elevated temperature erosive wear of metallic materials 2006 roy

8/3/2019 Elevated Temperature Erosive Wear of Metallic Materials 2006 ROY

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 39 (2006) R101R124 doi:10.1088/0022-3727/39/6/R01

TOPICAL REVIEW

Elevated temperature erosive wear ofmetallic materials

Manish Roy

Defence Metallurgical Research Laboratory, PO Kanchanbagh, Hyderabad 500 258, India

Received 6 July 2005, in final form 9 December 2005Published 3 March 2006Online at stacks.iop.org/JPhysD/39/R101

AbstractSolid particle erosion of metals and alloys at elevated temperature isgoverned by the nature of the interaction between erosion and oxidation,which, in turn, is determined by the thickness, pliability, morphology,adhesion characteristics and toughness of the oxide scale. The mainobjective of this paper is to critically review the present state ofunderstanding of the elevated temperature erosion behaviour of metals andalloys. First of all, the erosion testing at elevated temperature is reviewed.This is followed by discussion of the essential features of elevatedtemperature erosion with special emphasis on microscopic observation,giving details of the erosionoxidation (EO) interaction mechanisms. TheEO interaction has been elaborated in the subsequent section. The EOinteraction includes EO maps, analysis of transition criteria from one

erosion mechanism to another mechanism and quantification of enhancedoxidation kinetics during erosion. Finally, the relevant areas for futurestudies are indicated.

Nomenclature

E Erosion rate

En Erosion rate at nth exposure

Mn Mass loss suffered due to erosion on nth exposure

Mn Mass gain experienced due to oxidation on

nth exposuret Time of exposure to eroding conditions

mn Cumulative mass of erodent for nth exposure

V Impact velocity

K1 Constant

p Velocity exponent of erosion rate

Kop Parabolic rate constant

m Incremental mass gain due to oxidation

Ao Arrhenius constant

Q Activation energy for oxidation

R Universal gas constant

T Absolute temperature

Z Thickness of the oxide scale

C ConstantKp Scaling constant

o Density of the oxide

F Particle flux rate

L Depth to which the deformed zone extends

W Indentation diameter

Constant

H Hardness

U Crater volume

mp Mass of single erodentr Radius of the erodent

N Number of erodents

Semi included angle of conical erodent

tb Time between impacts

tim Time of impact

Zb Thickness of the oxide scale grown between two

successive impacts

1. Introduction

Erosive wear or solid particle erosion is sometimes known

as impact wear. Solid particle erosion is defined as material

degradation due to the impact of particles travelling withsome significant velocity. It is mechanistically different from

other forms of erosion such as liquid impact erosion, slurry

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Topical Review

Table 1. Examples of systems subjected to elevated temperature erosion.

System Components References

Combustion systems Burner nozzles, reheater, super heater, [5,6]economizer tube banks, boiler heat exchanger,in bed tubes, tube banks, etc

Coal gasification systems Turbine, lock Hopper valves [7]Coal liquefaction system Valve to throttle the flow of product stream [7]Gas turbines Blades [8]

erosion and cavitation erosion, etc. A tribosystem suffering

from erosive wear can be characterized as an open system.

In such a system, the counterbody is continuously replaced.

Another important feature of solid particle erosion is that

the wear of the counterbody is completely uninteresting. In

solid particle erosion, the contact time between the erodent

and the target material is only momentary. In this respect,

erosion is different from other related processes such as sliding

wear, abrasive wear, grinding and machining, etc, in which thecontact between the tool/abrasive and the target/work piece is

continuous.

Room temperature erosion is an important problem

in several engineering applications. Rocket motor trail

nozzles [1], the engine of a helicopter operating in dusty

terrain [2], equipment in oil and mining industries [3, 4],

etc are subjected to solid particle erosion at ambient

temperature. Several engineering components are degraded

dueto solid particle erosion at elevated temperature. A number

of industrial systems, which undergo erosion at elevated

temperature, are summarized in table 1 [58]. At the same

time, it should be mentioned that erosion could also be used

constructively, as in the case of shot peening, sand blasting,mining, rock drilling and cutting applications, etc [911].

Erosion behaviour of metallic materials at room

and elevated temperatures has been reviewed in several

publications [1220]. The information contained in these

reviews will be repeated only to the extent of impressing upon

the readers the essential features of the erosion processes. It

is not the purpose of this work to provide a comprehensive

review of the status of research in the field of solid particle

erosion at elevated temperature; rather the aim is to discuss

and review some of the recent results which have enhanced

our understanding in the areas of elevated temperature erosion

of metallic materials. This review will be confined only to

metals and alloys. Elevated temperature erosion of ceramics,glasses and composites will not be considered here.

For the purpose of convenience, the review is divided into

several sections. Section 2 consists of elevated temperature

erosion tests. The salient features of elevated temperature

erosion constitute section 3. Examination of the eroded

surfaces is consideredin section 4. Section 5 dealswitherosion

oxidation interaction. Erosion enhanced oxidation kinetics

is analysed in section 6. Future research areas for elevated

temperature erosion arediscussed in section 7. Thisis followed

by concluding remarks in section 8.

2. Elevated temperature erosion test

Over the last few decades several test techniques or

methodologies have been developed for studying mechanisms

and assessing the extent of erosion. These tests can broadly

be divided into two categories, namely (1) those simulative

tests that are designed to simulate a specific type of erosion

and (2) those that are intended to be used for fundamental

studies. The main problem of the simulative test is that it is

very expensiveandit is difficult toconduct fundamental studies

for material development or for understanding the mechanisms

of erosion. In order to avoidtheseproblems, various laboratory

tests have been developed.The most common laboratory test involves blasting a

stream of airborne particles against the target as standardized

by G 76-83 [21]. In this type of test, a known quantity

of erodent is fed into an air stream, accelerated through a

converging nozzle and directed towards the test specimen. A

cleaned andweighed sampleis exposed to theparticle-laden air

streamfora predetermined time andweighed after interrupting

the test. The ratio of the weight loss suffered by the sample to

the weight of erodent gives the dimensionless erosion rate.

The air jet-type elevated temperature erosion rigs can be

classified into two broad groups. The first group comprises

erosion rigs, which are designed in such a fashion that both

the fluid stream carrying the particles and the eroding target

are heated to the same test temperature. These are called

isothermal erosion rigs. On the other hand, the second group

of erosion rigs, called non-isothermal rigs, have the facility to

heat the target material alone, while the fluid stream with the

particles is not preheated before allowing it to enter the erosion

chamber. In these types of rigs, the colder fluid stream cools

the target materials to some extent on impact. Nevertheless,

the target material still attains a steady-state temperature. The

above classification of the erosion rig as isothermal and non-

isothermal is largely artificial for reasons stated below.

1. The test specimen attains a constant temperature and

remains at that temperature throughout the test in both

types of rigs.2. The erodents are not heated and the cold particles impinge

thetarget material. However, unlikein thecase ofabrasion

or sliding wear, the contact between the particles and the

target materials in the case of solid particle erosion is

momentary, and hence negligible transfer of heat takes

place between the particles and the substrate. Thus, it is

immaterial as to whether theparticlesarepreheatedor not.

The non-isothermal erosion rigs are easy to fabricate but

they fail to simulate the erosion conditions. Hence, such rigs

are not popular at present. In contrast, isothermal type erosion

rigs can simulate erosion conditions but they are difficult to

fabricate. The schematic diagram of one such erosion rig,

fabricated by the author at DMRL, is shown in figure 1. Theunique feature of this rig is its ability to alter the particle

feed rate by over 100 times. Its particle feeding system is

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Air Heating

System

Temperature

Controller

For Air Heating

System

Air Pressure Indicator

Air Flow Rate IndicatorFluidized

Chamber

Test

Chamber

Sample

HoldingDevices

Particle Feeding System

Temperature

Indicator of

Hot Air

Temperature

Indicator of

The Sample

Outlet

Inlet

Air Heating

System

Temperature

Controller

For Air Heating

System

Air Pressure Indicator

Air Flow Rate IndicatorFluidized

Chamber

Test

Chamber

Sample

HoldingDevices

Particle Feeding System

Temperature

Indicator of

Hot Air

Temperature

Indicator of

The Sample

Outlet

Inlet

Temperature

Controller of the

Test Chamber

Figure 1. Schematic representation of jet type elevated temperature erosion rig [67].

a miniaturized conveyer belt system and particle feed rate is

controlled by controlling the speed of the motor of the system.

Further description of this rig is available elsewhere [22].

The test procedure involves heating the compressed air

to the required temperature and then heating and soakingthe test sample to that temperature. The heated samples are

then exposed to compressed, heated, fluidized and accelerated

air streams carrying the particles. The elevated temperature

erosion test is a multiple specimen test procedure. Cleaned,

dried and weighed samples are exposed to erodents for various

time intervals (say t1, t2, t3, . . . , where t1 < t2 < t3)

corresponding to various mass of erodents (m1, m2, . . . , mn).

