-(paper No.46).1-

Aspects of High Temperature Corrosion of boiler tubes

M. Spiegel and W.Bendick

(Salzgitter-Mannesmann-Forschung GmbH, Duisburg, Germany

34. MPA-Seminar

“Werkstoff- & Bauteilverhalten in der Energie- & Anlagentechnik”

9. und 10. Oktober 2008, Stuttgart

-(paper No.46).2-

Abstract

The development of new boiler steels for power generation has to consider

significant creep strength as well as oxidation and corrosion resistance. High

temperature corrosion of boiler materials concerns steam oxidation as well as fireside

corrosion of parts, in contact with the flue gas. It will be shown that depending on the

quality of the fuel, especially chlorine and sulphur are responsible for most of the

fireside corrosion problems. Corrosion mechanisms will be presented for flue gas

induced corrosion (HCl) and deposit induced corrosion (chlorides and sulfates).

Especially for the 700 °C technology, deposit induced corrosion issues have to be

considered and the mechanisms of corrosion by molten sulfates ‘Hot Corrosion’ will

be explained. Finally, an overview will be given on the selection of suitable materials

in order to minimise corrosion relates failures.

1 Introduction

High temperature corrosion in combustion plants can occur by a variety of

mechanisms including passive scale degradation with subsequent rapid scaling, loss

of adhesion and scale detachment, attack by molten ashes or salts via fluxing

reactions and intergranular corrosion. As fireside corrosion is a big task for materials

selection in waste- and biomass fired plants, the topic was considered to be less

deleterious in coal fired stations, compared to steam oxidation and creep. With the

development of 700 °C technology and co-firing concepts, fireside corrosion

problems are expected to occur much more frequently. The intention of this paper is

to give an overview about the most severe corrosion processes possible on the boiler

side of the tube surface.

The risk of fireside corrosion failures strongly depends on the composition of the fuel

i.e. waste, biomass or coal and its conversion to form corrosive species like for

example HCl, SO2 and H2S, as well as alkali salts (halides and sulfates) within ash

deposits. Table 1 gives an overview on the composition of commonly used fuels,

regarding their content of corrosive species like chlorine, sulphur, alkali-elements and

heavy metals. The most corrosive fuel with a high chlorine content is waste,

especially in addition with heavy metals and alkali elements. Biomass (wood)

contains also potassium as corrosive species, in combination with chlorine and

sulphur. Looking at coal, the chlorine content of lignite is in the range of wood,

however, its sulphur content is higher. Char may contain also high amounts of

chlorine, as well as a high concentration of sulfur.

-(paper No.46).3-

Table 1.1 Corrosive species within the most common fuels i.e. waste, wood and

coal.

Cl

wt.%

S

wt.%

Na2O

wt.%

K2O

wt.%

CaO

mg/kg

Pb

mg/kg

Zn

mg/kg

Waste 0.5 0.2 1.5 0.3 3.0 1000 1500

Used wood 0.01-0.44 0.02-0.58 0.05 0.7 4.5 40-700 170-950

Wood 0.005-

0.02

0.02-0.1 0.02-0.05 0.2-0.5 0.35-1 <5 <35

Lignite 0.005-

0.012

0.1-2 0.01-0.3 0.002-0.1 2-3.5 1.5-5 20-30

Char <0.2 0.5-1.1 0.01-0.1 0.05-0.4 0.05-5.5 20-80 20-120

Upon combustion, chlorine is converted to HCl and tied up to sodium and potassium

to form alkali-chloride vapour and aerosols. By reaction with heavy metals, chlorine

forms highly volatile compounds. Sulfur is converted to SO2/SO3 and tied up to

sodium and potassium to form alkali-sulfates or, under reducing conditions, to H2S.

Fireside corrosion failures often exhibit a general picture, as shown in figure 1.1.:

Figure 1.1. Fireside corrosion attack of a superheater tube of 15 Mo 3, coated with

Alloy 625. The highest metal loss is observed on the front side of the

tube in contact with the flue gas stream.

-(paper No.46).4-

The area of highest metal loss is located directly in contact with the flue gas stream

and, therefore, deposition of volatile alkali-salts from the flue gas is most effective.

This commonly leads to the formation of deposits on the tube surface, as shown in

figure 1.2.

