Metal Dusting Corrosion of Ni-Based High-Cr Alloys in Colossal Carbon Supersaturation
Authors: Chun Charlie 
Published: December 2024

Metal dusting is a high-temperature carbon-induced corrosion phenomenon that metals and alloys undergo at temperatures in the range of 450~850°C in gaseous environments that are supersaturated with respect to carbon. Since most currently available high-temperature alloys are prone to metal dusting, understanding and thereby controlling metal dusting corrosion of structural materials is critical to many refining and petrochemical process operations and technologies, such as the production of syngas and the conversion of light hydrocarbons to high value products. The corrosion process involves the breakup of the bulk metallic structure into powder or “dust” consisting of metal particles, oxides, and carbides. Metal dusting is always accompanied by coking whereas coking can occur in process streams without metal dusting.
A great deal of effort has been expended by major alloy manufacturers to develop new structural alloys that are resistant to metal dusting in the syngas processing environments. These efforts have been focused primarily on Ni-based alloys having a significant amount of Cr (~30 wt.%) and additionally small amounts of Al, Si, Cu, Mo, etc. The composition of such commercial alloys is given in Table I. All these alloys rely on the formation of Cr2O3 film on the surface to provide metal dusting resistance.

Table 1. Commercially available metal dusting resistant alloys

Manufacturer

Name

Nominal composition (wt.%)

Special Metals

Inconel 693

Balanced Ni:4.5Fe:29.0Cr:3.2Al:1.5Nb, others (Si, Mn, Ti, Cu)

NSSMC

Alloy 696

Balanced Ni:3.0Fe:30.0Cr:2.0Cu:2.0Mo:1.5Si, others (Mn, Ti)

Haynes Int’l

HR235

Balanced Ni:1.0Fe:31.0Cr:5.6Mo:3.8Cu, others (Si, Mn, Al)

VDM Metals

Alloy 699XA

Balanced Ni:1.0Fe:28.0Cr:2.5Al, others (Si, Mn, Ti, Nb, Cu)

The metal dusting environment is characterized by carbon activities higher than unity (ac>1) and very low oxygen partial pressures. Generally, the carbon activity decreases with increasing temperature while the oxygen partial pressure increases with temperature. There are differences in syngas mixtures used by different investigators. However, the reaction that transfers carbon to the metal surface to trigger metal dusting can be written as
CO + H2 = H2O + C
By considering the pseudo-equilibrium of this reaction, where carbon is not allowed to precipitate out, one can calculate the activity of carbon from the equilibrium constant, K1
of this reaction assuming the formation of about 1 vol.% H2O. Thus,
ac = PCO
×PH2
/(K1
×PH2O
)
Since syngas process streams often contain significant amounts of oxygen-containing molecules such as H2O and CO2, the oxygen partial pressure of the environment is in favor of forming a protective Cr2O3 film.
In contrast some petrochemical processes that deal with light hydrocarbons, often being considered as those with 1 to 6 carbon atoms, are nearly free of any oxygen-containing molecules. Under these process conditions, the activity of carbon can be calculated in a following manner.
CxHy = xC + (y/2)H2

ac = [(K3
×PCxHy
)/PH2

(y/2)](1/x)

The calculated carbon activities in the light hydrocarbon environments are in the order of thousands, which is about two orders of magnitude higher than those in the syngas environments.
Therefore, it becomes important to understand how commercially available metal dusting resistant alloys developed mainly to control corrosion in the syngas environments behave in the light hydrocarbon environments. To address this question cyclic metal dusting tests were performed.
A 49CO:49H2:2H2O gas mixture was used at 650°C and ambient pressure. Equilibrium carbon activity of the gas mixture calculated by the above equation is 39, and oxygen partial pressure is 2.2x10-26. Alloy samples were suspended within the reactor and the reaction gas mixture was flowed through the reactor. An electrically heated furnace was then driven upwards until the alloys were in its hot zone at 650°C. After 45 minutes of exposure, the furnace was driven downwards, the gas flow maintained, and the samples cooled to less than 100°C. The furnace remained in this position for 15 minutes. The entire cycle (45 min at 650°C and 15 min cooling) was repeated for 1,440 times.
Similarly, a gas mixture of 20nC5H12:20H2:60CH4 was used to understand metal dusting of the same alloys at 600°C and ambient pressure. Equilibrium carbon activity of the gas mixture calculated by the above equation is 2,525, and oxygen partial pressure is 1.1x10-38 assuming presence of 100 ppm steam.
In the present paper, based on the microstructure of metal-dusted Ni-based high-Cr alloy samples in both syngas and light hydrocarbon environments, a mechanistic picture of alloy corrosion is presented which also attempts to explain the role of the alloying elements.