Alkali-induced corrosionCurrent understanding

&Research methods

Juho LehmustoChemistry in combustion processes II

March 13, 2018

1) Introduction

2) Corrosion chemistry

3) Experimental approach

Contents

Biomass--A part of future’s power production --

Replacement for fossil fuel

CO2 neutrality

Availability

Regulatory benefits

20

35

0 1 2 5

5

10

10

PEAT

BARK

WOOD BIOMASS

DEMOLITION WOOD

CHIP-BOARD

POLYOLEFIN

PLASTICS

(PE, PP, PC...)

COLORED

OR PRINTED

PLASTICS,

CLEAN

MIXED

PLASTICS

REF

PLY-

WOOD PVC

RDF

MSW

PVC

CONSUMER REF II - III

MIXED PLASTICS

PAPER &WOOD

BROWN COAL,

LIGNITE

PETROLEUM COKE

CHICKEN

LITTER

DEINKINGSLUDGE SEWAGE

SLUDGE

BIO &FIBERSLUDGE

REF

PELLETS

WOOD &PLASTICS

COW

MANURE

REF I

COMMERCIAL &

INDUSTRIAL

BITUMINOUS

COAL

ANTRACITE

COAL

20

35

0 1 2 5

5

10

10

WOOD BIOMASS DEMOLITION WOOD

CHIP-

BOARD

COLORED

OR PRINTED

PLASTICS,

CLEAN

COLORED

OR PRINTED

MIXED

PLASTICS

REF

-

PVC

RDF

MSW

PVCREF II - III

MIXED PLASTICS

PAPER &

WOOD

PETROLEUM COKE

MULTIPLE

CHALLENGES

SOME

CHALLENGES

STANDARD

DESIGN

LITTER

DEINKINGSLUDGE SEWAGE

SLUDGE

FIBERSLUDGE

REF

PELLETS

WOOD &

PLASTICS

COW

MANURE

REF I

COMMERCIAL &

INDUSTRIAL

OIL SHALE

BITUMINOUS

COAL

ANTRACITE

COAL

BIO &

BROWN COAL,

LIGNITE

PEAT

BARK

WOOD

PLY-

AGRO

BIOMASS

HE

AT

ING

VA

LU

E,

MJ/k

g

Courtesy of Amec Foster Wheeler

Fossil fuel(Coal)

Biomass(Wood)

Biomass(Agriculture)

Waste

O2(vol-%) ~ 3% ~ 5% ~ 5% 6-11%H2O(vol-%) ~ 10% 20-25% 20-25% 20-25%

Alkali chlorides Low Medium High Highp(SO2) High Low Low Lowp(HCl) High Low Medium High

Steam temperature(present maximum)

650ºC 550ºC 450ºC 450ºC

Superheater corrosion

Courtesy of Valmet Technologies

Cr-containing steel Ability to form a thin, yet remarkably dense oxide

layer.

Flue gas,

ash particles

Tube wall

Cr2O3-layer

Overheated steam

Source: Frandsen et al.

Slagging, fouling, and corrosion

Source: Froitzheim et al.

What can be done? Lower steam temperature

Lower power production efficiency

High-quality fuelIncreased process costs

Higher alloyed materialsIncreased materials costs

What can be done?

Better understanding on corrosion chemistryTheoretical approachField and laboratory studies Complemented reaction mechanisms More pieces to the puzzle Tools for material designers,

operators, etc.

1) Introduction

2) Corrosion chemistry

3) Experimental approach

Contents

Corrosion Corrosion is a natural process, which converts a refined metal to a more stable

form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical reaction with their environment.

Alloy MetalOxide (MO)

Gas

M M2+

e-

O2(g)

M → M2++ 2e- M2+ + O2-→ MO

O2 + 2e-→ O2-

• Aqueous corrosion• Atmospheric corrosion• High-temperature corrosion• Automotive corrosion• Aircraft corrosion• Aerospace corrosion• Microbiological corrosion• Glass corrosion• Polymer corrosion

High- temperature corrosion The prediction of reaction products when a certain alloy is exposed to a

certain atmosphere at a certain temperature and pressure is desired.

