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FILE NOTE ON
METAL DUSTING
TECP - 015 27 January 2006 Nitin Joseph
DOC. No. Date Prepared By Approved By
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FILE NOTE ON METAL DUSTING
Introduction:
Metal Dusting is a high temperature corrosion phenomena well known with the process industry , specially in
connection with CO rich gases & high temperatures. This phenomena is a catastrophic form of carburization
which can result in rapid metal wastage, producing pits and grooves as the affected metal disintegrates into a
mixture of powdery carbon and metal particles. It has been observed in chemical & petrochemical industries, in
reformers & direct-reduction plants, in processes that generate syngas and in other processes where
Hydrocarbons or other strongly carburizing atmospheres are present.
Metal dusting is a process of highly accelerated metal wastage that is preceded by the saturation of a metal with
carbon. The phenomena is typified by the disintegration of material(iron or nickel base) to a mixture of carbon
dust, metal particles & possibly oxides & carbides. Metals when exposed to gases at temperatures (450-850 oC)
with a high carbon potential are frequently observed to disintegrate into fine metal & metal oxide particles mixed
with carbon. The powder may be transported away by the gas leaving pits, grooves & holes in the metal surface.
The appearance of the affected surface may vary from an almost perfect surface with tiny pits to a very rough
cauliflower like surface. This depends on the alloy, the processing of the material, the temperature &
composition of the gas. Many times more than one mechanism is involved. This is usually a localized form of
attack resulting in pits and grooves. It occurs at temperatures of 400-800oC , but this type of corrosion is possible
at any temperature at which the carbon activity (ac) in the gas phase is greater than 1. Such high carbon activities
are prevelant in several chemical processes, such as methanol production, hydrocarbon and ammonia synthesis,
hydrogen production & syngas generation. In all these systems having CO-H2-H2O mixtures, carbon activity is
the thermodynamic driving force for metal dusting. This carbon activity increases with decreasing temperature.
The report describes the mechanisms causing metal dusting & the stages involved in the same. The
factors responsible for metal dusting, & the suitability of various materials in the metal dusting environment are
also discussed.
Mechanism of Metal Dusting:
The phenomena was first documented authoritatively in the 1950’s and although extensive studies have been
carried out in later decades there are still a number of anomalous observations that are not adequately explained
in published literature. The theory of metal dusting explained by Hochman & Grabke is as follows (shown in
figure 1)
Fig 1: Schematic Representation of
Metal Dusting.
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1. Rapid saturation & supersaturation of carbon in the metal matrix.
In typical reform gas environments some of the typical reactions that occur are:
2CO = CO2 + C (Boudouard Reaction) ……..(i)
H2 + CO = H2O + C ……..(ii)
CH4 = 2H2 + C ……..(iii)
H2O + CO = CO2 + H2 ……..(iv)
H2O + CH4 = CO + 3H2 ……..(v)
Reactions 1,2 & 3 produce carbon. Reaction 4 is the water gas shift reaction & reaction 5 describes the
steam methane reforming process. As methane is not expected to crack in the temperature band that causes
metal dusting, carbon is mostly generated by reaction 1&2. Reaction 1&2 are known to have equilibrium
constants greater than 1 at around 750oC. At higher temperatures, a reaction of H2 with deposited carbon
would generate sufficient amounts of methane, but this reaction is sluggish within the metal dusting
temperature range so that only low levels of CH4 are produced.
In a typical environment any metal is likely to be carburised or decarburised depending upon the carbon
activity in the environment and the carbon activity in the metal.
Carbon activity is defined as ac = (wt%C) * T
Where T is the activity coefficient, that is so chosen so that ac=1 for an amount of carbon in a solution
that is in equilibrium with graphite.
The metal is likely to be carburised when carbon activity in the environment is greater than the carbon
activity in the metal & the metal is likely to be decarburized if the carbon activity in the environment is less
than the carbon activity in the metal. The thermodynamics of both the reactions (i & ii) are similar & hence
more research has been carried out on the Boudouard reaction. The boudouard reaction is catalysed strongly
by iron, nickel & cobalt, & thus surfaces containing these elements have potentially favoured sites for carbon
deposition if the carbon activity is > 1. These elements become an important factor for further reactions once
metal dusting has begun. High carbon activity is the driving step. Lower H2O/ H2 Ratios, in combination
with higher CO/ CO2 ratios result in lower oxygen partial pressures & higher carbon activities.
The carbon activity is calculated using the following equation:
LogKp = Log(aH2O * ac / aCO * aH2) = - dG / (2.303 * R * T)
Where Kp is the equilibrium constant & dG is known as Gibbs free energy change.
