Metal Dusting | Materials And Corrosion Control

Damage Description

·         In low alloy steels, the wastage can be uniform but usually is in the form of small pits filled with a crumbly residue of metal oxides and carbides.

·         The corrosion product is a voluminous carbon dust containing metal particles and sometimes metal oxides and carbides. Frequently, this dust will be swept away by the flowing process stream, leaving behind only the thinned or pitted metal.

·         In stainless and high alloy steels, the attack is frequently local, appearing as deep, round pits.

·         Metallography will show that the metal is heavily carburized under the attacked surface.

Critical factors include:

·         Process stream composition, operating temperature and alloy composition are critical factors.

·         Metal dusting is preceded by carburization and is characterized by rapid metal wastage.

·         Metal dusting involves a complex series of reactions involving a reducing gas such as hydrogen, methane, propane or CO.

·         It usually occurs in the operating temperature range of 900°F to 1500°F (482°C to 816°C). Damage increases with increasing temperature.

·         The mechanism of metal dusting is considered to be:

o   Saturation of the metal matrix by carburization;

o   Precipitation of metal carbides at the metal surface and grain boundaries;

o   Deposition of graphite from the atmosphere onto the metal carbides at the surface;

o   Decomposition of the metal carbides under the graphite and metal particles; and

o   Further deposition of graphite catalyzed by the metal particles on the surface.

·         In high nickel alloys, it is thought that metal dusting occurs without the formation of metal carbides.

·         Metal dusting can also occur under alternating reducing and oxidizing conditions.

Materials & Equipment

Low alloy steels, 300 series stainless steels, nickel base alloys and heat resisting alloys. There is currently no known metal alloy that is immune to metal dusting under all conditions.

Equipment:

·         Primarily fired heater tubes, thermowells and furnace components operating in carburizing environments.

·         Catalytic reforming unit heater tubes, coker heaters, gas turbines, methanol reforming unit outlet piping and thermal hydrodealkylation furnaces and reactors.

Control Methodology

·         There is currently no known metal alloy that is immune to metal dusting under all conditions. Materials selection must be made based on the specific application and environment.

·         An aluminum diffusion treatment to the base metal substrate can be beneficial in some applications.

Monitoring Techniques

·         RT techniques can be employed to look for pitting and wall thinning.

·         If internal surfaces are accessible, visual inspection is effective.

·         Filtering the cooled furnace or reactor effluent may yield metal particles that are a tell tale indication of a metal dusting problem upstream.

Inspection Frequency

KPIs

Reference