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HIGH TEMPERATURE CORROSION OF STEELS USED
IN PETROLEUM REFINERY HEATERS
A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES
OF MIDDLE EAST TECHNICAL UNIVERSITY
BY
ABDELRAHMAN SULTAN
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF MASTER OF SCIENCE IN
METALLURGICAL AND MATERIALS ENGINEERING
JULY 2005
Approval of the Graduate School of Natural and Applied Sciences
_____________________ Prof. Dr. Canan Özgen
Director I certify that this thesis satisfies all the requirements as a thesis for the degree of Master of Science.
_____________________ Prof. Dr. Tayfur Öztürk Head of Department This is to certify that we have read this thesis and that in our opinion it is fully adequate, in scope and quality, as a thesis for the degree of Master of Science.
___________________________ Prof. Dr. İshak Karakaya
Supervisor Examining Committee Members Prof. Dr. M. Timuçin (METU, METE ) _____________________ Prof. Dr. İ. Karakaya (METU, METE ) _____________________ Prof. Dr. Y. Topkaya (METU, METE ) _____________________ Assoc. Prof. Dr. K. Aydınol (METU, METE ) _____________________ Asst. Prof. Dr. Mustafa Übeyli (TOBB, ETU.) _____________________
iii
I hereby declare that all information in this document has been obtained and
presented in accordance with academic rules and ethical conduct. I also declare that,
as required by these rules and conduct, I have fully cited and referenced all material
and results that are not original to this work.
Name, Last name: Abdelrahman Sultan
Signature:
iv
ABSTRACT
HIGH TEMPERATURE CORROSION OF STEELS USED IN PETROLEUM
REFINERY HEATERS
Sultan, Abdelrahman
M.S., Department of Metallurgical and Materials Engineering
Supervisor: Prof. Dr. İshak Karakaya
July 2005, 60 pages
The oxidation of three different steels used in the construction of petroleum refinery
heaters was investigated by using thermogravimetric analysis technique (TGA). C-5,
P-11, and P-22 steel samples were tested in two different oxidizing environments; air
and CO2+N2+H2O (that simulates the combustion products of natural gas) at two
different temperatures; 450oC and 500oC. In air oxidation P-22 had the best oxidation
resistance among the three steels at two temperatures. In CO2+N2+H2O environment,
C-5 possessed better oxidation resistance than P-22 and P-11. Analyses of oxidation
products by using optical microscopy, X-ray diffraction (XRD) and scanning
electron microscopy (SEM) were carried out to correlate TGA results to oxide
composition and morphology. Lower oxidation rate of P-22 in air was explained with
reference to the formation of Cr-O phase. Analytical rate equations showed that all
the steels obeyed parabolic rate equation during oxidation and no transition was
observed.
Keywords: High temperature, corrosion, petroleum refinery heaters,
thermogravimetric analysis, oxidation, kinetics
v
ÖZ
PETROL RAFİNERİ ISITICILARINDA KULLANILAN ÇELİKLERİN
YÜKSEK SICAKLIK KOROZYONU
Sultan, Abdelrahman
M.S., Metalurji ve Malzeme Mühendisliği Bölümü
Danışman: Prof. Dr. İshak Karakaya
Temmuz 2005, 60 Sayfa
Termogravimetrik analiz (TGA) yöntemi kullanılarak, petrol rafinerileri ısıtıcılarının
yapımında kullanılan üç değişik çeliğin oksitlenmesi incelenmiştir. C-5, P-11, ve P-
22 çelik örnekler iki değişik oksitleyici ortamda; hava ve CO2+N2+H2O karışımı
(doğal gaz yanma ürünlerine benzer) ve iki değişik sıcaklıkta; 450oC and 500oC test
edildi. Havada oksitlenmede P-22 üç çelik arasında iki sıcaklıkta en yüksek
oksitlenme direnci gösterdi. CO2+N2+H2O ortamında C-5, P-22 ve P-11’den daha iyi
oksitlenme direnci gösterdi. TGA sonuçlarını oksit kompozisyonu ve morfolojisi ile
ilişkilendirebilmek için, oksitlenme ürünleri optik mikroskop, X-ışını kırınımı (XRD)
ve tarama elektron mikroskobu kullanılarak analiz edildi. P-22 çeliğinin havada
düşük oksitlenme hızı, Cr-O fazının oluşumuna bağlanarak açıklandı. Analitik hız
denklemleri, bütün çeliklerin oksitlenme sırasında parabolik hız denklemine
uyduklarını ve hiçbir geçiş olmadığını gösterdi.
Anahtar kelimeler: Yüksek sıcaklık, korozyon, petrol rafineri ısıtıcısı,
termogravimetrik analiz, oksitlenme, kinetik
vi
To my parents,
who always support me in all aspects of my life
to my wife
for her patience in my study
to my children
Taha, Ala, Saleh, Mohammed
vii
ACKNOWLEDGEMENTS
Firstly, and mostly, I thank the almighty ALLAH for his mercy and grace, which
enabled me to complete this work.
Secondly, I would like to express my sincerest thanks to Prof. Dr. İshak
KARAKAYA for his guidance, support and valuable contributions throughout the
preparations for this thesis.
I express my deepest gratitude to my parents, my mother Saeida and my father Saleh
for their encouragements throughout my education life, and to my wife and children
for their patience during my study.
The Libyan secretariat of higher education is highly appreciated for its financial
support during my study period.
viii
TABLE OF CONTENTS
PLAGIARISM……………………………………………………………..………. iii
ABSTRACT ………………………………………………………….…………...iv
ÖZ………………………………….……………………………….……….…..……...…..v
ACKNOWLEDGEMENTS ……………..……………………………………….vii
TABLE OF CONTENTS…………………………...……………...……………...viii
LIST OF FIGURES……………………………………………………………….....x
LIST OF TABLES.................................................................................................... xii
CHAPTER
1. INTRODUCTION..........................................................................................1
2. OXIDATION..................................................................................................5
2.1. Introduction...............................................................................................5
2.2. Oxidation Thermodynamics............................................................... ......6
2.3. Oxide Properties and Oxidation......................................................... ......8
2.4. Oxidation of Iron and Fe-C Alloys .................................................... ......9
2.4.1. Oxidation of Pure Iron ................................................................. ......9
2.4.2. Oxidation of Fe-C Alloys............................................................. ....12
2.4.2.1. Effect of Alloying elements on Oxidation of Iron
and Fe-C steels……………………………………………….. 12
2.4.2.2. Effect of Atmosphere on the Oxidation of Iron
and Fe-C steels ………………...……………………………..14
2.4.2.3. Other factors can affect the Oxidation of Iron
and Fe-C alloys ……………………………...……………….17
2.5. Oxidation kinetics……………………………...………………………18
2.6. High Temperature Oxidation Testing………………………………….21
2.6.1. Gravimetric Method ......................................................................21
3. EXPERIMENTAL ........................................................................................24
3.1. Introduction ............................................................................................24
3.2. Material ..................................................................................................24
ix
3.3. Sample preparation ................................................................................25
3.4. Thermogravimetric Analysis (TGA) .....................................................26
3.5. Identification of Oxidation Products ......................................................29
4. RESULTS ........................................................................................................31
4.1. Introduction ............................................................................................31
4.2. Oxidation in air …....................................................................................31
4.3. Oxidation in CO2+N2+H2O ....................................................................33
4.4. Effect of oxidizing atmosphere ..............................................................35
4.5. Analysis of Oxidation Products .............................................................40
5. TREATMENT OF DATA AND DISCUSSION .........................................48
5.1. Introduction ............................................................................................48
5.2. Kinetics of Oxidation .............................................................................48
5.3. Thermodynamic Consideration ..............................................................53
5.4. Microscopic and Other Consideration ..........................................….....54
6. CONCLUSIONS ...........................................................................................55
REFERENCES ..................................................................................................57
APPENDIX A ..............................................................................................…..61
x
LIST OF FIGURES
FIGURES
1. Schematic cross section of a horizontal furnace used at Kırıkkale petroleum
refinery plant…………………….………………………………………………3
2.1. Iron-oxygen phase diagram at 1 atm ..................................................................10
2.2. Mechanism of the oxidation of iron in atmosphers containing H2O and CO2
as suggested by Rahmel and Tobolski……………………………………........16
2.3. Oxidation film growth curves for linear, parabolic, and logarithmic
rate equation……………………………………………………………………20
2.4. Schematic of experimental arrangement for use with an automatic
recording balance............................................................................…………...22
3.1. Schematic draw of the samples sets……………………………………………25
3.2. Schematic representation of setup for air oxidation test ....................................28
4.1. Oxidation of the three steels in air at 450oC .....................................................32
4.2. Oxidation of the three steels in air at 500oC .....................................................33
4.3. Oxidation of the three steels in CO2+N2+H2O at 450oC……………………….34
4.4. Oxidation of the three steels in CO2+N2+H2O at 500oC……………………….35
4.5. Oxidation of C-5 steel in air and CO2+N2+H2O at 450oC……………………..36
