High Temperature Erosion-Corrosion behavior of HVAF- & HVOF-Sprayed Fe-based Coatings

Esmaeil Sadeghimereshta, Sudharshan Ramanb, Nicolaie Markocsana, Shrikant Joshia

a Department of Engineering Science, University West, 46153 Trollhättan, Swedenb Nanyang Technological University, 639798 Singapore, Singapore

Motivation & backgroundExperiments High velocity air-fuel (HVAF) spraying High velocity oxy-fuel (HVOF) spraying Exposures

• Corrosion• Erosion

Results and discussionConclusions

Outline

Motivation & background

Boiler industry

T/P Thermal/electrical efficiency

CO2 emission Biomass/waste fuels Corrosive-erosive ashes Cost

Possible solutions

Environment-wise1. Additives (Sulfur, etc.)

2. T Efficiency

Material-wise1. Advanced materials Costly & time consuming

2. Coatings How could coatings slow down the corrosion and erosion-corrosion of boiler components?

fluidized bed combustors, coal gasifiers, compressor blades, boiler tubes, steam and gas turbines

Experiments

Substrate Feedstock powders Coating methods

16Mo3; a carbon steelin wt%:

0.01Cr-0.3Mo-0.5Mn-0.3Si-0.15C-Bal. Fe

Fe-based powdersin wt%:

30Cr-11Ni-3.4B-1.5Si-0.6C-0.1V-Bal. Fe

High velocity air fuel (HVAF)Uniquecoat M3TM gun

High velocity oxy fuel (HVOF)DJ2600 Hybrid gun

Corrosion test Erosion test

ASTM G76Ducum air-jet erosion tester Al2O3 particles:~50 μm Time: 10 min, 5 g/min, 90°

Ambient air at 600 °C up to 168hWith and without KCl

Free standing coatings

Högänas A.B., Sweden‐36 +20 μm for HVAF‐53 +20 μm for HVOF

ASTM G76 Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets

Highly erosive environment

Corrosion control in biomass boilers

High Velocity Air FuelH V A F

Substrate

Coating

Thermal spray coatings

Air Plasma Spray

High velocity oxy-fuel

High velocity air-fuelH

ighe

r par

ticle

infli

ght

tem

pera

ture

Higher particle inflight velocity

Inflight oxidation

Inter-splat cohesion

Change in feedstock phase and chemical composition

200 μm 200 μm

HVAFHVOF

Porosity ≈ 3.1%Hardness (HV0.3) ≈ 571 ± 83Roughness (Ra) ≈ 8.5 ± 0.7

300 μm

Results & discussion

Matrix: γ-Fe, & Ni

Hardneing phase(s):(Fe,Cr)2B, Fe3C, Cr7C3, VC10

Oxide(s):SiO2

Porosity ≈ 1.4%Hardness (HV0.3) ≈ 815 ± 43Roughness (Ra) ≈ 5.2 ± 0.4

Interconnected porosity, oxides, unmelted particles, etc.!

200 μm

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 24 48 72 96 120 144 168

Wei

ght c

hang

e (m

g/cm

2 )

Time (h)

HVAF coating exposed without KClHVOF coating exposed without KClHVAF coating exposed under KClHVOF coating exposed under KCl

Weight change after corrosion test

Reasons for the drop: Vaporization (FeCl2) KCl sintering Spallation Reaction of KCl with water vapor (100ppm) and KOH formation

Reasons for the weight increase: Oxide formation

1) HVAF corroded for 168h in ambient air

Oxide thickness: ~1.5 µm

Oxide phases:Fe2O3 ,(Fe,Cr)3O4

20 µm 2 µm

A dense, continuous and thin oxide

2) HVAF corroded for 168h in ambient air + KCl

30 µm 5 µm

Oxide thickness: ~17 µm

Oxide phases:K2CrO4, Fe2O3, (Fe,Cr)3O4,FeCl2, CrCl2

Continuous, but not thin or dense

Formation of chromate and metallic chlorides!

Non-protective oxide

How could K2CrO4 form?