The value of G can be obtained from the graph below.
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Fig: 2 The variation of free energy of formation v/s Fig:3 Equilibrium constants for reactions which may
temperature for nickel, iron & manganese produce carbon in metal dusting environments.
carbides of M3C type.
The figure 3 shows the reaction constants for various reactions over the range of temperatures at which
metal dusting is likely to occur. The diagram provides partial explanation why the metal dusting phenomena
is restricted to a specific range. From around 600oC to about 700oC reactions i, ii & & reverse of iii & v are
favoured. However above this temperature reactions iii & v & reverse of i & ii become favoured.
For carbon deposition to occur at a significant rate, assuming it is favoured thermodynamically, active
sites of iron, nickel or cobalt must be available to the reacting gas. Surface oxide films are effective barriers
to deposition, and metal dusting will not occur on surfaces which support stable oxide films which are able to
repair themselves once damaged. Uniform carbon deposition & thus metal dusting will occur on surfaces
which cannot restrain a protective oxide film demonstrated in the Figure below.
Fig:4 Illustration of the equidistant diffusion of carbon from a localized defect in the protective
oxide scale that results in the saturation of a hemispherical region with carbon
Localized deposition/ dusting will occur on surfaces which carry a pre existing oxide film which is unable to
repair itself if locally damaged. In such cases an induction period is likely to precede the onset of localized
metal dusting till the active metal surface is exposed. In contrast, if there are no stable films, dusting will
initiate immediately upon exposure to the CO rich gas.
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2. Formation of Cementite at the surface and at the grain boundaries.
The mechanism of metal dusting for iron alloys begins with the saturation of the alloy matrix with carbon/
carbides usually in a localized manner. Once the carbon is deposited it penetrates the alloy, preferentially at
grain boundaries, but ultimately more generally into the metal surface, resulting in considerable super-
saturation & hardening. In the case of iron based alloys the formation of metastable cementite is thought to
play a significant role in the carbon entry process, which then decomposes back to atoms and active metal
particles which in turn promote further carbon deposition. The cementite is formed as per the reaction
3Fe + C Fe3C
Fig:5 Illustration of diffusion of carbon & further formation of Cementite.
The cementite layer continues to grow and eventually acts as a barrier to further uptake of carbon. It is
relevant to note that the diffusivity of carbon in Fe3C has been determined to be about 5 orders of magnitude
lower than in a ferrite matrix. Continued carbon transfer leads to graphite nucleation and growth in
cementite surface. This reduces the local carbon activity to unity.(ac = 1) but Fe3C is metastable(stable at
ac>1 only).
Precipitation of Carbon in Chromium & Nickel based alloys:
A different mechanism exists for nickel based alloys, which does not involve formation of metastable
carbide. Such a mechanism begins in the same way as for nickel base alloys with saturation of the alloy
matrix with carbon. The Ni3C carbide is not predicted to be stable, having a slightly positive energy of
formation through out the metal dusting temperature range.
Figure 6 shows one interpretation of the crystallizing process for carbon during dusting of nickel (essentially
same process for iron). In the initial stage, single carbon atoms are deposited on the surface of the nickel;
they then either dissolve in the nickel or accumulate to form small carbon particles. Carbon particles
accumulate on the surface & form poorly crystalline carbon particles if the Ni lattice does not help the
crystallization process of carbon. However highly crystalline carbon is formed (as shown in the right branch)
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if carbon diffuses through nickel & if the nickel lattice helps the crystallization of carbon. This can be
explained as follows:
There are dangling bonds on the surface and many defects, such as vacancies and distorted bonds in the small
particles. The surface and internal defects cause an increase in the free energy of these particles compared to
that of well-
Fig:6 Described methodology for carbon crystallization from Syngas.
crystallized graphite. At higher temperature, where the carbon atoms have enough energy to migrate, the
carbon crystallizes from small distorted particles to large well-crystallized graphite. However, because
the C-C bond is very strong (the melting temperature of carbon is 4492°C), the crystallization process
requires a higher temperature. The rate of carbon crystallization is dramatically increased if nickel acts as a
catalyst. When carbon dissolves in nickel, the Ni-C bond is much weaker than the C-C bond, and transport of
carbon atoms is greatly facilitated. Therefore, the poorly crystallized coke transfers through nickel and
eventually achieve improved crystallinity
Fig:7 Illustration of Carbon
Crystallistion Process leading to
Metal Dusting in Nickel Alloys.