4.6. Oxidation of C-5 steel in air and CO2+N2+H2O at 500oC……………………..36
4.7. Oxidation of P-22 steel in air and CO2+N2+H2O at 450oC…………………….37
4.8. Oxidation of P-22 steel in air and CO2+N2+H2O at 500oC…………………….38
4.9. Oxidation of P-11 steel in air and CO2+N2+H2O at 450oC…………………….39
4.10. Oxidation of P-11 steel in air and CO2+N2+H2O at 500oC…………………...39
4.11. X-ray of P-11 oxidized in CO2+N2+H2O at 450oC …………………..………..41
4.12. X-ray of C-5 oxidized in air at 450oC ………………………..………………41
4.13. X-ray of P-22 oxidized in air at 500oC …..……………….…………………...42
4.14. X-ray of P-22 oxidized in CO2+N2+H2O at 500oC …………………………..42
4.15. Optical Photograph Of C-5 steel oxidized at 500oC
xi
in CO2+N2+H2O ……………………………………………………………..43
4.16. Optical Photograph Of P-22 steel oxidized at 500oC
in CO2+N2+H2O ……………………………………………………………..44
4.17. Optical Photograph Of P-11 steel oxidized at 500oC
in CO2+N2+H2O ……………………………………………………………..44
4.18. Optical Photograph Of C-5 steel oxidized at 450oC in air……………………45
4.19. SEM micrograph of C-5 steel oxidized in CO2+N2+H2O at 500oC…………..45
4.20. SEM micrograph of P-22 steel oxidized in air at 500oC…………………..….46
4.21. SEM micrograph of P-11 steel oxidized in CO2+N2+H2O at 500oC ..……… 46
4.22. SEM micrograph of P-22 steel oxidized in CO2+N2+H2O at 500oC ……..… 47
4.23. SEM micrograph of P-22 steel oxidized in CO2+N2+H2O at 450oC ………...47
5.1. First 3 hours oxidation data for P-11 steel oxidized
in air at 500oC………………………………………………………………….49
5.2. First 3 hours oxidation data for P-22 steel oxidized
in CO2+N2+H2O at 500oC……………………………..……………………….49
5.3. First 3 hours oxidation data for C-5 steel oxidized
in CO2+N2+H2O at 500oC……………………………………………………...50
5.4. Plot of log (y) vs. log (t) for all steels oxidized in air at 450oC ……………….51
5.5. Plot of log (y) vs. log (t) for all steels oxidized
in CO2+N2+H2O at 450oC……………………………………………………...51
xii
LIST OF TABLES
TABLE
3.1. Chemical Composition of Steels………………………………..……………...24
3.2. Gas flow rates used in CO2+N2+H2O environment at 450oC and 500oC ……..27
4.1. Weight gain after 35-hour oxidation period in air and CO2+N2+H2O
gas mixture at the two temperatures………………………………….………..40
5.1. Numerical values of k and n at 450oC................................................................52
5.2. Numerical values of k and n at 500oC................................................................52
A.1. Fe2O3 fractions for the oxidized steels ……………………………..…..……..61
A.2. Results of penteration depth calculation for air oxidation ................................62
A.3. Results of penteration depth calculation for CO2+N2+H2O oxidation .............63
1
CHAPTER 1
INTRODUCTION
Corrosion is defined as the destruction or deterioration of a material because of
reaction with its environment. It has been classified in many different ways: One
method divides corrosion to low-temperature and high-temperature corrosion.
Another separates corrosion into direct combination (oxidation) and electrochemical
corrosion. And some classifies corrosion to wet corrosion and dry corrosion. Wet
corrosion occurs when a liquid is present. This usually involves aqueous solutions or
electrolytes and accounts for the largest amount of corrosion by far. A common
example is corrosion of steel by water. Dry corrosion is most often associated with
high temperatures. An example is attack on steel by furnace gases.
High temperature corrosion plays an important role in the selection of construction
materials of industrial equipments. The corrosion forms that can be considered as
high temperature corrosion are: oxidation, sulfidation, halogen corrosion,
carburization, metal dusting, etc. Industries like chemical processing, refining and
petrochemical industries, automotive, ceramic, pulp and paper, fossil fuel power
generation, coal gasification, etc are faced with high temperature corrosion.
Environments are frequently classified in terms of oxygen activity, as either
“oxidizing or reducing’’. An oxidizing atmosphere is an environment that contains
molecular oxygen (O2), such as air or a combustion atmosphere with excess “free’’
oxygen. Oxygen activity in this case is very high and is controlled by the
concentration of molecular oxygen. A reducing atmosphere is generally produced by
combustion under stoichiomentric or substoichiomentric conditions (combustion
products are generally comprised of CO2, CO, H2O, H2, and products of impurities
coming from fuel and/or feedstock, such as H2S) with no excess oxygen. The
2
oxygen activity is very low in this case and is controlled by CO/CO2 or H2/H2O. The
reducing environment is generally more corrosive for many corrosion modes, such
as sulfidation, carburization, nitridation, and ash/salt deposit corrosion.
Oxidation is the most important high temperature corrosion reaction. In most
industrial environments, oxidation often participates in the high temperature
corrosion reaction, regardless of the predominant mode of corrosion. In fact, alloys
often relay upon the oxidation reaction to develop a protective oxide scale to resist
corrosion attack such as sulfidation, carburization, and ash/salt deposit corrosion.
During service in high temperature plants, tubing, piping, and other steel
components are exposed to corrosive environments; as a result, their service lifetime
may be limited by creep, fatigue or oxidation. Traditionally materials designed for
use at high temperature have been developed primarily for their mechanical
properties, but there is now a growing realization that oxidation may limit lifetime,
either directly through metal wastage or indirectly through raising local temperatures
(and consequently reducing creep-controlled lifetimes) due to the lower thermal
conductivity of the oxide scale.
The petroleum refining industry converts crude oil into more than 2500 refined
products, including liquefied petroleum gas, gasoline, kerosene, aviation fuel, diesel
fuel, fuel oils, lubricating oils, and feedstocks for the petrochemical industry. The
petroleum refining industry employs a wide variety of processes. The arrangement
of these processes will vary among refineries according to the composition of the
crude oil feedstock and the chosen state of petroleum products.
Heaters (furnaces) are used extensively in refineries to supply the heat necessary to
raise the temperature of feed materials to reaction or distillation level. They are
designed to raise petroleum fluid temperatures to a maximum of about 510°C
(950°F). The fuel burned may be refinery gas, natural gas, residual fuel oils, or
combinations, depending on economics, operating conditions, and emission
requirements. Heaters may be vertical or horizontal furnaces. Figure 1 shows
3
schematic cross section of a horizontal furnace used at Kırıkkale petroleum refinery
plant.
Figure 1 Schematic cross section of a horizontal furnace used at Kırıkkale
petroleum refinery plant.
In this study the oxidation of three different steels (C-5, P-11 and P-22) used in the
construction of Kırıkkale petroleum refinery heaters was investigated by using
thermogravimetric analysis technique. Two different environments; air and gas
mixture that simulate the combustion product of natural gas, and two different
temperatures (450oC and 500oC) were employed. Experiments involved continuous
4
recording of the weight change of the oxidized specimen as a function of time. By
using X-ray analysis, optical and scanning electron microscopy (SEM) the oxidation
products were studied to check and support the findings of thermogravimetric
analysis results.
5
CHAPTER 2
OXIDATION
2. 1 Introduction
With the exception of gold, no pure metal (including platinum) is stable in air at
room temperature. When a metal is exposed at room temperature or elevated
temperatures to an oxidizing gas (e.g. oxygen, sulfur or halogens), corrosion may
occur in absence of a liquid electrolyte. This sometimes called ‘’dry’’ corrosion
where a solid reaction-product film or scale (a scale is a thick film) forms on the
metal surfaces. The exposure of a metal to gaseous oxygen results in the formation
of an oxide as:
baOMObaM →+ 22 (2.1)
When a clean metal surface is exposed to oxygen, it is oxidized following the
sequence; (1) adsorption of oxygen on the metal surface, (2) formation of oxide
nuclei and (3) growth of a continuous oxide film. Oxide film frequently constitutes
protective layers, which separate the metal from the gaseous oxygen, thereby,
inhibiting further oxide formation. If the oxide film covering the metal surface is
tightly coherent in the sense that the film is free of cracks and macroscopic pores,
then additional chemical reaction usually requires diffusion of metal or oxygen
through the oxide layer. The rate of chemical reaction in such cases is time
dependent, being rapid in the early stages but decreasing markedly as the thickness
of the oxide layer increases.
6
2. 2 Oxidation Thermodynamics
Thermodynamically, an oxide is likely to form on a metal surface when the oxygen
potential in the environment is greater than the oxygen partial pressure in
equilibrium with the oxide. The equilibrium oxygen partial pressure can be
determined from the standard Gibbs energy of formation of the oxide. Consider the
reaction:
22 MOOM ⇔+ (2.2)
⎟⎟⎠
⎞⎜⎜⎝
⎛−=∆
2
2
.ln..
OM
MOo
Paa
TRG (2.3)
Assuming the activities of the metal and the oxide are unity, eq. (2.3) becomes:
2
ln.. Oo PTRG =∆ (2.4)
Then:
( )RTGO
o
eP ∆=2
(2.5)
When the environment is ‘‘reducing’’ (i.e., the environment contains no measurable
molecular oxygen, such as the one generated by stoichiometric or substoichiometric
combustion), the oxygen potential is controlled by OHH PP22
/ and/or 2
/ COCO PP . The
oxygen potential can be determined by the reaction:
OHOH 222 22 ⇔+ (2.6)
The standard Gibbs energy of formation is related to the partial pressures of
hydrogen, oxygen, and water vapor as:
7
⎟⎟⎠
⎞⎜⎜⎝
⎛−=∆
22
2
.ln.. 2
2
OH
OHo
PPP
TRG (2.7)
Rearranging the eq.2.7 results in:
( )
( )222
2
OHH
RTG
O PPeP
o∆
= (2.8)
Thus the oxygen partial pressures at various temperatures can be determined as a
function of OHH PP22
.
