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Figure 1: Schematic representation of the diffusion

processes and phase boundary reactions during the

oxidation of iron in oxygen, according to Hauffe [11]

Rajesh Ranjan Sinha, Assistant Professor, Department of Chemistry, NETGi, Ranchi, India

Dr. Brahm Deo Tripathi, Principal, GGSITM, Kamre, Ranchi, India

Dr. Mahendra Yadav, Associate Professor, Department of Applied Chemistry, Indian School of

Mines, Dhanbad, India

Corresponding author: Rajesh Ranjan Sinha, e-mail: rrsinha79@gmail.com:

Effect of rare earth additions on high-temperature oxidation

resistance of V and Mo alloyed steels

Rare earths (RE) have been used to improve the high temperature oxidation resistance of low alloy steel containing

elements like Cr, Al, V and Mo. Further, the RE can be added either to the alloy or by applying as an oxide coating

to the alloy surface. In this study the high temperature oxidation resistance of rare earth (RE) oxide coated 1Cr-

0.3Mo-0.25V alloy was determined. This paper presents the influence of surface additions of nano-crystalline oxides

CeO2 on the isothermal oxidation behavior of 1Cr-0.3Mo-0.25V alloys at temperatures ranging from 600 °C to 900

°C. The oxidation rate of RE oxide coated1Cr-0.3Mo-0.25V was significantly lower than that of the uncoated alloy.

The improvements in oxidation resistance are the reduced oxidation rates and the increased oxide scale adhesion.

Scanning electron microscopy (SEM), X-ray diffractometry (XRD), and electron probe micro analyzer (EPMA)

were employed for these analyses. The scale formed in the presence of RE oxides was very thin, fine grained and

adherent.

_____________________________________________________________________________

Introduction

Chromia scales that form on low-chromium content Fe-

Cr alloys at elevated temperatures grow relatively

slowly and usually provide good oxidation resistance.

Addition of small amounts of elements or their chemical

compounds, such as, cerium or their oxide dispersions

to these alloys greatly increases their oxidation

resistance under isothermal conditions. These so-called

reactive element effects have been extensively reviewed

by Whittle and Stringer [1] and more recently by Moon

and Bennet [2]. Oxidation resistance is also improved if

reactive element oxides are applied to the surfaces of

the alloys [3...7] or introduced by ion implantation [8;

9]. Rahmel et al. [10] showed that additions of 0.5 - 5.5

%Mo decrease the oxidation rate of iron by almost the

power of ten in the temperature range 500 - 1000 °C;

Fig. 1 schematically shows that the oxidation of iron

proceeds with increasing temperature, different layers of

oxide scales, i.e., FeO, Fe3O4 and Fe2O3 are formed. In

case of iron, wustite is formed above 600°C which is a

most ionic defective oxide. [11]

Very few definite conclusions can be drawn as to the

usefulness of the oxides of the rare-earth lanthanide

series. The thermodynamic properties of many of these

compounds have never been ascertained. Others have

been only estimated or assigned tentative values based

on qualitative or incomplete investigations. The

lanthanide series´ oxides, in general, appear to have

quite high melting points. Those known with some

degree of certainty are La2O3 (2305 °C), CeO2 (2600

°C), Nd2O3 (2270 °C), Sm2O3 (2300 °C), Eu2O3

(2050 °C), Gd2O3 (2300 °C), and Dy2O3 (2340 °C)

[12] Very favorable free energies of formation at high

temperatures (about – 418.4 KJ at 2000 K) [13] indicate

that these lanthanide oxides range among the most

stable oxides. Carbides of this series are also known to

exist, but data on the stability of these compounds have

not been reported. It is known that they have dicarbide

stoichiometry and that they are decomposed by water

vapor [14]. The use of reactive elements is well known,

especially rare earths (RE) to improve high temperature

oxidation resistance of chromium dioxide and alumina

forming alloys. The improvements are in the form of

reduced oxidation rates and increased scale adhesion

[15; 16].

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Figure 2: Weight gain per unit area vs. time plot of 1Cr-0.3-Mo-0.25V

steel at 600 °C, 700 °C , 800 °C, 900 °C

Figure 3: Weight gain per unit area vs. time plot of

CeO2 coated 1Cr-0.3-Mo-0.25V steel at 600 °C, 700

°C , 800 °C, 900 °C

In aqueous solution both vanadium and cerium oxides

can form ionic species:

Ce+4 + H2O ↔ (CeOH)+3 + H+ [17] (1)

2 (CeOH)+3 ↔ (Ce-O-Ce)+6 +H2O [17] (2)

V2O5 ↔ V+4 ↔ V4 +-OH[18] (3)

Starting from CeO2 and V2O5, a new phase CeVO4 is

formed via an intermediate state V2O5-X/CeO2-y in

which an unpaired electron is trapped at high

temperatures being responsible for the reducibility of

Ce4+ to Ce3+ and for the active sites on a ceria

supported vanadia catalyst [19...21].