A similar number of samples are again exposed to an air stream

without carrying particles. IfM1, M2, . . . , M n represent the

mass loss suffered by n samples when exposed to erodents for

time intervals oft1, t2, . . . , t n and ifM

1, M

2, . . . , M

n are the

massgainsexperienced by n samples when exposed to theplain

air stream without erodents for time intervals of(t1, t2, . . . , t n)then the incremental erosion rate E1, E2, . . . , En can be

computed as

En =(Mn M

n) (Mn1 M

n1)

mn mn1. (1)

This procedure is repeated until En1 is equal to En2 and

this E is considered to be the incremental erosion rate.

The main problem of the jet type of erosion rig is the

measurement of impact velocity. There are three different

types of velocity measurement techniques available. In the

first method, known as the photographic method, a high-

speed camera is used to photograph the successive positionsof a single particle as a function of time and thus compute

the velocity. The second method, known as the rotating

disc method, was developed by Ruff and Ives [23]. In this

method velocity is determined by estimating the time of flight

of particles between two discs fixed on a common shaft

rotating at some specified velocity. A modified version of

the rotating disc is the paddle wheel technique [24]. Thismethod gives a more reliable and statistically more accurate

velocity.

The third method uses the laser Doppler velocitimeter

(LDV). This technique is an accurate, non-interactive and

on-line velocity measuring device. The LDV uses the well-

known Doppler effect to measure the velocity of the particles.

When light is scattered from a moving object, the stationary

observer will see a change in the frequency of scattered light

proportional to the velocity of the object. A laser is used as a

light source because it is easily focused and is coherent.

In many applications, for example, pipe bends in a slurry

transportation system, the impact is primarily by big particles

with very low impact velocity. To simulate such a system,samples are impacted by dropping particles under gravity.

Such a system was first introduced by Bitter [25], a schematic

presentationof which is shown in figure 2. The system consists

of a ball dispenser unit, the velocity measuring system, ball

counting unit and the sample holder. The ball dispenser and

the sample holder can be moved up and down so as to alter the

height over which the eroding particle falls. Before the steel

ball impacts thesampleit passesthrough a multiple photodiode

unit, which measures the velocity of the passing ball and,

in addition, keeps track of the total number of balls passing

through.

Whirling arm rigs were developed to enable tests to be

carried out at precisely controlled velocity over a range ofimpact conditions. The target specimens are attached to the

tips of the rotor arms and whirled through a certain or a

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Ball

Feeder

Timer

Sample

Holder

Velocity MeasuringSystem

Figure 2. Schematic diagram of low velocity erosion rig.

Figure 3. Schematic diagram of whirling arm erosion rig.

narrow band of erosive particles. These rigs are very noisy

and consume considerable power. A whirling arm erosion

rig is schematically shown in figure 3. These kinds of rigs

simulate the degradation conditions prevalent in the fluidized

bed combustor. The whirling arm erosion rig was initially

developed by Tilly and Sage [26]. One of the advantages of

this type of rig is that the erodent velocity can be controlled

precisely as the velocity is governed by the rotating speed of

the arms. It also permits testing to be carried out over a widerange of impact velocities. It makes efficientuse of theerodent

as theentire amountof erodent is delivered to thetarget sample.

3. Salient features of elevated temperature erosionof metallic material

Over the last decade or so, a substantial amount of work on

elevated temperature erosion of metals and alloys has been

carried out [2762]. A compilation of these investigations,providingdetails of materialssubjectedto elevatedtemperature

erosion and the test conditions, is made in table 2. These

tests cover a wide range of test conditions and test materials.

Based on the work of the investigators compiled in table 2, the

important factors which influence the solid particle erosion

behaviour of metallic materials at elevated temperature are

described below.

3.1. Effect of temperature

The variation of erosion rate with temperature for a number

of metals and alloys is depicted in figures 4 and 5 [27, 28].

The erosion data presented in these figures pertain to highimpact velocities and mostly with angular particles. The

observed temperature dependence of the erosion rate can be

classified into three groups. In the first group, the erosion

rate initially decreases with the increase of temperature,

reaches a minimum and then starts increasing with increasing

temperature. Materials such as 5.0Cr0.5Mo, 17-4 PHSS,

410 SS, Alloy 800, Ti-6Al-4V, and tungsten belong to this

group. The second group comprises metals such as Ta and

lead (for oblique impact) andalloyssuch as 310SS (for oblique

impact), 1018 steel and 1100 aluminium (for normal impact)

which exhibit a temperature independent erosion rate up to a

critical temperature followed by an increase of the erosion rate

with increasing temperature. Finally, group three materialsshow a monotonically increasing erosion rate with increasing

temperature. Inco 600, carbon steel, 12Cr1MoV steel, and

2.25Cr1.0Mo steel, lead and 20245 Al are some of the typical

examples in this group.

3.2. Effect of impact velocity

The velocity dependence of erosion rate (E) is characterized

by the velocity exponent, p, given by

E = K1Vp, (2)

where K1 is a constant and V is the impact velocity. The

velocity exponents (p) obtained by various investigatorsare plotted in the velocitytemperature regime in figure 6.

The velocity exponent decreases with an increase in erosion

test temperature for 304 SS to values as low as 0.9 at low

impact velocities. At relatively higher impact velocities p

appears to lie in the range 23. Levy and Man [29, 30]

reported the influence of erodent size on the velocity exponent

for erosion of 9Cr1Mo steel at 923K using angular SiC

particles.

3.3. Effect of impact angle

The available data related to erosion rate and impact angle at

different temperatures are presented in this section. However,most of the metallic materials, irrespective of temperature of

erosion, exhibit a ductile behaviour, i.e. a maximum erosion

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Table 2. Compilation of the work of various investigators on elevated temperature erosion of metallic material.

Materials Name of the investigators Test conditions

310 SS, 304 SS, 1018, 2.25Cr1.0Mo, Levy et al [27] Nitrogen gas, 240 m SiC, V = 30ms1,5.0Cr0.5Mo, 410 SS, 17-4 PHSS T = RT to 1173K, = 30 and 90

1100 Al, 310 SS Finnie et al [32] Nitrogen gas, 250 m SiC, V = 3060 m s1,

T = RT to 0.8 Tm, = 30

9Cr1.0Mo, 2.25Cr1.0Mo, 5.0Cr10.5Mo, Levy and Man [29, 30, 47] Air, 130 m SiC and SiO2, V = 35ms1,

304 SS, 410 SS T = 9231123K, = 90 and 30

410 SS Levy et al [45] Air, 5 m fly ash, 130 m Al2O3,T = 1223K, V = 5 m s1, = 45

5.0Cr0.5MoSi Levy and Wang [40,48] Air, 90 m alumina, V = 30, 40 and 70m s1,T = 1123K, = 9

Inco 600 Tabakoff and Vittal [34] Air, 100800 m quartz, V = 60250ms1,T = 544, 666 and 846K, = 1070

Ti6Al-4V, 2074 Al, 410 SS, W, Ta, Pb Gat and Tabakoff [28] Air, 86 m quartz, V = 180300m s1,and 304 SS T = 3001023K, = 1590 also 164 m quartz,

V = 120ms1, T = 300483K, = 20, 60 and 90

Ni, Co Kang et al [50] Nitrogen, air, 20 m alumina, V = 90 and 140m s1,T = 9231073K, = 90

Ni, Co Chang et al [38, 47] Air, 20 m alumina, V = 140, 123 and 70m s1,

T = 8731053K, = 90

, 60

, 30

and 20

AISI 303 Shayler and Yee [31] Air, 4766 m fly ash, V = 150300 m s1,T = 300773K, = 35

304 SS, alloy 800, C-steel, 2.25Cr1.0Mo, Shida and Fujikawa [42] Argon gas, 120 m quartz, V = 40120m s1,12.0Cr1.0MoV T = 673923K, = 2090

304 SS, 316 SS and 410 SS Singh and Sundararajan [43] Air, 160 m SiC, V = 55110ms1,T = 300773K, = 30, 60 and 90

In 738, X 40, MA 754 and HA 188 Barklow et al [51] Burner rig, 20 m alumina, V = 200275 m s1,T = 1148 K

Stellite 6B and 1, In-100, MA-754, HA 8077, Wright et al [52] Argon gas, air, 12 m alumina, V = 43ms1,AISI 446 SS, FeCrAlY, AISI 446 T = 1033K, = 30

9.0Cr1.0Mo, 304 SS Sethi and Wright [53] Air, 1 m alumina, V = 2.7 and 4.3 m s1,T = 7331100K, = 30

304 SS, 416 SS, 430 SS and 17-4 PH SS Zhou and Bahadur [35] Air, 120 grit SiC, V = 65ms1,T = 9231073K, = 30

2.25Cr1.0Mo Sethi and Carey [54] Air, 1 m alumina, V = 2.7 m s1,T = 763863K, = 30

Alloy 800 HT, 310 SS Stott et al [55] Air, 4766 m fly ash, V = 150300 m s1,T = 300773K, = 35

Haynes 188, Waspaloy Chinadurai and Bahadur [56] Air, 150 m SiC, V = 50ms1,T = 3001073K, = 30

Ti6Al-4V Zhou and Bahadur [57] Air, 4766 m fly ash, V = 50ms1,T = 3001073K, = 1090

Carbon steel Xie and Walsh [58] Air, 43 m coal ash, V = 530ms1,T = 423673K

FeCrC cast steel Drotlew et al [59] V = 65ms1, T = 723K, = 1560

Ni, Ni20Cr Manish Roy et al [60] Air, 200 m SiO2, V = 35, 65 and 105m s1,

T = 3001073K, = 30, 45, 60 and 90

Fe3Al base alloy, Ni, Co Yu et al [61] SO2, air, 2050 m SiO2, V = 160190 m s1,

T = 8731073K, = 090

FeCrC Hayashi et al [62] Air, 23 m cold silica, V = 80ms1, T = 973K

Note: T = test temperature, V = impact velocity and = impact angle.

rate at oblique impact angles (1030). The universality

of such an observation is shown in figures 7 and 8,

wherein data from a large number of investigators have been

compiled [3136].