The equilibrium reaction for an environment whose oxygen potential is controlled
by2COCO PP is:
22 22 COOCO ⇔+ (2.9)
The corresponding oxygen potential is:
( )222 COCO
RToG
PPe
Op⎟⎠⎞⎜
⎝⎛ ∆
= (2.10)
The Gibbs energy change of the system as determined from 2OP of the environment
is the driving force of the reaction. When the initial pressure of oxygen corresponds
to the partial pressure of oxygen as represented in the equilibrium constant, there is
no driving force for the reaction, and the oxide and metal is then equally stable. If
the pressure is lowered below this value the oxide will dissociate. If several oxides
are formed on a metal, e.g. Fe2O3, Fe3O4, FeO, each one will dissociate at different
pressures and the oxide that is richer in oxygen will usually dissociate to an oxide
containing less oxygen not to a metal directly. Thermodynamically the oxide will be
formed only if the ambient oxygen pressure is larger than the equilibrium value.
8
2.3 Oxide Properties and Oxidation
Electron donation and acceptance occur in several types of chemical bonding during
oxidation. Metal oxides, sulfides, etc. exhibit ionic bonding. Every oxide has a
definite crystallographic structure in which anions and cations are distributed at
different specific sites. Oxides are composed of grains like metals. An oxide can
crystallize, exhibit grain growth and at high temperatures it may deform plastically.
Metal oxides are commonly in the class of semiconductors; this means their
conductivity lies between insulators and metallic conductors. Conductivity increases
with a slight shift from stoichiometric proportions of metal and oxygen and with an
increase of temperature. There are two types of semiconducting oxides; p-type and
n-type (p means positive carrier and n negative carrier). Cu2O, NiO, FeO, Cr2O3, and
Fe3O4 are examples for p-type oxides and this type shift of stoichiometric
proportions takes the form of a certain number of missing metal ions in the oxide
lattice called cation vacancies. At the same time, to maintain the electrical neutrality,
an equivalent number of positive holes form (sites where electrons are missing). For
n-type oxides, excess of metal ions exist in interstitial positions of the oxide lattice.
These metal ions migrate with the electrons during the oxidation to the outer oxide
surface. Examples of n-type oxides are: ZnO, CdO, TiO2, Al2O3, and Fe2O3.
The oxidation rate of an alloy will be minimized if the oxide film has a combination
of properties that include [1]:
1. The film should have good adherence, to prevent flaking and spalling.
2. The melting point of the oxide should be high.
3. The oxide should have low vapor pressure to resist evaporation.
4. The oxide film and metal should have very close thermal expansion
coefficients.
5. The film should have high temperature plasticity to accommodate differences
in specific volumes of oxide and parent metal and differences in thermal
expansion.
6. The film should have low electrical conductivity and low diffusion
coefficients for metal ions and oxygen.
9
2. 4 Oxidation of Iron and Iron alloys
In many industrial applications, the oxidation in atmospheres containing free
oxygen, carbon dioxide, water vapor and nitrogen takes place. The oxidation
behavior of steel in ambient air differs significantly from that in a mixed –gas
atmospheres [2]. Commercial steels usually contain impurity elements, in addition
to intentional alloying elements. Each of the elements present in the steel may
behave differently from iron. Generally the existence of chromium, aluminum, and
silicon, which are less noble than iron, provides a certain level of oxidation
resistance for steel, but the protective effect becomes insignificant if their levels are
very low [3]. Residual elements, such as copper, nickel, and tin those are more noble
than iron, are usually accumulated at the scale-substrate interface [4] and have little
effect on the steel oxidation behavior. High carbon steels may suffer from
decarburization during oxidation [5].
2.4.1 Oxidation of pure iron
Numerous studies have been conducted to examine the high-temperature oxidation
behavior of pure iron in air or oxygen [6 - 8]. The reactions between the iron and
oxygen are exothermic in nature and, as a result, an over temperature phenomenon is
present during the initial oxidation stage, because the heat generated by the rapid
initial reactions cannot be quickly conducted away when small samples are used [9,
10]. Despite the rapid initial reactions, the longer–term oxidation rate under
isothermal-oxidation conditions is quite steady and usually follows the parabolic rate
law.
As shown in Fig.2.1, oxidation of iron is complicated by formation of as many as
three distinct layers of iron oxides; Wüstite (FeO): very defective p-type
semiconductor which is only stable above 570oC, Magnetite (Fe3O4): a p-type
semiconductor oxide which has a much lower conductivity than wüstite. It has a
spinel structure and sometimes it is represented as FeOFe2O3, and Hematite (Fe2O3):
n-type oxide with anion defects has two structures: α, which has a rhombohedral
10
structure, and γ, which has a spinel structure similar to Fe3O4 with which it forms a
limited solid solution. The proportions of these layers change as the temperature or
oxygen partial pressure changes.
Figure 2.1 Iron-oxygen phase diagram at 1 atm [11]
Davies et al. [12] studied the oxidation of iron at temperature range of 700 to
1250oC and they found that the scale developed comprises an extremely thin
outermost hematite layer, a thin intermediate magnetite layer, and a thick inner
11
wüstite layer. This portion of oxide phases reflects the fact that the diffusion
coefficient of iron in wüstite is much greater than in magnetite and that the diffusion
of oxygen and iron through the hematite layer is extremely slow [13].
In the same study Davies et al., concluded that the higher oxides (Fe3O4 and Fe2O3)
did not form a considerable portion of the scale at temperatures above about 625oC
[12]. In another study, Paidassi et al. [14] examined the oxidation behavior of iron at
470 to 625oC in air and found that at 604oC, wüstite formed readily on the iron
surface, whereas, at 585oC, wüstite did not form for up to 24 hours. Cablan et al.
[15] found that at temperatures below 570oC, the wüstite is thermodynamically
unstable, and the scale is reported to comprise two layers of oxides, namely an outer
hematite layer and an inner magnetite layer. The wüstite formed at high temperature
is actually expressed as Fe1-xO, which implies an iron–deficient crystal structure.
The value of x increases with the distance from the scale/base metal interface.
Normally wüstite contains 5-16% of such defects. Below 570oC, it will decompose
as follows:
4Fe1-x O → Fe3O4 + (1-4x) α-Fe (2-11)
Chaudron et al. [16] reported that the optimum temperature to attain the maximum
decomposition rate is 470oC, while that reported by Fisher et al. [17] is 400oC. This
difference, of course comes from different oxidation and decomposition
atmosphere[11].
Goswami et al. [18] showed that a TEM analysis of oxide scale formed on thin films
of iron revealed that the oxidation of iron in air at 350 to 450oC proceeded in the
following stages:
Fe → Fe3O4 → α-Fe2O3 (2-12)
12
2.4.2 Oxidation of carbon steel and Fe-C alloys
Carbon is the most important alloying element in carbon steels and its level in steels
depends on the required mechanical properties [2]. The main effect of carbon on the
oxidation rates is to make them more erratic. Carbon diffuses to the scale/base metal
interface and reacts with iron oxide to evolve CO gas and develop a gap. In high
carbon steels at high temperatures, the gas pressure in the gaps cause gross cracking
so that the atmosphere gains access to the core and the oxidation rate is increased
[11]. For example, Boggs et al. [19] found that the oxidation rate of Fe-C alloy in 10
torr O2 at 500oC increased when its carbon content increased from 0.0 to 0.99 wt %.
Caplan et al. [20] also found that Fe-0.5 wt % C and Fe-1.0 wt % C alloys oxidized
faster than Fe-0.1 wt % C alloy in oxygen at 1 atm pressure at 500oC. On the other
hand, the oxidation rates of Fe-0.5 wt % C and Fe-1.0 wt % C alloys in 1 atm
oxygen at 700oC were slower than that of pure iron [21]. Malik et al. [22] also found
that Fe-C alloys with 0.1 to 1.2 wt % C oxidized slower than pure iron in 1 atm
oxygen at 600 to 850oC. Lower oxidation rates for Fe-C alloys were explained [21]
with reference to residual graphite left at the scale/base metal interface. Residual
graphite causes poor contact between the scale and the base metal, hinders the
transport of iron or oxygen ions, and hence reduces the oxidation rate.
2.4.2.1 Effect of alloying elements on oxidation of iron and iron alloy steels
The alloying elements are added to the steel not only for strengthening, but also for
modifying other properties including the oxidation behavior. Addition of aluminum
to iron reduces the oxidation rate through the formation of an aluminum-rich layer at
the scale/base metal interface and the retardation of iron ion diffusion. The exact
nature of such aluminum-rich layer appears to vary with aluminum content of the
steel, the temperature, and the oxidizing atmosphere. Saequsa et al. [23] studied the
effect of temperature on the oxidation of Fe-1wt % Al alloy in 1 atm O2. He found
that the aluminum-rich layer was probably Al2O3 after the oxidation at 500 to 700oC,
13
whereas, such layer was FeAl2O4 spinel after the oxidation in the same atmosphere
at 700 –900oC.
Lower oxidation rates of Fe-Si alloys arise from prefenctial formation of silicon-
rich layer at the scale/base metal interface due to its less noble behavior than iron.
