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

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

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

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

TYPES OF HIGH TEMPERATURE CORROSION Oxidation : Pure metal oxidation Alloy oxidation

Corrosion in Mixed Environment: Sulphidation Carburization

Hot Corrosion Coal ash deposit corrosion

HIGH TEMPERATURE OXIDATIONSimplified Mode for High Temperature Oxidation

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.

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

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

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

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

Oxidation of IronFormation of multi-layer scale

Oxidation Mechanism of Iron

Oxidation of ChromiumScale thinning by CrO3 evaporationScale buckling as a result of compressive stress development CrO2Cr2O3

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

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

Oxidation of MolybdenumVolatilization of oxides at high temperatures and high oxygen pressureUnlike Cr, which develops a limiting scale thickness, complete oxide volatilization

Thermal stress : difference in thermal expansion coefficient of the oxide and the metalE : elastic modulusa : coefficient of thermal expansiont : thickness

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

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

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

Fe-5CrFe-15CrFe-10CrFe-20CrSchematic Diagram of the Scale Morphologies on Fe-Cr alloys

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

Synergistic effect of Cr on the producing of Al2O3 scales; gettering duringtransient oxidation of Ni-15Cr-6Al at 1000 C

Fig. Comparison of the oxidation of various alloys in 1 atm O2 at 1200 C

CORROSION IN MIXED ENVIRONMENTFe3O4FeSFeFeO Fe-O-S System

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

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)

Stability Diagram for Ni-O-S System

Schematic Diagram illustrating the Na2SO4 induced Hot Corrosion

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

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

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

Schematic diagram of the reaction mechanism of low temperaturehot corrosion of Ni in O2+SO2 and/or SO3 atmospheres.

Schematic illustration of the reaction mechanism of low temperaturehot corrosion of Ni-Cr alloy in SO3-containing gases

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.

The Ni-S Phase Diagram

The NiSO4-Na2SO4 Phase Diagram

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

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-

Schematic Illustration of a Utility BoilerCorrosion of Heat Exchanger Materials(590-620C)(400-425C)

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

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

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

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.

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

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

- 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

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

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.

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

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

Limitations of Coating Oxides - Al2O3 Easy acidic fluxing

- Cr2O3 Evaporation at high PO2

- SiO2 Evaporation at low PO2