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Corrosion : Spontaneous reaction occurring in the direction of
lowering G
G1 < G2 < G3 > G1 Metal Ore Metal Extraction Forming
& Shaping Rust Oxide Blast F'ce Rolling, Forging Metal oxide
Sulfide Extrusion, etc. (Chemical Energy) (Mechanical Energy)
Aqueous Corrosion : Most low temperature corrosion
Electrochemical reaction in nature
High Temperature Corrosion : Most high temperature corrosion
Gas/Metal reaction in nature CORROSION OVERVIEW
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CORROSION STUDYCorrosion Mechanism Study- Liquid/Metal-
Gas/Metal- Mechanical effect- Irradiation effectCorrosion
ControlDesign- Process design- Process development- System design-
Alloy design- Coating : painting, plating conversion coating-
Inhibitor- Electrochemical protection
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Topics for Presentation - Principle of High Temperature
Corrosion - Experimental Methods for High Temperature Corrosion -
Corrosion of Heat Exchanger Materials - Case Study on Heat
Exchanger Materials
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Requirement for High Temperature Materials - High mechanical
strength - Dense and protective oxide formation by reaction between
the substrate material and environment - Good stability of oxide
under thermal and mechanical stresses - Cost effective
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TYPES OF HIGH TEMPERATURE CORROSION Oxidation : Pure metal
oxidation Alloy oxidation
Corrosion in Mixed Environment: Sulphidation Carburization
Hot Corrosion Coal ash deposit corrosion
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HIGH TEMPERATURE OXIDATIONSimplified Mode for High Temperature
Oxidation
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FUNDAMENTALS OF HIGH TEMPERATURE CORROSIONThermodynamic
FundamentalsElligham diagram plots of the standard free energy of
formation versus temperature for the compounds of type, e.g.
oxides, sulphides, carbides, etc.
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Stability Diagram: useful in interpreting the condensed phases
which form in the environment containing more than one
oxidantsVapor Species Diagram: most suited for presentation of
vapor pressure data in oxide systems are log pMxOy, for a fixed T,
versus log pO2Cr-O system at 1250 KNi-O-S system at 1250 K
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Reaction Rate Fundamentals1. Linear Rate Law W = kt Ex) Heat
treatment in partial vacuum
2. Parabolic Rate Law W2 = kt + A Ex) Diffusion controlled
oxidation through fairly thick oxide
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3. Logarithmic Rate Law W = k log(t + to) + A Ex) Oxidation
occurring typically at low temp. up to 400C
4. Inverse Logarithmic Rate Law 1/W = B - k log(t) Ex) Electric
field induced transport of ions through thin oxide films
5. Paralinear Rate Law W = (kp/kl) ln(1/1-(klx/kp)) + kl(f - 1)-
t Ex) Hot corrosion through the inner compact and outer porous
layers
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OXIDATION OF PURE METALS1. Systems forming single layer scales
Ni/NiO Zn/ ZnO
2. Systems forming multiple scale layers Fe/FeO/Fe3O4/Fe2O3
Co/CoO/Co3O4
3. Systems forming volatile species Cr/Cr2O3/CrO3 Mo/MoO2/MoO3
W/WO2/WO2.7/WO3 Si/SiO/SiO2 Pt/PtO2 Rh/RhO2
4. Systems with significant oxygen solubilities in the metal
Ti-O system
5. Systems with significant scale cracking Nb-O system Ta-O
system
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Oxidation of IronFormation of multi-layer scale
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Oxidation Mechanism of Iron
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Oxidation of ChromiumScale thinning by CrO3 evaporationScale
buckling as a result of compressive stress development
CrO2Cr2O3
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Vapor Species Diagram most suited for presentation of vapor
pressure data in oxide systems are log pMxOy, for a fixed T, versus
log pO2Cr-O system at 1250 K
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Mechanism of High Temperature Oxidation
- Stress Development in the Oxide Growth stress : volume
difference between the oxide and the metalV (per metal ion in
oxide)V (per metal atom in metal)PBR>1 : compressionPBR
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Oxidation of MolybdenumVolatilization of oxides at high
temperatures and high oxygen pressureUnlike Cr, which develops a
limiting scale thickness, complete oxide volatilization
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Thermal stress : difference in thermal expansion coefficient of
the oxide and the metalE : elastic modulusa : coefficient of
thermal expansiont : thickness