Figure 1.2. Deposits formed on the flue gas side of a superheater bundle.

The overall corrosion process is a complex reaction cycle, consisting of reactions of

flue gas components (i.e. HCl, SO2/SO3, O2,…) with the material, of reactions of

alkali-salts (chlorides, sulfates) within the deposits with the material and reactions of

components of the flue gas with components of the deposit.

In the following, some fundamental aspects of corrosion mechanisms due to HCl,

solid chlorides and molten sulfates are described.

2 Effect of chlorine

2.1 Corrosion by HCl

It is well known that the presence of chlorine in gases and deposits causes

accelerated attack of metal elements and alloys. Investigations on the corrosion

behaviour have been performed in chlorine-containing atmospheres at low-oxygen

partial pressure as well as at high-partial pressures on several metals and aIloys.

Early work [1-3] already showed that small amounts of chlorine in the reaction gas

leads to failures and spallation in preformed oxide scales. Boettger and Umland [3]

attributed this effect to a chemical transport reaction via volatile metal-chloride,

leading to the formation of porous oxides. Hancock et al. studied the oxidation of

-(paper No.46).5-

steels in the presence of NaCl vapour [4,5]. The authors observed a sudden

accelerated attack in the temperature range 550 – 800 °C. After the experiments,

metal-chloride were detected at the metal/oxide interface. The oxide was grown

voluminous and spalled partially after the tests. Spallation of oxide scales was also

observed by Smeggil with the use of thermogravimetric experiments at 900 and

1050 °C [6]. In this investigations, air was enriched with NaCl vapour by passing it

over a heated crucible filled with NaCl salt. Similar observation were made by McKee

et al. [7], who observed accelerated corrosion of different metals at 750 °C in an Cl2,

SO2 and HCl containing oxidising carrier gas. Also in this case, the gas was enriched

in NaCl by passing it over a crucible containing NaCl.

The commonly accepted model describing this corrosion mechanisms is the so called

’Active Oxidation’, as firstly described by McNallan and coworkers [8]. In this model,

chlorine, produced by catalytic oxidation of HCl, is supposed to diffuse through

cracks and pores of the oxide scale to the metal interface and condensed metal-

chlorides are formed. By subsequent evaporation, the metal-chloride vapour diffuses

outward towards the scale/gas interface, thereby oxidation to either hematite or also

magnetite occurs by reaching regions of higher p(O2):

4FeCl2(g) + 3O2 = 2Fe2O3 + 4Cl2 (1)

3FeCl2(g) + 2O2 = Fe3O4 + 3Cl2 (2)

In addition to this reaction, the oxidation to form FeCl3(g) is also possible, which is

subsequently oxidised to Fe2O3:

9FeCl2(g) + 3O2 = 6FeCl3 + Fe3O4 (3)

2FeCl3(g) + 3/2O2 = Fe2O3 + 3Cl2 (4)

The growth of the hematite/magnetite in the cracks and pores destroys the scale and

a porous, non-passivating oxide is formed. As chlorine is not consumed during this

process it has a catalytic effect and the reaction proceeds although the original

chlorine source may be not active any more. The rate determining step in this

-(paper No.46).6-

mechanism is the outward diffusion of the volatile metal chloride, which depends on

its vapour pressure. Figure 2.1 gives a schematic overview of the processes involved

in the active oxidation process.

Figure 2.1 Schematic view of the process of ‘Active Oxidation’. Chlorine penetrates

the oxide scale and forms metal-chloride at the metal scale interface.

By evaporation a re-oxidation porous non-passivating scales are

formed.

Reese and Grabke as well as Bramhoff et al. studied in depth the active oxidation of

2.25Cr-1Mo- steel in the presence of HCl [9] as well as beneath NaCl deposits [10].