Levy [37] obtained a higher erosion rate at normal impact

than at oblique impact for 9Cr1Mo steel at 1123K using

rounded Al2O3 (130 m) erodent for a range of impact

velocities (3070 m s1), as shown in figure 9. But at a low

impact velocity of 20 m s1, a maximum in the erosion rate

occurred at oblique impact angle. Observations by Chang et al

[33, 38] shown in figure 10 indicate that the peak erosion rateof Co at a test temperature of 1053K occurs at an impact angle

of 60 when impacted with 20 m angular alumina particles at

impactvelocities of 70140m s1. However, when the erosion

test is carried out at 873 K and at impact velocity of 140m s1,

the erosion rate peaks at an impact angle of 30. Thus, there

exist apparently conflicting observations regarding the erosion

rateimpact angle behaviour.

3.4. Effect of particle size

Tabakoff and Vittal [34] carried out erosion tests on Inco 600

alloy using quartz particles having sizes between 70 and

800 m. Their work indicates that the erosion rate increasesmarginally with the increase of particle size, as shown in

figure 11. Zhou and Bahadur [35] investigated the effect

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of the particle size of SiC on the erosion rate of 304 SS

at 923 K (impact angle: 30 and impact velocity: 65 m s1).

These results indicate that erosion rate increases with the

increase in particle size up to 40 m and thereafter it becomes

independent of the particle size (figure 12). Levy et al

[29, 30, 37, 39], however, noted only the increase of erosion

rate with the increase of particle size for 9Cr1Mo steel eroded

at a temperature of 923K and that for 1018 steel eroded

at 723 K.

0

20

40

60

80

100

120

0 200 400 600 800 1,000 1,200

Temperature (K)

ErosionRatex

106(

Kg/Kg)

310 SS

304 SS

410 SS

2.25Cr-0.5Mo

17-4 PHSS

1018 Steel

5.0Cr-0.5Mo

Impact Velocity : 30 m/s

Impact Angle : 30o

Erodent : SiC (250 m)Gas : Nitrogen

Figure 4. Variation of erosion rate with temperature for a number ofalloys [27].

20

30

40

50

60

70

80

90

100

110

120

283 303 323 343 363 383 403 423

Temperature (K)

ErosionRatex104(

Kg/Kg)

Lead

Impact Angle : 20o

Impact velocity : 120 m/s

Erodent : Quartz ( 138 - 164 m)

0

20

40

60

80

100

120

140

283 303 323 343 363 383 403 423

Temperature (K)

ErosionRatex104(

Kg/Kg)

Impact Angle : 90o


Erodent : Quartz ( 130 m)

Lead

Tungsten

0

1

2

3

4

5

6

7

8

9

10

283 333 383 433 483

Temperature (K)

Erosionratex104

(Kg/Kg)

2024 Al

Ta

Ti-6Al-4V

410 SS

Impact Angle : 20o


Erodent : Quartz ( 138 - 164 m)2

2

3

3

4

4

5

283 333 383 433 483

Temperature (K)

ErosionRatex104(

Kg/Kg) Ta

2024 Al

410 SS

Ti-Al-4V

Impact Angle : 90o


Erodent : Quartz ( 138 - 164 m)

Figure 5. Effect of test temperature on the erosion rate of a number of metals and alloys [28].

3.5. Effect of particle shape

Levy et al [29, 30, 37, 39, 40] investigated the erosion rate of

a number of Cr containing steels at 1123 K with angular SiC

and spherical Al2O3 as erodent particles. A typical result for

9Cr1Mo steel is presented in figure 13. The erosion rate is

significantly higher when SiC is used as the erodent.

3.6. Effect of particle feed rate

Zhou and Bahadur [41] have investigated the influence of the

particle feed rate on the erosion rate of 304 and 430 SS over

a large temperature range. The reported results as illustrated

in figure 14 suggest that up to a temperature of about 773 K

increasing the feed rate by a factor of 16 has no influence on

the erosion rate. However, beyond 773 K, a lower feed rate

results in a substantially higher erosion rate. Roy et al [60]

also noted a decrease of erosion rate with the increase of the

particle feed rate, especially at low impact velocity.

3.7. Effect of eroded material characteristics

The interpretation of the available data on the effect of eroded

material characteristics on its erosion rate is complicated by

the fact that the behaviour of the oxide scale under erosion

conditions needs to be considered in addition to the behaviour

of the metallic material per se. The oxidation characteristic

of the eroded material plays a more important role than the

mechanical properties of the eroded materials at elevated

temperature.

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P values given alongside data points

Figure 6. Velocity exponent obtained by various investigators,plotted in velocitytemperature regime.

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70 80

Impact Angle (Degrees)

ErosionRatex104

(Kg/K

g)

850 K

644 K

766 K

Material : INCO 600


0

1

2

3

4

5

6

10 20 30 40 50 60 70 80 90


ErosionRatex104(

Kg/Kg)

Material : 303 SS


291 K

1023 K

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0 20 40 60 80 100


MaximumThicknessLoss(m/s)

Test Temperature = 573K

Impact Velocity = 120 m/sErodent = quartz (120 m)

304 SS

Cr-Mo-V

C Steel

0

1

2

3

4

5

6

0 20 40 60 80 100


ErosionRatex104(

Kg/Kg)

1255 K

293 K

293K

1093 K

558 K372 K

743 K

Impact Velocity = 61 m/sImpact Velocity = 31 m/s

Erodent = SiC (250 m)Fluidizing Gas = Nitrogen

Al

310 SS

ab

cd

Figure 7. Variation of erosion rate with impact angle at various test temperatures: (a) INCO 600 [34], (b) 303 SS [31], (c) CrMoV, carbonand 304 SS [42] and (d) aluminium and 310 SS [32].

Even at elevated temperature, if one considers the erosion

behaviour of metallic material at high impact velocities and

feed rates, the oxidation plays an insignificant role and the

erosion behaviour is essentially metal erosion behaviour. The

experimental data of Shida and Fujikawa [42] pertaining to

1.25 Cr1MoV, 2.25CrMo, 12Cr1MoV and plain carbonsteel (up to 923 K), that of Singh and Sundararajan [36, 43]

pertainingto 304, 316, 410 stainless steel (up to773 K)and that

of Levy et al [44, 45] pertaining to 2.25Cr1.0Mo steel, 5Cr

0.5Mo steel, 1018steel, 304 SS, 310 SS, 410 SSand17-4PHSS

can be considered elevated temperature erosion of metals with

minimum or negligible oxidation. Under such conditions the

dependence of the strength of the material on temperature is a

reasonable indicator of the temperature dependence of erosion

resistance of the materials. Erosion data further indicate that

austenitic stainless steels have superior resistance to elevated

temperature erosion than ferritic steels. The 410 stainless

steel having a tempered martensitic matrix exhibits an erosion

resistance comparable to that of austenitic stainless steels.The oxidation effect is reported to be important for

elevated temperature erosion tests conducted at low impact

velocities and using rounded Al2O3 as an erodent. The

influence of Cr content on the erosion rate of steel is

illustrated in figure 15. It is indicated that the erosion rate

decreases to a very low value when Cr content in the steel

exceeds 1012% [35]. In the case of steel having Cr less than

10%, thick Fe2O3 scale was formed during erosion leading to

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0

5

10

15

20

25

30

35

40

45

10 20 30 40 50 60 70 80 90


ErosionRatex104 (Kg/Kg)

Material : 304 SS


1173 K

873 K

0

2

4

68

10

12

14

16

10 20 30 40 50 60 70 80 90


ErosionRate

x104(

Kg/Kg) Material : 304 SS

Impact Velocity : 183 m/s1173 K

873 K

0

1

1

2

2

3

3

0 20 40 60 80 100


ErosionRatex104(

Kg/Kg)

773 K

297 K

Material : 304 SS


4

5

6

7

8

9

10

11

12

13

14

20 30 40 50 60 70 80 90 100


ErosionRatex104(

Kg/Kg)

316 SS

410 SS


763 K

RT

548 K

a b

c

d

Figure 8. Influence of impact angle and test temperature on erosion rate of (a) 304SS, (b) 304SS, (c) 304SS [35] and (d) 410SS and316SS [43].

high erosion rates [46]. Theexperimental results of Chang etal

[47], presented in figure 16, also indicate the importance of

the nature of the scale that forms during erosion at elevatedtemperature. It is also noted that materials with high scaling

rate such as nickel and cobalt exhibit the highest erosion

rates, while the alumina-forming alloys such as CoCrAlY and

NiCrAlY exhibit intermediate erosion rates. The superiority

of the Al2O3 forming alloy stems from the fact that the Al2O3forming scale forms much more slowly than the Cr2O3 scale.