Dilute Fe-Si alloys were subject to internal oxidation [24] and complex scales were
found. A SiO2–rich layer apparently formed with alloys containing 2-3 wt % Si or
more [25] and this transition from internal to external SiO2 formation might result in
a markedly reduced oxidation rate. The slower oxidation rate was probably due to
the lower diffusion rate of silicon through the oxide layers and the hindered iron ion
diffusion through the SiO2 layer.
The major protection action of chromium is due to the formation of chromium-
containing layer at the scale/base metal interface. Wood et al. [26] reported that the
additions of 0.16 to 0.2 wt % of Cr to iron increased the initial oxidation rate in
oxygen at 1000oC. This is probably because Cr3+ ions increased the number of cation
vacancies in the major phase FeO. However, the oxidation rate was reduced
subsequently due to the suppression of FeO formation by the presence of chromium,
as a result of formation of more protective Fe3O4 and Fe2O3. The oxidation rate was
reduced progressively by addition of chromium greater than 1.25 wt %. If the
chromium content in the steel is sufficiently high, the formation of wüstite is
prevented above 570oC [27]. Hammar et al. [28] found that additions of less than 2
wt % Cr suppressed the oxidation properties of iron in oxygen at 625 and 675oC,
while at 500 and 575oC such effect was very little. It was explained that below
570oC if the oxidized iron contains only small amounts of chromium, it would be
dissolved as Cr+3 in the magnetite phase, the ions occupying octahedral sites. There
will, therefore, be no difference in the vacancy concentration in the oxide phase due
to the chromium ions.
In contrast to the foregoing alloying elements (Al, Si, and Cr), nickel is more noble
than iron. Consequently, the iron matrix of nickel steel is selectively oxidized and
nickel is rejected at the oxide/base metal interface [29]. Since the diffusion
14
coefficient of nickel in iron is low, nickel does not diffuse rapidly back into the core
results in the concentration of nickel at interface becomes higher than in the core.
Even in relatively dilute Fe-Ni alloys, the nickel concentration just ahead of this
interface can be very high. This selective oxidation of iron and concentration of
nickel in a thin layer results in interpenetration of the oxide and metal at the
interface and produces a tight mechanical oxide-metal bond and substantially
increased oxidation resistance.
The resistance of iron to oxidation between 500 and 900oC was shown to be
increased upon the presence of small amounts of phosphorus (<0.5 Wt %), while at
1000oC a destructive effect was found [30,31].
With the exception of certain free-cutting steels with high sulphur contents, sulphur
at the levels normally present in steel has no significant effect alone [32]. The effect
of sulphur (0.006-0.5 wt %) on the oxidation properties of iron was also to be very
slight [33].
Manganese is another important element in carbon steels; however, very little
attention has been focused on the effect of manganese on steel oxidation.
Molybdenum is more noble than iron and seems to behave like copper. Inokuchi et
al. [34] reported that addition of 0.013 wt % of Mo to silicon steel produced a
smooth surface and good oxidation resistance due to molybdenum concentrated near
the surface and/or due to the fine molybdenum sulphide particles preventing grain
boundary cracking.
2.4.2.2 Effect of atmosphere on the oxidation of iron and iron alloy steels
The atmosphere for high temperature oxidation of steels usually consists of
combustion products, such as N2, H2O, CO2, CO, H2, SO2, etc in proportions that
depend on the air to fuel ratio, the composition of the fuel, and the temperature of
15
the gas next to the steel. Nitrogen is inert and its principal effect on the oxidation is
dilution of other effective gaseous species only [11]. H2O and CO2 are oxidizing
gases and they react with iron as follows:
( ) 2432 , HOFeFeOOHFe +→+ (2-14)
( ) COOFeFeOCOFe +→+ 432 , (2-15)
On the contrary, H2 and CO are reducing gases and may reduce oxides according to:
( ) OHFeHOFeFeO 2243, +→+ (2-16)
( ) 243, COFeCOOFeFeO +→+ (2-17)
In high temperature oxidation the outward movement of iron ions from the metal
through the scale to the reaction site often induces gaps at the scale/base metal
interface. When the oxidation occurs in pure oxygen or in dry mixtures of oxygen
and inert gases; the rate decreases because of the throttling action of the gaps upon
the flux of iron ions through the scale. However, if sufficient water vapor or CO2 is
present in the atmosphere, the oxidation rate is maintained in spite of the gaps in the
scale. The most probable explanation of the effect of H2O and CO2 is that these
compounds transport oxygen across the gaps from the inner surface of the scale to
the metal surface, where it dissociates. The oxygen ions released by the dissociation
are adsorbed on the metal surface and react to form new scale.
adsorbedOHOH +→ 22 (2-18)
or
adsorbedOCOCO +→2 (2-19)
16
The hydrogen or carbon monoxide released by the dissociation migrates out to the
inner surface of the scale. Here H2 or CO reduces iron oxide according the following
reaction:
−+ ++→+ eOHFeHFeO 22
22 (2-20)
or
−+ ++→+ eCOFeFeOCO 22
2 (2-21)
H2O or CO2 is returned to the atmosphere in the gap to repeat the cycle. The iron
ions produced by the reaction diffuse by means of lattice defects in the scale towards
the scale/gas interface, site of the primary oxidation reaction. A schematic
representation of the mechanism as suggested by Rahmel and Tobolski [35] is
shown in Figure 2.2.
Figure 2.2 Mechanism of the oxidation of iron in atmospheres containing H2O and
CO2 as suggested by Rahmel and Tobolski [35]
17
Presence of sulphur dioxide in oxidizing gas environments tends to increase the
oxidation rate. The main mechanism appears to be the formation of a liquid phase
such as FeS in the scale and enhanced ionic transport through the scale. However, in
the amounts normally present in industrial applications its effect is marginal. In
oxidizing atmospheres increasing the air to fuel ratio dilutes the SO2 and minimizes
its effect on the scale.
2.4.2.3 Other factors can affect the oxidation of Fe and its alloys
There are some experimental variables known to affect, sometimes markedly, the
oxidation or scaling of Fe and its alloys. These include (a) the gas flow rate (b)
surface preparation (c) pre-treatment (d) the temperature [36]. The investigation of
the effect of the flow rate of the oxidizing atmosphere has produced surprisingly
wide differences in the results. Some authors have found that the oxidation rate
increases with increase in gas flow rate up to a critical velocity and thereafter no
further increase in the oxidation rate occurs [37,38]. Other workers have observed a
peak in the oxidation rate vs. gas velocity, the peak shifting to higher gas velocities
with increasing temperature. Still other workers have observed no change in
oxidation rate with increase in gas flow rate [39].
Several workers have been concerned with the effects of surface finish (and cold
work), specimen shape and pre-oxidative treatments on oxidation behavior [40, 41,
42]. Such work has been primarily concerned with specimens that have received the
same preparation treatment upon all faces and edges. It is known that radically
different oxidation rates are possible with specimens finished in different ways.
Surface finish has a long-term effect on oxidation behavior that extended beyond the
time of oxidation of the deformed (or cold-worked) layer, which, for a fine surface
finish, extended only 5µ below the original metal surface [43]. It should also be
borne in mind that stress relieving and recrystallization of such deformed layers will
occur within a short time at 700oC or higher temperatures and therefore only a small
portion of the initial cold-work layer should remain to have any effect.
18
2.5 Oxidation Kinetics
The mechanism in which a pure metal or alloy is oxidized at elevated temperatures
can be thought of as series of stepwise processes broken down as follows:
1- Chemisorption of a gaseous component.
2- Dissociation and electron transfer by the gaseous molecule.
3- Nucleation and crystal growth.
4- Diffusion and transport of cations, anions, and electrons through the scale.
The reported kinetics data for steel oxidation in air and oxygen are spares, with most
being derived from oxidation of low-carbon steel. The measurements of the
oxidation rate will show the slowest step, which will be the controlling step. The
most important parameter of metal oxidation from an engineering viewpoint is the
reaction rate. Reaction rates and corresponding rate equations for the oxidation of
metals are function of a number of factors such as temperature, oxygen partial
pressure, elapsed time of reaction, surface preparation, and pretreatment of the
metal. It has been established that the chemical reaction rate constant and the
diffusion coefficient increases exponentially with temperature. For this reason, it
may be expected that oxidation rate will also vary exponentially with temperature.
Temperature dependence of oxidation rate constants (k) at constant ambient oxygen
pressure obeys an Arhenius-type equation,
⎟⎠⎞
⎜⎝⎛ ∆−
=TREkk o .
exp. (2.22)
Where:
∆E: activation energy (J/mole)
R: gas constant
0k : constant (independent of temperature)
19
Since the oxide reaction product is generally retained on the metal surface, the rate
of oxidation is usually measured and expressed as weight per unit area. Empirical
rate laws sometimes observed during oxidation of various metals under different
conditions are illustrated in Figure 2.3. They usually are given as a plot of weight
gain per unit area versus time. The simplest empirical relationship is the linear law,
tkW t .= (2.23)
Where W is weight gain per unit area, t is time, and kt is the linear rate constant.
Linear oxidation is characteristic of metals for which a porous or cracked scale is
formed so that the scale does not represent a diffusion barrier between the two
reactants.
In 1933, C. Wagner showed that the ideal ionic diffusion-controlled oxidation of
pure metals should follow a parabolic oxidation rate law [44],
ctkW p += .2 (2.24)
Where kp is the parabolic rate constant, and c is a constant. The form of the
parabolic oxidation equation is typical of non-steady-state diffusion-controlled
reactions. The logarithmic empirical reaction rate law,
( )AtckW e += .log. (2.25)
Where: ek , c, and A are constants. Logarithmic oxidation behavior is generally
observed with the thin oxide layers (e.g., less than 1000Ao) at low temperatures.
