Development of novel coatings to resist fireside corrosion in biomass-fired

power plants

Supervisors:Dr. Nigel Simms

Prof. John Nicholls(Dr. Tanvir Hussain)

Industrial Supervisor:Colin Davis,

E.ON Technologies (Ratcliffe) Ltd

6th of October 2015

Dominika Orlicka

Aim

To develop a range of coating compositions resistant to fireside corrosion in biomass-fired power plants using a multi-target magnetron

sputtering technique

Objectives

Exposure conditions:• To understand stability of salts (KCl, NaCl, K2SO4, Na2SO4) at high

temperatures and chose a deposit for fireside corrosion testing

Coating development:• To use the combinatorial model alloy development methodology

by using two-target magnetron sputtering• To study the influence of Cr, Al and Fe on the coatings properties

and their role in chloride-based corrosion• To investigate the best coating compositions in the fireside

corrosion tests and to understand their behaviour in differentenvironments

• To evaluate the alternative methods of applying the best coatingcompositions on the boiler tubes

Experimental design

Mass Flow

Controller

(O2/SO2/N2/HCl/Air)

Vent

NaOH

scrubber

Alumina crucibles

with samplesAlumina

liner

Stainless steel

furnace tube

-5°C

+5°C

-5°C

H

o

t

z

o

n

e

Alumina tube

Safety gas

(N2) N2 vent

Schematic of a controlled atmosphere furnace (for salt stability and coating testing)

Salt stability tests

Matrix of salt compositions

020406080100

100

80

406080100

0

20

40

60

80

100

806040200

100806040200

020

60

40

20

0

100

40

60

20

20

0

60

0

100

40

80

K2Cl2

Na2Cl2

80

100

Na2SO4

K2SO4

Test 1 Test 2 Test 3 Test 4

Temperature

[°C]600 550 550 550

Gas composition

7% O2, 0.01% SO2,

0.035% HCl,

balance N2

7% O2, 0.01% SO2,

0.035% HCl,

balance N2

0.035% HCl,

balance N2

0.035% HCl,

balance N2

Total flow rate

[cc/min]99.8 99.8 109.5 109.5

Duration

[hour]50 50 50 50

Electron images of salt crystals

60% KCl + 40% K2SO4

beforea

fter

35% KCl + 20% NaCl+ 15% Na2SO4 + 30% K2SO4 100% K2SO4

100% KCl 80% KCl + 20% NaCl 100% Na2SO4

beforea

fterElectron images of salt

crystals (1)

Key issues for deposit stability tests

a) Test 1 (600ºC, HCl + SO2 environment, 22 mixtures, 50 hours)• conversion to sulphates• evaporation of chlorides• only chlorine left was in deposits that started with 50% at of Cl

b) Test 2 (550ºC, HCl + SO2 environment, 22 mixtures, 50 hours)• conversion to sulphates• evaporation of chlorides• only chlorine left was in 40% KCl+60% NaCl mixture and 100% NaCl

c) Test 3 (550ºC, HCl environment, mixtures (pure salts, KCl + NaCl, KCl + K2SO4), 50hours)• chlorides contaminated by sulphates• evaporation of chlorides• chlorine left was in each sample

d) Test 4 (550ºC, HCl environment, mixtures (pure chlorides, KCl + NaCl), 50 hours)• slight evaporation of chlorides• chlorine left was in each sample

Magnetron sputtering

Target 1

Target 2

Chromium target

Deposition chamber

Sample holder and substrates

Sample holder

Sapphire disc

Schematic of a controlled atmosphere furnace (1)

Mass Flow

Controller

(HCl/Air)

Vent

NaOH scrubber

Alumina crucibles

with samples Alumina liner

Stainless steel

furnace tube

-5°C

+5°C

-5°C

H

o

t

z

o

n

e

Alumina tube

Safety gas

(N2) N2 vent

Water pumpDeionised water

Coating compositions

Cr + Fe30Al

Fe50Cr + Fe20Al

Cr + Fe20Al

Experimental conditions

Test 1 Test 2 Test 3 Test 4

Temperature[°C]

550 550 550 550

Gas composition

Air315 ppm HCl,balance air

315 ppm HCl,balance air

344 ppm HCl, balance air,

10% H2O

Total flow rate[cc/min]

