High Temperature Oxidation and Electrochemical Investigations on Nickel-base Alloys

182 Pages·2011·13.5 MB·English

High Temperature Oxidation and Electrochemical
Investigations on Nickel-base Alloys
(Hoch-Temperatur Oxidation und elektrochemische
Untersuchungen an Nickel-Basis Legierungen)

Der Technischen Fakultät der
Universität Erlangen-Nürnberg
zur Erlangung des Grades

DOKTOR-INGENIEUR

vorgelegt von
Frau Dipl.-Ing. Georgia Obigodi-Ndjeng

Erlangen - 2011
Als Dissertation genehmigt von
Der Technischen Fakultät der
Universität Erlangen-Nürnberg

Tag der Einreichung: 10.03.2011
Tag der Promotion: 31.05.2011
Dekan: Prof. Dr.-Ing. R. German
Berichterstatter: Prof. Dr. sec. Techn. S. Virtanen, Prof. Dr.-Ing. U. Glatzel
Abstract
This study examined high-temperature oxidation behavior of different Ni-base alloys.
In addition, electrochemical characterization of the alloy’s corrosion behavior was
carried out, including comparison of the properties of native passive films grown at
room temperature and high temperature oxide scales.
PWA 1483 (single-crystalline Ni-base superalloy) and model alloys Ni-Cr-X (where X
is either Co or Al) were oxidized at 800 and 900 °C in air for different time periods.
The superalloy showed the best oxidation behavior at both temperatures, which
might be due to the fact that the oxidation growth function is subparabolic for the
model alloys and parabolic for the superalloy at 800 °C. At higher temperatures,
changes in the kinetics are induced, as the oxides grow faster, thus only PWA 1483
growth follows the parabolic law. Different scales in a typical sandwich form were
detected, with the inner layer comprised of mostly Cr O , the middle layer was
2 3
mixture of different oxides and spinels, depending on the alloying elements, and the
oxide at the interface oxygen/oxide was found to be NiO. The influence of sample
preparation could also be shown, as rougher surfaces change the oxidation kinetics
from parabolic and subparabolic for polished samples to linear.
The influence of moisture on the oxidation behavior of the 2nd generation single
crystal Ni-base superalloys (PWA 1484, PWA 1487, CMSX 4, René N5 and
René N5+) was studied at 1000 °C after 100 h oxidation period. It was found that the
moisture increased the oxidation rate and mostly the transient oxides growth rate.
The water vapor content in air also influenced the behavior of these alloys, as they
showed a higher mass gain in air + 30% water vapor than in air + 10% water vapor.
The alloys PWA 1484 and CMSX 4 showed respectively the worst and best behavior
in all the studied atmospheres. The addition of reactive elements, such as Yttrium,
Hafnium and Lanthanum is likely to enhance the oxidation behavior of PWA 1487
and CMSX 4, but does not show any influence on René N5+.
Furthermore, the oxidation resistance of the newly developed ASTRA alloys (ASTRA
00, ASTRA 02, ASTRA 20 and ASTRA 22) was studied at 950 and 1050 °C. The
addition of Ruthenium in the alloy ASTRA 02 increased the mass change, whereas
the addition Rhenium in ASTRA 20 showed a better oxidation resistance compared
to ASTRA 00. The alloy containing both Re and Ru, ASTRA 22, shows poor oxidation
resistance at 950 °C, whereas at 1050 °C, the scale s formed on all alloys show
cracks and spalls during oxidation and presented a severe spalling after cooling.
Those alloys therefore present a poor adhesion of the oxides mostly due to the
absence of active elements such as Yttrium, Hafnium, Lanthanum, etc. in the alloys.
A thin alumina layer was formed at the metal/oxide interface − a middle of which is
composed of different spinels – that could be detected and the top layer is NiO with a
columnar structure.
Electrochemical studies were performed on PWA 1483 and the model alloys Ni-Cr-X
and Ni-Cr-X-Y (X = Co or Al and Y is Ta) in different electrolytes. The Ni-base
superalloy showed good corrosion resistance in borate buffer (pH 8.4) and against
pitting. The corrosion behavior depends strongly on the alloying elements as, for
example, the alloy Ni-Cr-Al-Ta shows good corrosion behavior in all the electrolytes.
The XPS and AES analysis on the formed passive films showed the presence of
different oxides and hydroxides (chromia, NiO, NiOOH, and Ni(OH) ). The scales
2
were formed in a structure comparable to the oxides formed at high temperature.
High temperature oxides formed at 800 °C after 4 and 100 hours were also
investigated by using electrochemical analysis. The scales show very good corrosion