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OXIDATION OF ALLOYS1. The metals in the alloy have different
affinities for oxygen
2. Ternary and higher oxides may form
3. A degree of solid solubility between the oxides may exist
4. Various metal ions have different mobilities in the oxide
phases
5. Various metals have different diffusivities in the alloy
6. Dissolution of oxygen into the alloy may result in internal
oxidation of one or more alloying elements
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Classification of Reaction Types1. Noble parent with base
alloying element (A) Noble parent: Au, Ag, Pt (B) Base metal: Cu,
Ni, Fe, Co, Cr, Al, Ti2. Base parent with base alloying element (A)
Parent: Ni, Co (B) Alloying element: Cr, Al, TiA - BA+BO
BOA+BOAOBOLow conc. of BLow conc. of BHigh conc. of BHigh conc. of
BA - BA - BA - BNBNBNBNB
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Internal Oxidation1. DGo of formation for the solute metal
oxide, BOn, must be more negative than DGo of formation for the
base metal oxide.
2. Base metal must have a solubility and diffusivity for oxygen
which is sufficient to establish the required activity of dissolved
oxygen at the reaction front.
3. The solubility concentration of the alloy must be lower than
that require for the transition from internal to external
oxidation
4. No surface layer must prevent the dissolution of oxygen into
the alloy at the start of oxidationOxygen diffusion into an alloy
causes sub-surface precipitation of the oxide of one or more alloy
elementsThe necessary condition for the occurrence of the internal
oxidation
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Fe-5CrFe-15CrFe-10CrFe-20CrSchematic Diagram of the Scale
Morphologies on Fe-Cr alloys
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Gettering EffectIn multi-component system, one alloy element aid
anotherto be selectively oxidized to form a stable external
oxideNi-Cr-Al system
1/3 Go(Al2O3) < 1/3 Go(Cr2O3) < G one)
Ni-9%Cr-6%Al produces continuous protective Al2O3 scales
Cu-Zn-Al system
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Synergistic effect of Cr on the producing of Al2O3 scales;
gettering duringtransient oxidation of Ni-15Cr-6Al at 1000 C
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Fig. Comparison of the oxidation of various alloys in 1 atm O2
at 1200 C
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CORROSION IN MIXED ENVIRONMENTFe3O4FeSFeFeO Fe-O-S System
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Definition: Accelerated form of high temperature oxidation by
fluxing action of the salt deposit
Types of Hot Corrosion
1. High Temperature Hot Corrosion (HTHC) Main mechanism: Basic
fluxing
2. Low Temperature Hot Corrosion (LTHC) Main mechanism: Acidic
fluxingHOT CORROSION
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High Temperature Hot CorrosionHTHC by Na2SO4 Deposit
Temperature: above melting point of Na2SO4 (884C) Atmosphere: SO2
containing environment Degradation mechanism: basic fluxing
Basic Fluxing MO + O2- = MO22- ex) 2NiO + O2- + 1/2 O2 = 2 NiO2-
(At constant basicity the solubility of NiO increases with
increasing oxygen pressure.) Cr2O3 + O2- = 2 CrO2- (at low oxygen
pressure) Cr2O3 + 2 O2- + 3/2 O2 = 2 CrO42- (at high oxygen
pressure) Al2O3 + O2- = 2 AlO2- (at low oxygen pressure)
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Stability Diagram for Ni-O-S System
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Schematic Diagram illustrating the Na2SO4 induced Hot
Corrosion
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Degradation of alloys caused by presence of liquid salt
depositDissolution and reprecipitation of
oxide5678910111213141516171234Co3O4NiOCr2O3Al2O3Fe2O3-Log a Na2OLog
Concentration (ppm)MetalOxideSaltCr + 3/2O2 = Cr2O3Cr2O3 + O2- =
2CrO22-2CrO22- = Cr2O3 + O 2- Basic Mechanism of Hot Corrosion
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Low Temperature Hot CorrosionLTHC by Na2SO4 Deposit Temperature:
below melting point of Na2SO4 (884C) Atmosphere: oxygen and SO3
containing environment Degradation mechanism: acidic fluxing
Acidic Fluxing MO = M2+ + O2- ex) NiO + SO3 = Ni2+ + SO42- Cr2O3
+ 3 SO3 = 2 Cr3+ + 3 SO42- Al2O3 + 3 SO3 = 2 Al3+ + 3 SO42- SO3 +
SO42- = S2O72- V2O5 + SO42- = 2 VO3- + SO3
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Comparison of theoretically estimated values of the critical SO3
pressuresneeded to form molten Na2SO4+NiSO4 and the lowest SO3
pressures at which formation of molten sulphate was observed
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Schematic diagram of the reaction mechanism of low
temperaturehot corrosion of Ni in O2+SO2 and/or SO3
atmospheres.