Extensive work by Levy andco-workers [37,39,44,48,49]

implies that the morphology of the oxide scale that forms

during erosion is important. Segmented scales have a better

erosion resistance than thick, continuous and dense scale since

the spalled area is confined to oxide crystalline only in the case

of erosion of the segmented scale. A striking illustration of the

abovefact is obtained when Si is added to steels. Addition of Sito low chromium steel results in the formation of a segmented

scale even at high impact velocity and thereby reduces the

erosion rate substantially as compared with the same steel

without Si [49].

The above discussion clearly brings out certain features

of elevated temperature erosion of metallic materials. Almost

all metallic materials exhibit ductile erosion response at room

temperature, whereas at elevated temperature both brittle and

ductile erosion responses are noted [31, 36, 37]. The velocity

exponents for metallic materials are 2.5 during ambient

temperature erosion. At elevated temperature the velocity

exponent of metals and alloys varies over a wide range from

0.9 to even more than 3.0 [3133]. It is established that theerosion rate at room temperature increases with the increase

of the particle size up to 50 m and beyond such magnitude

0

5

10

15

20

25

30

35

40

45

0 20 40 60 8 00


MetalThicknessLoss(m) 70 m/s

45 m/s

35 m/s

25 m/s

Figure 9. Variation of metal thickness loss with the impact angle for

9.0Cr1.0Mo steel at 1123 K [37].

the particle size has no effect on the erosion rate. Reported

literature indicates that the erosion rate increases with the

increase of particle size at high temperature [29,30,34,39]. At

ambient temperature, changing theparticle shape from angular

to spherical results in altering the erosion response from brittle

to ductile [63, 64]. At elevated temperature, brittle to ductile

response is noted irrespective of the particle shape [29,30,37].

Particle feed rate has a negligible effect on room temperature

erosion rate [6567]. A remarkable effect of the particle

feed rate has been noticed at elevated temperature [29]. The

characteristics of mechanical properties of eroding material

particles have nominal influence on room temperature erosionbehaviour [56, 6875]. In contrast, this aspect at elevated

temperature is hardly explored.

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 8 00


ErosionRate(x10

-1,Kg/m2sec) 1053 K, 140 m/s

873 K, 140 m/s

873 K, 70 m/s

Figure 10. Variation of erosion rate of Co with the impact angle at1053 K [38].

0

1

2

3

4

5

6

7

0 100 200 300 400 500 600 700 800 900

Particle Size (m)

ErosionRate(m3/Kg)x10

Figure 11. Influence of particle size on the erosion rate of Inco 600alloy [34].

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160

Particle Size (m)

ErosionRate(Kg/Kg)

Figure 12. Effect of particle size on the erosion rate of 304 SS at923 K [35].

4. Examination of eroded surfaces

At elevated temperature, the material removal is governed

by the synergistic effect of erosion and oxidation (EO).

Detailed examination of the eroded target material using a

scanning electron microscope (SEM), a transmission electronmicroscope (TEM) and an optical microscope (OM) has been

carried out. The advent of single particle experiment is

0

50

100

150

200

250

300

0 10 20 30 40 50 60 7 0

Impact Velocity (m/s)

WeightLossx10-6 (Kg/Kg)

9Cr 1 Mo Steel

Nozzle Tester Corrosion Erosion Air

Particle Size: 130 m

Test Temperature : 1123K

Test Time: 2 hr

SiC

Al2O3

30o

30o

90o

90o

Figure 13. Variation of weight loss with impact velocity for9.0 Cr1.0Mo steel at 1123K showing the effect of particle feedrate on erosion behaviour [37].

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1,00 ,200

Test Temperature (K)

ErosiveMassLossRate(Kg/Kg) Erodent Concentration

2.6x10-6

Kg/m2s

42.0x10-6

Kg/m2.s

Material:

304 SS

430 SS

Figure 14. Variation of erosion rate of 304 SS and 430SS with testtemperature showing the effect of feed rate on erosion rate [41].

particularly fruitful since such experiments allow a detailed

examination of large, individual craters. On the basis of

the extensive literature [57, 67, 7678], four different types of

EO mechanisms can be envisaged: in the first case, at low

temperatures, at high impact velocities and feed rates, there is

no oxide scale. Even if there is any oxide scale, it will be very

thin and it will be able to deform in the same manner as that

of the substrate target. Under such circumstances, erosion

takes place from the metallic surface and this mechanism

of erosion is called metal erosion. The erosion behaviourin this regime is similar to the ambient temperature erosion

behaviour of metallic materials. The erosion response in the

metal erosion regime is ductile, the velocity exponent of the

erosion behaviour is between 2 and 3 and the erosion rate

is independent of the particle feed rate. The metal erosion

mechanism is schematically shown in figure 17(a). In the

metal erosion regime, there are two modes by which materials

can be removed. These modes are ploughing and cutting. In

general, when a particle is in contact with a target at positive

rake angle, the cutting mode operates. On the other hand, the

ploughing mechanism operates at negativerake angle. Cutting

mechanisms result in generation of new surfaces while the

ploughing mechanism involvesthe displacement and extrusionof the material with no new surface generation. In addition,

the cutting mode is more efficient than the ploughing mode

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0

20

40

60

80

100

120

0 5 10 15 20 25 30

Chromium Content (wt %)

MetalThicknessLoss

(m)

Normal Impact

Oblique Impact

Test Temperature : 1123 K

Impact velocity : 35 m/sErodent: Al2O3, 130 mFluidizing Medium: Air

310 SS304 SS

410SS

9Cr

5Cr

2.25Cr

Figure 15. Influence of Cr content on the erosion rate of steel [35].

when considered in terms of energy consumed per unit volume

removal of the target material. The work carried out by a large

number of investigators [7982] has revealed that almost all

metals and alloys lose material by the formation of a lip and

its subsequent fracture.

Inmetalsand alloys, duringerosion,once thelip is formed,

it is fractured by several modes. In the case of ductile metals

like copper [83], brass [83], aluminium [84] and iron [85]

the lip fracture occurs by necking, and the resulting fracture

is ductile, as exemplified by the dimpled fracture surface.

This mode of lip fracture is shown in figure 18(a). In the

case of a high strength alloy such as CuBe in age hardenedcondition [83], 301 SS [85] and TD nickel [85] the lip removal

is greatly aided by the formation of adiabatic shear bands at

the base of the lip as shown in figure 18(b) and subsequent

easy separation/fracture across this band. In this case also

the fracture is ductile, as indicated by the presence of shear

dimples on the fracture surface. These two modes of material

removal considered above involve the fracture of pre-existing

lips. On the other hand, a new mode termed adiabatic

shear induced spalling involves the formation of intersecting

adiabatic shear bands at the base of the crater and subsequent

removal of chunks of material as illustrated in figure 18(c).

This mode of weight loss, which is highly efficient in terms of

energyexpendedper unit volumeof targetmaterial removed, isimportant only at normal impact where maximum resistance

is offered to curtail spreading of deformation. The erosion

response under such circumstances will be similar to that

observedforceramic materials. But theunderlyingmechanism

is entirely different. In the case of ceramic materials, material

removal occurs with the formation of intersecting cone or

radial cracks, which nucleate from pre-existing flaws once a

critical tensile stress is exceeded. Further, these cracks are

essentially brittle in nature. On the other hand, formation of

anadiabaticshearbandrequires critical strain[86]. In addition,

the fracture surface resulting from adiabatic shear induced

spalling exhibits sheardimples, implying an essentiallyductile

fracture.On the other extreme, at very high temperatures and low

velocities and particle feed rates, erosion takes place from

the oxide scale only, as shown in figure 17(b). Under such

conditions, a thick oxide scale is formed on the target material

during erosion and the deformed zone formed due to impact is

confined within the oxide scale. The erosion behaviour from

the oxide scale is characterized by a brittle erosion response,

strong velocity dependence and particle feed rate independentof the erosion rate. This erosion mechanism is termed oxide

