Metal dusting is a catastrophic form of carburization which leads to pitting and grooves as the affected metal disintegrates into a mixture of powdery carbon, metallic particles, and possibly oxides and carbides. This high temperature carburization mode is not yet well understood and while relatively infrequent, can be economically disastrous when it does occur in large and complex chemical and petrochemical process streams. References in the literature show that all classes of heat resistant alloys are prone to metal dusting, given the necessary and specific environmental conditions. These same references describe the environments that plague nickel- containing alloys and are used as the basis for postulation on the probable corrosion mechanisms responsible for metal dusting. Using alloy 800 and other nickel-containing alloys and metal dusting atmospheres, an effort is made to examine the steps in the metal dusting process and the temperature ranges over which metal dusting occurs.
The phenomenon labeled ?metal dusting? has been observed and documented since the 1950?s in a variety of industries where metals are exposed to atmospheres having high carbon activity (due to the presence of carbon monoxide or hydrocarbons) combined with low oxygen potential. The phenomenon continues to be a problem. Failures have been reported recently in ammonia plants? as reduced energy requirements result in a lower steam/H, ratio while CO/CO, ratios have tended to increase. Recent failures have also been reported in methanol reforming plants2. Numerous failures in other industries, such as refining and heat treating, have also been reported3.
Metal dusting is a process of highly accelerated material wastage which is preceded by the saturation of a material with carbon. The phenomenon is typified by the disintegration of a material (iron or nickel-base) to a mixture of carbon dust, metal particles and possibly carbides and oxides. This is usually a localized form of attack, resulting in pits or grooves. Metal dusting occurs at intermediate temperatures, 45OT (842°F) to 8OOT (1472?F). in environments which have carbon activity greater than one and low oxygen partial pressure. Important reactions which can occur in the typical metal dusting environment are:
(1) 2co = co, + c
(2) H,+ CO = H,O + C
(3) H,O + CO = CO2 + H,
(4) H,O + CH, = CO + 3H2
Reactions 1 and 2 can produce carbon, reaction 3 deals with the water gas shift and reaction 4 with methane-steam reforming. Figure 1 shows equilibrium constants for the reactions over a range of temperatures, Some insight into the temperature range which favors the metal dusting phenomenon can be gained from this information. Below about 649°C (12OOT) to 704T (13OO?F), reactions 1 and 3, and the reverse of reaction 4 are highly favored. Above this temperature range, reaction 4 and reverse of reaction 1 and 2 become favorable. Chromium oxide is stable in the presence of such reactants in the temperature range at which metal dusting occurs as are silicon oxide and manganese oxide (Figure 2). A continuous chromium or silicon oxide layer provides protection against metal dusting attack. Figure 3 shows the Cr-C-O phase stability diagram at 62 1 C (1150°F).
The mechanism for metal dusting of iron-base alloys proposed by Hochman4*? has been confirmed in recent years 6*78*9~?o. The mechanism is as follows:
(1) o
