V. Guttmann et al., DEFORMATION-CORROSION INTERACTIONS IN SELECTED ADVANCED HIGH-TEMPERATURE ALLOYS, Materials at high temperatures, 14(2), 1997, pp. 127-136
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.