ABSTRACT Metal dusting has been responsible for a number of material failures in the chemical processing, thermal treatment and ore-reduction industries. Increasing severity of metal dusting attack has been associated with increasing ratios of carbon monoxide-to-carbon dioxide and decreasing ratios of steam- to-hydrogen, which act to simultaneously increase carbon activity and decrease oxygen partial pressure. Testing has been performed to examine the effects of alloying elements and surface condition upon the metal dusting resistance of numerous alloy systems in a hydrogen-carbon monoxide gas mixture at 621°C. Measurements of mass-loss rate, pit-progression rate, and pit distribution have been utilized to rate alloy performance.
INTRODUCTION The phenomenon termed "metal dusting" 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. Processing chemical derivatives of a given refining process often involves the creation of synthesized gas streams having high ratios of carbon monoxide-to-carbon dioxide and low ratios of steam-to-hydrogen, resulting in high carbon activity and low oxygen partial pressure. Iron ore reduction and thermal treatment processes involve generation of gas mixtures having similar characteristics. When such gas mixtures are present in the process stream in the critical temperature range of about 400 ° to 750°C, metal dusting can be a severe corrosion problem, t'2
Reformer components, furnaces, chemical reactors, and steam generators can all suffer from this costly problem.
MECHANISMS AND THERMODYNAMIC CONSIDERATIONS
The production of synthesis gas (syngas), a mixture of CO, H2, C02 and H20, from natural gas via steam reforming is a common step to begin production of hydrogen, ammonia, methanol and liquid hydrocarbons. The need for greater energy efficiency has driven producers to reduce the amount of steam used for the reforming process, lowering the steam-to-hydrogen ratio. Trends towards higher front-end pressures have also increased the CO content of the syngas. Lower H20/H2 ratios in combination with higher CO/CO2 ratios result in lower oxygen partial pressures and higher carbon activities. This combination of factors greatly accelerates the propensity for metal dusting. Figure 1 shows the variation of the carbon activity, as a function of CO/CO2 and H20/H2 ratios, for the reduction of CO by hydrogen at 627°C and a pressure of 1 atmosphere, with the H20 content fixed at both 1% and 10%. The equation for the reduction of CO is as follows:
CO + H 2 = H 20 + C (1) a
and the carbon activity a~ is calculated using the following equation:
log Kp = log al~2°ac = AG acoan2 2.303RT (2)
where Kp is the equilibrium constant and zSG is the Gibbs free-energy change for reaction (1). At higher temperatures, the reaction of H2 with deposited carbon would generate significant amounts of CH4, but this reaction is sluggish within the metal dusting temperature range so that only low levels of CH4 are produced. The Boudouard reaction
2co = c o s + c (3)
is strongly catalyzed by the presence of metallic particles and may be more of a factor once the process of metal dusting has progressed. It is interesting to compare Figure 1 with a graph published by Parks and Schillmoller 3 ( Figure 2), relating not the carbon activity but the observed severity of metal dusting attack of alloys 800 and 304 to the CO/CO2 and H20/I-I2 ratios within critical zones of ammonia plant waste heat boilers. Consideration should be given, however, to the fact that the atmosphere in waste heat boilers