resistance as they show high impedances (R ~ 1 GΩ.cm2) and more anodic OCP
p
values. The presence of different oxides and defects such as pores could also be
proved by using this method.
Zusammenfassung
Hoch-Temperatur Oxidationsverhalten verschiedener Ni-Basis Legierungen wurde
untersucht. Darüber hinaus wurde das Korrosionsverhalten der Legierungen
elektrochemisch charakterisiert. Einschliesslich wurden auch die Eigenschaften der
bei Raumtemperatur gewachsenen Passivfilme mit den Hoch-temperatur Oxiden
verglichen.
Die einkristalline Ni-Basis Superlegierung PWA 1483 und Ni-Cr-X Modell-
Legierungen (wobei X =Co, Al) wurden bei 800 und 900 °C für unterschidiedliche
Zeiten an Luft oxidiert. Die Ni-Basis Superlegierung zeigt das beste
Oxidationsverhalten bei beiden Temperaturen. Die Modellegierungen zeigen ein
subparabolisches Oxidationswachstumsgesetz und die Superlegierung ein
parabolisches bei 800 °C. Veränderungen in der Kinetik durch die erhöhte
Temperatur wurden beobachtet, da die Oxide schneller wachsen, und einzig das
Oxidationsgesetz von PWA 1483 fast parabolisch ist. Es konnte auch gezeigt
werden, dass die Probenpräparation bezüglich der Oxidationskinetik eine Rolle spielt,
da die rauhen (geschliffenen) Oberflächen die Kinetik des Oxidationswachstums
ändern (von parabolisch oder subparabolisch für die polierten Proben zu linear für
die geschliffenen). Verschiedene Zunderschichten in der typischen “sandwich”
Morphologie wurden detektiert. Die innere Schicht ist meistens Cr O , die mittlere
2 3
eine aus verschiedenen Oxiden bestehende Schicht und Spinelle, die sehr von den
Legierungselementen abhängen, und die Schicht an der Phasengrenze
Sauerstoff/Oxid besteht aus NiO.
Der Feuchtigkeitseinfluss auf das Oxidationsverhalten von der zweiten Generation
einkristalliner Ni-Basis Superlegierungen (PWA 1484, PWA 1487, CMSX 4, René N5
and René N5+) wurde bei 1000 °C für 100 Stunden untersucht. Es wurde gezeigt,
dass die Feuchtigkeit die Oxidationsrate, insbesondere die Wachstumsrate der
transienten Oxide erhöht. Der Wasserdampfgehalt in der Luft beeinflusst stark das
Verhalten der analysierten Legierungen, da sie eine höhere Massenzunahme in Luft
+ 30% Wasserdampf als in Luft + 10% Wasserdampf aufweisen. Die Legierungen
PWA 1484 und CMSX 4 haben jeweils das schlechteste und beste Verhalten in allen
Medien. Die Beigabe (Zulegierung) von reaktiven Elementen wie Yttrium, Hafnium
und Lanthanum verbessert das Oxidationsverhalten von PWA 1487 und CMSX4,
jedoch aber zeigt keinen Einfluss auf René N5+.
Der Oxidationswiderstand der neu entwickelten ASTRA Legierungen (ASTRA 00,
ASTRA 02, ASTRA 20, ASTRA 22) wurden bei 950 und 1050 °C untersucht. Die
Zulegierung von Ruthenium in ASTRA 02 erhöht die Massenänderung, während
Rhenium Beigabe in ASTRA 20 einen verbesserten Oxidationswiderstand im
Vergleich zu ASTRA 00 zeigt. Die Superlegierung ASTRA 22 (legiert mit Re und Ru)
zeigt einen geringen Oxidationswiderstand bei 950 °C. Die bei 1050 °C
ausgebildeten Schichten platzen während der Oxidation auf, und blättern stark
während des Abkühlens ab. Diese Legierungen zeigen eine schwache Haftung der
Oxide, vorwiegend wegen fehlender aktiver Elemente wie z.B. Yttrium, Hafinium,
Lanthanum, etc… (in den Legierungen). Eine sehr dünne Aluminiumoxid Schicht
bildet sich an der Phasengrenze Metall/Oxid auf. Die mittelere Schicht besteht aus
diversen Spinellen und die obere Schicht aus NiO (mit einer kolumnaren
Mikrostruktur).
Elektrochemische Untersuchungen wurden auf PWA 1483 und Ni-Cr-X Modell-
Legierungen (X = Co, Al or Al and Ta) in verschiedenen Elektrolyten durchgeführt.
Die Ni-Basis Superlegierung zeigt einen guten Korrosionswiderstand im Borat Puffer
(pH 8,4) und gegen Lochkorrosion. Das Korrosionsverhalten hängt stark von den
Legierungselementen ab, z.B. Ni-Cr-Al-Ta zeigt einen guten Korrosionswiderstand in
allen studierten Medien. Die XPS und AES Analyse der Passivschichten weisen auf
verschiedene Oxide und Hydroxide (Chromia, NiO, NiOOH, Ni(OH) ) hin. Die
2
Passivschichten bestehen aus einer vergleichbaren Struktur wie die
Hochtemperaturoxide. Die bei 800 °C für 4 und 100 Stunden oxidierten Proben
wurden auch elektrochemisch untersucht. Die Schichten zeigen einen guten
Korrosionswiderstand, da sie sehr hohe Impedanzen (R ~ 1 GΩ.cm2) aufweisen und
p
sich die Ruhepotentialwerte im anodischen Bereich befinden. Die Anwesenheit von
verschiedenen Oxiden sowie Defekte wie z.B Poren konnte auch durch diese
Methode nachgewiesen werden.
Table of content

