DEFORMATION-CORROSION INTERACTIONS IN SELECTED ADVANCED HIGH-TEMPERATURE ALLOYS

The present work deals with investigations on the deformation-corrosio n interactions of the three advanced high temperature alloys MA 956, N icrofer 45 TM, and HR3C. The technical background relates to the corro sion conditions to be expected for heat exchanger tubes in coal gasifi cation plants. The atmosphere typical for a dry-feed entrained slaggin g gasifier, was composed of CO, H-2, CO2 and H2S, with medium S-and ve ry low O-potentials of about 10(-11) and 10(-28) bar, respectively, at a test temperature of 600 degrees C. The gas has been used in a non-e quilibrium condition corresponding to fast gas quenching from about 13 00 degrees C (gasifier) to 400-600 degrees C (tube walls). Possible de formation of the technical components have been taken into account by superimposed creep strains. The results have shown a very acceptable c orrosion resistance for all alloys. For 10 000 h exposure the internal attack remained clearly below 100 mu m. The external corrosion produc ts were composed of sulphides whereas a sulphide-oxide mixture formed internally. An interesting observation concerns the fact that for long er exposure times an increasing fraction of Cr2O3 formed at the intern al corrosion front. This was probably the reason for the sub-parabolic corrosion kinetics. For superimposed creep, the type and morphology o f the overall attack did not change. Differences for stress-free and s tressed samples however, concerned enhanced local attack. The critical creep value for the initiation of strain-assisted corrosion was about 1-2%. Approximately 2% or more strain was needed for the corrosion to clearly exceed that of stress-free samples. The mechanism of corrosio n enhancement was characterized by the formation of thin cracks in the compact internal corrosion zone whereas the external sulphides did no t play a role in loss of protection. Corrosion areas which formed at t he crack tip, composed of oxide-sulphide mixtures enabled the cracks t o extend further into the alloy under simultaneous corrosion-creep int eraction at the crack tip. Crack filling occurred mainly by iron sulph ides and therefore restoration of protection was not possible.