{"id":9553,"date":"2018-05-29T11:40:03","date_gmt":"2018-05-29T16:40:03","guid":{"rendered":"http:\/\/blog.wika.com\/us\/\/?p=9553"},"modified":"2025-02-03T14:37:33","modified_gmt":"2025-02-03T19:37:33","slug":"green-rot-affects-type-k-thermocouples","status":"publish","type":"post","link":"https:\/\/blog.wika.com\/us\/products\/temperature-products\/green-rot-affects-type-k-thermocouples\/","title":{"rendered":"How Green Rot Affects Type K Thermocouples"},"content":{"rendered":"

Green rot is most pronounced when type K thermocouples are exposed to temperatures between approximately 800\u00b0C\u00a0and 1,260\u00b0C (1,472\u00b0F to 2,300\u00b0F) and when there\u2019s a low concentration of oxygen. This problem causes negative drifts, inaccurate measurements, and even open circuits due to breakage caused by embrittlement.\u00a0<\/span>Applications where there is abundant oxygen supply or no oxygen at all, green rot does not occur. <\/strong><\/p>\n

Inexpensive, fast-responding, robust, and self-powered, industrial thermocouples are a top choice for temperature measurements<\/a> and are widely used in many applications. Thermocouples<\/a> comprise two wire conductors (legs) made of different metals (alloys), coming together at one end to create the measuring point (hot junction).<\/p>\n

Thermocouples come in several types, designated by a letter. Each type has a different temperature-versus-EMF (electromotive force) characteristic due to the particular pairing of conductors. Some thermocouples are better in lower temperatures, while others do not even register much of an EMF until much higher temperatures.<\/p>\n

The most commonly used thermocouple is type K, which works well within a broad temperature range \u2013 from between \u2212200\u00b0C to 1,260\u00b0C (\u2212328\u00b0F to 2,300\u00b0F). Its positive leg is made of a nickel-chromium (NiCr) alloy, while its negative leg is made of a nickel-aluminum (NiAl) alloy (ferromagnetic).<\/p>\n

Although type K thermocouples are rugged and long lasting, they \u2013 like all thermocouples \u2013 lose accuracy with age and depending on their operating conditions. \u201cGreen rot\u201d is one of the issues that can affect this general-purpose temperature measurement device.<\/p>\n

What is Green Rot?<\/h2>\n\n

Example of green rot<\/p><\/div>\n \n

Green rot is another name for the oxidation of chromium. It appears in high-temperature situations \u2013 typically between 800\u00b0<\/span>C and 1,260\u00b0<\/span>C (1,472\u00b0F and 2,300\u00b0F) \u2013 and when the process occurs in a low-oxygen environment. An oxide layer on the surface of the NiCr leg normally protects it from oxidizing. But in the presence of hydrogen or another reducing agent, chromium oxidation accelerates. The result is a layer of scaly green corrosion on the positive leg.<\/p>\n

As oxidation eats away at the chromium, the normally non-magnetic NiCr leg becomes magnetic, causing a decrease in the thermoelectric voltage at the reference junction (cold junction). The greater the corrosion, the larger the negative voltage drift becomes, leading to significant temperature measurement errors. Green rot also makes the NiCr leg more brittle. If the corrosion is not checked, the positive leg can break, opening the circuit and rendering the device inoperable.<\/p>\n

What to Do About Green Rot<\/h2>\n

Preventive measures can minimize the formation of green rot, thereby ensuring accurate temperature measurements and extending the lifetime of a type K thermocouple. These steps include:<\/p>\n