Chemical thermodynamics and especially phase equilibria are required. Chemical equilibrium in gas mixtures Chemical equilibria between solids and gases Chemical equilibria involving multiple solids Chemical equilibria of gases containing two reactants Equilibria between alloys and a single oxide Equilibria between alloys and multiple oxides

The stability of oxides Diffusion rates Corrosion rates

High- temperature corrosionOxide stability

Ellingham/Richardson diagram for free energies of formation of oxides as a function of temperature

Iron-oxygen phase diagram

Source: Ellingham Source: Dudziak

High- temperature corrosionCorrosion ratesDiffusion rates

Diffusion coefficient D of haemoglobin in red blood cells as a function of temperature.

Weight change of different materials as a function of time

Fick’s first law of diffusion

Source: Stadler et al. Source: Salas et al.

Factors affecting the corrosion rate Temperature and time

Water vapor Also i.a. SO2 and HCl (not addressed here)

Steel composition

Deposit composition

Temperature and time

The corrosion rate increases as functions of both time and temperature

Non-linear oxidation, roughly following the parabolic rate law Controlled by the diffusion of gaseous species

Reactions involving gaseous species are prone to the effect of flow rate

Source: Israelsson et al.

Water vapor

Virtually always relevant in biomass combustion

Accelerates corrosion compared to dry conditions

Enables thermodynamically favored reactions In theory, the rate of oxidation increases as a

function of humidity

Source: Jianian et al.

The complexity of simplicity Despite the detrimental effect of water vapor,

it appears to hinder chloride-induced corrosion

A salt-dependent phenomenon; not observed with carbonates

Originates most likely from HCl formation:

16 KCl(s,g) + 4 FeCr2O4(s)+ 9 O2(g) + 8 H2O(g) → 8 K2CrO4(s) + 2 Fe2O3(s) + 16 HCl(g)

The effect of water vapor on mass gain

Source: Pettersson et al.

Steel composition Classic improvers: Cr and Ni

Ni: tendency to form austenite results in a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion.

Cr: added to the steel to increase resistance to oxidation through Cr2O3 formation.

Others: Mn, Mo, and Nb Mn: an austenite forming element, which improves hot working properties and

increases strength, toughness and hardenability Mo: improves resistance to pitting corrosion, especially by chlorides and sulfur

chemicals Nb: carbide stabilization, which tends to minimize the occurrence of inter-

granular corrosion; strengthens steels and alloys for high temperature service

Source: Cha et al.

Increasing Cr and Ni content

Increasing material costs

Steel composition

Newer candidates: Al and reactive elements (RE) Al: Al2O3 forms instead of Cr2O3; potassium aluminate (KAlO2) is

thermodynamically less favored than K2CrO4. RE (Y, Ce, La, etc.): already very small concentrations improve the

corrosion resistance of binary FeCr or NiCr alloys remarkably.1) Oxide adherence is improved greatly2) Corrosion kinetics are reduced3) A lower Cr content is sufficient to stabilize an exclusive Cr2O3 scale4) Cr2O3 growth mechanism is changed from outward growth by cation (Cr3+) to

inward growth by anion (O2-) diffusion.

Steel composition Thermal Barrier Coatings

(TBC) Bond coat: MCrAlY (M=Ni, Fe,

or Co) TGO: α-Al2O3 Top coat: Y2O3 stabilized ZrO2 Superior performance at

temperatures above 1000 °C Vulnerable to cyclic

temperature changes and mechanical vibration/erosion

Steel composition Laser-cladded coatings Base material is coated with a powder coating by laser. Dense, metallurgically perfectly bonded coating. For now, a costly method. Reduced material costs in the future.

