Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers, answer questions about heat treating, brazing, sintering, and other types of thermal treatments, as well as metallurgy, equipment, and process-related issues. In this installment, Dan Herring examines the devastating effects of hot gaseous corrosion on furnace alloys: exploring the mechanisms behind metal dusting, the gas-solid reactions that drive catastrophic carburization, and the mitigation strategies to extend the life of heat treaters’ most valuable furnace components.
This informative piece was first released in Heat Treat Today’s January 2026 Annual Technologies To Watch print edition.
Have questions or feedback? We’d love to hear from you — reach out to our editorial team at editor@heattreattoday.com.
Corrosion is a concern experienced by everyone involved in manufacturing industrial products. While there is a plethora of data and information on the effects of corrosion on engineered materials available (sources provided in the references section of this column), most corrosion engineers are focused on aqueous corrosion. By contrast, heat treaters must understand the effects of hot gaseous corrosion, especially on our furnace alloys. Let’s learn more.
Corrosion Basics
It is important to understand that all materials are chemically unstable in some environments and corrosive attack will always occur. In the scientific world, it can often be modeled and its effects predicted by studying thermodynamic data and knowing which of the many corrosion-related chemical states are active. In our world, however, it is equally important to understand the various forms of corrosion, namely:
- Dezincification (aka selective leaching)
- Electrolytic
- Erosion
- Galvanic (or two metal) action
- General (aka uniform) attack
- Intergranular attack
- Pitting
- Stress corrosion
The greater the metal’s solubility, the greater the degree and severity of the corrosive attack. There are many important variations of these forms of corrosion; two of the most important are 1) localized corrosive attack (e.g. pits, intergranular attack, crevices) and 2) interaction with mechanical influences (e.g., stress, fatigue, fretting). These actions are frequently rapid and have catastrophic effects.
The number of ways to combat corrosion have been well-documented, including alloying to produce better corrosion resistance materials; cathodic protection (via sacrificial anodes); coatings (metallic or inorganic); organic coatings (e.g. paints); metal purification; alteration of the environment; and nonmetallic or design (i.e., physical) changes.
Heat Resistant Alloys
Furnace interiors contain numerous examples of heat-resistant nickel-chromium-iron (Ni-Cr-Fe) alloys, including radiant tubes, fans, heating elements, roller rails and rollers, thermocouple protection tubes, chain guides, and atmosphere inlet tubes, to name a few. Baskets, grids, and fixtures are other examples. These alloys are normally selected based on their strength (at temperature) rather than resistance to corrosive attack.
Since these heat-resistant alloy parts are often the most expensive furnace components, heat treaters must understand how they can be attacked and what can be done to extend their life by minimizing or preventing corrosion.
Gas-Solid Reactions
A chemical reaction involving a (non-equilibrium) gas or gas mixture and a solid is classified as a gas-solid reaction. Examples of intermediate and high temperature reactions of this type include oxidation, sulfidation, carburization, and nitriding. Effects of gases containing vapors of chlorine, fluorine, and effluents from deposits of various alkaline chemicals (from cleaning compounds) and even phosphates are also problematic. The principles are the same for all types — only the details differ. As heat treaters, our interest is in controlling, retarding, or suppressing these reactions to prevent unwanted corrosion, gasification, or embrittlement of the furnace alloy or materials being processed.
Examples of Catastrophic Carburization (a.k.a. Metal Dusting)
Metal dusting (Figure 1) is a hot gaseous corrosion phenomenon in which a metallic component disintegrates into a dust of fine metal and metal oxide particles mixed with carbon.
Generally, metal dusting occurs in a localized area, and how rapidly the disintegration progresses is a function of temperature, the composition of the atmosphere and its carbon potential, and the material. Other significant factors include the geometry of the system, reaction kinetics, diffusivities of alloy components, the specific-volume ratio of new and old phases, and the ultimate plastic strain.
Metal dusting usually manifests itself as pits or grooves on the surface, or as an overall surface attack in which the metal can literally be eaten away in a matter of days, weeks, or months. As an example, this writer has seen a 330-alloy plate mounted underneath a refractory-lined inner door of an integral quench furnace (where atmosphere passes underneath the door and into the quench vestibule) reduced in thickness from 12.5 mm (0.50 in) to less than 0.75 mm (0.03 in) in a little over two months.