20
Figure 2.3 oxidation film growth curves for linear, parabolic, and logarithmic rate
equation [44].
In some cases it may be found that the oxidation is initially parabolic and then the
reaction gradually becomes linear. This situation may arise if a compact scale
growing at a parabolic rate transforms at a linear rate to an outer porous and non-
protective oxide layer. This combination between parabolic and linear oxidation
called paralinear oxidation [45]. After long periods the oxidation becomes
essentially linear, and the concurrently the inner layer approaches a stationary
thickness; that is, it grows at the same rate at which it is consumed by the linear
depletion.
Sachs et al., [46] compared the oxidation kinetics of mild steel (equivalent to SAE-
AISI 1006) with those of pure iron in different atmospheres (including air and
oxygen) at 700 to 1100oC. It was found that the oxidation kinetics followed the
parabolic law and, in general, the oxidation rates of mild steel were lower than those
of pure iron in either air or oxygen. Raman et al. [47] found that the oxidation
kinetics, for steels that have similar compositions followed the parabolic law in air at
730 to 935oC for 60 minutes and at 1050oC for 30 minutes.
21
Abuluwefa et al. [48] studied the effect of oxygen level in O2-N2 mixture (1-15 %
O2) on the oxidation of similar steels (containing Fe-0.4% C-0.2% Mn-0.02% Si) at
1000 to 1250oC and found that the oxygen level significantly affected the initial
oxidation rates, but had no effect on the subsequent parabolic oxidation rates.
2.6 High Temperature Oxidation Testing
Most of the scientific theories on high-temperature corrosion have been developed
through laboratory study. Laboratory testing has also contributed significantly to the
wealth of corrosion data that allows engineers to make informed material selections
for various processing equipment. There are, however, several drawbacks. A
laboratory test cannot simulate exactly the operating environment and the conditions
of a processing system. Another drawback is the relatively short test duration
compared to the equipment’s design life. Extrapolation becomes necessary to make
materials behavior predictions. One major problem with extrapolation is the
unpredictability of breakaway corrosion. In many cases, metals and alloys rely on
the formation of protective scales (mostly oxide scales) to resist high temperature
corrosion attack. Although it is generally understood that the protective scale may
eventually break down, leading to breakaway corrosion, it is not currently possible
to predict the onset of breakaway corrosion. Thus laboratory tests are often
conducted under accelerated conditions (e.g., higher temperatures and/or more
corrosive environments) in order to increase the confidence level for the selected
alloy. The accelerated test is also frequently used for initial alloy screening to
narrow down the viable candidates for long term tests and/or field trials. Extreme
care should be taken if the results of short-term tests and/or accelerated tests are
used for life extrapolation.
2.6.1 Gravimetric Method
The gravimetric method is widely used to study oxidation and other forms of high
temperature corrosion. This test method involves measuring the specimen’s weight
as a function of time. Test apparatus that continuously monitors the specimen’s
22
weight during testing with a recording balance is very popular in academia for
conducting oxidation studies. Figure 2.4 shows a schematic of experimental
arrangement for use with an automatic recording balance. Continuous monitoring is
possible of the oxidation of a sample suspended from a recording balance in the
reaction chamber within a furnace. Oxidation in air can be measured easily using a
laboratory balance with a hole in the bottom through which the suspension wire is
directed. For corrosion in defined gas mixtures, closed systems are necessary and
microbalances are available which also can be used under vacuum conditions [49].
Figure 2.4 Schematic of experimental arrangement for use with an automatic recording balance [50].
23
The major advantage of a gravimetric apparatus with an automatic recording balance
is the continuous record of the reaction kinetics. The disadvantage is that only one
sample can be tested each time, so generating comparative data for a number
candidate alloys is a lengthy process. Therefore, the method is not suitable for
generating an engineering database [50].
24
CHAPTER 3
EXPERIMENTAL PROCERURE
3.1 Introduction
Continuous thermogravimetry is a common method to test materials in gaseous
corrosive environments at high temperatures and to elucidate kinetics and
mechanisms of high temperature corrosion. So thermogravimetric technique was
used in this research to study high temperature corrosion of steels used in the
construction of petroleum refineries heaters. Effects of oxidizing atmosphere and
temperature changes were studied. Oxidation products were analysed by using X-
ray, optical and scanning electron microscopes to correlate weight change data to the
nature of oxidation products.
3.2 Material
In this study, three different steels (C-5, P-11 and P-22) with different chemical
compositions (see Table 3.1) were used in oxidation experiments. These steels are
used in the construction of petroleum refineries heaters. These steels were delivered
by Kırıkkale petroleum refinery plant in tubular form. They were cut and machined
to obtain flat and thin steel pieces used to prepare the specimens.
Table 3.1 Chemical compositions of steels
Steel ASTM C% Mn% P% S% Si% Cr% Mo%
C-5 A 106 0.25 0.65 0.047 0.055 0.11 -- --
P-11 A 335 0.15 0.45 0.03 0.03 0.77 1.25 0.57
P-22 A 335 0.15 0.47 0.03 0.03 0.45 2.25 1.0
25
3.3 Sample Preparation
The samples were divided into two sets as seen in Figure 3.1. One of the sets was
large with dimensions of 50 mm x 25 mm x 2 mm to provide an accurate
experimental data. The other set was smaller, with 10 mm x 10 mm x 2 mm
dimensions, to be used for microscopic and X-ray analysis. Each large sample
contained two holes with 3 mm diameter at two ends. The one at the top was used to
suspend the sample from the electrobalance and the other at the bottom was used to
suspend the small sample, which also had a hole with 3 mm diameter. The edges of
the samples were rounded to avoid effects of corners and edges. Quartz hooks were
used to suspend the both samples.
Figure 3.1 Schematic draw of the samples sets.
The steel samples were ground by using SiC papers. The following grades were used
during grinding; starting from 120, 220, 320, 400 and 600 grades. Samples were
cleaned ultrasonically and degreased with alcohol to remove SiC and steel particles
left from grinding. Grinding without polishing is recommended in high temperature
26
corrosion studies. This provides a surface that favors nucleation of oxides and lead
to dense adherent scales. Accurate measurements of dimensions of both samples
were done by using a micrometer. Measured dimensions were used to obtain total
surface area of samples.
3.4 Thermogravimetric Analysis (TGA)
Thermogravimetric analysis was preformed on CAHN C-1000 electrobalance. The
balance was consisted of weighing and control units. The weighing unit is the unit
that detects the actual weight. While the control unit converts the weight to an
electrical signal (voltage) output. The test sample was suspended from the weighing
unit by means of a quartz hook. The output signal of the control unit was connected
to a computer via a data acquisition unit to record the weight change data on a
digital computer. With these two units the weight was zeroed by mechanical and
electronic taring at the beginning of each experiment. Suspended sample stayed in
the middle of a Pyrex reaction tube in the hot zone of the furnace. The reaction tube
had 5 cm diameter and 80 cm length and it was extended out of a vertical split
furnace from both ends. Upper end was connected to a hole, where suspension hook
was provided, at the bottom of the weighing unit. Lower end was open to the
atmosphere. Figure 3.2 shows a schematic representation of setup for air
oxidation[51].
The heat necessary for high temperatures was supplied by a LINDBERG
LHTF322C split type cylindrical furnace. It was connected to a Eurotherm
temperature controller. The desired temperature (±1oC) for the oxidation reaction
was achieved by adjusting the temperature controller.
Oxidation of the three different steels was performed at 450oC and 500oC in two
different atmospheres of air and (CO2+2H2O+7.52N2) mixture. The total flow rate of
the oxidizing gases during the oxidation tests was controlled at room temperature as
895 cc/min for 450oC and 837 cc/min for 500oC. Above flow rates are equivalent to
120 cm/min gas velocity in the reaction chamber. In the case of experiments
27
involving oxidation by air, above flow rates were delivered into the reaction
chamber by means of an air pump from the upper part (balance).
For experiments with CO2+2H2O+7.52N2 mixture, CO2 and N2 gases (with technical
grade) were used to prepare given gas composition. Both CO2 and N2 gases were
supplied from cylinders. Flowmeters were used to measure the flow rates of these
gases. To obtain the desired water vapor concentration in the gas phase, part of N2
gas was passed through water in a constant temperature bath. The temperature of the
bath at which it should be kept in order to get the desired water partial pressure for
the TGA tests was determined experimentally as 61oC for 450oC tests and 60oC for
500oC tests. H2O+N2 mixture was delivered into the reaction tube directly from the
side of the tube. Remaining gases were sent from the upper part. In this way, upward
movement of H2O+N2 mixture was minimized. To eliminate the condensation of the
water vapor outside the reaction tube, glass piping carrying H2O+N2 mixture was
covered by a heating tape. Table 3.2 gives the distribution and amounts of gases
used in the oxidation in CO2 +2H2O+7.52N2 environment at 450oC and 500oC.
