INTRODUCTION

ABSTRACT

The corrosion behavior of ferritic, stainless steel, iron-nickel-chromium and nickel-base alloys was investigated in H2-80% C0at 621°C. Mass change and rate of mass loss, pit depth progression rate and pit distribution were monitored and recorded. It was found that wastage rates and pit depth progression rates were generally much lower for nickel-base alloys than iron-base alloys. Pit depth did not necessarily correlate with area averaged mass change rate, Chromium, silicon and aluminum additions were found to be beneficial in producing an oxide scale which reduced or prevented wastage due to metal dusting.

Metal dusting is a catastrophic form of carburization which can lead to rapid wastage as the affected metal disintegrates into a mixture of powdery carbon and metal particles. The phenomenon occurs in gaseous atmospheres which are strongly carburizing in the temperature range of approximately 400Cto 800°C with a carbon activity greater than. Some important reactions which occur in typical metal dusting environments are:

2C0 = C02 + c (1)

H*+ CO= H20+C (2)

CH4=2HZ+C (3)

H20 + CO = C02 + H2 (4)

H2O+ CI&= CO + 3Hz (5)

Reactions 1,2 and 3 produce carbon, reaction 4 is the water gas shift reaction and reaction 5 describes the steam methane reforming process. Figure 1shows the equilibrium constants for reactions 1through 5 over the range of temperatures at which metal dusting is likely to occur, as well as reactions similar to reactions 3 and 5 but substituting ethylene for methane. This diagram provides a partial explanation of why the metal dusting phenomena are restricted to a specific temperature range. Below about 600°Cto 700°C, reactions 1, 2 and 4 and the reverse of reactions 3 (for methane but not ethylene) and 5 are favored. Above this temperature range, however, reactions 3 (for both methane and ethylene) and 5 and the reverse of reactions 1 and 2 become quite favorable. The kinetics for the production of carbon by reaction 3 are rather slow for methane in the metal dusting temperature range, but can be appreciable for other hydrocarbons. 1 The production of carbon monoxide by the reforming reaction, number 5, is also favored by the reaction of ethylene with steam than the reaction of methane with steam. Thus, metal dusting type attack has been known to occur in processes containing hydrocarbons, other than methane, and hydrogen, Within the typical metal dusting temperature range (400°C to 800°C), reactions 1,2 and 3 can potential y control the carbon activity.

The mechanism for metal dusting of iron and low alloys steels was proposed by Hochman as being related to cementite decomposition which has been confirmed by others more recently to proceed as follows: (a) oversaturation of the metal with carbon results in formation of cementite, FeSC,(b) graphite precipitates, reducing the carbon activity to one, and causes the FesC to become unstable and decompose, and (c) the formation of carbon and small metal particles, catalyzing further carbon deposition. The mechanism for nickel, nickel-base alloys and heat-resistant alloys having sufficiently high nickel/iron ratios has been shown to involve the direct formation of graphite and metal particles, without the formation of an intermediate NisC which is not thought to be thermodynamically favorable. First, a break in the scale provides a site for carbon ingress. This break maybe the result of a precipitate at the surface which disrupts the scale or a physical defect such as a scratch. Internal carbides form, radiating in a hemispherical pattern from the initiation site. As the internal carbon activity approaches one, direct internal growth of graphite begins, proceeding perpendi

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