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Hot Corrosion of Materials

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Title: Hot Corrosion of Materials


1
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

2
Carl Wagner
1951 Pd Medal Award
3
Marcel Pourbaix
  • 1975 Pd Medal Award

4
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

5
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.

6
Na-S-O Phase Stability Diagram at 1173K
7
EC Reference Electrodes to Measure Sodium and
Oxygen Activities and thereby Melt Basicity
8
Ni-Na-S-O Phase Stability Diagram at 1200 K
9
Solubility of NiO in Fused Na2SO4 at 1200 K
PO2 1 atm, Gupta
       
Basic dissolution
Acidic dissolution
10
Na-Cr-S-O Phase Stability Diagram at 1300K
11
Experimentally Measured Solubilities for Cr2O3 in
Fused Na2SO4 and 1 atm Oxygen Y. Zhang
12
Solubilities of Oxides in Na2SO4 at 1200K
13
Reprecipitation of Porous MO Oxide Supported by
Negative Solubility Gradient in Fused Salt Film
14
Cases of Sustained Hot Corrosion of a Pure Metal
(I is Oxide/Salt Interface, and II is Salt/gas
Interface)
15
Trace of Basicity and Oxygen Activity Measured
upon Polarization of a Pt Working Electrode C.
O. Park
16
Experimental Arrangement to Measure Activities at
Oxide/Salt Interface during Hot Corrosion of Ni
at 1200K N. Otsuka
17
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.
18
Effect on Preformed Oxide Thickness on Hot
Corrosion of Ni at 1200K
19
Effect of Ni Purity on Hot Corrosion at 1200K
20
Effect of Melt Basicity on Hot Corrosion of
Nickel at 1200K
21
Effect of Inhibiting Addition to Salt on Hot
Corrosion of Nickel at 1200K
22
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

23
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
24
Experimental Arrangement to Measure Kinetics of
Synergistic Dissolution of Fe2O3 and Cr2O3 at
1200K Y. S. Hwang
25
Rates of Fe2O3 Dissolution in Na2SO4 at 1200K Y.
S. Hwang
26
Rates of Cr2O3 Dissolution in Na2SO4 at 1200K Y.
S. Hwang
27
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

28
Schematic Illustration of the Role of Chromium in
Inhibiting Hot Corrosion Attackat some Grain
Boundaries or Other Defects
29
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.

30
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)

31
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.

32
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
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