TABLE IV Maximum Permissible Temperature as a Function of Wire Size	Diameter, mm in:
	1.0-3.0ºC 0.039-0.118ºF	>3.0ºC >0.118ºF
KANTHAL A-1 KANTHAL AF KANTHAL D ALKROTHAL NIKROTHAL 80 Plus NIKROTHAL 60 Plus NIKROTHAL 40 Plus NIKROTHAL 20 Plus	1225-1350 2240-2460 1225-1350 2240-2460 1100-1200 2010-2190 1000-1050 1830-1920 1075-1150 1970-2100 1000-1075 1830-1970 1000-1050 1830-1920 975-1025 1790-1880	1400 2550 1400 2550 1300 2370 1100 2010 1200 2190 1150 2100 1100 2010 1050 1920

Furnace Atmospheres As stated earlier, the life of a resistance-heating element depends on the continuous presence of a dense oxide layer completely coating the element surface. Corrosion results from interference with the formation or replenishment of the oxide layer by the presence of specific compounds in the atmosphere. The greater the interference, the shorter the element life. The effect of the corrosive compounds is often temperature dependent. Table V lists maximum permissible element temperatures for various furnace atmospheres. The values are intended for guidance only, since they may be affected by changes in gas composition and dew- point, or the presence of impurities. However, the figures given should not be exceeded, since the heating material will tend to deteriorate rapidly at higher temperatures than those indicated. Air. The possibility of using resistance alloys in air at high temperatures depends entirely on the protective oxide formed at the surface of the material. However, impurities in the atmosphere may disturb the oxide formation. Often fumes, gases, dust, etc. coming from the batch or the furnace insulation contaminate the atmosphere. Unless such furnaces are well ventilated, the gases may escape along the terminals, which may become excessively corroded and fail prematurely. The tendency for the oxide to spall is greater for NIKROTHAL alloys than for KANTHAL alloys under normal operating conditions. This must be kept in mind when heating goods with sensitive surfaces, for example, white porcelain. Furthermore, ceramic element supports may become contaminated to such an extent that creep-currents may cause premature element failure. Controlled atmospheres. In carbonaceous atmospheres of both endothermic and exothermic types, the alumina layer on the surface of KANTHAL alloys provides effective protection against the active components of these gas mixtures. Pre-oxidation in air at 1050º C 1920º F for seven to ten hours considerably lengthens the life of KANTHAL elements in such "protective" atmospheres. In order to secure the longest possible life, the elements should be re-oxidized at intervals, depending on the operating conditionsCarbon deposits may damage the elements. In exothermic and endothermic atmospheres, the oxide layer on NIKROTHAL 80 Plus is not protective. Instead a selective Cr-oxidation along the grain boundaries occurs ("green rot"). The tendency to "green rot" formation is strongest at low oxygen potential in the element temperature range 800 - 950º C 1470 - 1740º F. In such cases a KANTHAL alloy is recommended. Hydrogen and nitrogen atmospheres. Pure hydrogen is not harmful to KANTHAL or NIKROTHAL alloys. However, service life will be shortened if the gas mixture contains uncracked ammonia. Very dry nitrogen, deficient in oxygen, causes the formation of aluminum nitride, thus limiting the permissible temperature (KANTHAL A-1 1050º C 1920º F, KANTHAL AF 1100º C 2012º F). On the other hand, the strong affinity for oxygen tends to inhibit nitride formation in a furnace atmosphere of technical pure nitrogen, which often contains some oxygen. KANTHAL AF is relatively stable in a pure nitrogen atmosphere at temperatures up to 1250º C 2280º F, provided that controlled pre-oxidation is performed at the service temperature. Vacuum. The oxide formed on NIKROTHAL alloys is decomposed at temperatures exceeding 1000º C 1830º F when subjected to a high vacuum, and the components in the alloy may vaporize to an extent depending on pressure and temperature. The protective oxide on KANTHAL alloys is more stable, and pre-oxidized elements can be operated at lower pressures and higher temperatures. At 5 · 10-5 torr and 1100' C 2010º F the element life is very good. If the element temperature is 1150'C 2100 F the element should be re-oxidized after 250 service hours. At 1250º C 2200ºF re-oxidize after 100 hours (1050º C 1920º F at 5 hours).

