INTRODUCTION

ABSTRACT

Increases in syngas steam to carbon ratio are generally expected to reduce the tendency of high chromium nickel-based alloys to suffer from metal dusting corrosion. This might only be true for the initiation of metal dusting. Laboratory experiments have been conducted in a pressurized autoclave to indicate that sustained metal dusting degradation of alloy 601 (UNS N06601) may also be driven by the presence of steam (oxygen partial pressure). Oxidation of matrix carbides is proposed as a possible contributing factor in metal dusting corrosion.

Metal dusting corrosion, often described as a form of catastrophic carburization ~, continues to be a significant restriction on the optimum design and construction of syngas production plants. There appears to be a significant discrepancy between claims made by fundamental metal dusting research 2.3 and the ability of operating companies to solve practical metal dusting problems 4. This is aptly reflected in the 1999 launch of a new European research program on metal dusting 5 as well as the MTI/DOE Program on Metal Dusting 6

The reasons for the above are twofold. Firstly, fundamental research requires tight control of the test environment where only one process parameter is changed at a time. While this is considered a sound scientific approach, the results often do not reflect the interaction between various changing parameters encountered in an operating plant. Secondly, the process conditions selected for fundamental metal dusting research are often selected to yield fast results and ensures results within a given time- frame. Relating data generated in this manner to plant experience and materials selection for new equipment is extremely difficult, if not impossible.

The alloys used in syngas plants are exposed to significantly different gas atmospheres during startup, compared to stable operation. The current work has aimed to conduct research under realistic process conditions (gas composition, temperature & pressure). The high pressure exposure program therefore included a simulation of both dynamic and stable process conditions.

Observations made during high temperature steam exposure (starup or trip conditions) of pre- corroded alloy 601 (UNS N06601) will be reported in this paper.

DIFFERENT EXPLANATIONS OF THE METAL DUSTING MECHANISM

Extensive research into the fundamental understanding of the various mechanisms of metal dusting has been conducted during the past forty-five years. Notable contributions to the current understanding came from Dr. R.F. Hochman and his group at the Georgia Institute of Technology, Atlanta USA 7.8, and prof. H.J. Grabke and his coworkers at the Max-Planck-Institute of Iron Research (MPI) in Dtisseldorf, Germany 9-11

Several other groups have through the years researched the mechanisms of metal dusting. Most of these reported observations that are in agreement with the mechanisms proposed by Hochman and Grabke, with some modifications. Some of these modifications include:

A. Metal dusting corrosion assisted by unreleased stresses.

B. Metal dusting corrosion involving catalytic carbon deposition.

C. Metal dusting corrosion as internal oxidation and carburization.

Mechanism by R.F. Hochman - Georgia Institute of Technology

Most of the research conducted at the Georgia Institute of Technology was on the interaction of pure metals and alloys with carbon monoxide at high temperatures. Only a limited amount of research has been conducted using hydrocarbon or complex gas environments. A comprehensive summary of this research was published in 1973 7. This was followed up another publication in 1976, summarizing the

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