Figure 2.2 shows the mass gain of 2.25Cr-1Mo in an He-5 vol.% O2- 500 ppm HCl at

500 °C. In general, the addition of HCl leads to enhanced corrosion and a significant

accelerated mass gain is recorded due to the mechanism of ‘Active oxidation’. The

scales formed during this process are porous and non-protective and tends to spall

from the metal surface. As a consequence, the metal is no longer passivated. The

effect of temperature on the mass gain due to corrosion is shown in Figure 2.3. on

the same material in nearly the same atmosphere containing 500 ppm HCl. With

increasing temperature, an increasing corrosion rate is observed. The highest

increase is observed between 600 and 650 °C metal temperature. This can be

understood looking at the vapour pressures of common metal chlorides (figure 2.4)

-(paper No.46).7-

Figure 2.2 Mass gain vs. time for 2.25Cr-1Mo in He-5 % O2 and He-5%O2-500

ppm HCl at 500 °C. By addition of HCl, the corrosion is accelerated.

Figure 2.3 Mass gain vs. time for the corrosion of 2.25Cr-1Mo in He-O2-500 ppm

HCl for different temperatures.

-(paper No.46).8-

Figure 2.4 Vapour pressure of common metal chlorides as a function of

temperature. With increasing temperature, the vapour pressure

increases.

As a rule of thumb, chlorine induced corrosion starts to be significant, if the vapour

pressure of the chloride in question exceeds 10-5 bar. This pressure is reached for

FeCl2 at 500 °C. At 700 °C, all common metal-chlorides evaporate significantly, so

chlorine induced corrosion becomes an important issue looking at 700 °C

technologie.

In conclusion, chlorine induced corrosion is detrimental in the presence of HCl, even

at a low amount of 500 vppm.

2.2 Chloride salts

Active oxidation also takes place if the sample is covered with solid chlorides, for

example NaCl or KCl being present in deposits of ashes from waste-, biomass and

coal combustion. Figure 2.5 shows results of experiments with 2.25Cr-1Mo-steel

without and after deposition of 5 mg NaCl in He-133mbar O2 at 500 °C.

-(paper No.46).9-

Figure 2.5 Mass gain vs. time of 2.25Cr-1Mo with and without deposition of 5 mg

NaCl in He-133 mbar O2 at 500 °C.

The corrosion rate is accelerated, shortly after deposition of the chloride. The main

mechanism observed here is also active oxidation, initiated by chlorine which is

produced by reaction of the salts and the oxide scale of the metal:

Fe2O3 + 2KCl + ½ O2 = K2Fe2O4 + Cl2 (5)

Fe2O3 + 2NaCl + ½ O2 = Na2Fe2O4 + Cl2 (6)

and

Cr2O3 + 2KCl + 2O2 = K2Cr2O4 + Cl2 (7)

Cr2O3 + 2NaCl + 2O2 = Na2Cr2O4 + Cl2 (8)

-(paper No.46).10-

As the corrosion mechanism is the same as for HCl induced corrosion, the corrosion

rate increases with increasing vapour pressure resp. temperature of the formed metal

chloride.

3 Corrosion by molten sulfates

3.1 General considerations

Extensive work on hot corrosion beneath Na2SO4 was based on gas turbine blade

failures as a consequence of Na2SO4 deposition. It has been established that

deposition of Na2SO4 formed by salt from the air and sulphur from the fuel is

responsible for these kinds of attack. Especially work by Goebel and Pettit [11] as

well as Bornstein and DeCrescente [12,13] recognized that the formation of sulfides

on the metallic engine parts results from the presence of a thin salt film on the metal

surface. As a consequence, extensive research was conducted on alloys, coated

with Na2SO4 and reacted in different environments.

In the following, some general principles on the reaction mechanism will be

discussed. The corrosion kinetics of metals is significantly enhanced by several

orders of magnitude compared to the oxidation without any salt as shown in Figure

3.1 for nickel, reacted beneath a Na2SO4 deposit at 900 °C in an O2-SO2 containing

atmosphere [14]. The corrosion mechanisms have to be distinguished between Type

I and Type II Hot Corrosion. Type I usually occurs at temperatures higher than the

melting point of Na2SO4 (T> 884 °C) und die oxide scale is dissolved in the salt melt

due to basic fluxing (see chapter 3.2). Type II Hot Corrosion occurs at temperatures

below the melting point of Na2SO4. One example for this is the corrosion of nickel

beneath a solid Na2SO4 salt deposit in SO3 containing gas. In the early stages of

corrosion a Na2SO4-NiSO4 solid solution is formed:

NiO + SO3 + Na2SO4 = Na2SO4 - NiSO4 (ss) (9)

With proceeding reaction, the amount of NiSO4 increases and the Na2SO4-NiSO4

mixture starts to melt and accelerated corrosion is observed. The melting point of the

mixture strongly depends on p(SO3) in the gas phase i.e.with increasing p(SO3) the

melting point of the Na2SO4-NiSO4 mixture decreases [15,16], figure 3.2.