Table V. Maximum Permissible Temperatures in various Atmospheres	KANTHAL A-1		KANTHAL AF		KANTHAL D		NIKROTHAL 80 Plus		NIKROTHAL 60 plus		NIKROTHAL 40 plus
	ºC	ºF	ºC	ºF	ºC	ºF	ºC	ºF	ºC	ºF	ºC	ºF
Oxidizing:
Air, dry	1400	2550	1400**	2500	1300	2370	1200	2190	1150	2100	1100	2010
Air, moist	1200	2190	1200	2190	1200	2190	1150	2100	1100	2010	1050	1920
Neutral
N2, Nitrogen*	1200/1050	2190/1920	1250/1100	2280/2010	1150/1000	2100/1830	1250	2280	1200	2190	1150	2100
Ar, Argon	1400	2500	1400	2550	1300	2370	1250	2280	1200	2190	1150	2100
Exothermic
10 CO, 15 H2,
5 CO2, 70 N2	1150	2100	1150	2100	1100	2010	1100***	2010	1100	2010	1100	2010
Reducing:
Endothermic:
20 CO, 40 H2,
40 N2	1050	1920	1050	1920	1000	1830	1100***	2010	1100	2010	1100	2010
H2, Hydrogen	1400	2500	1400	2550	1300	2370	1250	2280	1200	2190	1150	2100
Cracked
ammonia:
75 H2, 25 N2	1200	2190	1200	2190	1100	2010	1250	2280	1200	2190	1150	2100
Vacuum:
10-3 torr	1150	2100	1200	2190	1100	2010	1000	1830	900	1650	900	1650
Table V. Maximum Permissible Temperatures in various Atmospheres	KANTHAL A-1		KANTHAL AF		KANTHAL D		NIKROTHAL 80 plus		NIKROTHAL 60 plus		NIKROTHAL 40 plus
	ºC	ºF	ºC	ºF	ºC	ºF	ºC	ºF	ºC	ºF	ºC	ºF
*The higher values apply for pre-oxidized material. **For maximum element life above 1300ºC 2370ºF we recommend KANTHAL A-1 do to better oxide properties. ***Please note risk of "green rot" formation in carburizing atmospheres. Use KANTHAL AF or NIKROTHAL 60 Plus.

Corrosion Resistance Typical furnace atmospheres have been discussed above, and their effect on element life detailed in Table V. Other corrosive or potentially corrosive constituents can also enter the furnace atmosphere; in addition, corrosion can be caused by mounting or supporting materials or contamination.

Gaseous Impurities Steam. Steam affects the formation of the protective oxide, making it porous and non-adhering, and thus shortens element life. This effect is more pronounced on KANTHAL alloys than on NIKROTHAL alloys. Halogens. Halogens (fluorine, chlorine, bromine and iodine), even in trace quantities, severely attack all high-temperature alloys at fairly low temperatures. Sulphur. In sulphurous atmospheres KANTHAL iron-base alloys have considerably better durability than nickel-base alloys. KANTHAL is particularly stable in oxidizing gases containing sulphur; although reducing gases with a sulphur content diminish its service life. NIKROTHAL alloys are sensitive to sulphur. Sulphur reacts with nickel to form nickel sulphide, which has a low melting point and prevents the formation of the protective chromium oxide.

Salts and Oxides, Metals Salts and oxides. The salts of alkaline metals, boron compounds, etc. in high concentrations may interfere with oxide formation, and are therefore harmful to heating alloys. Metals. Some molten metals, such as zinc, aluminum and copper, react with the resistance alloys. Moreover, these metals readily combine with oxygen to produce corrosive oxides. The elements should therefore be protected from splashes.

Ceramic Support Materials For electric furnaces, special attention must be paid to the ceramic supports that come in direct contact with the heating element. Firebricks for element support should have an alumina content of at least 45%. In high-temperature furnaces, the use of sillimanite and high-alumina firebricks is often recommended. In general, the free silica (uncombined quartz) content should be kept low, since silica may react with the surface oxide at high temperatures. Iron oxide content (Fe, O,) must be as small as possible, preferably below 1%. The content of alkali oxides (Na2, O, K, O, etc.) must be kept at very low levels (below 0.1%). Water glass is sometimes used as a binder in cements. This compound, however, has a detrimental effect on resistance heating material and any contact must be avoided. Leakage and creep currents occurring at high temperatures attack the points of contact between the ceramic support and the heating element, often leading to premature failure of the element. This means that support material must have as high an insulating resistance as possible at the service temperature.

Embedding Compounds Most embedding compounds including ceramic fibers are suitable for KANTHAL and NIKROTHAL if composed of alumina, alumina-silicate, magnesia and zirconia and if the conditions under "Ceramic support material" are observed. Acceptable commercial products are generally available. If moistened cement is used with KANTHAL alloys as in heating panels, immediate drying is necessary to prevent the possibility of corrosion caused by sulphuric impurities in the cement. Distilled water is preferred as moistening agent, since fluorinated and chlorinated tap water may cause corrosion. Similarly, degreasing solvents containing chlorine must be re- moved after cleaning the element coils. Certain cements may attack resistance-heating materials. In a closed environment such as in embedded elements, even traces of sulphur-containing contaminants may cause severe attack on NIKROTHAL wires at elevated temperature. Boron compounds at- tack KANTHAL and NIKROTHAL alloys above 900 C 1650º F. Corrosion tests for embedding compounds should always be performed before they are specified for use.