The role of electric heating chromium:

The role of electric heating chromium:

Chromium is a key element to improve the high temperature oxidation resistance of nickel-chromium and nickel-chromium-iron electric heating alloys. The protective oxide film formed by the alloy at high temperature is mainly composed of Cr2O3. The oxide film mainly made of Cr2O3 is denser and has strong adaptability, which can ensure long-term use of the alloy at high temperatures. The oxidation rate of the alloy decreases as the chromium content increases. The relationship between chromium content, temperature and oxidation rate in Fe-Cr system is shown in Figure 3-2.

In order to maintain the good oxidation resistance of the electric heating alloy at high temperature, it is required that the oxidation weight gain of the alloy at this temperature is not more than 2.0*10-2g.cm-2.h-1. According to this value, nickel-chromium and nickel-chromium-iron electric heating alloys containing 15%~18%, 19%~23% and 28%~31% of Cr, the highest oxidation resistance temperature is correspondingly: 900~950℃, 1000~1100℃ And 1000~1200℃.

High resistivity value:

Electric heating alloy materials should first have relatively high resistivity. The electric heating element made of high resistivity material has high electrothermal conversion efficiency; the volume of the electric heating element is small; alloy material is saved; high-power rapid heating can be realized. It is especially important when designing an electric heating furnace with high temperature, high power and small size.

The room temperature resistivity of commonly used electric heating alloys is 1.00～1.60μΩ•m. The resistivity of the iron complex aluminum alloy is higher than that of the nickel-chromium and nickel-chromium-iron alloys. The room temperature resistivity of typical electric heating alloys is listed in Table 5-1. Different manufacturers combine their alloy grades to give corresponding room temperature resistivity values.

Stable stage of electrothermal oxide film:

When a continuous oxide film is formed on the surface of the alloy, the oxygen penetration rate into the alloy continues to decrease. At this time, there is still a small amount of oxygen that can oxidize through the oxide film with the alloy elements enriched under the film to form an internal oxide layer.

At this stage, the alloy is oxidized at a very slow rate. After a long time of high temperature, the density and thickness of the oxide film increase. The oxidation of the alloy has a linear relationship with time. The oxide film becomes a two-layer double structure inside and outside. The longer the stable phase of the oxide film is, the longer the life of the alloy.

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