Understanding Oxidation in High-Temperature Heating: When Heat Becomes Destructive

Understanding Oxidation in High-Temperature Heating: When Heat Becomes Destructive

A high-temperature furnace is equipped with a brand-new cartridge heater. The initial weeks of operation are uneventful. The temperature of the process then starts to fluctuate. Setpoint is reached more slowly by the heater. The system eventually breaks down. The sheath has a flaky, rough surface after the heater is removed. Some of it is now greenish-gray or black. This isn't a manufacturing flaw. Anyone involved in industrial heating must comprehend this process of oxidation.


The chemical reaction that occurs when a metal and oxygen come into contact at high temperatures is called oxidation. There is a temperature threshold for every metal above which oxidation speeds up significantly. That barrier is between 550°C and 600°C for common stainless steel grades like 304 or 316. The chromium oxide layer that typically shields the surface becomes unstable above these temperatures. It gradually becomes porous, thickens, and spalls off, leaving new metal vulnerable to additional assault. The sheath becomes thinner and less capable of transferring heat as the process feeds on itself.

Specifically designed to withstand this kind of deterioration is an Incoloy600 single head electric tube heater. About 15–17% chromium and 72% nickel make up the alloy. The chromium creates a thick, sticky film of Cr2O3 when it is exposed to high-temperature oxidising conditions. This layer is incredibly resilient even when subjected to heat cycling because of its high nickel concentration. The exposed alloy instantly starts to regenerate the protective scale if the surface is scraped or mechanically damaged. The heater can run continuously in oxidising environments at temperatures as high as 1095°C (2000°F) thanks to this self-healing feature.

However, even this sophisticated alloy has its limitations. The rate law of oxidation is temperature and time dependent. The protective layer stays thin and the reaction is slow at lower temperatures. The reaction rate exhibits an exponential curve as the temperature rises. After thousands of hours, an Incoloy600 single head electric tube heater running at 600°C could exhibit very little oxidation. Over time, a visible oxide layer will form on the same heater running at 900°C. The sheath will eventually shrink sufficiently to jeopardise structural integrity at 1050°C, when the reaction rate accelerates dramatically.

Oxidation is also influenced by the surroundings of the heater. Air or pure oxygen speeds up the process. The kind of oxide that forms can be altered by environments that contain sulphur compounds, carbon dioxide, or water vapour. Although it is not impervious, Incoloy600 functions effectively in the majority of industrial environments. Sulfidation, for instance, can happen in settings with high sulphur contents at temperatures higher than 500°C. This results in the formation of nickel sulphide compounds, which are liquid at working temperatures and cause quick intergranular attack. Alternative materials like Incoloy825 or Inconel 601 might be more suitable in these situations.

The impact of thermal cycling on oxidation is another element that is frequently disregarded. The oxide scale expands at a different rate than the underlying metal each time the heater heats up. This differential expansion has the potential to fracture the scale across numerous cycles, exposing new metal to oxygen. A tiny quantity of metal from the sheath is consumed with each cycle as the new scale develops. Even an Incoloy600 single head electric tube heater will eventually undergo detectable oxidation in applications that cycle regularly, like packing sealers or hot runner systems. The oxidation rate can be decreased by maintaining the sheath temperature as low as feasible for the necessary power output by keeping the watt density between 5 and 7 W/cm².

In practical terms, how may oxidation be reduced? Maintaining the sheath temperature as low as possible while yet reaching the necessary process temperature is the best course of action. This entails choosing the appropriate watt density for the intended use. For the majority of conduction-heated applications with incoloy600 sheaths, a watt density of 5 to 7 W/cm² is a safe starting point, as was covered in earlier articles. The sheath must run at a greater temperature due to larger watt densities, which speeds up oxidation. Although they might be acceptable, lower watt densities could lead to slower heat-up times.

Oxidation is also influenced by how well the heater fits the hole. Because of the air gap caused by a loose fit, the sheath must operate at a higher temperature to keep the process temperature constant. The oxidation rate is directly increased by the greater sheath temperature. By minimising the temperature differential between the heater and the target material, proper installation with a tight clearance fit keeps the oxide layer thinner and the sheath cooler.

A prevalent misperception is that a discoloured sheath always indicates a failure. Particularly after extended use at high temperatures, some discolouration is typical. It usually appears as a light brown or golden colour. On the other hand, dark grey or greenish-black scaling suggests that the sheath has been working at temperatures higher than what was anticipated. A significant warning indicator is flaking or peeling scale, which indicates that the metal is being consumed more quickly and that the protective coating is no longer adhering.

The importance of material selection is demonstrated by field data from industrial heat treatment applications. In one instance, sheath oxidation necessitated replacing conventional stainless steel cartridge heaters every 8 to 10 weeks in a continuous annealing furnace running at 800°C. The replacement period increased to more than 18 months after switching to an Incoloy600 single head electric tube heater with the same wattage and dimensions. The labour and downtime benefits outweighed the little initial cost difference.

Regular visual examination is an easy-to-use yet powerful tool for maintenance crews. Check the sheath for indications of severe oxidation whenever a heater is removed for any reason. The operating temperature or watt density may be too high for the selected material if the surface is uneven, pitted, or exhibits scaling. The heater has merely reached the end of its natural service life if the oxidation is homogeneous and the sheath still has acceptable mechanical integrity.

It's also important to remember that there are other breakdown mechanisms at high temperatures besides oxidation. Performance can also be impacted by creep, thermal fatigue, and grain boundary development. Although the Incoloy600 alloy is made to withstand these processes, it is not a long-term fix. Every heater has a limited lifespan, which is influenced by environmental factors, cycle frequency, and operating temperature. The thermal requirements of many industrial processes vary greatly, from the melting of aluminium to the production of semiconductors. The appropriate heater material and design are chosen for the task thanks to a careful evaluation of the operating environment.