Metal Dusting in Ammonia plants
What is metal dusting?
Metal dusting is an erosion process 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 metal thickness can be literally reduced to that of foil.
For ammonia plants metal dusting is a corrosion phenomenon which was known before and came into focus after the introduction of exchanger reformer technology and the operation of steam superheaters in the hot process gas downstream of the secondary reformer waste heat boiler. The corrosion mechanism can be summarized in the below points:
- Conventional carburization is a familiar problem with high-temperature alloys in steam reforming furnaces caused by inward migration of carbon leading to formation of carbides in the metal matrix.
- It happens at high temperatures, typically above 800 “C, and the carbon originates from cracking of hydrocarbons. In contrast, metal dusting occurs at 500 – 800 “C on iron -nickel and iron – cobalt based alloys with gases containing carbon monoxide.
- The Roudouard reaction, strongly catalyzed by iron, nickel, and cobalt, is generally regarded as the source of the carbon in this case.
- It is assumed that thermodynamically favored sites exist for these elements at the surface and enhance the carbon deposition if the gas composition corresponds to a carbon activity> 1.
- As the name implies, the affected material disintegrates into fine metal and metal oxide particles mixed with carbon. Depending on the defects in a protective oxide film on the metal surface and the ability of the material to sustain this film, an induction period may be observed until metal dusting manifests itself as pitting or general attack.
- A possible mechanism was proposed by GRABKE and HOCHMANN.
- At least from a theoretical point of view, alloys formulated to form chromium, aluminum, or silicon oxide films should exhibit an increased resistance.
- Efforts to findsolutionsfor this problem are continuing, but at present the following situation must be accepted:
- Virtually all high-temperature alloys are vulnerable to metal dusting.
- Higher steam/carbon ratios reduces this sort of corrosion.
- Improvements may be expected by additional surface coating (for example with aluminum). With materials such as Inconel 601, 601H, 625 and similar alloys. It is at least possible to reduce the attack to a level which is tolerable in practical operation.
- The active sites at the metal surface which catalyze the Boudourd reaction can be poisoned by H2S, thus inhibiting the initiation of metal dusting.