Property of Yanmar Co., Ltd.

Deposit composition

Highly complex system with all the components having an influence on one another.

T0, T15, T75, and T100

Temperature gradient

Temperature gradient

Source: Engblom et al.

Temperature gradient

Source: Lindberg et al.

First melting temperatures

Deposit T0 (°C) Deposit T0 (°C)

KCl 771 K2CrO4-K2Cr2O7 393PbCl2 501 KCl-PbCl2 406ZnCl2 318

K2CrO4 980K2Cr2O7 398 31ZnCl2-69KCl 430

KCl-K2CrO4 650 48ZnCl2-52KCl 250KCl-K2Cr2O7 366 68ZnCl2-32KCl 230

Reaction mechanisms

1) Active oxidation

2) Chromate-dependent corrosion

3) Chloride ion corrosion

The starting point

Cr2O3-layer

KCl

Alloyed steel (Fe, Cr, Ni)

KCl(s) + Cr2O3(s)/FeCrNi(s) → ? → ?

KCl

Air flow

Steel

Oxide

Deposit

p(O2)increases

p(Cl2)increases

Cr

Active oxidation

KCl, NaCli)

ii) Cl2

iii) Cr + Cl2 → CrCl2(s,g)

iv) CrCl2(g)

v) 4CrCl2(g) + 3O2(g) → 2Cr2O3(s)+ 4Cl2(g)

Active oxidation First and most cited reaction mechanism Initiation through chromate (CrO4

2-) formation Cation (K+, Na+) plays an active role

A cyclic reaction of molecular chlorine Chlorine acting as a catalyst

? Driving force for the chlorine diffusion? The molecular sizes of O2, CrClx and Cl2

Chromate-dependent corrosion4Cr(s) + 12MCl(s) → 4CrCl3(s,g) + 12M(s)

4CrCl3(s,g) + 3O2(g) → 2Cr2O3(s) + 6Cl2(g)

6Cl2(g) + 12M(s) → MCl(s)

4Cr(s) + 3O2(g) → 2Cr2O3(s)

Chromate-dependent corrosion A two-stage reaction Initiation through metal chloride (CrClx) formation

Chlorine (Cl2) plays an active role Continuation through chromate (CrO4

2-) formation Cation (K+, Na+, etc.) plays an active role

? Controversial role of chromate formation? Later published carbonate-induced corrosion

Corrosion involving chloride ions

Source: Pettersson et al.

Corrosion involving chloride ions A two-stage reaction Initiation through chromate (CrO4

2-) formation Cation (K+) plays an active role Degradation of the protective oxide

Continuation through hematite (Fe2O3) formation Anion (Cl-) plays an active role

? Passivity of chromium oxide? Further reactions of potassium? Driving force for anion diffusion

What do we know

On a general level The effect of various process parameters

On a more detailed level The species involved The overall course of the corrosion reaction

What do we not know

On a general level The relationship between the process parameters More accurate values for the process parameters

On a more detailed level The initiation of the corrosion reaction Solid equation for the corrosion reaction For example, the faith of potassium

What do we not know

Universal theory, that could be applied to whatever conditions with whatever materials

Almost there...

Cr2O3-layer

KCl

Alloyed steel (Fe, Cr, Ni)

KCl

KCl(s) + Cr2O3(s)/FeCrNi(s) → → Fe2O3(s)?

1) Introduction

2) Corrosion chemistry

3) Experimental approach

Contents

Lab-scale vs. full-scale

Laboratory studies

Field studies

Corrosion mechanisms• How does it happen?• Why does it happen?

The effect of corrosion• What happens?

