Content:
Operating life
Corrosion resistance
Operating life
Element life is dependent on the alloy used, element temperature, element design, ambient atmosphere, heating cycle, type of element support, etc.
When heated, resistance-heating alloys form an oxide layer on their surface, which reduces further oxidation of the material. To accomplish this function, the oxide layer must be dense and resist the diffusion of gases as well as metal ions. It must also be thin and adhere well to the metal under temperature fluctuations. By carefully balancing the alloying elements, alloys with longer life, higher maximum operating temperatures, and improved high-temperature mechanical strength can be achieved.
The protective oxide layer on Kanthal® alloys, formed at temperatures above 1,000°C (1,830°F), consists mainly of alumina. The color is light grey, while at lower temperatures (below 1,000°C or 1,830°F), the oxide color becomes darker. The alumina layer has excellent insulating properties and good chemical resistance to most compounds.
Even with good adherence, some spalling (flaking off) of the oxide layer cannot be avoided. Damages to the oxide layer are repaired by the spontaneous formation of new oxide.
The oxide formed on Nikrothal® alloys consists mainly of chromium oxide. The color is dark, and its 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 forms on Kanthal® alloys.
For a round wire, the ratio of volume to surface area is proportional to the diameter. Practically, this means that as the wire diameter increases, more alloying element is available per surface unit to form a new oxide. Thus, at a given temperature, thicker wires have longer lifetimes than thinner wires. Similarly, for strip elements, increased thickness leads to longer life.
To estimate the relative quality of a resistance alloy, a test method must be chosen that considers both oxidation rate and spalling. The method used at Kanthal is the Bash-test (ASTM B-76 and B-78). A 0.7 mm (0.0276 in) wire is electrically heated to a standardized temperature and cycled on and off every two minutes. The time to failure is recorded.
Results of such tests are given in the table below, for Kanthal® and Nikrothal® alloys. In the table, the durability of Kanthal® A-1 wire at 1,200°C (2,190°F) is set at 100%, and the durability of the other alloys is related to that figure.
Numerous practical applications also show a much longer life for Kanthal® elements than for elements equipped with NiCr(Fe) wire.
The life of the resistance heating alloy is dependent on several 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.
Corrosion resistance
Corrosive or potentially corrosive substances can significantly reduce the lifespan of resistance-heating wires. Corrosion can be caused by perspiring hands, mounting or supporting materials, or various contaminants.
Steam
Steam is particularly detrimental to wire life, with a more pronounced effect on Nikrothal® alloys than Kanthal® alloys.
Halogens
Halogens (fluorine, chlorine, bromine, and iodine) aggressively attack all high-temperature alloys, even at relatively low temperatures.
Sulfur
Sulfurous atmospheres also pose a threat; however, Kanthal® alloys demonstrate considerably better durability than nickel-based alloys in these environments. Kanthal® alloys are especially stable in oxidizing gases containing sulfur, but their service life is reduced in sulfurous reducing gases. Conversely, Nikrothal® alloys are more sensitive to sulfur.
Salts and oxides
The salts of alkaline metals, boron compounds, etc. in high concentrations can harm resistance heating alloys.
Metals
Molten metals such as zinc, brass, aluminum, and copper can react with resistance alloys, necessitating protection from splashes of these metals.
Ceramic support material
Ceramic support should be carefully selected when in direct contact with heating wires. Firebricks used for wire support should contain at least 45% alumina, and in high-temperature applications, sillimanite or high-alumina firebricks are recommended. The free silica (uncombined quartz) content should be minimized, and iron oxide (Fe2O3) content should be kept as low as possible, ideally below 1%. Additionally, water glass as a binder in cement should be avoided.
Embedding compounds
Most embedding compounds including ceramic fibers are suitable for Kanthal® and Nikrothal® is composed of alumina, alumina-silicate, magnesia, or zircon. For detailed information, refer to the table on “Maximum Wire Temperatures.”
Relative durability values in % Kanthal® and Nikrothal® alloys (ASTM-test wire 0.7 mm (0.028 IN))
|
Alloy |
1,100°C |
1,200°C |
1,300°C |
|---|---|---|---|
|
Kanthal® A-1 |
340 |
100 | 30 |
|
Kanthal® AF |
465 | 120 | 30 |
|
Kanthal® D |
250 | 75 | 25 |
|
Nikrothal® 80 |
120 | 25 |
– |
|
Nikrothal® TE |
130 | 25 |
– |
|
Nikrothal® 60 |
95 | 25 |
– |
|
Nikrothal® 40 |
40 | 15 |
– |
Maximum wire temperatures as a function of wire diameter when operating in air
| Alloy | Diameter | |||||||
| 0.15 – 0.40 mm | 0.0059 – 0.0157 in | 0.41 – 0.95mm | 0.0161 – 0.0374 in | 1.0 – 3.0 mm | 0.039 – 0.18 in | >3.0 mm | >0.118 in | |
| °C | °F | °C | °F | °C | °F | °C | °F | |
| Kanthal® AF | 900 – 1,100 | 1,650 – 2,010 | 1,100 – 1,225 | 2,010 – 2,240 | 1,225 – 1,275 | 2,240 – 2,330 | 1,300 | 2,370 |
| Kanthal® D | 925 – 1,025 | 1,700 – 1,880 | 1,025 – 1,100 | 1,880 – 2,010 | 1,100 – 1,200 | 2,010 – 2,190 | 1,300 | 2,370 |
| Nikrothal® 80 | 925 – 1,000 | 1,700 – 1,830 | 1,000 – 1,075 | 1,830 – 1,970 | 1,075 – 1,150 | 1,970 – 2,100 | 1,200 | 2,190 |
| Nikrothal® TE | 925 – 1,000 | 1,700 – 1,830 | 1,000 – 1,075 | 1,830 – 1,970 | 1,075 – 1,150 | 1,970 – 2,100 | 1,200 | 2,190 |
| Nikrothal® 60 | 900 – 950 | 1,650 – 1,740 | 950 – 1,000 | 1,740 – 1,830 | 1,000 – 1,075 | 1,830 – 1,970 | 1,150 | 2,100 |
| Nikrothal® 40 | 900 – 950 | 1,650 – 1,740 | 950 – 1,000 | 1,740 – 1,830 | 1,000 – 1,050 | 1,830 – 1,920 | 1,100 | 2,010 |