In another example, a metallographic investigation performed by this writer on a failed wrought 330 alloy radiant tube (Figure 2) was conducted. Optical microscopy of the inside (Figure 3) and outside diameter surfaces in the attacked area revealed evidence of massive carbides. These carbides are formed by the reaction of carbon with chromium, depleting the matrix of chromium in regions adjacent to the carbides. Grain detachment and subsequent failure by erosion then occurred.
How Does It Occur?
In general, catastrophic carburization of ferrous alloys proceeds via the formation and subsequent disintegration of metastable carbide. The first step in the process is absorption of the gaseous phase on the surface of the metal; the more reactive this phase, the easier it decomposes or is catalytically decomposed (in the case of iron) on the surface. This step is followed by diffusion of carbon atoms from the surface into the bulk metal.
As a result, there is a continuous buildup of carbon within the surface layer. As this layer becomes saturated with carbon, a stable carbide, metastable carbide, or an active carbide complex forms, which then grows until it reaches a state of thermodynamic instability, at which point it rapidly breaks down into the metal plus free carbon.
It’s at this stage that the metal disintegrates to a powder as the result of plastic deformation and subsequent fracture in the near-surface layer. The process is controlled by internal stresses due to phase transformation; in other words, competition between stress generation and relaxation exceeds the ultimate strength in this near-surface layer and causes fracture to occur.
In Ni-Cr-Fe alloys, the phenomenon occurs slower (but does not stop) since the disintegration leads to larger metal particles, which are less active catalysts for carbon deposition than the fine iron particles that form with ferrous metals. Therefore, the mass gain from carbon depositing onto high-nickel alloys is much lower. Also, the decomposition of high-nickel alloys occurs by graphitization and not via unstable carbides.
Pourbaix-Ellingham Diagrams
Thermodynamics can be applied to solid-gas reactions to obtain equilibrium dissociation pressures below which no reactions occur. Data and diagrams are available for the free energies of formation versus temperature for most metallic compounds. An interesting use of Pourbaix diagrams (generally reserved for mapping out possible stable equilibrium phases of an aqueous electrochemical system) as a predictor of stable alloy systems is found by superimposing the various elemental constituents. These diagrams are read much like a standard phase diagram (with a different set of axes).
In Summary
Hot gaseous corrosion should be an area of focus for every heat treater to extend the life of alloy components, reduce downtime, and save money. Mitigation in the form of alloy selection, equipment design, type of atmosphere, process/cycle selection, and idling temperatures will play a huge role in extending the life of our furnace alloys, baskets, and fixtures.
References
ASM International. 1971. Oxidation of Metals and Alloys.
ASM International. 2003. ASM Handbook. Vols. 13A–C.
Fontana, Mars G., and Norbert D. Greene. 2008. Corrosion Engineering. New York: McGraw-Hill.
Herring, D. H. 2003. “What to Do About Metal Dusting.” Heat Treating Progress, August.
Herring, Daniel H. 2015. Atmosphere Heat Treatment. Vol. 2. Troy, MI: BNP Media Group.
Javaheradashti, Raza. 2008. Microbiologically Induced Corrosion. Berlin: Springer-Verlag.
NACE International. www.nace.org.
Nateson, K. 1980. Corrosion–Erosion Behavior in Metals. Warrendale, PA: Metallurgical Society of AIME.
National Bureau of Standards. 1978. Gas Corrosion of Metals.
Pourbaix, Marcel. 1974. Atlas of Chemical and Electrochemical Equilibria in Aqueous Solutions. Houston, TX: NACE International.
Pourbaix, Marcel. 1998. Atlas of Chemical and Electrochemical Equilibria in the Presence of a Gaseous Phase. Houston, TX: NACE International.
Schweitzer, Philip A. 1996. Corrosion Engineering Handbook. New York: Marcel Dekker.
Staehle, R. W. 1995. “Engineering with Advanced and New Materials.” Materials Science and Engineering A 198 (1–2): 245–56.
Stempco, Michael J. 2011. “The Ellingham Diagram: How to Use It in Heat-Treat-Process Atmosphere Troubleshooting.” Industrial Heating, April.
Uhlig, Hubert H. 2008. Corrosion and Corrosion Control. Hoboken, NJ: Wiley-Interscience.
Fabian, R., ed. 1993. Vacuum Technology: Practical Heat Treating and Brazing. Materials Park, OH: ASM International.
The Boeing Company. n.d. “Practical Vacuum Systems Design Course.”
About the Author
“The Heat Treat Doctor”
The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
For more information: Contact Dan at dherring@heat-treat-doctor.com.
For more information about Dan’s books: see his page at the Heat Treat Store.
