Corrosion of heat exchanger alloys exposed to a non-equilibrated CO-based sulfidizing environment at 550 degrees C

This paper presents results from a series of experiments involving the inte rmittent exposure of several steels to a CO-based gas mixture containing 0. 1% H2S for periods of up to 1000 h. The flow of the gas was maintained at a rate sufficiently high to prevent equilibration of the mixture occurring a t the reaction temperature of 550 degrees C, thereby simulating conditions found in commercial coal gasification plants. Five materials were selected for this study, i.e. three commercial alloys c ontaining 9, 12 and 20% Cr and two specially cast 12% Cr "model" alloys wit h additions of 1% and 2% Si. Several specimens of each alloy were exposed a t the same time; some samples were also coated with a Cl-containing ash mix ture prior to exposure in order to study the influence of fly-ash deposits upon degradation. In addition, after intermittently cooling the samples to room temperature, half were desiccated dry whilst the remainder were placed in moisture-saturated air at 30 degrees C in order to simulate conditions arising during periods of plant down-time, i.e. down-time corrosion (DTC). Corrosion kinetics are presented in terms of weight change and metal loss m easurements indicating the significant improvement accompanying the additio n of 1-2% Si to the basic 12% Cr composition. The contributions of ash depo sits and periods of moist down-time are also discussed.