Oxidation of Ni-10Cr-9Al alloy at 1000 0 C Pressure Dependence of SO - - PowerPoint PPT Presentation

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Aug 24, 2022 195 likes •371 views

Oxidation of Ni-10Cr-9Al alloy at 1000 0 C Pressure Dependence of SO 2 Reaction with SO 2 is tempertaure and Pressure Dependent At 1 atms. SO 2 pressure at 500-600 o C 7Ni + SO 2 = 4NiO + Ni 3 S 2 ------- [1] At this pressure as per Phase

SLIDE 1

Oxidation of Ni-10Cr-9Al alloy at 10000C

SLIDE 2

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Pressure Dependence of SO2

Reaction with SO2 is tempertaure and Pressure

Dependent

At 1 atms. SO2 pressure at 500-600oC

7Ni + SO2 = 4NiO + Ni3S2 ------- [1]

At this pressure as per Phase stability Diagram, NiSO4 is also stable, because of which there is additional reaction

9Ni + NiSO4 = 8NiO + Ni3S2 ------- [2]

Because of which the rate of reaction is very fast. As the pressure is reduced from 1 atm to 0.1 atms. NiSO4 is no longer stable and reaction 2 does not occur and rate falls.

SLIDE 5

Phase Stability Diagram Of Ni

SLIDE 6

Temperature Dependence of SO2

NiSO4 is stable upto 600oC, hence no possibility of

reaction [2].

At 635oC Ni-S liquid starts becoming stable. The reaction

from 700-900oC are linear and rates are proportional to SO2 pressure.

The rate decreases further with increase in temp. above

800oC because of instability of Ni3S2 above 800oC.

The sulphide formation becomes thermodynamically

unstable at high temperatures and at low SO2 pressure

Under these conditions, the corrosion product is is only

NiO and no sulphide thus corrosion rate falls drastically.

That is why rate of corrosion of Ni in SO2 at 1000oC is

almost same as rate of oxidation of Ni.

SLIDE 7

Sulphidation of Ni Vs Co

SLIDE 8

Sulphidation of Ni in SO2 + O2 NiO (s) + SO3 (g) = NiSO4 (s) SO2 (g) + O2 (g) = SO3 (g)

SLIDE 9

Oxidation of Binary Alloys of Ni-Cr

SLIDE 10

Sulphidation of Fe-Cr, Ni-Cr and Co-Cr Alloys

SLIDE 11

Effect of Pre-Oxidation on Sulphidation

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Mechanism of SO2/SO3 Reaction

Reaction starts with the sulphide formation of most stable element

and further growth occurs mainly by lattice diffusion of ions thru sulphide scales.

Formation of mixed oxide scale occurs even when the partial

pressure of oxygen or sulphur is dominant at one particular point. – this happens because of continuous increase in conc. Of other gas below the main scale interface.

How the sulphur partial pressure in craeses below oxide scale

(a) Dissolution of SO2 – not sound as s disolution is very low. (b) Gas dissolution thru’ pores and cracks and hence establishes con. Gradient. (c )Direct reaction of SO2 which can produce S C + SO2 = S + CO2

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