erosion. In oxide erosion, material removal occurs with

the formation of intersecting cones and radial cracks, which

nucleate from pre-existing flaws once a critical tensile stress

is exceeded. At an intermediate temperature, impact velocity

and particle feed rate, an oxide scale of intermediate thickness

is formed. However, the depth of the deformed zone extends to

the metallic substrate beyond the oxide scale. Consequently,

the oxide scale beneath the eroding particle tends to crack, gets

pushed down into much softer base material and in the process

the softer base material gets squeezed out onto the top surface

through the cracks in the oxide scale. Over a period of time,

the repetition of such a process during each impact causes the

formation of a composite layer comprising the bulk metal and

broken pieces of oxide scale. Erosion takes place from this

composite layer. This mechanism, presented schematically

in figure 17(c), is termed oxidation affected erosion. The

interesting aspect of oxidation affected erosion is that the

volumefractionof theoxidein thecomposite layer isa function

of erosion conditions such as temperature, impact velocity

and particle feed rate. As a result, the erosion behaviour

in the oxidation affected erosion regime can vary from a

ductile to a brittle response depending on the amount of oxide

scale present in the composite layer. Further, unlike in the

case of metal erosion or oxide erosion, the oxidation affectederosion rate depends strongly on the test temperature and

particle feed rate. The final erosion mechanism is oxidation

controlled erosion and this is illustrated in figure 17(d). At

relatively higher temperaturesandlowerparticle feed rates and

impact velocities, the oxide scale that forms during erosion

is brittle and non-adherent. In such cases, the oxide scale

gets removed after it attains a critical thickness. The erosion

behaviour in this regime exhibits a brittle erosion response,

weak velocity dependence and particle feed rate dependent

erosion rate. Figures 19 and 20 show SEM images of the

morphologies of the eroded surfaces and transverse sections

of the eroded surfaces, obtained after exposing commercially

pure Ni to elevated temperature erosion. All the four

mechanisms described schematically in figure 17 can be seen

under SEM.

5. Erosion oxidation interaction

As mentioned previously, the erosion behaviour of metallic

materials at elevated temperature is governed by the nature

of interaction between erosion and oxidation. The nature of

interaction between erosion and oxidation in turn depends on

the thickness, morphology, adherence and the toughness of

the oxide scales that form in these materials. Before goinginto the details of mechanisms it is important to deal with the

theoretical aspects.

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 2 3 4 5 6

ErosionRate(Kg/m2s)x106

Co

MA-754Ni-30Cr

CoCrAlY

Ni-20Al



Impact Angle : 30o

Erodent : Al2O3 (20 m)

0.0

0.5

1.0

1.5

2.0

2.5

0

1 2 3 4 5 6

ErosionRate(Kg/m2s)x106

Ni

Co

MA-754 Ni-30Cr

CoCrAlY

Ni-20Al



Impact Angle : 30o

Erodent : Al2O3 (20 m)

Ni

(a)

3.(b)

Figure 16. Bar diagram showing the erosion rates of various alloys forming different types of scale under oblique impact at (a) 1073 K,(b) 873K [48].

5.1. Theoretical aspects

The details of the interrelationship between erosion conditions

and erosion mechanisms has been presented earlier. Furtherdiscussion on this interrelationship will be carried outsubsequently. Thus, only the salient theoretical aspects willbe considered in the following section.

5.1.1. Steady state oxide scale thickness. If it is assumed

that the oxide scale which forms on the eroding materialduring erosion is adherent and sufficiently ductile to withstandrepeated impacts without developing cracks, steadystateoxidescale thickness can be defined. It can be assumed that theoxidationof theeroding material follows the parabolickineticsgiven by equation (3)

m2 = Kop t, (3)

where m is the mass gain experienced by the metal per unitarea due to intake of oxygen to form oxide scale, Kop is the

parabolic rate constant and t is the time of exposure. Theparabolic rate constant is usually expressed in the form:

Kop = Ao expQRT

, (4)

where Ao is the Arrhenius constant, Q is the activation energy

for oxidation, R is the gas constant and T is the absolute

temperature.

In order to represent EO interaction in mathematical

terms one needs the rate of growth of the oxide scale thicknesswith time rather than the weight gain given by equation (3).

As noted by Lim and Ashby [87], once the composition of the

oxide scale is known, equation (1) can be transformed to give

Z2 = 2Kpt, (5)

Kp = 0.5 C2Ko

p

, (6)

where C is a constant for a given oxide composition (unit ism3 kg1). Kp is usually referred to as the scaling constant.

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Metal

Deformed

Region

Erodent

Lip

Lm

Metal Erosion with No Oxide Scale

Metal

Deformed

Region

Erodent

Lip

Lm

Metal Erosion with No Oxide Scale

Metal

Deformed

Region

Erodent

Lip

Lm

Metal Erosion with Thin Adherent Ductile Oxide Scale

V

Oxide

Scale

Metal

Deformed

Region

Erodent

Lip

Lm

Metal Erosion with Thin Adherent Ductile Oxide Scale

V

Oxide

Scale

a (Metal Erosion)

Metal

OxideZ Lo

Damaged

Zone

V

Metal

OxideZ Lo

Damaged

Zone

V

b (Oxide Erosion)

Metal

Deformed

Region

ErodentV

Oxide

Scale

Metal

Deformed

Region

ErodentV

Oxide

Scale

Metal

Erodent

V

Oxide

Scale

Lip

Metal

Erodent

V

Oxide

Scale

Lip

Metal

Composite LayerV

Metal

Composite LayerV

c (Oxidation Affected Erosion)

Metal

Oxide

Scale

Metal

Oxide

Scale

Metal

Newly Formed

Scale

Metal

Newly Formed

ScaleOxide

Debris

Oxide

Scale

Metal

Oxide

Debris

Oxide

Scale

Metal

d (Oxidation Controlled Erosion)

Figure 17. Schematic presentation of various erosion mechanisms at elevated temperature.

A value appropriate to the erosion conditions should be chosen

for Kp since Levy etal [88] have clearly demonstrated that the

oxide scales grow much more rapidly under erosion conditions

as compared with static conditions. From equation (5),

the rate of increase of oxide scale thickness with time is

given as

dZ

dt=

Kp

Z. (7)

IfEo is assumed to be the erosion rate of oxide scale and

F be the particle flux rate given by the ratio of particle feed rate

(f ) to the eroded area then the rate of decrease of the oxide

scale thickness due to its erosion is obtained as

dZ

dt=

EoF

o, (8)

where o is the density of the oxide.

Finally, a situation will arise when the oxide growth byoxidation (equation (7)) will be equal to the oxide removal by

erosion (equation (8)). Under such conditions, the steady state

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a

Figure 18. Schematic diagrams of various metal removal mechanisms during metal erosion.

Figure 19. SEM images showing different morphologies of eroded surfaces of Ni exposed at elevated temperature: (a) metal erosion,(b) oxidation affected erosion, (c) oxidation controlled erosion and (d) oxide erosion [67].

oxide thickness (Zss) can be obtained as

Zss =Kpo

EoF. (9)

Hence, the steady state oxide thickness increases with

increasing temperature(through Kp), decreasing oxide erosion

rate and decreasing particle flux rate. It should be mentioned

that Eo in equation (9) represents theerosion rate of pure oxide.

5.1.2. Depth of deformed zone in oxide and metal. A largenumber of experiments have consistently shown that the depth

to which the deformed zone extends is usually of the order of

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Figure 20. SEM images of the transverse section of the eroded surfaces of Ni shown in figure 19. (a) Metal erosion, (b) oxidation affectederosion, (c) oxidation controlled erosion and (d) oxide erosion [67].

indentation diameter, or in other words

L = W , (10)

where W is the indentation diameter and is a constant of

the order of unity. The magnitude ofW depends on whether

the erodent is spherical or angular. When a conical particle

of mass mp as shown in figure 21 impacts an eroding material

of hardness H with an impact velocity V, then the incident

energy of the impacting particle is given as 0.5 mpV2 and the

energy consumed in forming the crater of volume U is H U

where H is the hardness of the target material. Equating these

two energies, we get

H U = 0.5 mpV2. (11)

For a spherical erodent having radius r , U is given by

U = W4

64r(12)

since crater depth can be considered to be considerablysmallerthan the diameter of the erodent. Substituting equations (12)

in equation (11) and solving for W one obtains

W = 2.56 rV1/2

H

1/4, (13)

where is the density of the erodent. Hence, the depth of

deformation for a spherical erodent can be obtained by putting

equation (13) in equation (10) as

L(Sphe) = 2.56r V1/2

H

1/4. (14)

For conical particles having a half angle of = 30, U is

given asU =

24

W3tan

. (15)

Substituting equation (15) in equation (11) and solving for

W one obtains

W =2r1/3V2/3

H1/3. (16)

Thus the depth of deformation for a conical particle can

be obtained by putting the value ofW in equation (10) as

L(con) = 2.0r1/3V2/3

H1/3. (17)

Equations (14) and (17) are valid irrespective of whether the

target material is metallic material or oxide scale. However,

it should be mentioned that the hardness H in equations (14)

and (17) represents the hardness of the oxide scale Ho in the

case of thick oxide scale as target material and the hardness of

the base metal in cases where the thickness of the oxide scale

is very small.

5.1.3. Critical oxide thickness (Zc). An important factor

that should be considered to understand the EO interactionis that the oxide scale usually exhibits a ductile to brittle

transition as a function of both thickness and temperature.