Decohesion and loss of thickness

Is the Cr-rich oxide protective?

Transformation of Cr-rich layers into volatile species through their reactions with chlorides

Bulk

Salt deposit: 4KCl(s)+Cr2O3(s)+5/2O2=2K2CrO4(s)+2Cl2(g)

Fe(s)+Cl2(g)=FeCl2(s) or/andCr(s)+3/2Cl2(g)=CrCl3(s) or/and

FeCl2(s)=FeCl2(g) or/andCrCl3(s)=CrCl3(g) or/and

2FeCl2(g)+3/2O2= Fe2O3(s)+2Cl2(g) or/and2CrCl3(g)+3/2O2= Cr2O3(s)+3Cl2(g) or/andwherever O2 is available (high pO2)

Cl2(g)

Cl2(g)

2 3

4

1

Oxide scale

Cl2(g)Cl2(g)

wherever O2 is less available (low pO2) At temperature > 400 °C

Active corrosion mechanism

4 → 2 2 ∆ 73.8 / 600

3) HVOF corroded for 168h in ambient air

30 µm 2 µm

Oxide thickness: ~2 µm

Oxide phases:Fe2O3, (Fe,Cr)3O4

Dense, continuous and thin oxide, similar to HVAF

4) HVOF corroded for 168h in ambient air + KCl

30 µm 5 µm

Oxide thickness: ~20 µm

Oxide phases:K2CrO4, Fe2O3, (Fe,Cr)3O4,FeCl2, CrCl3

Continuous, but not thin or dense Non-protective oxide

Formation of the metallic chlorides!

Weight change after erosion test

Reasons for similar behavior: Presence of a thin oxide scale Severe erosion test (long time or high feed rate) Errodants reached the coatings

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 24 48 72 96 120 144 168

Mas

s lo

ss (g

)

Oxidation exposure time (h)

HVAF coating exposed under KCl

HVOF coating exposed under KCl

Very short time between successive impacts of the erodent particles does not allow oxidation to come into play!

7) HVAF eroded-corroded for 168h in ambient air + KClTopography

12

34

5

Zone 1 Zone 3Zone 2

Presence of not well-adherent oxides facilitate the abrasive wear mechanism

Zone 4 Zone 5

Zones1. Highly porous 2. Cracks3. Partly affected4. Small pores5. No effect

1

Substrate

Coating

Oxide

Erosion crater 1 23 4

5

pitting 

7) HVAF eroded-corroded for 168h in ambient air + KClCross section

Zone 2

Zone 3

Zone 5

Zone 4

Affected zone

1 2 34 5

Zone 1

Depth of attack = 56 ± 11 μm

8) HVOF eroded-corroded for 168h in ambient air + KClTopography

1

23 4

5

Zone 3Zone 2Zone 1Similar behavior to HVAF

Zone 4 Zone 5

Zones1. Highly porous 2. Cracks3. Partly affected4. Small pores5. No effect

Substrate

Coating

Oxide

Affected zone

1 2 34 5

8) HVOF eroded-corroded for 168h in ambient air + KClCross section

Zone 2

Zone 5Zone 1

Zone 4

Zone 3

Depth of attack = 41 ± 9 μm

HVAF eroded-corroded for 168h in ambient air + KClZone 3

TopographyCross section

• Presence of the (Fe, Cr)-rich oxide• Non compact

1

Substrate

Coating

Oxide

Erosion crater 1 23 4

5

Less porous coatings with small splats have been reported to be favorable for better erosion resistance!

Conclusions Fe-based coatings are economically favoured but not highly protective in a harsh corrosive

environment

The coatings may find application in boiler tubes and other structural materials attacked by slow

moving particles

Similar behaviour of HVAF and HVOF

Low corrosion-erosion resistance due to inherent features of the coatings and formed oxide scales

The high Cr content (30 wt%) did not improve as it vaporized in form of CrCl2

Future works High temperature corrosion-erosion tests

Erosion tests in a milder environment with ashes

Ni-based coatings

As‐sprayed   Oxidized Eroded Eroded‐oxidized

Thank you for the attention!

Questions?