Content:
Operating life
Oxidation properties
Corrosion resistance
Maximum wire temperatures as a function of wire diameter when operating in air

Operating Life

The life of the resistance heating alloy is dependent on a number of factors, among them the most important are:

• temperature
• temperature cycling
• contamination
• alloy composition
• trace elements and impurities
• wire diameter
• surface condition
• atmosphere
• mechanical stress
• method of regulation

Since these are unique for each application it is difficult to give general guidelines of life expectations. Recommendations on some of the important design factors are given below.

Oxidation Properties

When heated, resistance heating alloys form an oxide layer on their surface, which slows down further oxidation of the material. To accomplish this function the oxide layer must be dense and resist the diffusion of gasses as well as metal ions. It must also be thin and adhere to the metal under temperature fluctuations. The protective oxide layer on Kanthal® alloys formed at temperatures above 1000°C (1830°F) consists mainly of alumina (Al2O3). The color is light gray, while at lower temperatures (under 1000°C (1830°F)) the oxide color becomes darker. The alumina layer has excellent electrical insulating properties and good chemical resistance to most compounds.

The oxide formed on Nikrothal® alloys consists mainly of chromium oxide (Cr2O3). The color is dark and the electrical insulating properties are inferior to those of alumina.

The oxide layer on Nikrothal® alloys spalls and evaporates more easily than the tighter oxide layer that is formed on Kanthal® alloys.

Results of several life tests according to ASTM B 78 (modified) are given in a table for Kanthal® and Nikrothal® alloys. In the table, the durability of Kanthal® A-1 wire at 1200°C (2190°F) is set at 100%, and the durability of the other alloys is related to that figure.

Corrosion Resistance

Corrosive or potentially corrosive constituents can considerably shorten wire life. Perspiring hands, mounting or supporting materials or contamination can cause corrosion.

Steam

Steam shortens the wire life. This effect is more pronounced on Nikrothal® alloys than on Kanthal® alloys.

Halogens

Halogens (fluorine, chlorine, bromine and iodine) severely attack all high-temperature alloys at fairly low temperatures.

Sulphur

In sulfurous atmospheres Kanthal® alloys have considerably better durability than nickel-based alloys.

Kanthal® is particularly stable in oxidizing gasses containing sulfur, while reducing gasses with a sulfur content diminish its service life.

Nikrothal® alloys are sensitive to sulfur.

Salts and oxides

The salts of alkaline metals, boron compounds, etc. in high concentrations are harmful to resistance heating alloys.

Metals

Some molten metals, such as zinc, brass, aluminum and copper, react with the resistance alloys. The elements should therefore be protected from splashes of molten metals.

Ceramic support material

Special attention must be paid to the ceramic supports that come in direct contact with the heating wire. Firebricks for wire support should have an alumina content of at least 45%. In high-temperature applications, the use of sillimanite and high-alumina fire bricks is often recommended. The free silica (uncombined quartz) content should be held low. Iron oxide (Fe2O3) content must be as small as possible, preferably below 1%. Water glass as a binder in cements must be avoided.

Embedding compounds

Most embedding compounds including ceramic fibers are suitable for Kanthal® and Nikrothal® if composed of alumina, alumina-silicate, magnesia or zircon.

Maximum wire temperatures as a function of wire diameter when operating in air