Material and Methods

Boiler quality low alloy steel i.e. 1Cr-0.3Mo-0.25V

were hot rolled to 2.5 mm thickness. The plates were

obtained from RDCIS,

SAIL plant for oxidation

tests. These plates were de-

scaled and cut to the size of

2.5 • 15 • 25 mm3, approx.

The percentage of

chemical composition of

steel is Fe-97.63, C-0.12,

Mn-0.5, Si-0.20, S-0.015,

P-0.018, Cr-0.99, Mo-0.28,

V-0.24. Both the surfaces

of the specimens were

mechanically polished

using SiC papers of up to

600 grit. The prepared

samples were thoroughly

washed in distilled water

and rinsed with acetone,

then dried and stored in

desiccators. 2.5 g of cerium oxide

powder (ASTM mesh size -230 +325)

were mixed with 50 ml ethyl alcohol

to prepare the slurry. The samples

were dipped into the slurry to be

coated and the slurry was constantly

stirred to prevent the reactive oxide

particles from sedimentation. The

samples were dried in air at a

temperature of 100 °C. The dipping

and drying process was repeated for

10 to 15 times until a uniform

thickness of the coating layer was

achieved on both the surfaces. The

difference between the initial and final

mass of the samples was measured and

the coating thickness was calculated to

range between 5 • 10-3 and 7 • 10-3

cm. Isothermal oxidation tests were

carried out in dry air under

atmospheric pressure by hanging all

the specimen with and without coat in

a vertical quartz tube reactor. The

reactor tube was closed with glass wool. The

temperature accuracy in the reactor tube was ± 5°C. The

atmosphere in the reaction zone was natural air. A Pt-

Pt13%Rh thermocouple was inserted from the top of the

furnace to measure the temperature of the reaction zone.

This thermocouple connected to a PID controller was

placed inside the annular space between the furnace and

reactor wall. Electronic digital balance having an

accuracy of ±0.1 mg was used for measuring the mass

changes of the sample during oxidation. A timer was

used for a fixed time interval to continuously record the

weight gain of the sample during oxidation.

Evaluation of the results

Kinetic studies. The oxidation kinetics data for 1Cr-

0.3Mo-0.25V low alloy steels with and without cerium

oxide coatings were conducted in the temperature range

of between 600 °C and 900 °C for 24 h. The weight

gains as a function of time for the above mentioned

three steels were plotted for different temperatures.

Figs. 2, 3 show the curves

for 1Cr-0.3Mo-0.25V

steel without coating and

CeO2 coated in the

temperature range from

600 °C to 900 °C,

respectively. Further, the

data were plotted as log

(weight gain per unit area)

vs. log t at temperatures

600 °C to 900 °C,

respectively, which is

shown in fig. 4. Further,

the slope vs. 1/T plot of

steel at 700°C, 750 °C and

800 °C is shown in fig.5.

The kinetics data and rate

constants for the above

mentioned steels are also

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Figure 4: Log-log plot of 1Cr-0.3-Mo-0.25V steel at 600 °C, 700

°C, 800 °C, 900 °C

Figure 5: Slope vs. 1/T plot of 1Cr-0.3-Mo-0.25V

steel at 700 °C, 750°C, 800 °C

available in the book [22]. Gibb’s free energy of

uncoated and CeO2 coated oxidized steels were 86.31

KJ/mol and 136.86 KJ/mol respectively.

SEM, EPMA & XRD analyses. All the specimens

subjected to the different tests were weighed after each

cycle and their surfaces were examined in a scanning

electron microscope (SEM) and electron probe micro

analyzer (EPMA). The oxide scales were also analyzed

by X-ray diffraction (XRD). These analytical techniques

were used to understand the oxidation process in terms

of scale morphology and chemistry in the presence or

absence of ceria coatings. These techniques were also

used for analyzing the scale composition, the nature of

scale adhesion and for identifying the various phases

occurring in the oxide scale. SEM was used to study the

alloy/scale cross-sections of the uncoated and coated

oxidized steels which were oxidized in the temperature

range from 600 °C to 900 °C. SEM micrographs of the

above-mentioned steels at 800 °C are represented in fig.