1 INTRODUCTION.................................................................................................1
2 FUNDAMENTALS...............................................................................................3
2.1 PRINCIPLES OF METAL OXIDATION....................................................................3
2.2 OXIDATION OF ALLOYS....................................................................................8
2.3 NI-BASE SUPERALLOYS..................................................................................12
2.4 CHARACTERISTICS OF PROTECTIVE OXIDES: CHROMIA AND ALUMINA................15
2.5 EFFECT OF WATER VAPOR ON OXIDATION........................................................18
2.5.1 Oxidation kinetics and mechanisms.........................................................19
2.5.2 Influence of Protons (hydrogen ions) incorporation.................................22
2.5.3 Volatility of metal hydroxide.....................................................................23
2.5.4 Effect of water vapor on the plasticity of oxide scales..............................24
2.6 RESEARCH OBJECTIVES: OXIDATION...............................................................25
2.7 PASSIVITY AND ANODIC OXIDES......................................................................26
2.7.1 Thermodynamics of aqueous corrosion...................................................27
2.7.2 Kinetics of aqueous corrosion..................................................................29
2.7.3 Passivity ..................................................................................................34
2.7.4 Electronic properties of oxide films on metals..........................................36
2.7.5 Passivity breakdown................................................................................38
2.7.6 Passivity and localized corrosion of Ni-base alloys..................................40
3 EXPERIMENTAL WORK...................................................................................41
3.1 MATERIALS...................................................................................................41
3.1.1 Nominal composition...............................................................................41
3.1.2 Microstructural characterization...............................................................43
3.2 OXIDATION EXPERIMENTS...............................................................................43
3.2.1 Isothermal oxidation.................................................................................43
3.2.2 Preparation of cross-sections..................................................................46
3.2.3 Characterization methods........................................................................46
3.3 ELECTROCHEMICAL INVESTIGATIONS...............................................................47
3.3.1 Experiments.............................................................................................47
3.3.2 Characterization methods........................................................................49
4 RESULTS...........................................................................................................50
4.1 MICROSTRUCTURE OF THE STUDIED ALLOYS....................................................50
Table of content
4.2 OXIDATION OF THE SINGLE CRYSTAL NI-BASE SUPERALLOY PWA 1483 AND NI-
CR-X MODEL ALLOYS AT 800 AND 900 °C..................................................................53
4.2.1 Oxidation at 800 °C..................................................................................53
4.2.2 Oxidation at 900 °C..................................................................................64
4.2.3 Summary: Oxidation of PWA 1483 and model alloys at 800 and 900 °C.75
4.3 OXIDATION OF 2ND GENERATION NI-BASE SUPERALLOYS AT 1000 °C.................76
4.3.1 Oxidation in air.........................................................................................76
4.3.2 Oxidation in air and moisture...................................................................80
4.3.3 Summary: Influence of moisture on the oxidation behavior of 2nd
generation single crystal Ni-base superalloys....................................................88
4.4 OXIDATION OF POLYCRYSTALLINE NI-BASE SUPERALLOYS ASTRA...................92
4.4.1 Oxidation at 950 °C..................................................................................92
4.4.2 Oxidation at 1050 °C................................................................................96
4.4.3 Summary: Oxidation of ASTRA alloys.....................................................98
4.5 ELECTROCHEMICAL INVESTIGATIONS ON NI-BASE ALLOYS................................99
4.5.1 Passivity of Ni-base alloys and electronic properties of films formed in
borate buffer (pH 8.4).........................................................................................99
4.5.2 Passivity of Ni-base alloys and electronic properties of films formed in
sulfuric acid......................................................................................................116
4.5.3 Effect of chloride ions on the corrosion behavior of Ni-base alloys........121
4.5.4 Electrochemical behavior of high temperature oxides...........................125
5 DISCUSSION...................................................................................................131
5.1 OXIDATION OF PWA 1483 AND NI-CR-X MODEL ALLOYS................................131
5.1.1 Effect of alloying elements.....................................................................131
5.1.2 Effect of microstructure and surface preparation...................................134
5.1.3 Effect of temperature.............................................................................136
5.2 OXIDATION OF THE 2ND GENERATION SINGLE CRYSTAL NI-BASE SUPERALLOYS.136
5.2.1 Effect of moisture...................................................................................136
5.2.2 Effect of alloying elements.....................................................................140
5.3 OXIDATION OF ASTRA ALLOYS....................................................................144
5.3.1 Influence of temperature........................................................................144
5.3.2 Effect of microstructure..........................................................................145
5.3.3 Effect of Rhenium and Ruthenium.........................................................146
Table of content
5.4 ELECTROCHEMICAL BEHAVIOR OF NI-BASE ALLOYS AND PWA 1483 AT ROOM AND
HIGH TEMPERATURES..............................................................................................146
5.4.1 Passivity and localized corrosion...........................................................146
5.4.2 Characterization of high temperature oxides with electrochemistry.......149
6 CONCLUSIONS...............................................................................................151
7 OUTLOOK (FUTURE WORK).........................................................................153
8 BIBLIOGRAPHY..............................................................................................154
Chapter 1: Introduction