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Schematic illustration of the reaction mechanism of low
temperaturehot corrosion of Ni-Cr alloy in SO3-containing gases
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The temperature dependence of the corrosion rate of Ni-30%Cr
coated with Na2SO4in 1 atm of O2+1%(SO2+SO3) and of Ni-20%Cr (no
salt deposit) in 1 atm of SO2:O2=1:1.
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The Ni-S Phase Diagram
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The NiSO4-Na2SO4 Phase Diagram
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Comparison of Na2SO4-induced hot corrosion of Ni-30%Cr
andCo-30%Cr in O2+1%(SO2+SO3) and O2+0.15%(SO2+SO3),
respectively
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Alloy Induced Acidic FluxingIn high Mo containing alloys Ex:
B-1900 (Ni-8Cr-6Al-6Mo-10Co-1Ti-4.3Ta-0.11C-0.15B-0.072Zr)
1. Initially attacked by basic fluxing, as the protective scale
is broken up, then, the alloy is oxidized Mo + 3/2 O2 = MoO32. MoO3
gradually dissolves in the melt as Na2 MoO4 MoO3 + SO42- = MoO42- +
SO3 This reaction changes the acidity of the melt to acidic
fluxing3. MoO3 also dissolves the protective alumina scale Al2O3 +
3 MoO3 = 2 Al3+ + 3 MoO42-
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Schematic Illustration of a Utility BoilerCorrosion of Heat
Exchanger Materials(590-620C)(400-425C)
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Boiler OperationPeak Flame Temperature: 1550 - 1750 CMetal
Temperature of Steel Tube: 400 - 450 CSuperheater: The water-free
steam is heated to 650C. at 16.5MPa for subcritical boiler at
24.0MPa for supercritical boiler (Max. at 650C, 34.5MPa)Economizer:
The condensed water from steam is preheated before recycling to the
boiler. Gas temp in the region: 800C in, 300 C out
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Oxidation LimitsMaterial ASME Spec. Temp.(C)Carbon Steel SA-178,
SA-210, SA-192 454Carbon + 0.5%Mo SA-209-T1 4821.25%Cr + 0.5%Mo
SA-213-T11 5522.25%Cr-1%Mo SA-213-T22 57918%Cr-10%Ni SA-213-321H
816
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Effect of Alloying ElementsSi: added to all steels as a
deoxidizing element mild ferrite strengthener when added in amount
up to 2.5%, the strength is increased without any adverse effects
on ductilityMo: ferrite strengthener increases the strength without
any loss of ductility mild carbide stabilizer retards the formation
of graphite upon prolonged heating graphitizing temp.: carbon steel
- 420C, carbon-Mo steel - 455CCr: greatly enhances the oxidation
resistance oxidation limit: T-11(1.5%Cr) - 550 C, T-22(2.25%Cr) -
580 C improves the high-temp. strength and creep properties carbide
stabilizer: Cr-Mo steels do not form graphite under any
condition
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Coal Ash Deposit Problem1. Heat transfer to the working fluid
either water or steam is reduced due to low thermal conductivity of
the deposit.