-(paper No.46).11-

Figure 3.1 Comparison of the reaction kinetics of pure nickel with and without

deposit of Na2SO4 in 1 atm. O2 - 4 % SO2 at 900 °C.

Figure 3.2 Comparison of the theoretical values of the critical SO3 pressure needed

to form a Na2SO4 + NiSO4-melt with the lowest pressures where a

molten sulphate was observed and ‘Hot Corrosion’ was initiated.

5 0

1 0

4 0

2 0

3 0

50

0 1 00

1 0 0

2 0 0

3 0 0

900 °C , Ni

O2 + 4% SO2 , 1atm

without Na2SO4

TIME , h

WE

IGT

GA

IN ,

mg/c

m2

TH

ICK

NE

SS

OF

NiO

, m

m

with Na2SO4

-(paper No.46).12-

At temperatures lower than 700 °C the sulfate mixture is already molten, if p(SO3) is

in the range of 10-4 atm.

3.2 Corrosion mechanisms in sulfate melts

Sulfate melts are melts with SO42--oxyanions. According to the model of Flux and

Flood, Forland and Motzfeld [17] the SO42--anion dissociates, giving SO3 and O2- -

ions:

SO42- = SO3 + O2- (10)

As also cations (for example Na) being present in the melt, the equilibrium (10) can

be written as:

Na2SO4 = Na2O + SO3 (11)

The basicity of the melt is defined by the p(O2-) value:

pO2- = -log aO2- (12)

According to Equation 12 a small pO2- value is given by a high O2- activity and the

melt is defined to be basic. Accordingly, a high pO2- value is given by a small O2-

activity and the melt is defined to be acidic.

The overall basicity of the melt can be changed by gas phase composition. From

Equation 11 it becomes clear that:

a(Na2O) ≅ p(SO3)-1 (13)

-(paper No.46).13-

Hence, by variation of p(SO3) in the gas phase, a(Na2O) is increased.

Metal surfaces are protected by a dense and adherent oxide layer, formed upon high

temperature oxidation. The corrosive attack of metals and alloys in molten salts is

due to the solubility of oxide scales by basic and acidic dissolution whereas the

solubility of oxides is a function of melt basicity and depends on the chemical

composition of the passive layer.

Basic solubility of oxide scales occurs by oxide ions, being present in the molten salt

from the dissociation of the sulfate ion according to reaction:

SO42- = SO2 + ½ O2 + O2- (14)

By the oxidation of the bare metal surface by SO42- in a basic melt, metal oxide and

additional oxide ions are formed according to:

Me + SO42- = MeO + O2- + SO2 (15)

Hence, the O2- concentration on top of the metal-oxide is higher than in the entire

melt and dissolution of oxide takes place according to:

MeO + O2- = MeO22- (16)

In the case of basic dissolution, the metal-oxide is dissolved as a complex oxide ion.

Acidic solubility occurs by SO3, which is dissolved in the molten sulfate as S2O72-. In

principle, the following reaction takes place:

MeO + SO3 = Me2+ + SO42- (17)

-(paper No.46).14-

The oxides are dissolved as metal-ions in the sulfate melt and metal sulfates are

formed.

In order to investigate solubilities of oxides in molten salts as a function of melt

basicity experiments were carried out by Rapp et al.[18] in molten Na2SO4 at 1200 K.

The melt basicity was fixed by a variation of the gas atmosphere i.e. p(SO2) and

p(O2) and by addition of Na2O2 to the molten salt. Figure 3.3 shows solubility plots of

various oxides in molten Na2SO4 at 1200 K as a function of melt basicity e.g. –log

a(Na2O). The characteristic shape of the plots represents the amphoteric dissolution

behaviour of oxides in molten sulfates. Basic solubility occurs at high a(Na2O) (left

hand side of each curve), acidic solubility occurs at low a(Na2O) (right hand side of

each curve). A characteristic minimum is reached, where the dissolution mechanism

changes from basic to acidic.

