HOT CORROSION HOT CORROSION Metals and alloys experience accelerated - - PowerPoint PPT Presentation

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Jan 05, 2024 417 likes •606 views

Relationship between pS 2 and pSO 2 HOT CORROSION HOT CORROSION Metals and alloys experience accelerated oxidation / sulphidation when their surfaces are covered with a thin film of fused salt in an oxidizing gas atmosphere at elevated

SLIDE 1

SLIDE 2

Relationship between pS2 and pSO2

SLIDE 3

HOT CORROSION

SLIDE 4

HOT CORROSION Metals and alloys experience accelerated oxidation / sulphidation when their surfaces are covered with a thin film of fused salt in an oxidizing gas atmosphereat elevated temperatures

Temp. range 850-950oC (m.p.
f Na2SO4 884°C)
Initial attack on protective
xide film
Chromium depletion,
xidation of the base material

accelerates

Type I (HTHC)

Temp. range 650-800oC
Formation of low melting

mixtures Na2SO4-NiSO4 eutectics

High partial pressure of SO3

(g) –required

Typical pitting

Type II (LTHC)

Types of Hot Corrosion

SLIDE 5

HIGH TEMPERATURE HOT CORROSION

Ref: Introduction to High Temperature Oxidation and corrosion, A.S.Khanna, IIT Bombay, ASM International, 2002

Occurs at high temperature
Salt deposit on metal / alloy in liquid state
Very high corrosion rate
Kinetics - mainly linear
Corrosion of Ni in presence of Na2SO4

at 900 oC

Melting point of Na2SO4 - 880 oC

SLIDE 6

LOW TEMPERATURE HOT CORROSION

Ref: Introduction to High Temperature Oxidation and corrosion, A.S.Khanna, IIT Bombay, ASM International, 2002

Reaction rate at beginning – slow till certain point – incubation period
Later rate increases suddenly and follow linear kinetics
Occurs at lower temperatures – lower than melting temperature of salt
Initial stages - Na2SO4 solid

– corrosion reaction involves

xidation of Ni to NiO
NiO + SO2 = NiSO4
Melting point of NiSO4 - 671
C

SLIDE 7

FLUXING MECHANISM OF HOT CORROSION

Oxyanion sodium sulphate melt similar to acid-base chemistry of aqueous solutions Na2O + SO3 = Na2SO4 log K (1200 K) = -16.7 log aNa2O = melt basicity log aSO3 = melt acidity

Ref: Robert A. Rapp, Hot corrosion of materials: a fluxing mechanism? Corrosion Science 44 (2002) 209±221

Dissolution on NiO can be represented by the following equations: Basic dissolution: 2NiO + O2- (Na2O) + ½ O2 = 2NiO2- (NaNiO2) Acidic dissolution: NiO = Ni2+ + O2- Further O2- + SO3 = SO4

2- (NiSO4)

SLIDE 8

SUSTAINED ACIDIC DISSOLUTION

Basic dissolution 2MO + O2- (Na2O) + ½ O2 = 2MO2- (NaNiO2) melt gradually consumes the salt gets saturated with NaNiO2 reaction subsides Acidic dissolution MO = M2+ + O2- Further O2- + SO3 = SO4

(NiSO4) molten Na2SO4 – solvent for NiSO4 which reacts with nickel migrating

utwards through the inner layer with a

continuous sulphide network Na2SO4 is not consumed and the reaction continues for extended periods

f time

SLIDE 9

Crucible Test Schematic of Bead Test Isothermal Tests Cyclic Tets

SLIDE 10

Schematic of a Burner Rig

SLIDE 11

Hot Corrosion of Nickel

SLIDE 12

FORMATION OF EUTECTIC MIXTURE

SLIDE 13

SLIDE 14

SLIDE 15

Temperature dependence of the corrosion rate of Ni-30% Cr coated with Na2SO4 (2.5 mg/cm2) in latm. of O2 + 1% (SO2 + SO3) and Ni-20% Cr (with no salt deposit) in 1 atm. of SO2 : O2 : 1 : 1.

SLIDE 16

Na2SO4 induced hot corrosion of Ni-30% Cr and Co-30% Cr in O2 + 1% (SO2 + SO3) and O2 + 0.15% (SO2 + SO3) exposed for 24 h.

SLIDE 17

ROLE OF ALLOYING ELEMENTS

Improves mechanical properties
Presence makes alloy highly susceptible to hot

corrosion Vanadium and molybdenum

increase the hot corrosion resistance.

Titanium, aluminum, and niobium