Fig. 1. Typical appearance of a used metallic heating element

Question:
I’ve heard the term “green rot.” What is it, and how can it be avoided?

Fig. 2. Photomicrograph of a failed 80% Ni/20% Cr heating element (Note: Microstructure shows evidence of severe oxidation along the grain boundaries; evidence of carburization as well)  

Answer:
Historically, the corrosion phenomenon that took place in 80% nickel/20% chromium heating elements became known as “green rot.” It is preferential oxidation of the chromium present in the element material by a furnace atmosphere having a high oxidation potential such that the atmosphere was oxidizing to the chromium but reducing to the nickel content of the material. Furnace atmospheres that are sufficiently reducing to both nickel and chromium (e.g., hydrogen) or that are oxidizing to both elements (e.g., air) do not cause this type of attack.  

To complicate matters, many people confuse “green rot” with the gray/greenish surface appearance (Fig. 1) found on most heating elements as a result of their exposure to an air atmosphere under normal operating conditions.  

Fig. 3. Higher magnification image of a failed heating element (Note: Photomicrograph showing significant oxidation and carbide formation along grain boundaries)

The symptoms of green rot are a highly embrittled material that will break if bent exhibiting a fracture surface having heavy green oxide scale below the surface and one in which the microstructure (Figs. 2-3) shows evidence of oxidation and grain-boundary attack. The heating element typically becomes strongly magnetic with a blistered (i.e. “lumpy” or “warty”) surface appearance.

Green rot takes place in a narrow temperature range, from approximately 870-980°C (1600-1800°F). The phenomenon is particularly acute in the 925-980°C (1700-1800°F) temperature range.

One solution for this problem is the substitution of alloys with higher nickel contents. For example, a 35% nickel/15% chromium material does not exhibit this phenomenon up to 1040°C (1900°F). In general, Ni-Cr alloys should not be used in furnaces having endothermic (carburizing) atmospheres and enriched exothermic gas atmospheres because of the concern over preferential oxidation.