- 47 47 105

Duration[hour]

Up to 450 Up to 150 150 Up to 300

KCl - - + +

Techniques used

• SEM/EDX• XRD • FEG-SEM • Cross-section• FIB • IC • TGA• Mass change

Selected results

Mass change (Cr + Fe30Al)

Mass change graphs for the test Air + HCl without KCl (left) and with KCl (right) after 150 h

Lowest mass change E, F: 50-80 at.% Cr, 12-29 at.% Fe, 8-22 at.% Al

Lowest mass change F, G: 32-63 at.% Cr, 22-40 at.% Fe, 15-28 at.% Al

Mass change (Cr + Fe20Al)

Mass change graphs for the test in Air (left) and Air with HCl (right) after 150 h

Lowest mass change E, e, F: 39-68 at.% Cr, 22-42 at.% Fe, 10-19 at.% Al

Lowest mass change E, e, F: 39-68 at.% Cr, 22-42 at.% Fe, 10-19 at.% Al

Surface composition (Cr + Fe30Al)

Composition of unexposed coatings

Composition after the test in Air with HCl (with KCl)Composition after the test in Air with HCl (without KCl)

ESEM images (Cr + Fe30Al, Air with HCl and Air with HCl + KCl)

50 µm

Surface morphology of an as deposited coating (GSE)

Post-exposure surface

morphologies of the coatings (GSE). A4, D4, E4, F4, H4, K4 – without KCl; A3, D3, E3, F3, H3,

K3 – with KCl

Surface morphologies (Cr + Fe20Al and Fe50Cr + Fe20Al)

Post-exposure surface morphologies of the coating E (GSE) after 150 hours. First picture: Air exposure, followed by Air with HCl, Air with HCl + KCl and Air with HCl + KCl + 10% moisture (last picture)

Post-exposure surface morphologies of the coating A (GSE) after 50 hours. Left picture: Air exposure, right picture: Air with HCl

Cr + Fe20Al

Fe50Cr + Fe20Al

50 µm50 µm 50 µm50 µm

50 µm 50 µm

XRD analysis for Cr + Fe30Al

XRD spectra of Fe-Cr-Al coatings after 150 h exposure in Air with HCl (left) and Air with HCl (+ KCl) (right)

Cross-sections and FEG-SEM analysis

(Cr + Fe30Al)Test without KCl Test with KCl

1: low O content2: lower O and higher

Cr content than atthe top

1: high O and Cr content

2: depletion in O and Cr;higher KCl contentthan at the top(exception of sampleF)

3: 61 at% O, 28 at% Cr,9.5 at% Fe, 1 at% Al,0.5 at% KCl

4: 41.5 at% O, 36 at% Cr,7.5 at% Fe, 7 at% Al,8 at% KCl

25 µm

25 µm

25 µm

25 µm

25 µm

D4

E4

F4

D3

E3

sapphire

disc

sapphire

disc

sapphire

disc

2

1

25 µm

F3

1

2 2

1

2

3

2

1

4

1

2

2

11

FIB-sections (Cr + Fe20Al and Fe50Cr + Fe20Al) -

examples

E2, Cr+Fe20Al, Air, 150h

B3, Fe50Cr+Fe20Al, Air with HCl, 50h

f1, Cr+Fe20Al, Air with HCl, 150h

D2, Fe50Cr+Fe20Al, Air, 50h

Conclusions

• The magnetron sputtering was successfully used to producea range of Fe-Cr-Al coatings

• The presence of HCl in gas (and no KCl) did not result in any significantchanges compared to air alone

• KCl strongly accelerated the corrosion rate• The weight change data showed the smallest values for samples D – G with a composition range of 32-80 at.% Cr, 12-40 at.% Fe and 8-28

at.% Al for Cr + Fe30Al coatings E – F with a composition range of 39-68 at.% Cr, 22-42 at.% Fe, 10-19 at.%

Al For Cr + Fe20Al coatings• XRD analysis showed the presence of Cr2O3 and (Fe, Cr)2O3 oxides• No signs of chromates or chlorides were detected• The investigation of samples is still in progress

Thank you!