One of the proposed catalytic crystallization process states that, carbon dissolves on the surface of the
nickel and crystallizes at the defects of the bulk nickel. Because the free energy of poorly crystallized carbon is
probably higher than that of well-crystallized graphite, the saturating concentrations for poorly crystallized
carbon and graphite will differ.
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Poorly crystallized carbon exhibits a slightly higher saturating concentration than graphite; hence, the
saturating concentration of poorly crystallized carbon will be oversaturating for graphite; therefore, poorly
crystallized carbon could dissolve in nickel and form well-crystallized graphite. The free energy difference
between poorly crystallized carbon and the well-crystallized graphite drives carbon to transfer through the nickel
particles and precipitate inward via the defects or grain boundary of nickel. The accumulation of carbon in an
alloy causes the metal to separate. The metal particles finally become nano-size powder in this process until they
are too small to help carbon grow to large crystals. The nano-size nickel particles in coke continue to interact
with carburizing gas. These particles also work as catalyst to help the deposition of carbon. However, the
average size of a nickel particle in coke is only 48 nm. These nickel particles are too small to help carbon grow
to large crystals. The surface areas of nano-particles are large. Therefore, carbon has more chance to accumulate
on the surface of nano-size nickel particles according to the left branch in Figure 6, and formed poorly
crystallized particles. Since coke contains these poorly crystalline carbon particles, its crystallinity is worse than
that of the carbon in the adhering carbon layer of the nickel specimen.
3. Decomposition of Cementite
Deposited carbon atoms penetrate the alloy, preferentially at grain boundaries, but ultimately more generally
into the metal surface, forming cementite & resulting in considerable supersaturation & hardening. The
cementite is stable only at high carbon activities (ac > 1) & can decompose at the outer surface, like coke
being deposited locally from the gas atmosphere. But at spots where cementite has already diffused, the
carbon activity decreases to unity as the (ac = 1) & the cementite becomes unstable. In case of iron shaped
alloys, the formation of metastable cementite is thought to play a significant role in the carbon entry process
which then decomposes back to atoms and active metal particles, which in turn promote further carbon
decomposition. The following mechanism is suggested for decomposition of iron:
Fe3C 3Fe + C
The decomposition reaction is independent of partial pressures & carbon activity, its activation energy is
about 170 KJ/mole. The free metal particles act as catalysts for further coke deposition. The volume changes
associated with these transformations generate high internal stresses & subsequently result in disintegration
of the surface into a loose mixture of powdery & filamental carbon with metal particles. The process of
cementite formation & decomposition continues until the supersaturated region is consumed. When the
reaction products are removed the gas erosion starts all over again. The repeated attack leads to significant
material consumption & formation of pits and grooves.
Fig:8 Illustration of Decomposition of Cementite in Fe Alloys.
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The methodology involved in the decomposition is different in case of Fe alloys & Ni based alloys. The
figure 8 shows ,metal dusting of Fe based alloys without decomposition of Fe3C. Both carbon filament growth
and metal dusting involve catalytic deposition of carbon and catalytic growth of graphite on the metal surface.
The deposition and precipitation process may favor different faces. Therefore, carbon deposits on one face of
metal or carbide and precipitates on another face. The carbon transport through carbide or metal particles causes
the movement of metal particles and the growth of carbon filaments. Both processes have the same driving force,
which is the free energy difference between poor and good crystalline carbon. For large-size carbon fiber, the
decrease in interlayer plane distance decreases the free energy of the carbon fiber. The metal dusting process
ultimately results in separation of very small particles when carbon is continuously inserted into defects in
metals.
Nickel carbide does not form in the metal dusting process since it is unstable. Therefore, the mechanism for
nickel has been considered to be different from that for iron. However, if the carbon catalytic crystallization
process through iron carbide and nickel is similar, their metal dusting mechanism should be similar, although the
process details may be slightly different. Metal dusting of both iron and nickel is caused by the catalytic
crystallization of carbon and has a similar driving force. Carbon deposits easily on some faces and may
precipitate from the other face. The deposited carbon into and out of different faces causes the nickel particles to
move and separate into dust. When carbon continues to transfer from one face of Ni to another face, the
accumulation of carbon leads to the formation of carbon fibers or tubes. This process is similar to the formation
of carbon fibers or tubes through Fe3C. Figure 9 shows the proposed process for metal dusting in nickel.
Fig:9 Illustration of Decomposition of Cementite in Ni Alloys.
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Factors Affecting Metal Dusting:
• Metal Dusting is very sensitive to gas composition and local temperature. Minor changes in process
conditions can cause the onset or arrest of metal dusting or move the attack to another location in the
system.
