The role of sodium in the accelerated oxidation phenomenon termed sulfidation

Hot Corrosion of Na[sub 2]SO[sub 4]-Coated Ti[sub 3]AlC[sub 2] in Air at 700-1000°C

“…6 Metallographic inspection of failed parts often showed sulfides of nickel and chromium, so the mechanism was initially called sulfidation. However, studies by Goebel and Pettit 7 and by Bornstein and DeCrescente 8,9 showed that sulfide formation indeed resulted from the reaction of the metallic substrate with a thin film of fused salt of sodium sulfate base rather than with Na 2 SO 4 vapor. Because corrosion by a thin electrolyte film bears some common features with ''atomospheric corrosion'' by an aqueous film at room temperature, the phenomenon has been renamed ''hot corrosion''.…”

Studies on the hot corrosion of a nickel-base superalloy, Udimet 700

“…A comparison of the amount of Na2S04 reacted after 4 hr of corrosion at 950°C (this time corresponds to 1 hr after the beginning of the second breakaway oxidation period) shows the amount of Na2S04 reacted to be the same for all the doses of Na2S04. This is to be expected because (1) most of the conversion of Na2S04 takes place at the very beginning of the breakaway oxidation periods and (2) the extent of sulfidation at the beginning of the breakaway oxidation period is dependent on the extent of alloy depletion plus the thickness of the depleted zone, but not on the amount of Na2S04.…”

“…For examp"'e, for a dose of 3.5 mg/cm 2 Na2S04, the maximum amount of sulfur that can be introduced in the alloy is 0.788 mg/cm 2 . Assuming all the sulfides are Cr2S3' the oxidation reaction for the sulfides can be written as (1) Assuming that all the sulfur is conserved in the alloy, the maximum 02 pick up due to the oxidation of all the sulfides would be 0.788 mg/cm 2 . Similarly, assuming that all the sulfides are CrS, the maximum oxygen pick up would be 0.591 mg/cm 2 ; The weight gain, as determined from the kinetics, represent the 02 pick up and ther~fore the preferential oxidation of sulfides, by itself, cannot account for the total weight gain during the second breakaway oxidation for the heavier doses (about 5 mg/cm 2 for a dose of 3.5 mg/cm 2 Na2S04).…”

Hot corrosion behavior of HVOF-sprayed CoNiCrAlYSi coatings in a sulphate environment

“…7, the Al-oxide scale formed in the hot corrosion environment (for 10 cycles, about 480 hours at 900 °C) had smaller grain size and was thicker than the pure alumina scale formed in a lab-air oxidation process (for 500 hours) which had a typical columnar structure. The morphology of the small-grain Al-rich oxides may have formed due to a fluxing process of alumina [17,18]. It is also worth mentioning that multi-layers of the Al-rich oxides could be observed in some areas where the coating was locally heavily attacked, indicating that buckling and reformation of alumina scales had occurred during the cyclic hot corrosion testing.…”