Hot Corrosion of Materials

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Hot Corrosion of Materials

• Robert A. Rapp
• Materials Science Engineering Dept
• The Ohio State University
• Olin Palladium Award Address
• The Electrochemical Society
• October 19, 2005

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Carl Wagner
1951 Pd Medal Award
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Marcel Pourbaix

• 1975 Pd Medal Award

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Introduction

• During Viet Nam War, hot gas turbine engine
  components suffered extreme corrosion attack
  sulfides were found in Ni-base alloys faulted
  protective oxides failed to protect the alloys
  porous oxide/salt products were seen.
• An NMAB committee examined the phenomenon, but a
  detailed mechanism was lacking. The problem was
  called Sulfidation

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Progress in Understanding

• Goebel and Pettit, Bornstein and DeCresente
  showed that the problem was tied to a thin
  Na2SO4-base fused salt film coating the external
  surface. The geometric similarity to aqueous
  atmospheric corrosion provided the new name Hot
  Corrosion.
• As a qualitative mechanism Fluxing/dissolution
  of protective scale leads to a reduction of the
  salt by the alloy (forming sulfides) with
  precipitation of non-protective oxide particles
  in the salt-coated surface film.

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Na-S-O Phase Stability Diagram at 1173K
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EC Reference Electrodes to Measure Sodium and
Oxygen Activities and thereby Melt Basicity
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Ni-Na-S-O Phase Stability Diagram at 1200 K
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Solubility of NiO in Fused Na2SO4 at 1200 K
PO2 1 atm, Gupta
       
Basic dissolution
Acidic dissolution
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Na-Cr-S-O Phase Stability Diagram at 1300K
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Experimentally Measured Solubilities for Cr2O3 in
Fused Na2SO4 and 1 atm Oxygen Y. Zhang
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Solubilities of Oxides in Na2SO4 at 1200K
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Reprecipitation of Porous MO Oxide Supported by
Negative Solubility Gradient in Fused Salt Film
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Cases of Sustained Hot Corrosion of a Pure Metal
(I is Oxide/Salt Interface, and II is Salt/gas
Interface)
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Trace of Basicity and Oxygen Activity Measured
upon Polarization of a Pt Working Electrode C.
O. Park
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Experimental Arrangement to Measure Activities at
Oxide/Salt Interface during Hot Corrosion of Ni
at 1200K N. Otsuka
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Trace of Basicity and Oxygen Activity Measured on
a Preoxidized 99Ni Coupon Covered with a Na2SO4
Film at 1173K in O2-0.1SO2 Gas. Numbers
Designate Reaction Time in Hours.
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Effect on Preformed Oxide Thickness on Hot
Corrosion of Ni at 1200K
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Effect of Ni Purity on Hot Corrosion at 1200K
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Effect of Melt Basicity on Hot Corrosion of
Nickel at 1200K
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Effect of Inhibiting Addition to Salt on Hot
Corrosion of Nickel at 1200K
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Basic Fluxing Mechanism

• Ni metal reacts with fused Na2SO4 to form NiO.
• The oxygen activity decreases and sulfur activity
  increases.
• Formation of liquid nickel sulfide causes an
  increase in local salt basicity (sodium oxide
  activity).
• Normally protective NiO scale is dissolved/fluxed
  to form a basic solute, nickelate ions.
• With a negative solubility gradient,
  non-protective NiO particles are precipitated
  within the salt film.
• These results support a basic fluxing reaction,
  i.e. corrosive attack by forming a basic solute
  of the protective scale.
• 4Ni Na2SO4 Na2O 3 NiO
  NiS

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Synergistic Dissolution
Fe2O3 Dissolution
Cr2O3 Dissolution
 Basic Dissolution
 Basic Dissolution
Acidic Dissolution
Acidic Dissolution
Superposed Solubility Diagrams for Cr2O3 and
a-Fe2O3 in Fused Na2SO4 at 1200K and 1 atm O2 Y.
S. Hwang
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Experimental Arrangement to Measure Kinetics of
Synergistic Dissolution of Fe2O3 and Cr2O3 at
1200K Y. S. Hwang
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Rates of Fe2O3 Dissolution in Na2SO4 at 1200K Y.
S. Hwang
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Rates of Cr2O3 Dissolution in Na2SO4 at 1200K Y.
S. Hwang
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Protective Behavior of Chromium

• Why is chromium the most effective alloying
  element to combat corrosion by sodium sulfate?
  From the Cr2O3 solubility study, the answer lies
  in the oxygen-pressure-dependence for the basic
  dissolution of Cr2O3.
• The valence of Cr is increased from 3 to 6 upon
  Cr2O3 dissolution to form CrO42- ions, so the
  basic solubility of Cr2O3 increases with
  increasing oxygen activity
• Cr2O 3 2 Na2O 3/2 O2(g) 2 Na2CrO4
• (d log Na2CrO4 / d log PO2)
  3/4

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Schematic Illustration of the Role of Chromium in
Inhibiting Hot Corrosion Attackat some Grain
Boundaries or Other Defects
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Protective Behavior of Chromium

• Any thin salt film is more reducing at the
  metal/salt interface than toward the oxidizing
  gas. Therefore, chromate solute experiences a
  positive solubility gradient.
• Consumptive reprecipitation of oxide in the salt
  film will not occur. Rather, the chromate ion
  will deposit the oxide, satisfying a reduction in
  its solubility, at the most reduced sites, e.g.
  at the flaws or grain boundaries of the scale.
• In this way, the Cr serves as a protective
  component to plug flaws in the scale.

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Other Hot Corrosion Studies at OSU

• Studies of the effect of vanadates of oxide
  solubilities
• Mechanism for vanadate hot corrosion
• Complex impedance studies of salt conductivities
• Transient ec experimentation to identify species
  involved in oxidation/reduction reactions (cyclic
  voltammetry, chronoamperometry, chronovoltametry)

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Other Types of Hot Corrosion

• Type 1 High-Temperature Hot Corrosion-Acidic
  Fluxing High-valent refractory metal components
  in the alloy or the salt complex with oxide ions
  to greatly increase the acidic solubilities of
  oxides. Pettit, et al. Again, oxide
  dissolution/reprecipitation.
• Type 2 Low-Temperature Hot Corrosion-Far below
  melting point of Na2SO4. Corrosion products of
  the base metal or coating form a multi-component
  salt with low liquidus temperature. Negative
  solubility gradient criterion still applies.
  Shores and Luthra.

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Acknowledgements

• This research was principally sponsored by NSF.
• Roger Staehle who taught me Pourbaixs
  methodology
• Students and Post-Docs responsible for careful
  measurements and clever interpretations D.
  Gupta, C. O. Park, N. Otsuka, D. Z. Shi, J. Nava,
  Y. S. Hwang, J. Kupper, W. C. Fang, K. S. Goto
• Especially Y. Zhang