NiCr and FeCr materials
The formation of a protective oxide layer is critical in ensuring extended service life for nickel-chrome and iron-chrome-aluminum wire heating elements. Other factors do play a part in determining heating element life such as atmosphere, temperature, chemical attack and mechanical shock, so it is difficult to accurately predict the service life of wire elements
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This information has been sourced, reviewed and adapted from materials provided by Thermcraft.
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