Table 3.2 Gas flow rates used in CO2+N2+H2O environment at 450oC and 500oC
N2 CO2 H2O Flow rate
(cm3/min) U L T U L T U L T
450oC 136.2 570.8 707 62.7 - 62.7 - 125.2 125.2
500oC 127.4 533.9 661.3 58.6 - 58.6 - 117.2 117.2 U: upper side, L: lower side, T: total
28
29
The reaction tube, including the balance part, was flushed through N2 gas during
heating up the furnace. This provided an inert atmosphere before reaching the
reaction temperature. After reaching the reaction temperature and before starting the
experiment, sufficient volume of reaction gases was supplied through the reaction
chamber for about 5 minutes. This was to fill in the apparatus with desired gas
composition quickly. This eliminated a small weight change of the sample due to
density differences of gases. Then, flow rates of gases are maintained at above levels
during the course of each experiment.
Weight change of suspended samples subjected to oxidation was recorded by PC-
LD 711 analog-to-digital (A/D) converter used with a computer to collect data. A 10
milligram recording range (with an accuracy of 0.1% of recording range), which
could be expanded up to 50 milligrams, was used in this study. Over all accuracy
was ± 0.2 mg. A computer program was used to collect data for every 2 minutes.
Each run was lasted for about 35 hours and then the supply of oxidizing gases was
cut. A continuous flow of N2 gas was maintained through the reaction tube until the
system cooled down. After the furnace was cooled, the samples were taken out from
the furnace and kept in desiccators for subsequent analysis of oxidation products.
3.5 Identification of Oxidation Products
X-ray, optical and electron microscope were used for phase identification and
structural investigation of the oxidation products. The 10 mm x 10 mm x 2 mm
specimens were mounted in bakelite. Mounting was done in such a way to have
cross section of the oxide film perpendicular to the specimen surface. They were
ground on SiC papers going from grade 120 to grade 600. They were then
investigated under OLYMPUS optical microscope. The same samples were
examined by scanning electron microscope (JEOL JSM 6400).
X-ray diffraction (XRD) was done by using Rigaku D/MAX2200/PC with Cu Kα
target, λ=1.54056 Ao. Since the depth of oxide scales were very small to be removed
30
mechanically and ground to powder, the large specimens themselves were used for
XRD analysis by placing the largest surface against the diffraction beam. The
quantative X-ray diffraction analyses was not considered because the relative
intensities of the oxide phases may not reprsent the true relative intensities since
oxide scales formed on the metal surface might have preferred orientation of the
metal grains. Furthermore, diffractions from the metal under oxide films may
interfere with intensities of diffractions coming from oxide layers.
31
CHAPTER 4
RESULTS
4.1 Introduction
Figures 4.1 through 4.4 show the thermogravimetric test results of the three steels at
450oC and 500oC in two different oxidizing atmospheres. The results are represented
as plots of weight gain per unit surface area in (mg/cm2) vs. time in (hr). The data
was drawn for a period of 35 hours as point for every one hour to clearly show the
oxidation behavior of the steels.
4.2 Oxidation in Air
Thermogravimetric test results at 450oC and 500oC are shown in Figure 4.1 and
Figure 4.2, respectively. From these figures, steels with increasing weight gain, can
be ranked as: P-22, C-5 and P-11 at both temperatures. At the end of 35-hour
oxidation period, steels; P-22, C-5 and P-11 gained 0.197, 0.312, and 0.47 mg/cm2
of weight respectively at 450oC. Whereas, P-22, C-5, and P-11 gained 0.44, 0.554,
and 0.725 mg/cm2 of weight respectively at 500oC.
The effect of temperature on the oxidation behavior of steels can be noticed by
comparing the curves for all samples in Figures 4.1 and 4.2. The oxidation rates for
tests done at 500oC were higher than those done at 450oC and this was expected
since the oxidation was controlled by the diffusion of ions, which is faster at higher
temperatures.
Although there is no definite correlation on the effect of chemical composition on
oxidation rates, a qualitative comparison can be done with reference to Table 3.1.
From the table, it can be seen that oxidation rates decreased with increasing
32
chromium and molybdenum contents for P-11 and P-22 steels. Lower oxidation
rates, relative to P-11 steel, were obtained for C-5 that has higher carbon and
manganese contents and lower silicon content.
0
0,1
0,2
0,3
0,4
0,5
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
P-22P-11C-5
Figure 4.1 Oxidation of the three steels at 450oC in air.
33
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
P-22P-11C-5
Figure 4.2 Oxidation of the three steels at 500oC in air.
4.3 Oxidation in CO2+N2+H2O Atmosphere
The thermogravimetric test results of oxidation of the three steels in CO2+N2+H2O
environment at two different temperatures are shown in Figures 4.3 and 4.4. In this
environment, C-5 steel has the lowest weight gain of 0.377 mg/cm2 at 450oC and
0.879 mg/cm2 at 500oC, then come P-22 with 0.467 mg/cm2 at 450oC and 0.927
mg/cm2 at 500oC and P-11 has the highest weight gain at both temperatures after a
35-hour oxidation period with 0.66 mg/cm2 at 450oC and 1.147 mg/cm2 at 500oC.
By looking at the two figures the effect of temperature on the oxidation behavior of
the steels can be noticed. As expected all the steels were oxidized more at the higher
temperature.
34
Comparison of Figures 4.3 and 4.4 with Table 3.1 does not indicate any definite
correlation on the effect of chemical composition on oxidation rates. From the table,
it can be seen that oxidation rates decreased with increasing chromium and
molybdenum contents for P-11 and P-22 steels. The lowest oxidation rates were
obtained for C-5 that has the highest carbon and manganese contents and the lowest
silicon content.
00,10,20,30,40,50,60,7
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
P-22P-11C-5
Figure 4.3 Oxidation of the three steels in CO2+N2+H2O at 450oC
35
00,20,40,60,8
11,21,4
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
P-22P-11C-5
Figure 4.4 Oxidation of the three steels in CO2+N2+H2O at 500oC
4.4 Effect of Oxidizing Atmospher
The oxidation behavior of C-5 steel in the two oxidizing environments at two
temperatures is illustrated in Figures 4.5 and 4.6. C-5 steel has lower weight gain in
air than in CO2+N2+H2O atmosphere, where it gained 0.312 and 0.554 mg/cm2 in 35
hours at 450oC and 500oC respectively in air. It gained 0.377 mg/cm2 at 450oC and
0.879 mg/cm2 at 500oC in CO2+N2+H2O within 35 hours period.
36
00,050,1
0,150,2
0,250,3
0,350,4
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
Figure 4.5 Oxidation of C-5 steel in air (diamonds) and in CO2+N2+H2O environment (Triangles) at 450oC
0
0,2
0,4
0,6
0,8
1
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
Figure 4.6 Oxidation of C-5 steel in air (diamonds) and in CO2+N2+H2O environment (Triangles) at 500oC
37
Oxidation behavior of P-22 steel at 450oC in the two atmospheres can be seen in
Figure 4.7. The more oxidizing atmosphere was CO2+N2+H2O mixture. The
recorded weight gain was 0.467 mg/cm2 in this environment in 35-hours oxidation
period. On the other hand, it gained only 0.197 mg/cm2 in air.
The oxidation behavior of the same steel at 500oC in the same atmospheres is shown
in Figure 4.8. CO2+N2+H2O was again more oxidizing atmosphere. The weight gain
in this atmosphere (0.927 mg/cm2) was about two times that in air ( 0.44 mg/cm2 )
in 35 hours.
00,05
0,10,15
0,20,25
0,30,35
0,40,45
0,5
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
Figure 4.7 Oxidation of P-22 steel in air (diamonds) and in CO2+N2+H2O environment (Triangles) at 450oC
38
0
0,2
0,4
0,6
0,8
1
0 5 10 15 20 25 30 35time(hr)
g.w
.(mg/
cm2 )
Figure 4.8 Oxidation of P-22 steel in air (diamonds) and in CO2+N2+H2O environment (Triangles) at 500oC
The oxidation behavior of P-11 steel in the two oxidizing environments at 450oC is
shown in Figure 4.9. Similar to the other two steels P-11 steel has got lower weight
gain in air than in CO2+N2+H2O atmosphere. It gained 0.47 mg/cm2 in air and 0.66
mg/cm2 in CO2+N2+H2O mixture in 35 hours. With 1.147 mg/cm2 weight gain in
CO2+N2+H2O and 0.725 mg/cm2 in air at 500oC (see Figure 4.10), CO2+N2+H2O
environment can be considered as more corrosive one for this steel also.
39
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
Figure 4.9 Oxidation of P-11 steel in air (diamonds) and in CO2+N2+H2O environment (Triangles) at 450oC
0
0,2
0,4
0,6
0,8
1
1,2
1,4
0 5 10 15 20 25 30 35
time(hr)
g.w
.(mg/
cm2 )
Figure 4.10 Oxidation of P-11 steel in air (diamonds) and in CO2+N2+H2O environment (Triangles) at 500oC
40
To summarize above findings Table 4.1 was prepared. It shows the amount of
weight gained for all steels in (mg/cm2) in both environments at the two
temperatures.
Table 4.1 Weight gain after 35-hour oxidation period in air and in CO2+N2+H2O gas mixture at the two temperatures
Weight gained in air for 35 hr
in (mg/cm2)
Weight gained in CO2+N2+H2O
for 35 hr in (mg/cm2) Steel
450oC 500oC 450oC 500oC
C-5 0.312 0.554 0.377 0.879
P-22 0.197 0.440 0.467 0.927
P-11 0.470 0.725 0.660 1.147
4.5 Analysis of Oxidation Products
X-ray diffraction and microscopic analysis were performed on the oxidation
products to identify the nature of each product on the oxidized steel surface. Figures
4.11- 4.14 show X-ray diffraction patterns. The selected samples of these X-ray
diffraction patterns are for the three steels used in this study. They are selected to
give effects of temperature and corrosive gas environments. X-ray diffraction studies
for the other samples were also done. They are not shown here because they yielded
very similar diffractions to those given above. As it can be seen from above figures,
41
most of the samples show Fe2O3 and Fe3O4 as oxide phases. In addition to above
oxides, P-22 steel revealed the presence of a Cr-O phase. Fe peaks and Fe-Cr peaks
are belived to be coming from steel samples under oxide films.
