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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
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Metal Dusting Corrosion (MDC)
Disintegration of metals and alloys
into a dust of graphite and metal particles
after carbon ingress and over-saturation.
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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
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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
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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
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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
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aC
carbon activity
Ki
equilibrium constant
Pi
partial pressure
Potential for Metal Dusting Corrosion
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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)
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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
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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
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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
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SMR Outlet System – Reducer Sockets
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Manifold, Transfer Line
Gas Barriers
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Process Gas Boiler
„Cold“ Bypass Design
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Process Gas Boiler
Flow and temperature distribution
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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
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Inserts in PGB tubes for temp. control
WHB - Temperatures Cooling Fluid / Tubewall
WHB - Temperatures Cooling Fluid / Tubewall
1000
400
950
800
Wall inside
380
Temperature (°C)
850
Temperature (°C)
390
Feedgas
Wall medium
Steam Cooling
900
750
700
650
600
550
500
Wand outside
370
Steam Cooling
360
350
340
330
450
320
400
310
350
300
300
0
2000
4000
6000
8000
Distance from Inlet (mm)
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10000
0
2000
4000
6000
8000
Distance from Inlet (mm)
10000
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
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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
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Literature References
R.T. Jones, K.L. Baumert; Metal Dusting – An Overview of
Current Literature; Corrosion 2001; No. 01372
H.J. Grabke, E.M. Müller-Lorenz; Occurrence and
Prevention of Metal Dusting on Stainless Steel; Corrosion
2001; No. 01373
F. Hohmann; Improve Steam Reformer Performance;
Hydrocarbon Processing; 03/1996; p. 71-74
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