Figure 3.3: Solubility of different oxides oxides in a Na2SO4 - melt at T = 1200 K as a

function of Na2O activity.

In conclusion, sulphate induced corrosion is markedly enhanced if the salt becomes

liquid. In energy conversion systems, liquid sulfates are usually formed as eutectic

mixtures in ashes on superheater tubes. The main constituents of such systems are

CaSO4, Na2SO4 and K2SO4, depending on the chemistry of the coal (see table 1).

4

3

2

1

NiO

Fe2O

3

Al2O

3Co3O

4

Cr2O

3

5 6 7 8 9 10 11 12 13 14 15 16 17

SiO2

(900°C)

-log a Na O2

log

Conc.

pp

mm

ole

Met

al I

ons

mo

le N

a2S

O4

(

)

-(paper No.46).15-

Eutectic mixtures usually have lower melting point than the pure substances. This

becomes detrimental, if metal components starts to dissolve in the solid mixture,

thereby decreasing the liquidus temperature.

4 Examples for corrosion failures

4.1 Corrosion by sulfates

Figure 4.1 shows a metallographic cross section of a thermal spray coating as

applied to a superheater tube of a biomass combustion plant with an estimated

surface temperature of 550 °C. The picture shows the coating in the lower part and

the corrosion scale with sulphate deposits (dark gray) and oxide inclusions (bright

phase).

Figure 4.1: Metallographic cross section of a thermal spray coating as applied to a

superheater tube of a biomass combustion plant (details see text).

The overall scale is porous and, therefore, non-protective and the corrosion and

oxidation process proceeds very quickly. Detailed analysis of the corrosion products

have shown that obviously K3(SO4)3 has formed (dark grey phases) in which oxide

particles are embedded. The sulfate salt can react with the oxide scale of the coating,

thereby dissolve parts of the oxide in the salt according to:

3K2SO4 + SO3 + Fe2O3 = 2K3Fe(SO3)4 (18)

-(paper No.46).16-

This is a typical example for a sulphate induced corrosion process without melt

formation but rapid conversion of the protective oxide scale into porous non-

protective products.

Chlorine induced corrosion failures are much more difficult to detect because the

volatile metal chlorides evaporate and are no more detectable. Typical chlorine

induced corrosion failures show a significant zone of internal attack, as shown in fig.

4.2. on a weld overlay of Alloy 625.

Figure 4.1: Metallographic cross section of a corroded Alloy 625 weld overlay.

Massive internal attack by chlorine is observed.

5 Summary and Conclusions

Fireside corrosion problems in combustion plants are mainly due to amounts of HCl

in the flue gas and deposition of chlorides and sulfates from the flue gas stream on

boiler tube surfaces. The corrosion mechanisms are identified as ‘active oxidation’ by

chlorine or reactions with solid and molten sulfates with the oxide scale of the metal

surface. The risk of corrosion depends on the composition of the fuel, especially the

chlorine and sulfur content.

The corrosion rates observed by ‘active oxidation’ strongly depend on temperature

i.e. significantly increasing at T > 450 °C and above. This can be correlated with the

vapour pressures of the metal-chlorides, formed by reaction of the tube metal with

100 µm

100 µµµµm

-(paper No.46).17-

chlorine. At 600-700 °C, most metal-chloride of the common alloying elements (Fe,

Ni, Cr) start to evaporate significantly. In practice, increasing the operating

temperature increases the risk of chlorine induced corrosion. The experimental

investigations have shown that 500 ppm HCl in the flue is quite enough to accelarte

the corrosion process.

Sulfur-rich fuels tend to the formation of sulfates and SO2/SO3 in the combustion gas

as long as the overall combustion conditions are oxidising. Most critical is the

formation of liquid phases in the sulfate deposits, by forming low melting eutectic

mixtures and/or by reaction of sulfates with oxide scales.

In conclusion, the application of co-combustion processes and the 700 °C

technologie requires the full knowledge of the fuel composition with respect to

corrosive species, as well as knowledge on their chemical conversion upon

combustion. In order to judge the corrosion risk of the particular combustion plant the

knowledge of the composition of the fuel is mandatory and helpful for alloy selection.

-(paper No.46).18-

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[18] R. A. Rapp: Corrosion 42, (1986), 568