02000400060008000
100001200014000
20 30 40 50 60 70 80 902theta(deg.)
Inte
nsity
(a.u
)
Fe2O3
Fe3O4
Fe
Figure 4.11 X-ray diffraction pattern of P-11 oxidized in CO2+N2+H2O at 450oC
0
2000
4000
6000
8000
10000
20 30 40 50 60 70 80 90
2theta (deg.)
Inte
nsity
(a.u
.)
Fe2O3Fe3O4Fe
Figure 4.12 X-ray diffraction pattern of C-5 oxidized in air at 450oC
42
0
2000
4000
6000
8000
10000
12000
14000
20 30 40 50 60 70 80 90
2 theta (deg.)
inte
nsity
(a.u
.)
Fe2O3
Fe-CrCr-O
Figure 4.13 X-ray diffraction pattern of P-22 oxidized in air at 500oC
0
5000
10000
15000
20000
25000
30000
20 30 40 50 60 70 80 902theta (deg.)
Inte
nsity
(a.u
)
Fe2O3
Fe3O4
Fe
Figure 4.14 X-ray diffraction pattern of P-22 oxidized in CO2+N2+H2O at 500oC
43
Photographs from optical microscope for selected specimens of P-11 and C-5 are
shown in Figures 4.15 through 4.18. Fe3O4 and Fe2O3 phases can be distinguished in
these photographs, due to darker color tone of Fe3O4 relative to Fe2O3. Examination
of these photos showed that Fe2O3 was the thin outer layer of the scale at the
gas/oxide interface and Fe3O4 was the inner layer at the metal/oxide interface.
Optical photographs are used to calculate approximate relative thicknesses of oxide
phases. Relative thickness values are used in the calculation of the approximate
weight of metal loss as given in Appendix A.
Figure 4.15 Optical photograph of C-5 steel oxidized at 500oC in CO2+N2+H2O (2000x)
Bakelite
Oxide
Steel
44
Figure 4.16 Optical photograph of P-22 steel oxidized at 500oC in CO2+N2+H2O (2000x)
Figure 4.17 Optical photograph of P-11 steel oxidized at 450oC in CO2+N2+H2O (2000x)
Steel Bakelite
Oxide
Steel
Bakelite
Oxide
45
Figure 4.18 Optical photograph of P-11 steel oxidized at 450oC in air (2000x)
SEM analysis was performed on the oxidized samples to get information about the
morphology of the oxides formed. Micrographs of selected samples are given in
Figures 4.19- 4.23.
Figure 4.19 SEM micrograph of C-5 steel oxidized in CO2+N2+H2O at 500oC
Steel
Bakelite
Oxide
46
Figure 4.20 SEM micrograph of P-22 steel oxidized in air at 500oC
Figure 4.21 SEM micrograph of P-11 steel oxidized in CO2+N2+H2O at 450oC
47
Figure 4.22 SEM micrograph of P-22 steel oxidized in CO2+N2+H2O at 500oC
Figure 4.23 SEM micrograph of P-22 steel oxidized in CO2+N2+H2O at 450oC
48
CHAPTER 5
TREATMENT OF DATA AND DISCUSSION
5.1 Introduction
In this section kinetic models proposed for oxidation reaction will be used to
correlate the numerical data obtained from thermogravimetric tests. In addition,
oxidation mechanisms, constitution and morphology of oxidation products will be
discussed with reference to thermodynamic, microscopic and other considerations.
5.2 Kinetics of Oxidation
The oxidation behavior of the steels in the initial period cannot be seen clearly from
the graphs presented in chapter 4, because each point in those figures corresponds to
1-hour period. In order to show the details of oxidation data for the first 3 hours for
the selected three samples, figures are repeated here (Figures 5.1, 5.2 and 5.3 ). Each
point in these figures (indicated as “exp.” in figures) corresponds to 2 minutes
oxidation period. The lines are representing values calculated by parabolic rate
equation (indicated as “model.” in figures).
49
00,020,040,060,08
0,10,120,140,16
0 50 100 150 200
time(min)
g.w
.(mg/
cm2 )
modelexp.
Figure 5.1 The first 3 hours oxidation data for P-11 steel oxidized
in air at 500oC
0
0,05
0,1
0,15
0,2
0,25
0 50 100 150 200time(min)
g.w
.(mg/
cm2 )
exp.model
Figure 5.2 The first 3 hours oxidation data for P-22 steel oxidized in CCOO22++NN22++HH22OO at 500oC
50
0
0,05
0,1
0,15
0 50 100 150 200
time(min)
g.w
.(mg/
cm2 )
exp.model
Figure 5.3 The first 3 hours oxidation data for C-5 steel oxidized in CCOO22++NN22++HH22OO at 500oC
From these figures, it can be seen that all steels have similar kinetic behavior in both
environments. In other words, all steels seem to follow parabolic forms during
oxidation and no transition in the kinetic can be observed.
Figures 5.4 and 5.5 are the representations of log (y) vs. log (t) graphs of Figures 4.1
and 4.4 respectively, where (y) is the gained weight per unit surface area in
(mg/cm2) and (t) time in hours. Above representation resulted in straight lines with
values of (R2) squares of the associated correlation coefficients above 0.97 for all
data obtained in this study. This indicates that the oxidation kinetics of all steels
studied in given environments follow the equation:
ntky *= ( 5.1)
where (k) is the rate constant, (n) is the exponent of time in the rate law.
Table 5.1 shows the numerical values of k and n for all the cases at 450oC, and
Table 5.2 at 500oC. The smooth curves presented in Figures 5.1-5.3 represent the
plot of equation 5.1 with the corrosponding values of k and n for each steel.
51
-1,8
-1,6
-1,4
-1,2
-1
-0,8
-0,6
-0,4
-0,2
00 0,5 1 1,5
log (t)
log
(y) P-22
P-11C-5
Figure 5.4 Plot of log (y) vs. log (t) for all steels oxidized
in air at 450oC
-0,8
-0,7
-0,6
-0,5
-0,4
-0,3
-0,2
-0,1
0
0,10 0,5 1 1,5
log (t)
log
(y) P-22
P-11C-5
Figure 5.5 Plot of log y vs. log t for all steels oxidized in CO2+N2+H2O at 500oC
52
Table 5.1 Numerical values of k and n at 450oC
Air CO2+N2+H2O Steel
k n k n
P-22 0.027 0.559 0.127 0.366
P-11 0.087 0.512 0.147 0.386
C-5 0.057 0.478 0.077 0.448
Table 5.2 Numerical values of k and n at 500oC
Air CO2+N2+H2O Steel
k n k n
P-22 0.056 0.582 0.192 0.443
P-11 0.078 0.599 0.252 0.375
C-5 0.096 0.523 0.156 0.415
53
5.3 Thermodynamic Consideration
During the oxidation in air the first formed oxide was Fe2O3 according to the
following reaction:
322232 OFeOFe →+ (6.1)
This was shown by equilibrium computations [51]. And then Fe3O4 is formed at the
Fe2O3/Fe interface according the following reaction:
4332 34 OFeFeOFe →+ (6.2)
Futher oxidation of Fe3O4 to Fe2O3 takes place at Fe2O3/Fe3O4 interface as result of
the following reaction:
32243 3212 OFeOOFe →+ (6.3)
Continued growth of Fe3O4 film takes place by the diffusion of Fe to Fe2O3/Fe3O4
interface due to reaction 6.2.
In the case of P-22 steel oxidized in air, X-ray diffraction of oxidized surface did not
revealed the presence of Fe3O4 instead, it showed the presence of Cr-O phase. The
presence of Cr-O attributed to the reduction of diffusion of Fe. As a result less
Fe3O4 formation according to reaction 6.2 and lower corrosion rate was observed in
this study.
On the other hand, when steels are exposed to CO2 containing environments Fe3O4
was formed immediately on the steel surface as stated by Rahmel and Tobolski [35]
according to the following reaction:
COOFeCOFe 443 432 +→+ (6.4)
This was also shown by the equilibrium computations[51].
54
Formation of Fe2O3 layer on steel samples exposed to CO2 containing environments
takes place as a result of oxidation of Fe3O4 by CO2 according to:
COOFeCOOFe +→+ 32243 32 (6.5)
Growth of Fe3O4 layer continues as a result of reaction 6.2 at Fe3O4/Fe2O3 as well as
Fe/Fe3O4 interfaces.
5.4 Microscopic and Other Considerations
From the thermogravimetric results it was observed that the oxidation rate decreased
as the oxide formed became thicker. This can be explained as the result of longer
path, which the ions follow. Since the oxidation is controlled by the diffusion of
ions, longer diffusion path slows down the oxidation process.