High-temperature corrosion

Laboratory studies Mimicking the genuine conditions, but focusing on a

specific point of interest. Well-controlled conditions Simplified environments → well-defined correlations

CAUTION! Application of the results Extrapolation of the results

High-temperature exposures1. Pretreatment2. Tube furnace exposure3. Sample preparation for SEM/EDXA4. Analyses

The effect of KClAv

erag

e oxi

de th

ickne

ss (µ

m)

0

20

40

60

80

100

120

140

500 550 600

Temperature (oC)

Dry conditions

500 550 600

10 CrMo304LAlloy 625

Humid conditions

< 1 µm < 1 µm

10CrMo--K2CO3--

168 h, K2CO3, dry conditions , 600°C

20 μm

K O

Fe Cr

304L--K2CO3--

168 h, K2CO3, dry conditions, 600°Clocal corrosion

Alloy 625--K2CO3--

20 μm

K O

Fe Cr

Ni

168 h, K2CO3, dry conditions, 600°C

Alloy 625--K2CO3--

Steel

Oxide

Epoxy

1 µmGrain boundaries

168 h, K2CO3, dry conditions, 600°C

Scanning Electron Microscopy (SEM) Interactions between the electron beam and atoms Fast with resolution up to 1 nanometer Information about surface topography and composition

o Unable to distinguish valence stateso Non-optimal depth resolution regarding composition analyseso Poor sensitivity to elements present in low abundances

Scanning Electron Microscope

Reference N2 + H2O Air

KCl + N2 KCl + N2 + H2O KCl + N2 + H218O

KCl + Air KCl+ Air + H2O KCl + Air + H218O

Focused-Ion Beam Scanning Electron Microscopy (FIB-SEM)

Interactions between the ion beam and atoms A micro- and nano-machining tool Sample preparation for i.a. transmission electron microscope,

atom probe, and nanoscale secondary ion mass spectrometer

Transmission Electron Microscopy(TEM)

Interactions between the electron beam and atoms Electrons are transmitted through a specimen with a

resolution up to 50 pm Information about inner fine structure of the sample

Digging even deeper into the detailsAtom Probe (AP) and Nanoscale Secondary Ion MassSpectrometry (nano-SIMS)

Detailed studies on the diffusion and reaction chemistry within the oxide.

X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS)

Interactions between X-rays and electrons Kinetic energy and number of electrons are measured Information about elemental composition, chemical or

electronic states of elements Surface sensitive (5 nm)

o Large analytical area (100 μm2)o Ultra high vacuum requiredo Time consuming

X-ray photoelectron spectroscopy (XPS)

0 1 2 3 4 5 6 7 80

10

20

30

40

50

60

Ele

me

nta

l co

nce

ntr

atio

n [%

]

Sputtered time [min]

O

Fe

Cr

Ni

K

Synthetic air + KCl

0 1 2 3 4 5 6 7 80

10

20

30

40

50

60

Ele

men

tal conce

ntr

atio

n [%

]

Sputtered time [min]

O

Fe

Cr

Ni

K

Synthetic air + KCl + H2O

Surface characterization-- XPS --

Surface around the salt pill

Under the salt pill

The unaffected surface

The edge of the salt pill

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

Interactions between primary ion beam and ejected secondary electrons

Information about elemental, isotopic, or molecular composition of the sample surface

Surface sensitive (1-2 nm)

o Ultra high vacuum requiredo Generally, no quantitative results obtainedo Very time consuming

Surface analysis-- ToF SIMS --

18O-16O map K-Fe map

Stainless steel with KCl in synthetic air (16O) + H218O

18O

16O

Fe

K

Positron Emission Tomography (PET ): specific specimens/reactants can be marked with isotopes producing strong gamma radiation, which in turn can be identified on-line in 3D as a function of time.

PET

15O: 2 min

11C: 11 min

18F: 120 min

82Br: 2118 min

Some isotopes must be produced close to where to be used

Better to, for example, produce CO from 11C than from 15O

18F is very reactive, suitable for many applications

Half lives of isotopes

Alternative isotopes

Among halides KCl and KF turned out to be challenging to begin with

Half life and decay type of K82Br seemed appropriate High cost of the traditional production process Demand for a large radiation source

Neutron-activation of standard suprapur KBr in a cyclotron.

