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Jan-2019

Resisting metal dusting corrosion

Metal dusting corrosion can limit operating conditions, but recent developments in alloys are helping to combat the phenomenon

RAMESH VENKAT
Tubacex
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Article Summary
Metal dusting occurs in environments containing carbon monoxide and hydrogen in the temperature range 350-800°C. Several industrial applications are subject to metal dusting corrosion such as plants producing hydrogen by steam reforming processes in industries including oil refining and ammonia and methanol production. Other examples could be coal gasification plants, synthetic gas production and direct iron reduction plants.

The extent of corrosion in terms of general loss of material and/or pitting corrosion is severe, and pre-mature failure of tubes and pipes can lead to loss of revenue and jeopardise the safety of equipment and plant. Lots of research activity has taken place and several studies are still in progress to mitigate metal dusting corrosion. Some of the ways and means developed by industry to increase the life of tubes and pipes from metal dusting include use of gas phase inhibitors and diffusion coatings. This article is not going to discuss the pros and cons of these alternatives and we believe each solution could be effective in given circumstances. The article will discuss nickel alloys and their performance and effectiveness in combating metal dusting corrosion.

Mechanism
Metal dusting involves the disintegration of bulk metals and alloys to metal particles, oxides and graphite at high temperatures in environments that are supersaturated with carbon. It is generally believed that the phenomenon is most widespread in the temperature range 400-800 °C. Such corrosion has been observed in processes in chemical and petrochemical industries where hydrocarbons or other strongly carburising atmospheres are encountered. It is a catastrophic form of corrosion that occurs when materials are exposed to environments with high carbon activity. It breaks up bulk metal to metal powder (see Figure 1).

The suspected mechanism is firstly the deposition of carbon on the surface of the metal. The carbon is, for instance, derived from carbon monoxide in a gas with a carbon activity much higher than 1. This carbon forms iron carbides in the case of an iron alloy or diffuses into the metal in the case of a nickel alloy and, in both cases, after super saturation the matrix decomposes to carbon and fine metal particles (and carbides). In a general understanding of the chemistry, at lower temperatures the rate of reaction is too low to be significant, and at much higher temperatures carbon activity and deposition decrease. A critical item of equipment that encounters metal dusting corrosion is a boiler where the operating temperature zone is in the range 400-800°C.

Nickel alloys
The use of high nickel, high chromium alloys is preferred as they have much better corrosion resistance to metal dusting than normal austenitic steels. While alloying has enabled an increase in resistance to metal dusting corrosion, it also leads to other non-desirable properties such as difficult workability and weldability.

Importance of alloys in metal dusting corrosion resistance
VDM Alloy 699XA, an alloy developed by VDM and Tubacex in partnership, can produce seamless tubes and pipes. Three key requirements for steels in this application besides resistance to metal dusting corrosion are creep strength, weldability and workability.

Corrosion resistance
A high percentage of chromium is important in any stainless steel and in nickel alloys for metal dusting corrosion resistance as it provides a stable passive layer of chromium oxide. As can be seen in Table 1, the latest alloys developed to fight metal dusting, such as VDM Alloy 699XA, have a maximum chromium content of 30%, which makes it one of the better alloys to be used in this application.

However, any localised rupture of the chromium oxide layer can lead to significant intake of carbon from the process atmosphere into the alloy. Here, the addition of aluminium leads to the formation of a protective aluminium oxide scale or sub-scale. From Table 1 we can see that VDM Alloy 699XA has an aluminium content as high as 2%. A much higher aluminium content, such as 3%, would reduce workability.

Increasing chromium to about 30% together with a low iron content is necessary for high metal dusting corrosion resistance in nickel alloys.

This combination of alloying elements makes VDM Alloy 699XA one of the best alloys for resistance to metal dusting corrosion in combination with its good workabilty. Laboratory results of metal loss through metal dusting in a highly carbonising gas (37% CO, 9% H2O, 7% CO2, 46% H2, ac=163, p(02)= 2.5*10-27 bar at 600°C, 20 bar) shows that normalised maximum mass loss for VDM Alloy 699XA was close to 0.001 after 4000 hours of exposure. Also, no pitting showed in a total exposure time of 5692 hours; the time to first pit was significantly higher than 602CA, 601 and 690.

Weldability, workability and creep
VDM Alloy 699XA has good weldability under argon. Welding recommendations and requirements need to be followed. Workability at room temperature is comparable to 601. Also creep resistance is similar to or better than 601.

Conclusion
Refiners would like to run their plant under peak operating conditions to take advantage of cost and efficiency but corrosion of steel alloys under such harsh conditions has been a deterrent factor. While a lot of effort is being put into the research and development of alloys for metal dusting, recently developed alloys have so far shown promising results.
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