• Metal dusting phenomena is also related to the steam to carbon ratio in the process gas. Increasing the
steam content is an excellent way to reduce metal dusting, but this solution is very expensive.
• From material standpoint all high temperature alloys containing nickel & cobalt are attacked quite easily.
Low nickel or nickel free alloys & high chrome steels, particularly those containing Si & Al are more
resistant. However these steels embrittle markedly at high temperature& hence cannot be used for parts
requiring ductility & toughness, but can be used as cladding to protect the alloys. Ferrules must be made
of material resistant to metal dusting like Inconel, Ceramic etc.
• Formation of carbon is an important step for metal dusting. It is influenced by ac in the gas mixture & the
availability of catalytic surface. Both these factors play an important role in the metal dusting
phenomena.
• Metal dusting is also determined by the oxide scale development and the access of the virgin metal
surface to the carbon deposit. The presence of an oxide scale may not prevent metal dusting but can delay
its initiation, there by slowing its overall attack. The defects in the oxide film also play a large role in
initiation. Oxide scaling may not occur if the ac >>1 and /or if the H2O content in the environment is very
low. Laboratory experiments have indicated a pronounced effect of gas chemistry (in particular the H2O
content ) in the scaling, carbon deposition, and dusting initiation of the alloy. The results show that
environment in reformers is high enough in pO2 that a Cr-rich alloy can develop a chromia scale (given
enough exposure time) before the carbon deposition.
• Oxide coatings also minimize the carbon producing reactions and can act as a barrier to minimize the
carbon ingress and pitting of the substrate alloy. Short term experiments showed virtually no carbon in
pre-oxidised layers of Al, Cr, & Si enriched layers that were subject to metal dusting environments.
• Avoiding of the bypass and instead having the gas flow through pipes of different cooling capacity, will
help curb metal dusting.
• Partial pressures of CO/CO2 & H2O/H2 ratios play a significant role in the severity of the metal dusting
attack. Figure 10 shows a graph by Parks & Schillmoller for critical zones of ammonia plant waste heat
boilers.
• Metal dusting & Boudourd reaction are a function of the ratio of partial pressure of CO2 to CO &
temperature of the gas. The relationship is clearly shown in figure 11.
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Fig:10 Graph published by Parks & Schillmoller relating to the observed severity of
metal dusting attack of alloys 800 & 304 to the CO/CO2 & H2O/H2 ratios
within critical zones of Ammonia plant waste heat boilers.
Fig:11 Graph showing Carbon Deposition as a function
of pCO2/p2CO & temperature of gas
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Materials Suitable for Metal Dusting Environments.
A lot of research has been carried out to study the suitability of materials in metal dusting environments. Some
of the common materials used are given below with their composition (figure 12). The figures 12 show the
composition of some of the metals tested.
Fig:12 Composition of some of the alloys tested for Metal Dusting.
The graph (fig. 13)& table (fig. 14) shows the results of the tests carried out by Special Metals Ltd. UK. Which
compares the pitting progression rate for a reducing atmosphere (20%CO 80% H2 at a temperature of 621oC.
This environment is known to induce metal dusting at elevated temperatures.
Fig:13 Mass loss rate v/s exposure time for samples exposed to Co-20%, 80% H2 at 621oC
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Fig:14 Pit Progression Rate for samples exposed to Co-20%, 80% H2 at 621oC
A similar test was carried out by Chiyoda Corporation, Japan to evaluate metallic materials on susceptibility to
metal dusting. They checked eight groups of materials:
1. C- steel & Cr-Mo steel
2. Ferritic Stainless steel
3. Austenitic Stainless Steel
4. Fe based high alloy
5. Ni based alloy
6. Heat resisting spun cast tube
7. Pure metal
Composition of each of the steels of a particular group along with their susceptibility are specified in figure 15.
The susceptibility of metal dusting was investigated as follows: (Gas Composition was of 66% H2, 17% CO, 1%
CO2, 17% H2O & Steam to Carbon Ratio of 2.Temp of 695oC)
(A) No metal dusting
(B) Only slight Carburisation
(C) Metal Dusting & / or severe carburization
(D) Damaged by oxidation even if no metal dusting.