Stephenson et al [89] have demonstrated this phenomenon

under impact conditions. Saunders and Nicholls [90] have

also noted similar ductile to brittle transition for chromia and

alumina coatings. Importantpointsrelated to this phenomenon

are given as follows.

(1) At all temperatures, the scale becomes brittle beyond a

critical thickness (Zc) and thus can be removed easily

by spalling or by cracking and chipping due to particle

impacts.

(2) This value of Zc changes discontinuously over narrow

temperature ranges usually in the range of 700800

C.(3) Belowandabovethis temperature range Zc is independent

of temperature.

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ts

/

Figure 21. Variation of the calculated ratio of the time between impacts to the time of impact with hardness.

5.1.4. Time between the impacts and time of impact. If a

spherical erodent of radius r and mass m is considered, then

the number of particles impacting the unit area of the eroding

material every second equals F /m where F is the particle feed

rate. If it is assumed that each impact event causes damage

over an area A, where A is the impact crater area and is a

constant of the order of unity, then the number of particles (N )

impacting an area ofA every second is given by

N=

F

m

A (18)

or

N=

F

m

0.25 W2, (19)

where A = 0.25 W2, W is the impact crater diameter. The

time between two impacts can now be obtained as

tb =4mp

F W2. (20)

Substituting the value ofW obtained from equation (13)

in equation (20) and noting that mp=

4/3 r3

, tb is obtainedas

tb(sph) =0.82r1/2H1/2

F V. (21)

In the case of conical particles with a hemispherical top

of size 2r, mass m and half angle , the time between impacts

can be obtained from equation (20) if is 30. This is because

the mass of such a particle is equal to that of a sphere of

diameter 2r . For conical particles W is given by equation (16)

and substituting equation (16) in equation (20) and putting

m = 4/3 r3, tb is obtained as

tb(con) =4r1/3H2/3

3F V4/3. (22)

In the above expression it is assumed that the rebounding

particle does not interfere with the incident particle. This

assumption is reasonable because the erosion oxidation

interaction becomes prominent only at a low particle feed rate.

It is also important to consider the contact duration

between the particle and the eroding material during each

impact. This duration known as time of impact (tim) is given

as [91, 92]

tim(sph) =1.28r

H1/2, (23)

where r is the radius of the spherical particle having hardness

H and density . Thus for spherical particles the ratio of

time between impacts and time of impact can be obtained by

dividing equation (21) by equation (23) as

tb

tim(sph) =

0.64H

V F. (24)

In the case of conical particles the time of impact can be

given to a good approximation as

tim(con) =2.8r1/3

H1/3V1/3. (25)

Thus the ratio oftim and tb is obtained from equations (22)

and (25) as

tb

tim(con) =

0.48H1/3

V F. (26)

In the above equations H represents the hardness of the

eroding material. The calculated values of the ratios tb/tim in

the case of spherical (continuous lines) and conical erodents

(broken lines) are illustrated in figure 21 as a function of

hardness of the eroding material and for four values of the

product of impact velocity and feed rate. It can be noted from

figure 21 that the time between impacts is several orders of

magnitude higher than the time of impact. Hence for theerosion process at elevated temperature, time of impact has

negligible influence compared with time between impacts.

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5.1.5. Oxide scale growth between impacts. On the

assumption that oxidation is governed by a parabolic law,

the growth of oxide scale thickness (Z) is described by

equation (5). Thus the thickness of the oxide scale (Zb) grown

between two successive impacts is given by

Z2b = 2Kptb. (27)

Substituting the value oftb from equations (21) and (22)

in equation (27), one obtains the following equation for Zb for

spherical and conical particles.

Zb(sph) = 1.26

Kpr

V F

1/2(H)1/4 (28)

and

Zb(con) = 1.63Kpr

F

1/2

1/6H1/3

V2/3 . (29)

While deriving the above two equations it is assumed

that the growth of the oxide scale is controlled by the metal

and/or the oxygen ion diffusion through the scale. Thus, it

is assumed that the oxide scale does not crack as it grows,

thereby providing an easy path for the oxygen diffusion to

the metal oxide interface. This assumption is reasonable

because it concerns the growth of the oxide scale at a very local

region surrounding the impact point wherein prior impact has

removed the scale. Thus, the scale needs to be adherent and

uncracked only in this local region for the parabolic kinetics

to be valid. It does not matter that the scale is heavily cracked

or spalled at the macroscopic level.ThemagnitudeofZb incomparisonwithsteadystateoxide

scale thickness (Zss) has an important bearing on the erosion

oxidation interaction. Therefore, the variation ofZss and Zbwith impact velocity is given in figure 22 for two different

temperatures and two different feed rates. The magnitude of

Zb is indicated for both spherical and conical erodents. The

values of various parameters used for calculating Zb and Zssare given in table 3. Since most of the work on elevated

temperature erosion is done with steel, the values appropriate

to steel are chosen for various parameters. A value of 105 for

Ao and 210kJ mol1 for Q are chosen on the basis of the data

reported by Quinn [93]. The value ofC is calculated based

on the formation of Fe2O3 scale as observed by Levy and co-

workers [29, 45, 59] for a variety of ferritic steel undergoing

erosion oxidation degradation. The hardness of the oxide

scale and the metal is assumed to be 3.0 GPa and 2.0GPa,

respectively. Zc iskeptwithintherangeof110 m, consistent

with values reported by various investigators [89, 90]. A

perusal of the erosion literature indicates that the particle feed

rate mostly lies in the range 0.110 kg m2 s1. The erosion

rate of the oxide scale is assumed to be proportional to V3. The

test temperature is chosen in the range of 8731173 K since the

oxidationeffect becomes importantat these temperaturesin the

case of steels. It is clear from figure 22 that Zss decreases much

more rapidly with impact velocity than Zb. Thus a transitionvelocity beyond which Zss < Zb can be obtained for a given

temperature and feed rate.

Table 3. Assumed values of various important parameters.

Constant/variable Symbol Values Units

Hardness of oxide Ho 3.0 GPaHardness of metal Hm 2.0 GPaParabolic rate constant Kop kg

2 m4 s1

Arrhenius constant Ao 105, 104, 103 kg2 m4 s1

Activation energy Q 210 kJ mol1

Erosion rate of oxide Eo kgkg1

Erosion rate constant Eoo 105107

Reference velocity Vo 10 m s1

Velocity exponent n 3Density of oxide o 5400 kg m

3

Density of particles p 3200 kg m3

Erodent radius r 100(10) mCoversion factors for C 1.3 104 m3 kg1

transforming Kop to KpParticle flux rate F 0.1, 1.0, 10.0 kgm2 s1

Particle shapeCritical oxide thickness Zc 1, 10 mImpact velocity V 5100 m s1

Test temperature T 8731173 K

5.2. Conditions for prevalence of various EO mechanisms

If there is no oxide scale on the metal surface or if the

thickness of the oxide scale is very small compared with the

depth to which the deformation extends in the metal surface,

the dominant erosion mechanism will be metal erosion. On

the other extreme, if the steady state thickness (Zss) of the

oxide scale is less than the critical thickness of spalling (Zc)

and the depth of deformation is lower than the steady state

thickness, oxide erosion is the prevalent erosion mechanism.

If, however, Zss is lower than Zc and the depth of the

deformed zone is bigger than Zss, the oxidation affectederosion mechanism is observed. Finally, if the steady state

thicknessof theoxidescaleis greater than thecriticalthickness,

the oxide scale will never attain steady state thickness. The

erosion will take place by spalling of the scale and oxidation

controlled erosion will be operative.

Roy et al [94], employing a new methodology, examined

thetransitioncriteria from themetalerosionregimeto theoxide

erosion regime. In their work, they eroded pre-oxidized Ni

samples having varying thicknesses of oxide scale at ambient

temperature. The ratio of the erosion rate of the oxide scale

(Eo) to the erosion rate of the substrate (E) is plotted against

the ratio of the initial thickness of the oxide scale (t) to the

depth of deformation (L) due to impact. Their observationis portrayed in figure 23. This figure reveals three distinct

regimes. In region one, the value oft/L is higher than 4.0.

The erosion rate of the oxide scale assumes a relatively high

but constant value. In region two, t /L is higher than 0.5 but

lower than 4.0. In this regime, there is a smooth change of the

relative erosion rate from a low value to a high value. Finally,

in regime three, t/L is lower than 0.5. In this regime, the rate

of change of the relative erosion rate is slow. In addition, there

appears tobea peak in therelative erosion rate atapproximately

t /L = 6.

Regime one can be deduced as an oxide erosion regime

on the basis of the erosion response, which is brittle, i.e.

higher erosion rate at normal impact and on the basis of thevelocity exponent, which is 3.0, of the erosion rate. Further,

the material in this regime was removed by brittle chipping.

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Figure 22. Variation of the calculated values of the steady state thickness and the thickness of the oxide scale grown between two successiveimpacts.

Figure 23. Variation of the ratio of the erosion rate of the oxide scale to the erosion rate of the substrate with the ratio of the thickness of theoxide scale to the depth of deformation [94].

