Title: Hot Corrosion of Materials
1Hot Corrosion of Materials
- Robert A. Rapp
- Materials Science Engineering Dept
- The Ohio State University
- Olin Palladium Award Address
- The Electrochemical Society
- October 19, 2005
2Carl Wagner
1951 Pd Medal Award
3Marcel Pourbaix
4Introduction
- 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
5Progress 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.
6Na-S-O Phase Stability Diagram at 1173K
7EC Reference Electrodes to Measure Sodium and
Oxygen Activities and thereby Melt Basicity
8Ni-Na-S-O Phase Stability Diagram at 1200 K
9Solubility of NiO in Fused Na2SO4 at 1200 K
PO2 1 atm, Gupta
Basic dissolution
Acidic dissolution
10Na-Cr-S-O Phase Stability Diagram at 1300K
11Experimentally Measured Solubilities for Cr2O3 in
Fused Na2SO4 and 1 atm Oxygen Y. Zhang
12Solubilities of Oxides in Na2SO4 at 1200K
13Reprecipitation of Porous MO Oxide Supported by
Negative Solubility Gradient in Fused Salt Film
14Cases of Sustained Hot Corrosion of a Pure Metal
(I is Oxide/Salt Interface, and II is Salt/gas
Interface)
15Trace of Basicity and Oxygen Activity Measured
upon Polarization of a Pt Working Electrode C.
O. Park
16Experimental Arrangement to Measure Activities at
Oxide/Salt Interface during Hot Corrosion of Ni
at 1200K N. Otsuka
17Trace 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.
18Effect on Preformed Oxide Thickness on Hot
Corrosion of Ni at 1200K
19Effect of Ni Purity on Hot Corrosion at 1200K
20Effect of Melt Basicity on Hot Corrosion of
Nickel at 1200K
21Effect of Inhibiting Addition to Salt on Hot
Corrosion of Nickel at 1200K
22Basic 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
23Synergistic 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
24Experimental Arrangement to Measure Kinetics of
Synergistic Dissolution of Fe2O3 and Cr2O3 at
1200K Y. S. Hwang
25Rates of Fe2O3 Dissolution in Na2SO4 at 1200K Y.
S. Hwang
26Rates of Cr2O3 Dissolution in Na2SO4 at 1200K Y.
S. Hwang
27Protective 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
28Schematic Illustration of the Role of Chromium in
Inhibiting Hot Corrosion Attackat some Grain
Boundaries or Other Defects
29Protective 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.
30Other 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)
31Other 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.
32Acknowledgements
- 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