6. This clearly shows the occurrence of internal

oxidation. Similarly, EPMA and XRD micrographs of

the alloy/scale cross-sections formed on the coated

steels are shown in figs. 7 and 8, respectively.

Discussion of the results

Kinetic consideration. For the 1Cr-0.3Mo-0.25V type

steels data exists [23] suggesting that V-additions may

have distinct benefits as far as hydrogen embrittlement

in service is concerned, and this seems to be linked to

greater trapping and lower diffusivities in the V-

modified steels. Other work [24], indeed, indicated that

the problem of disbonding at cladded interfaces can be

reduced in V-modified steels. The possible consequence

of reduced hydrogen diffusivity on welding procedures

is observed in the fabrication of V-modified Cr-Mo

steels.

When an oxidant comes into contact with a metal, the

reactions proceed by forming a stable scale at the metal-

gas interface due to combination of metal

ions with oxidant ions. The growth of

scale at the interface was found to be rate

controlled process. The mechanically

polished samples were improved

oxidation resistance in the presence of

CeO2 coatings. In all cases, the rate of

weight gain was initially fast but after a

certain time, it was observed to increase

at a steady rate. This can be seen in the

plots of weight gain vs. time as shown in

fig. 2 - 3. The weight gain vs. Time plot

for 1Cr-0.3Mo-0.25V steel at 800 °C

temperature shows the kinetics of

oxidation and slope were changed.

However, the curve was observed to be of

parabolic nature. In the presence of RE it

grows at the metal/oxide interface. The

result of Gibb’s free analysis shows that

the stability of different oxides depends

on their standard Gibb’s free energies of

formation ( ΔGoMO), oxygen partial

pressure pO2 and temperature. It is possible to evaluate

the stabilities of different oxides in different

atmospheres by calculating the oxygen partial pressure

where the oxide dissociates. Dissociation oxygen partial

pressure (pO2)dissoc for the metal oxide (MO) is

calculated by the equation:

log (pO2) dissoc = 2ΔG0MO /(2.303RT)

The oxygen partial pressure under atmospheric pressure

is greater than the dissociation oxygen partial pressure;

therefore, the oxide is thermodynamically stable. Gibb’s

free energy analyses clearly show the marked increase

in the oxidation resistance of the molybdenum and

vanadium alloyed steels compared to those coated with

RE oxides. For each oxide, care has been taken to

optimize the RE oxide grain size and shape, to

consequently increase coverage and also to optimize the

chemical potentials of those two RE oxides having ionic

radii very close to each other.

SEM, EPMA and XRD investigation. Figs. 6 – 8

schematically show the oxide growth in the presence of

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Figure 6: SEM micrograph of 1Cr-0.3-Mo-0.25V Figure 7: EPMA micrograph of 1Cr-0.3-Mo-0.25V

steel at 800 °C steel at 800 °C

RE. SEM and EPMA clearly suggest that the scales

formed on the bare steel were of coarse-grained, non-

uniform character and depleted the substrate, whereas

fine-grained, compact, well adherent and uniform scales

were found to be formed on the CeO2 coated steel.

XRD analysis of the scales showed the presence of

simple, complex and mixed oxides (spinals), including

CeO2, Fe2O3, FeCe2O4, (FeO•Ce2O3), CeFeO3,

Fe21.34O32, CrO2 and FeO•Cr2O3 for ceria coated

oxidized 1Cr-0.3Mo-0.25V steel samples. The formed

FeO•Ce2O3 is thermodynamically more stable than

CeO2 under the present testing conditions. A

comparison of XRD analysis of steel showed that the

steel coated with cerium oxide shows superior oxidation

resistance compared to the others investigated here.

Conclusions

[1] In the above-mentioned steel, the kinetics of

oxidation were faster for an initial period of 100 min

after which it was decreased. This showed that the

diffusion of anions and cations were also decreased due

to the formation of a certain scale thickness;

[2] SEM studies showed that the outer layer of the oxide

scale was compact and the micrograph of fractured

interface showed the crystalline structure;

[3] the Mo and V alloyed steels coated with CeO2

exhibited higher oxidation resistance compared to the

uncoated alloys;

[4] the effect of CeO2 coating on the oxidation behavior

of 1Cr-0.3Mo-0.25V showed that the CeO2 coating

prevents scale formation in the temperature range from

600 °C to 900 °C;

[5] X-ray diffraction patterns for the CeO2 coated steel

showed the presence of CeO2 in the scale.

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Figure 8: XRD micrograph of 1Cr-0.3-Mo-0.25V steel at 800 °C

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