1. Metal Dusting Corrosion in Steam Reforming Plants
J. Bohle, Dr. C. Beyer, U. Wolf, Dr. D Ulber
Steam Reforming Technology User Conference
Labuan, Malaysia
March 5-7, 2007

2. Metal Dusting Corrosion (MDC)
Disintegration of metals and alloys into a dust of graphite and metal particles after carbon ingress and over-saturation.

3. Overview Phenomena of Metal Dusting Corrosion (MDC)
Mechanism involved / Reactions
Material considerations
Examples of MDC in Steam Reforming Plants
Prevention of MDC
Influence of MDC on process design
Literature

4. Steps in Metal Dusting Corrosion
Diffusion of reducing / carburizing gas through oxide protection layers to metal surface
Formation & supersaturation of carbides
Dissociation into metal particles and graphite
Diffusion of catalytically active metal particles
Loss of carbon, metal, metal carbide, metal oxide

5. Phenomena involved in MDC
Gas-phase and gas-metal reactions (T, p, composition fi)
Diffusion of reducing gas, carbon and metals (T, fi)
Flow and temperature distribution (vel, T) – equipment design
Catalytically active components in gas and metal
Thermodynamic & mechanical stability of protective layers
Stresses / fractures imposed on surfaces by gradients
Metal crystallographic structure
Sulfidic components in gas

6. Reactions Carburization reactions
CO + H2 <-> C + H2O aC =K1*pCO*(pH2/pH2O)
2 CO <-> C + CO2 aC =K2*(p2CO/pCO2)
CH4 <-> C + 2 H2 aC =K3*(pCH4/p2H2)
Gas-phase reactions
H2O + CO <-> CO2 + H2
H2O + CH4 <-> CO + 3 H2
aC carbon activity
Ki equilibrium constant
Pi partial pressure

7. Potential for Metal Dusting Corrosion

8. Temperature range Promoting components
450°C < MDC temp. range < 800°C / Boudouard temp.
At [Fe/Ni] > ~ 2/3 mass-frac, metal dusting is retarded at the lower regimes of the metal dusting temp. range
MDC promoting gas components (negative impact on metal oxide protection layer)

9. Material Considerations
Surface oxide stability is enhanced by alloying elements such as Cr, Al, Si, Ti, Mo providing a barrier to carbon diffusion
Carbon and alloying elements diffusion is influenced by crystallographic structure and surface condition (e.g. grain size)
Surface coatings or surface finish (grinding) can provide added stability by influencing the carbon diffusion and/or physical resistance to mechanical and thermal effects

10. Material Considerations
Fe-based and Ni-based metals show different behaviour
Empirical equation of alloying material resistance to MDC (Parks & Schillmoller)
Crequiv. = Cr % + 3 x (Si % + Al %)
Inclusion of the effects of other alloying elements such as Ti, Mo, Ni outstanding
Preferred material Nicrofer® 6025 HT – alloy 602 CA
No alloy is MDC resistant under all conditions

11. Where can Metal Dusting Corrosion occur?
Reducer Sockets
Transfer Line at Reformer Outlet
Process Gas Boiler (PGB) Inlet chamber
PGB tube inlet section (ferrules) and tube sheet
PGB bypass tube, bypass flow control device
PGB outlet chamber
Heat Exchanger d/s of PGB, e.g. Feed Preheater

12. SMR Outlet System – Reducer Sockets

13. Manifold, Transfer Line Gas Barriers

14. Process Gas Boiler „Cold“ Bypass Design

15. Process Gas Boiler Flow and temperature distribution

16. Process Gas Boiler Flow and temperature distribution
Temperature distribution in outlet chamber mixing zone of a process gas boiler
Gastemperatures:
below 450°C
450 – 1000°C
Bypass open
Bypass closed

17. Inserts in PGB tubes for temp. control

18. Measures against MDC Pro & Cons
Avoid metal wall temperatures in MDC temperature range
Use of non-metallic materials in critical areas
Change gas atmosphere
Introduction of process gas (for purge) in critical areas
Catalytically activated refractory (Lurgi Patent)
Material science, Protection layers
Sulfidic compounds in gas
Design for easy maintenance / replacement

19. Influence of MDC on process design
Process efficiency / consumption figures
Steam to Carbon Ratio
Steam Superheater for Process Gas Cooling
Export Steam Value
Gas-heated Steam Reformer design

20. Literature References
R.T. Jones, K.L. Baumert; Metal Dusting – An Overview of Current Literature; Corrosion 2001; No
H.J. Grabke, E.M. Müller-Lorenz; Occurrence and Prevention of Metal Dusting on Stainless Steel; Corrosion 2001; No
F. Hohmann; Improve Steam Reformer Performance; Hydrocarbon Processing; 03/1996; p