Sufficient activity of K82Br

Thermogravimetry (TG)

The sample is heated up under well-defined conditions

Detects mass changes as functions of time and temperature

Source: Gallagher and Brown Handbook of thermal analysis and calorimetry: Volume 1 Principle and practice, Elsevier 1998

Differential thermal analysis (DTA)

Other physical and chemical changes

Reactions, melt formation, phase transitions…

Source: Gallagher and Brown Handbook of thermal analysis and calorimetry: Volume 1 Principle and practice, Elsevier 1998

Cr + KCl; 2°Cmin-1 → 700°C

100 200 300 400 500 600

80

90

100

110

120

130

-0,5

0,0

0,5

1,0

1,5

2,0

Re

lative

mas

s (%)

Temperature (oC)

Tem

pera

ture

diff

eren

ce (o C)

DTA

TG

Exo

Endo

Phenomenon without a mass change

100 200 300 400 500 600

80

90

100

110

120

130

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

Re

lative

mas

s / %

Temperature / oC

Tem

pera

ture

diffe

renc

e / o C

DTA

TG

Exo

Endo

K2CrO4 in synthetic air

High-temperature reaction kinetics

Reactions of metal salt vapors in real-time and in-situ at high temperatures

A tool to selectively detect trace concentrations of different salt vapors in the gas phase

New possibilities to determine the steps of different reactions paths together with their reaction rates.

Source: Tampere University of Technology

High-temperature reaction kinetics

t1/e = 30 ms

-> [O2] = 310 ppm

aLK = 0.3 & 1 cm

→ [K] = 30 ppb

Sample holder withKClKCl+Cr

Source: Tampere University of Technology

Electrochemical setup

Source: Sui et al.

Salt-generated reaction currents

0 20 40 60 80 1000,0

0,4

0,8

1,2

1,6

2,0

Norm

alize

d oxi

datio

n cu

rren

t [µA

]

Exposure time [min]

NaClKClK2CO3

Source: Sui et al.

Deposit formation

Source: University of Toronto

Test surfaces after 40 minutes100% rice huskDeposit mass = 10 mg

Rice/Eucalyptus 64/36 % Deposit mass = 13 mg

100% eucalyptus barkDeposit mass = 195 mg

Source: University of Toronto

Field studies

The possibility for studies under genuine conditions Complex environment Verification of modelled/lab-scale results

CAUTION! Interpretation of the results Conclusions based on the results

Full-scale measurements

Source: Vainio et al.

A ring of studied steel is located in the probe tip Ring temperature can be adjusted Weight loss → Annual tube wall thickness loss (mm/year) Cross section analyses → Corrosion and deposit chemistry

Corrosion probe

Corrosion probe

Probe

Sample rings

Gas analysis probe

On the corrosion studies

The value of both laboratory and field studies The role of new technology Various approaches needed

No super-method of super-everything exists! Contribution to the big picture

LiteratureGetting started P. Kofstad, High temperature corrosion, 1988, Elsevier Applied Science, London,

England and New York, USA D.J. Young, High temperature oxidation and corrosion of metals 2nd edition, 2016,

Elsevier Science, Amsterdam, Netherlands

Relevant theses S. Enestam, Corrosivity of hot flue gases in the fluidized bed combustion of

recovered waste wood, 2011, Åbo Akademi D. Bankiewicz, Corrosion behavior of boiler tube materials during combustion of

fuels containing Zn and Pb, 2012, Åbo Akademi J. Lehmusto, The role of potassium in the corrosion of superheater materials in

boilers firing biomass, 2013, Åbo Akademi H. Wu, Chemistry of potassium halides and their role in corrosion in biomass and

waste firing, 2016, Åbo Akademi

Thank You for Your attention!

© Vicza