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

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

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

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

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

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

Potential for Metal Dusting Corrosion

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)

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

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

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

SMR Outlet System – Reducer Sockets

Manifold, Transfer Line Gas Barriers

Process Gas Boiler „Cold“ Bypass Design

Process Gas Boiler Flow and temperature distribution

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

Inserts in PGB tubes for temp. control

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

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

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