Type K thermocouples, which consist of a chromel leg and an alumel leg, are widely used in industry for measuring process temperatures up to 1200°C. They are however, susceptible to preferential oxidation or “green rot” as it also more commonly known.
Green rot can occur when a Type K thermocouple is used in a reducing oxygen environment at a temperature between 800°C to 1,040°C. While they operate well in oxidizing atmospheres, if a reducing gas (such as Hydrogen) is present, a reducing atmosphere can result. Under these conditions, with only a very small amount of oxygen present, the chromium in the chromel alloy will start to oxidize. As the oxygen reacts with the chromium in the conductor, it changes to chromium oxide and a green scale starts to appear on the conductor. When this “green rot” corrosion occurs, the non-magnetic chromel wire will become magnetic. Checking the chromel leg with a magnet to see if it is magnetic is an effective test to determine if green rot has occurred.
Oxidation of the chromel reduces the emf output causing the thermocouple to read low, resulting in a temperature measurement error. Eventually, if left unchecked, it can corrode right through the chromel wire resulting in an open circuit and total loss of measurement.
To reduce the likelihood of oxidation of the chromel alloy occurring, the following preventive steps can be taken:
- Use Magnesium oxide (MI) cable type K thermocouple instead of a beaded thermocouple. The sheathing on the MI cable partially protect the chromel alloy from exposure to the reducing agent.
- Use a protection tube to isolate the type K thermocouple from the process.
- Thoroughly clean the inside of the protection tube to remove any traces of hydrocarbons that can become a reducing agent.
- Install an inert gas purge system onto the protection tube. By surrounding the thermocouple with an inert gas such as Argon, the reducing agent can be prevented from contacting the chromel leg.
- Use a Type K thermocouple that has a titanium ‘getter’ leg. The purpose of the titanium is to absorb the trace amounts oxygen first, thus keeping the chromel from oxidizing.
Even after these preventive steps have been taken green rot may still occur as under the right conditions hydrogen can diffuse through solid metals. To truly eliminate green rot, switching to a type N thermocouple, which has a similar operating temperature range as type K thermocouples but don’t suffer from oxidation, is recommended. It is important to note that a type N thermocouple has a different emf response than type K thermocouples. When changing to a type N thermocouple, the emf response curve for the type N must be used.