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Nominal Composition (%weight)
No. Group Material Evaluation at 695°C
Cr Ni Fe Others
1 C steel C steel 99 D
2 1.25Cr-0.5Mo 1.25 97 Mo D
3 2.25Cr-1Mo 2.25 95 Mo D
4 Cr-Mo steel 9Cr-1Mo 9 89 Mo D
5 Ferritic Type 405 13 86 Al A
6 Stainless 430 17 82 A
7 Steel 403 12 87 A
8 447 30 68 Mo A
9 Austenitic Type 304 18 8 71 C
10 Stainless 316 18 10 68 Mo C
11 Steel 321 17 10 70 Ti C
12 347 18 10 69 Nb C
13 309 24 14 60 C
14 310 25 21 53 C
15 Alloy 800H 20 32 46 Al, Ti C
16 Fe based HP-AA 28 33 35 Ti, Nb, Mo A
17 High Alloy HK4MS 24 36 37 Ti, Mo A
18 HK4M 25 25 47 Al, Ti, Mo C
19 HPM 25 39 30 Ti, Mo A
20 Inconel 600 15 75 9 B
21 Inconel 601 21 60 17 Al B
22 Ni based Inconel 625 21 62 4 Al, Ti, Nb, Mo A
23 Alloy Inconel 690 29 60 9 A
24 Inconel 617 22 54 3 Ti, Co, Mo A
25 Hastelloy G 22 51 19 Nb, Mo A
26 Hastelloy X 21 49 17 Mo A
27 HK40 25 20 53 C
28 Spun Cast IN519 24 24 49 Nb C
29 Tube HP-Nb 25 35 37 Nb B
30 HP-Nb-Ti 20 32 45 Nb B
31 27 34 36 Nb C
32 Pure Ni C
33 Metal Cr A
Fig:15 Material Composition & Susceptibility of various materials at 695oC with a gas composition of 66% H2,
17% CO, 1% CO2, 17% H2O & Steam to Carbon Ratio of 2.
After the above investigation the following factors were observed.
1. Effect of temperature on Metal Dusting
The figure 16 shows the varied nature of a gas with same composition but at different temperatures. Some
typical metals have been studied & values given in the same figure. It can be seen that for the same gas, the
temperature at which carburization & metal dusting begins to occur is different for each material.
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Temperature °C
Material
445 495 595 695 795
2.25Cr - 1Mo A B C D D
9Cr - 1 Mo A B C D D
SS 304 A A C C C
Alloy 800H A B C C C Fig:16 Result of Exposure Tests for typical
Inconel 601 A A B B B materials at each temperature in the
Atmosphere with Steam to carbon ratio of 2
Inconel 625 A A A A A
Hp-Nb, Ti A A C C C
2. Effect of Alloying Elements:
The effect of different alloying elements like Cr, Fe & Ni was found to be different for ferritic, Austenitic &
Nickel based alloys. The weight gain of the alloy by means of carburization is discussed below:
i. Effect of Cr content: The weight gain rapidly decreases as the chromium content increases.
ii. Effect of Fe content: The effect of Fe content is exactly in contrast with the effect of Cr content for
ferritic & austentic alloys.
iii. Effect of Ni content: For austenitic alloys & Ni base alloys, increasing of Ni contents decreases in
weight gain. In the range of more than 40% weight of Nickel there is little weight gain.
The final outcome of these tests were:
i. Cr & Cr-Mo steels were damaged with severe oxidation even if there was no metal dusting at more
than 695°C
ii. Ferritic stainless steels had excellent property namely lower susceptibility to metal dusting.
iii. Austenitic steels had higher susceptibility to metal dusting.
iv. Fe based high alloys which consist of less than 26% Cr content was very susceptible to carburization
& metal dusting. However Fe alloys with more than 27% Cr had good resistance to metal dusting.
v. Nickel based alloys had lower susceptibility to metal dusting. Especially Inconel 625& 690 had the
lowest susceptibility of metal dusting among the materials used in the exposure.
vi. Higher Ni content (more than 30%) & higher Cr content (more than 25%) of heat resisting spun cast
metals were good properties for carburization & metal dusting.
vii. Pure Ni had higher susceptibility to metal dusting even though Ni based alloys had good resistance to
metal dusting. Pure Cr had a lower susceptibility for metal dusting.
Summary:
The deposition of carbon from carbonaceous gaseous environments is prevalent in many chemical &
petrochemical processes, such as reforming plants, syngas production systems, iron reduction plants & others.
The report summarizes the process & the factors that influence metal dusting. In general the studies carried
out indicate that the major significant factors are:
i. Gas Composition
ii. Operating conditions.
Some of the measures taken by the industry are to choose materials that have less tendency to metal dusting.
In case of reform gas boilers, this phenomena is predominant in double compartment design. Studies are planned
to evolve suitable measures to reduce the damage possibility due to metal dusting phenomena.