In contrast, regime three is characterized by pure metallic

erosion because the thickness of the oxide scale wasnegligibly

low. In addition, the erosion response, the material removal

mechanisms and the velocity exponent of the erosion rate in

this regime are consistent with those of the erosion of metals

and alloys at ambient temperature. Regime two represents the

transition from the oxide erosion regime to the metal erosion

regime. It also shows that the transition is not sharp but rather

smooth. Thus as long as the depth of deformation is confined

within the oxide scale, erosion takes place from the oxidescale only and erosion behaviour similar to oxide erosion is

prevalent. When there is no oxide scale, metal erosion is

dominant. In the intermediate regime, erosion may take place

from the oxide layer, but the deformation will extend to the

substrate also.

5.3. Models for erosion oxidation interaction mechanisms

Several investigators [57, 61, 95103] have tried to identify

various possible mechanisms of interaction between erosion

and oxidation over the last few years. For example,

Hogmarketal [95] have identified six differentmechanismsof

interaction ranging from pure oxidation to pure erosion. Suchmechanisms are compiled in the report of Stack et al [96]

and Sundararajan and Roy [16]. Wellman and Nicholls [97]

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presented an excellent review on this aspect. Barklow and

Petit [98] have identified four different mechanisms such as

metallic erosion, oxidation modified erosion, erosion modified

oxidation and oxidation on the basis of the combined effect of

kinetic energy of the impacting particles, oxide growth rate

and particle flux rate. According to them the effect of thesubstrate becomes negligible when the particle impact energy

and particle flux rate are less. The influence of the substrate

becomes important at high particle flux rate and high impact

energy. Wright et al [52] classified the interaction between

corrosion and erosion into two regimes on the basis of kinetic

energies of the impacting particles. When the kinetic energy

is low, erosion behaviour exhibits brittle erosion mechanisms

characterized by a maximum erosion rate under normal impact

and a velocity exponent in excess of 3.0. When the kinetic

energy is high, erosion behaviour is ductile, maximum erosion

rate occurs at oblique impact angle and the velocity exponent

lies in the range 22.5. This classification was subsequently

modelled by Natesan and Liu [99] and later modified byStacket al [100].

Rishel et al [101] extended the idea originally proposed

by Kang et al [50] and Chang et al [102]. They defined

various mechanisms of erosionoxidation interaction using four

parameters: instantaneous scale thickness ( ), parabolic rate

constant under combined attack of erosion and oxidation Kce,

erosion rate constant of oxidation product K and parabolic

rate constant for oxidation (Kp) only. According to these

investigatorstherateof change of instantaneous scalethickness

is given byd

dt=

K(ce)

K . (30)

If d /dt is positive erosion of the oxide scale takes place.Erosion under such conditions will be characterized by a brittle

erosion response. If d/dt is negative oxidation affected

erosion becomes dominant, i.e. erosion takes place from a

composite layer consisting of oxide scale, metallic substrate

and erodent. If, however, d /dt is equal to zero erosion

enhanced oxidation plays a dominant role. Within this regime

three different modes are possible; Type I erosion enhanced

oxidation will be operative if K(ce) is equal to Kp. Type II

erosion enhanced oxidation can be seen ifK(ce) is greater than

Kp and Type III erosion enhanced oxidation occurs when the

oxide scale spalls. Finally, if there is no corrosion, metal

erosion will be prevalent. This model, in principle, canexplain

the effect of feed rate, particle size, etc on the observed erosionrate. However, the predictive capability of this model is

limited.

Sethi and Corey [54], with the help of oxide scale growth

kinetics, have demonstrated that the temperature dependence

of erosion rate shows three different regimes. These regimes

are (1) a low temperature regime where erosion rate is

independent of temperature, (2) an intermediate temperature

regime where erosion rate increases with the increase of

temperature and (3) a high temperature regime.

Stephenson and Nicholls [103] have defined three

different regimes on the basis of the ratio of contact radius (a)

and scale thickness (z). If this ratio is less than 0.1 a substrate

dominated regime can be seen. If this ratio is between 0.1and 1.0, oxide modified behaviour will be prevalent. If this

ratio is more than 1.0 oxidation dominated erosion will be the

operatingmechanism. Theyalso proposed the presenceof pure

oxidation in the case of the negligible presence of the erodent.

Another classification due to Stacket al [104] proposes

the presence of three regimes, namely, erosion dominated,

erosioncorrosiondominated and corrosion dominated. These

regimes are defined based on kinetic energy, temperature andcritical oxide thickness. The transition to corrosion dominated

behaviour can be attributed to the formation of critical oxide

thickness. Above the temperature at which this occurs, the

oxide formed in a given time interval cannot be removed from

the scale metal interface by erosion. Below the temperature at

which the critical scale thickness is attained, the oxide formed

can be removed during impact. The corrosion dominated

regime can be further divided into two more sub-regimes

depending on the velocity dependence of these regimes. The

corrosion dominated regimes can be corrosion dominated-1

and corrosion dominated-2 regimes. In the processes of

identifying these mechanisms, Stack assumed that oxidation

behaviour of the target material follows a parabolic oxidationbehaviour and the oxidation that occurs during each impact is

negligible.

Sundararajan [78] has broadly proposed two differ-

ent regimes of EO interaction, namely, erosion controlled

regimes and oxidation controlled regimes. In erosion con-

trolled regimes three different mechanisms can be envisaged.

These mechanisms are(a)metal erosion, (b)oxidationaffected

erosion and (c) oxide erosion. In oxidation controlled regimes

there are two different mechanisms, namely (a) oxidation con-

trolled erosion continuous and(b) oxidation controlled erosion

spalling. These zones can be identified on the basis of sev-

eral erosion related parameters. The most important feature of

this model is the ability of the model to predict the prevalentmechanisms of erosion once the conditions of erosion and the

thermo-physical properties of the erodent and target material

are known.

5.4. Erosion oxidation interaction map

While establishing the operative mechanisms for erosion

oxidation, it is noted that erosion conditions significantly

influence the oxidation. The factors which influence the

operating mechanisms are impact velocity, impact angle, feed

rate and temperature. Similarly, the particle size and shape

also have a profound effect on this map. The combined

influence of all these factors on erosion rate remainedqualitative for a considerably long time. Specific erosion

mechanismsfor variousmetallic materialshave beendescribed

by Hogmark et al [95], Wright et al [52], Kang et al

[50] and Sundararajan [78]. Barkalow and Petit [98] for

the first time have tried to organize such information in the

form of an erosion oxidation map where the prevalence of

various mechanisms is shown in the domain of the particle

kinetic energy and the scale growth rate. Sundararajan [78]

has attempted to organize erosion oxidation maps in the

domain of impact velocity and temperature. Stephenson

and Nicholls [77, 105] have plotted particle velocity versus

oxide thickness for a specific particle size and temperature

to construct such maps while Stack and Pena [106] plottedvelocity versus temperature. But the attempt has remained

limited with the consideration of theoretically postulated

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values of erosion rates. Numerous experiments conducted

by the author can be considered sufficient to attempt making

erosion oxidation interactionmapsfrom the experimental data.

Because of the significance of various erosion conditions

on erosion rate and erosion mechanisms, an erosion oxidation

interaction map can be represented in a multidimensionalconfiguration. A simplified representation of the erosion

oxidationmap canbe made in a two-dimensionalplot of impact

velocityand temperature fora givenimpact angle andfeed rate.

The influence of feed rate on the EO map at normal impact

for commercially pure Ni is shown in figure 24 whereas the

influence of impact angle on the EO map of Ni is illustrated

in figure 25. Similarly the effect of Cr addition on the EO

map can be noted in figure 26. In general all these maps

describe the transitional boundary between regimes of various

operative mechanisms of erosion at elevated temperature.

These maps are able to clearly delineate the metal erosion,

oxidation affected erosion, oxidation controlled erosion and

oxide erosion. The extent of each of these regimes depends ontemperature, feed rate, impact angle and impact velocity. An

examination of all these maps shows that:

(a) a low temperature imparts the metal erosion regime. With

an increase in temperature the metal erosion regime shifts

to the oxide erosion regime via the oxidation affected

erosion and oxidation controlled erosion regimes,

(b) a higher feed rate extends the metal erosion regime and in

turn alters theexistence andtheextentof theother regimes

and

(c) oblique impacts tend to reduce the extent of the metal

erosion regime and promote other regimes.

Theexistence andextent of a particular regime is governedby the oxide scale growth and the depth of deformation due to

impact. These two factors are opposing in nature with respect

to erosion conditions. Thus for a given condition, keeping

the impact velocity constant, as the temperature of erosion is

increased, the influence of oxide scale becomes dominant. As

a result, the oxide scale starts growing at a rate faster than

erosion. This leads to a situation where the thickness of the

oxide scale becomes significant compared with the depth of

deformation andat still highertemperaturewhere thethickness

of the scale attains critical thickness of spalling or attains a

thickness where the rate of growth of oxide scale is equal to

therate of erosion. This results in transition from metal erosion

to oxidation affected erosion or oxidation controlled erosionor even to oxide erosion. This can be seen in figure 25 at

impact angle of 30 and at impact velocity of 35 m s1 and at

feed rate of 0.2 g min1. Similarly keeping the temperature

of erosion constant as the impact velocity is increased, it not

only becomes more difficult for the oxide scale to grow but the

thickness of the oxide scale also becomes less compared with

the depth of deformation. This situation causes prevalence of

metal erosion in preference to oxidation affected erosion and

oxidationaffectederosion in preferenceto oxidationcontrolled

erosion, as shown in figure 24 (at impact angle of 90, feed rate

of 0.2 g min1 and temperature of 673 K).

