Data and information on the effects of corrosion on engineered materials are available in many forms and from many sources. The focus for most corrosion engineers is on aqueous corrosion, an important topic in and of itself. As heat treaters, however, the effects of hot gaseous corrosion in our heat-treat furnaces are of more immediate concern.

Fig. 1.  Pusher-furnace alloy fan and shaft assembly

Data and information on the effects of corrosion on engineered materials are available in many forms and from many sources.[1–15] The focus for most corrosion engineers is on aqueous corrosion, an important topic in and of itself. As heat treaters, however, the effects of hot gaseous corrosion in our heat-treat furnaces are of more immediate concern. Let’s learn more.  

Corrosion Basics

We begin with the realization that all materials are chemically unstable in some environments, and corrosive attack will occur. It can often be predicted or modeled by studying thermodynamic data and knowing which of the many corrosion-related chemical states are active. In the real world, however, it is important to recognize the various forms of corrosion:
  •   Uniform (or general) attack
  •   Intergranular attack
  •    Galvanic (or two-metal) action
  •   Erosion
  •    Dezincification (or parting)
  •    Pitting
  •    Stress corrosion
  •    Electrolytic (or concentration) cells
      When it comes to corrosion, the greater the metal’s solubility, the greater the degree and severity. There are many important variations on the above, and two of the most important are (a) localized corrosive attack (e.g., pits, intergranular attack, crevices, galvanic action) and (b) interaction with mechanical influences (e.g., stress, fatigue, fretting). These actions are frequently very rapid and have catastrophic effects.

    There are also a number of ways to combat corrosion, including: alloying to produce better corrosion resistance; cathodic protection (via sacrificial anodes); coatings (metallic or inorganic); organic coatings (e.g., paints); metal purification; alteration of the environment; nonmetallics and design (i.e. physical) changes.

Heat-Resistant Alloys

Furnace interiors contain numerous examples of heat-resistant iron-nickel-chromium (Fe-Ni-Cr) alloys for such items as radiant tubes, fans, heating elements, roller rails and rollers, 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) and resistance to corrosive attack.

    Since these heat-resistant alloy parts are often the most expensive furnace components, heat treaters should have an understanding of how they can be attacked and what can be done to extend their life by minimizing or preventing it.

Gas-Solid Reactions

A chemical reaction involving a solid and a non-equilibrium gas or gas mixture can be classified as a gas-solid reaction. Examples of intermediate and high-temperature reactions of this type include oxidation, sulfidation, carburization, nitriding, chloridation and the like. The principles are the same for all these 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.

Fig. 2.  330-alloy radiant tube removed after six months of use (rotary retort furnace)

Examples of Catastrophic Carburization (a.k.a. Metal Dusting)

Metal dusting (Fig. 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. 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, the 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 (0.5 inch) to less than 0.75 mm (0.03 inch) in a little over two months.

    In another example, a metallographic investigation of a failed wrought 330-alloy radiant tube (Fig. 2) was conducted. Optical microscopy of the inside (Fig. 3) and outside diameter surfaces in the attacked area revealed evidence of massive carbides. These carbides 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.

Fig. 3.  Microstructural view – catastrophic carburization

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 activated carbide complex forms. It 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 Fe-Ni-Cr 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 such as oxidation, carburization, sulfidation and nitriding to obtain equilibrium dissociation pressures below which no reactions occur. Diagrams are available of 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 aqueous, electrochemical systems) 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).  

Final Thoughts

Hot gaseous corrosion should be an area of focus in an effort to extend the life of alloy components, reduce downtime and save money. IH