2. The designed lifetime of the boiler tubes is shortened by the
very corrosive environment created by the deposit.
3. Slag accumulation on upper furnace walls can lead to the
formation of large deposits of frozen material which may impedes
gas flow.
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Melting Points of Complex SulfatesCompound Temperature, C
K3Fe(SO4)3 618K3Al(SO4)3 654KFe(SO4)2 694(a)Na3Fe(SO4)3
624Na3Al(SO4)3 646NaFe(SO4)2 690(a)Na2S2O7 401K2S2O7 300 (a) In
high SO3 atmosphere
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Corrosion of Steel by TrisulfatesMainly responsible for
corrosion of superheaters and reheatersin temperature range of 480
- 730C- Deposition of alkalies on exposed metal surface- Conversion
of alkali oxides to alkali metal sulfates by SO3 (Na or K)2O + SO3
(Na or K)2SO4- Formation of alkali trisulfates with adsorption of
SO3 and dissolution of transition metal oxides 3(Na or K)2SO4 +
Fe2O3 + 3 SO3 2(Na or K)3Fe(SO4)3 - Reestablishment of transition
oxides on metal surfaces Fe + 1/2 O2 FeO 3FeO + 1/2 O2 Fe3O4 3
Fe3O4 + 1/2 O2 2Fe2O3
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- Deposition of alkalies on exposed metal surface- Convection of
alkalies to Na2SO4 and K2SO4- Oxidation of SO2 in the bulk flue gas
to SO3 SO2 + 1/2 O2 SO3 (on catalytically active surfaces)-
Formation of pyrosulfates (Na or K)2SO4 + SO3 (Na or K)2S2O7-
Reaction of pyrosulfates with the scale on metal surfaces 3(Na or
K)2S2O7 + Fe2O3 2(Na or K)3Fe(SO4)3 which then dissociate 2(Na or
K)3Fe(SO4)3 3(Na or K)2SO4 + Fe2O3 + 3SO3- Further oxidation of the
metal to replace its scale 3Fe + 2O2 Fe3O4 (Temperature Range: 400
- 500C)CORROSION OF STEEL BY PYROSULFATES
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Corrosion of Steel by SulfidesDue to poor combustion, the
furnace wall tubes corrode by unoxidized sulfides- Condensation of
alkali sulfates on the oxidized tube surface- Unburned coal and
particles of pyrites (FeS2) adhere to the tube surface and form a
thick deposit- Pyrites gradually oxidize to FeS and Fe3O4 FeS2 FeS
+ 1/2S2 3FeS + 5 O2 Fe3O4 + 3 SO2- These sulfides, SO2 and SO3,
then form trisulfates, (Na or K)3Fe(SO4)3 in small quantities
leading to loss of metal
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Effect of ChlorineSource of Cl - refused-derived fuels (mostly
from PVC plastics) - NaCl from seawater during transport -
contained in the coal (i.e., Illinois coals) 2 Fe + 3 Cl2 = 2
FeCl32 Fe + 6 HCl = 2 FeCl3 + 3 H2Fe2O3 + 6 HCl = 2 FeCl3 + 3
H2O
FeCl3 melts at 280C, and it acts as a flux to promote the
formation of liquids within the ash deposit.FeCl3 may form even
lower melting point compounds by combining with other ash
constituents.
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Required Properties of High Temperature Coatings - Adherent,
crack- and pore-free - Microstructurally stable - Thermal expansion
compatible to substrate - Predictable reaction & interdiffusion
with substrate - No effect on mechanical and structural properties
of substrate - Locally repairable - Cost effective
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High Temperature Coating Principles - Dense and protective oxide
formation by reaction between the coating material and environment
- Barrier effect against inward diffusion of oxidants and outward
diffusion of cations - Slow growing oxides for common coatings
Al2O3, Cr2O3, SiO2
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Limitations of Coating Oxides - Al2O3 Easy acidic fluxing
- Cr2O3 Evaporation at high PO2
- SiO2 Evaporation at low PO2