First, selective oxidation
For iron-containing nickel-chromium alloys, if the oxygen partial pressure is lower than a specific value, then chromium, which has an affinity with oxygen, will undergo selective oxidation, which is the unique grain boundary oxidation of nickel-chrome alloys. — 1050 ℃范围内，体系内含有一氧化碳﹑氢气等还原性气体时，这时的K型热电偶的正极，更容易发生选择性氧化。 When the temperature is in the range of 800-1050 ℃, and the system contains reducing gases such as carbon monoxide and hydrogen, the positive electrode of the K -type thermocouple at this time is more likely to undergo selective oxidation. 型热电偶在热处理行业长期使用的限制因素正是因为铬含量降低而引起的热电势偏低所影响。 The limiting factor for the long-term use of K -type thermocouples in the heat treatment industry is precisely the lower thermoelectric potential caused by the reduced chromium content.
If you use a very pure gas, and the system does not contain oxygen, this can extend the life of the thermocouple . However, if there is an oxide layer on the surface of the thermocouple wire, it can also provide sufficient oxygen for the selective oxidation of chromium. Therefore, when used in a non-oxidizing atmosphere, clean, polished wire should be used. At the same time, try to avoid using in the gas containing a trace amount of oxygen or air with a low oxygen partial pressure. When the protective tube is very thin, due to the inability of air to circulate well, an oxygen-deficient state is formed, so that a small amount of residual oxygen can still provide conditions for the selective oxidation of chromium.
Countermeasures for selective oxidation
型热电偶引起劣化，除了在材质方面要特别注意外，还应该在热电偶的结构上采取相应的对策： In order to prevent or slow down the degradation of the K -type thermocouple caused by selective oxidation, in addition to paying special attention to the material, we should also take corresponding measures on the structure of the thermocouple :
① A metal that has a stronger affinity for oxygen than chromium is selected as the getter and sealed in the protective tube to prevent the selective oxidation of chromium. In addition, the diameter of the protective tube or the method of blowing air can also be used to increase the oxygen content.
② Fabricated thermocouples.
Third, the atmosphere of use
的稳定性，会因使用温度，气氛不同而各不相同。 The stability of thermocouples varies depending on the operating temperature and the atmosphere. 型热电偶的也会因为直径的不同而发生变化，而直径相同的K型热电偶也会因结构的不同，使得其稳定性也有很大不同。 For the same sensor, for example, at the highest operating temperature, the K -type thermocouple will change due to the different diameters, and the K -type thermocouples with the same diameter will also have different stability due to their different structures. When selecting a thermocouple, the following points must be considered due to the conditions of use:
抗振动性能 Anti-vibration performance 氧化还原等使用气氛 2. Redox atmosphere 常用温度及最高使用温度 3.Common temperature and maximum use temperature