The effect of the higher particle feed rate is analogous

to that of lower temperature. At a specific condition, higherparticle feed rate does not permit the oxide scale to grow as the

time intervals between impinging particles are shortened. This

(a)

(b)

Figure 24. Erosion oxidation interaction map for Ni, (a) for feedrate of 3.3 106 kg s1 and (b) for feed rate of 3.3 104 kg s1.

condition results in an expansion of the metal erosion regime

and the shifting of other erosion regimes to higher temperature

or lower impact velocities. The effect of impact angle can

be considered in terms of impact velocity. As the impact

angle isdecreasedthe normalcomponent of theimpact velocitydecreases. Thus, oblique impact gives rise to constricted metal

erosion regimes. Consequently, the oxidation affected erosion

regime and the oxidation controlled erosion regimes appear at

higher impact velocity and lower test temperature. At oblique

impact, it is possible to seethe presence ofoxideerosion, which

is not prevalent at normal impact in many cases.

It is noted in figures 2426 that the erosion rate tends

to increase and the erosion oxidation interaction mechanism

shifts from metal erosion to oxide erosion via oxidation

affected erosion and oxidation controlled erosion with the

increase of temperature. It can also be inferred that the erosion

rate is higher (1) in oxidation affected erosion than in metal

erosion, (2) in oxidation controlled erosion than in oxidationaffected erosion and (3) in the oxide erosion regime than in the

oxidation controlled erosion regime [107]. It is important to

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(b)

(a)

Figure 25. Erosion oxidation interaction map for Ni, (a) for impactangle of 30 and (b) for impact angle of 90.

mention that the erosion oxidation map has been constructed

experimentally for the first time. The influence of various

erosion conditions on such a map can be explained on the basis

of oxidation characteristics and mechanical properties of theeroding materials. Stacket al [108] constructed a similar map

previously. However, such a map was available only in the

low velocity regime. It also failed to highlight the influence of

other parameters such as particle feed rate, impact angle, alloy

composition, etc.

6. An analysis of enhanced oxidation kineticsdue to erosion induced roughness

The oxidation kinetics of metals and alloys are found to

alter during erosion or wear. The observation on enhanced

oxidation kinetics is primarily centred on iron base alloys. The

influence of sliding wear on oxidation was initially proposedby Quinn [109]. According to Quinn, the activation energy

for the parabolic rate constant (KP) remains the same during

(a)

(b)

Figure 26. Erosion oxidation interaction map for (a) Ni and(b) Ni20 Cr alloy.

static or wear induced oxidation. However, the magnitude

of KP of wear induced oxidation is higher than that for

static oxidation. A similar contention is also made by Lim

and Ashby [87] for wear induced oxidation and by Levyand co-workers [29, 45, 59] for erosion induced oxidation.

Roy et al [76] for the first time attempted to estimate the

altered oxidation kinetics during erosion and subsequently

tried to formulate a phenomenological model to explain such a

result.

The observations of Roy et al [76] are presented in

figures 27 and 28. The variations of mass gain per unit area

as a function of time at 1073 K for as-received and eroded Ni

are given in figure 27 whereas figure 28 depicts similar data at

1173 K for Ni20Cr alloy. It is clear that the oxidation rate of

Ni increases significantly for eroded samples when compared

with as-received samples. The oxidation rate increases with

the increase of impact velocity. The increase of oxidation ratewith increase of impactvelocityis higherat normalimpactthan

at oblique impact. With regard to the Ni20Cr alloy it is noted

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0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 10 000 20 000 30 000 40 000 50 000 60 000 70 000 80 000

Time (sec)

MassGain/UnitAreax103 (Kg/m

2)

As Received

V = 35 m/s, = 30oV = 65 m/s, = 30oV = 35 m/s, = 90oV = 65 m/s, = 90o

Material: Ni

V= Impact Velocity

= Impact AngleTemperature : 1073 K

Figure 27. Variation of mass gain per unit area of Ni with time at 1073 K [76].

0

2

4

6

8

10

12

14

16

18

0 10 000 20 000 30 000 40 000 50 000 60 000 70 000 80 000

Time (sec)

MassGainperU

nitAreax103

(Kg/

m2)

Material: Ni-20Cr

V= Impact Velocity

= Impact AngleTemperature : 1173 K

As ReceivedV = 35 m/s, = 30 oV = 65 m/s, = 30 oV = 65 m/s, = 90 oV = 35 m/s, = 90 o

Figure 28. Variation of mass gain per unit area of Ni20Cr alloy at 1173 K [76].

that the oxidation rate decreases with the increase of impact

velocity. Further, the influence of impact angle on oxidation

kinetics is negligible.

Theabove observation forNi is modelled by assuming that

the parabolic rate constant is related to the surface roughness

(Ra) or impact velocity as

K = KO1(Ra)x1 exp

Q

RT

, (31)

K = KO2 (V sin )x2 exp

Q

RT , (32)

where KO1 and KO2 are constants, V is the impact velocity and

is the impact angle, Q is activation energy, R is the universal

gas constant, T is test temperature andx1and x2 areroughness

exponent and velocity exponent, respectively. In order to

establish the relation between the parabolic rate constant

and impacting condition, the average activation energy for

the parabolic rate constant is determined by plotting ln K

(parabolic rate constant) against (1000 T1) i n K1. Using the

average activation energy, the average parabolic rate constants

at different eroding conditions are estimated. The natural

logarithm of the average parabolic rate constant is then plotted

against ln Ra and ln V sin . The slopes of best-fit straight

lines are then computed as constants KO1 and KO2 whereasthe intercepts of the lines with the ordinate are calculated as

exponents x1 and x2. The expressions for rate constants are

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y

Figure 29. Variation of the parabolic rate constant with (V sin )2.08

exp(Q/RT ) [76].

then obtained as

K = 4.67 109(V sin )2.08 exp

22 400

RT

, (33)

K = 9.62 107(V sin )4.26 exp

22 400

RT

. (34)

In order to assess the ability of the expression to predict

the parabolic rate constant, the estimated rate constants are

plotted against (V sin )2.08

exp(22 400/RT ) in figure 29.A straight line with a good fit suggests the suitability of the

expression.

A similar exercise cannot be carried out for Ni20Cr

alloy because of two factors. First, the nature of the oxide

scale that forms on Ni20Cr alloy can have a dual grain size

distribution [110]. Second, higher roughness in these alloys

results in lower sizes of the globular oxide grains. Hence even

though the roughness changes the amount of grain boundary

also gets altered and this in turn provides different extents of

short circuit paths. Thus the pre-exponential factors in the

Arrhenius type equation should be a function of the roughness,

grain size and the nature of the oxides that are present in the

oxide scales of Ni20Cr alloy. Hence a similar modelling forNi20Cr alloy cannot be achieved.

7. Areas of future research

In spite of significant progress in elevated temperature erosion

of metals and alloys, certain areas still need to be addressed.

The material flow behaviour during solid particle erosion is at

high strain, high strain rate, under adiabatic deformation and

undermultiaxial stress condition. The material flow behaviour

under such conditions, particularly at elevated temperature is

not well understood. The estimation of mechanical properties

of materials under such conditions is required for effective

modelling of the elevated temperature erosion behaviour ofmaterials. Thus there is a clear need to develop a simplified

test technique which will simulate the erosion conditions in a

test sample and evaluate the mechanical behaviour under such

conditions. For example the depth of deformation estimated

using equation (14) would be more accurate if dynamic

hardness is used instead of static hardness.

The deformation and fracture behaviour of the oxide scale

under erosion conditions is poorly understood. Further, thehigh strainrate flow behaviour of a layered structurecontaining

the oxide scale and the substrate material also needs to be

addressed. Such an understanding is required to model the

spalling behaviour of theoxide scale once it reaches thecritical

thickness of spalling. The dramatically different spalling

behaviour of the segmented scale and the compacted scale can

be explained more accurately only when concepts related to

such deformation behaviour get crystallized.

The kinetics of oxidation during erosion are an order of

magnitude higher than that under static conditions. An effort

to estimate the increased oxidation rate quantitatively either

through careful experiment or by rigorous theoretical analysis

is hitherto unexplored. All the reasons for such acceleratedoxidation are not yet revealed. Thus, there is a clear need

to carry out organized experiments to evaluate the oxidation

behaviour of materials under erosion conditions. Attempts

should be made to separate out the influence of possible factors

responsible for enhanc

elevated temperature erosive wear of metallic materials 2006 roy

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