1 Introduction
The lifetime of materials exposed to high temperatures is significantly decreased by
high temperature oxidation, as it affects the mechanical properties of these materials.
Ni-base superalloys are frequently used at high temperatures, due to their excellent
mechanical properties and oxidation resistance. Depending on the temperatures
required, different alloy classes are used. As high temperature oxidation takes place
in an oxidizing environment, Ni-base superalloy is a suitable choice of material, due
to its capability to resist this type of corrosion, which stems from the formation of a
slow-growing, homogeneous, adherent and with low defect concentration oxide film.
The oxides with such properties are mostly Cr O , Al O and SiO . The use of
2 3 2 3 2
chromia is limited by its evaporation (formation of the gas CrO ) at 1000 °C. At
3
temperatures above 1000 °C, α-alumina is the suitable oxide due the very slow
growth rate, and more importantly, the inherently slower transport of species through
the oxide layer than found in chromia. Silica, on the other hand, is used at much
higher temperatures.
The alloys are normally coated for service, but it is also important to have a substrate
material with good oxidation resistance. As the Ni-base superalloys are very complex
materials, comprised of more than eight alloying elements, their high temperature
oxidation is also complex, with each element influencing the oxidation behavior. The
oxidation behavior depends on the temperature, alloy composition, the properties of
oxidizing gas (flow rate and velocity) and the environmental thermal fluctuations,
which are generally not well defined. Thus, it is of paramount importance to
understand the oxidation behavior of these alloys, as well as to determine the
protective factors governing the oxidation resistance of superalloys and to
understand the role of the alloying elements.
The atmosphere in which the materials are used is generally not dry, as there is
always a certain amount of moisture in air, with the highest content of 100 % reached
in steam generators. The water vapor is known to affect the oxidation behavior of
metals by increasing their oxidation rate. Therefore, the presence of water vapor may
influence the oxidation behavior of Ni-base superalloys at high temperature. It has
been shown that steels oxidize faster in air or combustion gases containing water
vapor [1], particularly in the case of low Cr-containing steels [2]. The presence of
water vapor as reported in the literature to cause the cracking and spalling of oxides
1