Deformation-corrosion interactions in selected advanced high temperature alloys

The present work deals with investigations on the deformation-corrosion interactions of the three advanced high temperature alloys MA 956, Nicrofer 45 TM, and HR3C. The technical background relates to the corrosion conditions to be expected for heat exchanger tubes in coal gasification plants. The atmosphere typical for a dry-feed entrained slagging gasifier, was composed of CO, H-2, CO2 and H2S, with medium S-and very 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-equilibrium condition corresponding to fast gas quenching from about 1300 degrees C (gasifier) to 400-600 degrees C (tube walls). Possible deformation of the technical components have been taken into account by superimposed creep strains. The results have shown a very acceptable corrosion resistance for all alloys. For 10 000 h exposure the internal attack remained clearly below 100 mu m. The external corrosion products were composed of sulphides whereas a sulphide-oxide mixture formed internally. An interesting observation concerns the fact that for longer exposure times an increasing fraction of Cr2O3 formed at the internal corrosion front. This was probably the reason for the sub-parabolic corrosion kinetics. For superimposed creep, the type and morphology of the overall attack did not change. Differences for stress-free and stressed 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 corrosion enhancement was characterized by the formation of thin cracks in the compact internal corrosion zone whereas the external sulphides did not play a role in loss of protection. Corrosion areas which formed at the crack tip, composed of oxide-sulphide mixtures enabled the cracks to extend further into the alloy under simultaneous corrosion-creep interaction at the crack tip. Crack filling occurred mainly by iron sulphides and therefore restoration of protection was not possible.