Examination of the SEM micrographs for the steels those had the lowest weight gain
in both environments, C-5 in CO2+N2+H2O and P-22 in air (Figures 4.19 and 4.20)
indicated that the scales formed were less porous than the others. This could explain
why these steels had a better oxidation resistance. From micrographs given in
Figures 4.21, 4.22 and 4.23 it can be seen that the scales formed in CO2+N2+H2O
oxidation environment generally contained cracks and voids. The reason for that
may be the accumulation of the gases formed (CO and H2) during the oxidation. As
the gas pressure build up inside the scale, the gas mixture could try to escape from
the scale and cause cracking of the scales. Therefore, these cracks act like channels
for the oxidant gases to penetrate easier and expedite oxidation.
55
CHAPTER 6
CONCLUSIONS
From the thermogravimetric oxidation tests and the microstructural examinations the
main conclusions can be cited as follows:
1. In air oxidation, P-22 had the best oxidation resistance among the three steels
at two temperatures. Lower oxidation rate of P-22 in air was explained with
reference to the formation of Cr-O phase.
2. In CO2+N2+H2O environment, C-5 possessed better oxidation resistance than
P-22 and P-11. Higher oxidation rate of P-22 in CO2+N2+H2O environment
was attributed to the absence of Cr-O phase to retard diffusion process.
3. Oxidation rate increased with increasing temperature from 450oC to 500oC.
4. In both oxidation environments, thermogravimetric results showed that the
oxidation rate decreased as the oxide scale became thicker. This is due to the
increase in diffusion paths of ions responsible for oxidation.
5. Although the oxygen potential was higher in air, the oxidation rate was
higher in CO2+N2+H2O atmosphere. This was due to formation of gaps at the
metal/oxide interface and channels within the oxide phase that promoted
oxidation.
6. It was observed that all steels oxidized according to a parabolic equation in
all oxidation tests.
56
7. Since the oxide layers formed were very thin, only approximate information
was obtained for relative thicknesses of oxide layers.
57
REFERENCES
[1] Deny A.Jones Mancmillan, Principles and Prevention of Corrosion, New York,(1992).
[2] R.Y. Chen and W.Y.D. Yuen, Oxidation of Metals, 59, Nos. 5/6, June (2003).
[3] V.B.Ginzburg, Steel –Rolling Technology: Theory and Practice (Marcel Dekker, NewYork), (1989).
[4] H.J. grabke, V. Leroy, and Viefhaus, ISIJ Inter. 35,95 (1995).
[5] J. Kucera, M. Hajduga, J. Glowacki, and P. Broz, Z. Metall. 90, 514, (1999).
[6] R. J. Hussey, G. I. Sproul, D..Caplan, and M. J. Graham, Oxid. Met.11,65 (1977).
[7] N. Birk and G.H Meier, Introduction to High Temperature Oxidation of Metals (Edward Arnold, London), (1983).
[8] O Kubaschewski and B. E. Hopkins, Oxidation of Metals and Alloys (Butterworths,London),(1962).
[9] D. Caplan, J. Electrochem. Soc. 107,359 (1960).
[10] N.G. Schmahl, H. Baumann, and H. Schenck, Arch. Eisenhüttenwes. 29, 41 (1958).
[11] Yao-Nan Chang, J. Mate. Sci. 24,14-22 (1989).
[12] M.H Davies, M.T.Simand, and C.E.Birchenall,Trans.AIME , 193,1250 (1953).
[13] L. Himmel, R.Fmehl, and C.E.Birchenall,Trans.AIME 5,827 (1953)
58
[14] J.Paidassi, Acta. Metall. 6,184 (1958).
[15] D. Caplan and M. Cohen, Corros. Sci. 3,139 (1966).
[16] G. Chaudron and H. Forestier, Acad. Sci 178, 217 (1924).
[17] W.A Fisher,A.Hoffmann, and R.Shimada, Arch.Eisenh. 27, 521 (1956).
[18] A.Goswami, Indian J. Chem. 3, 385 (1965).
[19] W.E. Boggs and R.H. Kachik, J. Electrochem. Soc. 116, 424, (1969).
[20] D. Caplan, G. I. Speoule, R.J Hussey and M.J. Graham, Oxid. Met. 12 (1978).
[21] Idem, ibid, 13, 225 (1973).
[22] A.U. Malik and D.P. Whittle, Oxid. Met., 16, 339 (1981).
[23] F. Saequsa and L. Lee, Corrosion 22, 168 (1966).
[24] J.A Von Fraunhofer and G. A . Pickup, Anti-Corros. 17,10 (1970).
[25] L S Darken, Trans. AİME 150,157 (1942).
[26] G. C. Wood, İ. Wright, T. Hodgkiess, D. P. Whittle, Wekstoffe Korrosion, 21, 900 (1970).
[27] D.Mortimer and W.B. A. Sharp, Brit. Corros. J. 3, 61 (1968).
[28] B.Hammar and N.G. Vannerberg, Scand. J. Metall. 3,123 (1974).
[29] G.G. Brown, K.G.Wold, JISI, 207, 1457 (1969).
59
[30] N.G.Vannerberg and I.Svedung , Corros. Sci.11, 915 (1971).
[31] I. Svedung and N.G. Vannerberg. Scand. J. Metall. 1, 141 (1972).
[32] T. Smith, Steel times 210, 339(1982).
[33] B. Hammar and N.G. Vannerberg, ibid, 3, 173 (1974).
[34] Y. Inkuchi and Y. Ito. Bull., Jpn. Inst. Met. 23, 276 (1984).
[35] A. Rahmel and J.Toboski, Corros. Sci. 5, 333 (1965).
[36] J. A. Von Fraunhofer, and G. A. Pickup, Corros. Sci. 253, 10 (1970).
[37] C.W. Tuck and D.W. Wown, J. Iron Steel Ins. 205, 972 (1967).
[38] C. Upthegrove and D. W. Murphy, ibid. 21, 73 (1933).
[39] W. J.Tomilson and S. Catchpole, Corros. Sci. 8, 845 (1968).
[40] D. Caplan and M. Cohen, Corros. Sci. 7, 725 (1967).
[41] K.G. Eubanks, D. G. Moore and W.A. Pennington, J. Electrochem. Soc. 109,382 (1962).
[42] J. Romanski, Corros. Sci. 8, 67 (1968).
[43] L. E. Samuels, J. Aust. Inst. Metals,151 (1964).
[44] Mars G. Fontana, Corrosion Engineering, McGraw-HILL, International Edition, 3rd edition (1987).
60
[45] P. Kofstad, High-Temperature Oxidation of Metals, John Wiley & Sons, New York, 1966.
[46] K . Sachs and C.W. Tuck,Werkst. Korros. 11, 945 (1970).
[47] R.K. Singh Raman. Gleeson, and D. J. Young, Proc.13th Inter. Corros. Co , Australia, (1996), paper 297.
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61
APPENDIX A
The steels may also be compared by calculating the weight of metal lost during the
test period (depth of penteration). The relative amounts of oxides were calculated
from the optical photographs and Fe2O3 fractions were tabulated in table A.1.
Table A.1 Fe2O3 fractions for the oxidized steels
Fe2O3 fraction
in air
Fe2O3 fraction
in CO2+N2+H2O Steel
450oC 500oC 450oC 500oC
C-5 0.152 0.20 0.166 0.248
P-11 0. 218 0.248 0.248 0.27
P-22 1 1 0.20 0.23 Fe3O4 fraction = 1- Fe2O3 fraction
Since the results showed that we have only Fe2O3 and Fe3O4 oxides for C-5, P-11
and for P-22 oxidized in CO2+N2+H2O, so the gained weight belonges to the weight
of oxygen atoms that reacted with Fe. So from the Fe2O3 and Fe3O4 fractions given
in the Table A.1 above and the gained weight given in Table 4.1 we can calculate
the theoretical oxide scale thickness δ in centimeters as:
dy
*1000=δ
wher y is the gained weight after 35 hours oxidation period in mg/cm2 and
⎥⎦
⎤⎢⎣
⎡⎟⎠⎞
⎜⎝⎛+⎟
⎠⎞
⎜⎝⎛= NZMLd *
23264**
16048*
62
L is the gravity of Fe2O3 = 5.24 g/cm3
M is Fe2O3 fraction in the oxide layer
Z is the gravity of Fe3O4 = 5.18 g/cm3
N is Fe3O4 fraction in the oxide layer = 1-M
Then the amount of Fe oxidized to Fe2O3 in mg/cm2 is equal to
M*160112*24.5*δ
and the amount of Fe oxidized to Fe3O4 in mg/cm2 is equal to
N*232168*18.5*δ
neglecting other elements in the steels, the theoretical depth of penteration in (cm)
can be calculated by dividing the total Fe oxidized to both Fe2O3 and Fe3O4 with the
gravity of pure iron (7.86 g/cm3). Tables A.2 and A.3 give the depth of penteration
after 35 hours oxidation period for all steels in (cm).
Table A.2 results of penteration depth calculation for air oxidation
Pen.depth at 450oC Pen.depth at 500oC
Steel cm in
35-hrs ipy
cm in
35-hrs ipy
C-5 0.000102 .010 0.000180 .0177
P-11 0.000153 .0151 0.000235 .0232
P-22 0.0000558 .0055 0.000133 .0131
63
Table A.3 results of penteration depth calculation for CO2+N2+H2O oxidation
Pen.depth at 450oC Pen.depth at 500oC
Steel cm
in 35-hrs ipy
cm
in 35-hrs ipy
C-5 0.000123 .012 0.000285 .0281
P-11 0.000214 .0211 0.000371 .0365
P-22 0.000152 .015 0.000301 .0296
