thermodynamic and kinetic consideration on the corrosion of fe, ni and cr beneath a molten...

7/31/2019 Thermodynamic and kinetic consideration on the corrosion of Fe, Ni and Cr beneath a molten KCl–ZnCl2 mixture

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Thermodynamic and kinetic consideration onthe corrosion of Fe, Ni and Cr beneath a

molten KCl–ZnCl 2 mixture

Andreas Ruh, Michael Spiegel *

Max-Planck-Institut fu ¨ r Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 D €u sseldorf, Germany

Received 18 August 2004; accepted 3 February 2005Available online 12 May 2005

Abstract

Thermogravimetric (TG) experiments have been carried out to study the kinetics of hotcorrosion of Fe, Cr and Ni, covered by a molten KCl–ZnCl 2 mixture of a composition closeto the eutectic (50 mol% KCl–50 mol% ZnCl 2 ). Furthermore binary and ternary phase dia-grams were calculated in order to describe the corrosion process. The tests were conductedat a temperature of 320 ° C in an atmosphere consisting of argon and oxygen. For iron differ-ent stages are observed in a TG curve. They can be attributed to the different reaction steps of iron chloride formation (incubation phase), oxide precipitation (linear stage) and scale forma-tion (parabolic or logarithmic stage). Based on these observations a model, described by Spie-gel [A. Spiegel, Molten Salt Forum 7 (2003) 253], is conrmed. For Cr and Ni these stages arenot observed. At 8 vol% O 2 only slight oxidation of Cr and Ni was observed accompanied byevaporation of the salt deposit. At 16 vol% O 2 the rate of oxidation increases and the exper-iments yield a curve that is either parabolic or logarithmic for both Ni and Cr. As a result it isshown that the solubility of iron chloride in the KCl–ZnCl 2 melt is higher than the solubility of nickel chloride and chromium (III) chloride in the KCl–ZnCl 2 melt. This enables a higher dif-fusibility of iron chloride to the upper region of the melt where a higher oxygen partial pres-sure ( p(O 2 )) is present leading to a higher oxidation rate of iron.Ó 2005 Elsevier Ltd. All rights reserved.

0010-938X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2005.02.015

* Corresponding author. Tel.: +49 211 6792 915; fax: +49 211 6792 218.E-mail addresses: [email protected] (A. Ruh), [email protected] (M. Spiegel).

Corrosion Science 48 (2006) 679–695

www.elsevier.com/locate/corsci

mailto:[email protected]






Keywords: C. Hot corrosion; C. Thermodynamic diagrams; A. Molten salts; A. Iron; A. Nickel

1. Introduction

Operation of waste incinerators is associated with the production of corrosivegases and aerosols, which condense on the surface of boiler tube walls as salt deposits[2]. Heavy metal compounds are present that form eutectic salt mixtures withlow melting points. The corrosive gases and the molten salt deposit both result inaccelerated corrosion. In addition, economical aspects and political requirementsthat demand an increase of the quota of regenerative energy to 12% of the totalenergy production by the year 2010, require reliability and safety in operating suchplants.

Salt induced corrosion has initially been investigated for sulfate deposits (e.g. [3]).A review of sulfate melt-induced corrosion is given by Rapp [4]. The presence of molten sulfates on the steel surface leads to increased corrosion caused by the disso-lution of the passivating oxide layer in the sulfate melt.

As increased corrosion also takes place for steels covered with chloride deposits,corrosion studies for chloride systems have also been undertaken. Reese and Grabke[5,6] investigated the corrosive inuence of solid chlorides (KCl, NaCl, MgCl 2 andCaCl 2 ) on the high temperature corrosion of alloys. In this experiments, carriedout at 500 and 700 ° C in He–O 2 and He–O 2 –SO 2 atmospheres, ferrate or chromateformation as well as release of chlorine, caused by a solid state reaction of the saltwith the oxide scale of the preoxidised sample, could be observed. Chlorine diffusesthrough cracks and pores of the oxide scale to the metal/scale interface and reactswith the metal to form solid FeCl 2 . The vapour pressure of FeCl 2 reaches10

À4 bar at 500 ° C and the volatile iron chloride diffuses outward through the oxidescale. At the oxide/gas interface formation of iron oxide takes place, releasing furtherCl 2 . The growth of iron oxide inside cracks and pores of the oxide scale leadsto destruction of the scale and corrosive gases react with the unprotected metal. Agenerally non-passivating scale is formed on the metal substrate. For this reason

the mechanism initially described by Lee and McNallan [7] was originally calledÔ

ac-tive oxidation Õ. As no chlorine is consumed in this process, it plays a catalytic role.Reese and Grabke [5] concluded from thermogravimetric experiments that evapora-tion of FeCl 2 from the metal/scale interface is the rate determining step in NaCl in-duced corrosion. Spiegel [1] observed increased mass gain in TG tests investigatingthe oxidation of 2.25Cr1Mo at 500 ° C and 600 ° C using pure PbCl 2 and ZnCl 2

deposits in comparison to pure oxidation without deposit. At these temperaturesboth chlorides exist in liquid form. Enhanced mass gain is also discovered at lowertemperatures (350–400 ° C) when a eutectic chloride mixture (e.g. KCl–ZnCl 2 ) isused.

In recent years corrosion studies have also been carried out on pure Cr coveredwith solid and liquid chlorides. The KCl induced corrosion behaviour of pure Cr

680 A. Ruh, M. Spiegel / Corrosion Science 48 (2006) 679–695



was studied by Li et al. [8]. At 650 ° C strongly enhanced oxidation of pure Crcovered by a KCl deposit was observed in an atmosphere containing KCl vapourin comparison to a purely oxidising environment. Additionally the corrosion behav-iour of pure Cr interacting with a solid NaCl deposit was investigated at 500–700 ° Cby Shu et al. [9]. Kinetic experiments have shown that a solid NaCl deposit on thesurface signicantly accelerates the corrosion of pure Cr in air or oxygen mixedwith water vapour. However, corrosion of pure Cr in air or in O 2 mixed with watervapour at 500–700 ° C was not signicant because of the formation of protectiveCr 2 O 3 on the surface. It was pointed out that the reaction of NaCl with theoxide and metal substrate is a reason for the accelerated corrosion, as the protec-tiveness of Cr 2 O 3 scale was destroyed. Corrosion behaviour of pure Fe, Ni, Crand Fe-based alloys covered by ZnCl 2 –KCl has been tested by Li et al. [10].The experiments were carried out at 450 ° C in pure oxygen. This atmosphere is verydifferent from the atmosphere in waste incinerators. Since severe corrosion mayoccur even at lower temperatures, further experiments at lower temperature arenecessary with an oxygen content that is closer to the atmosphere inside wasteincinerators.

The working model for corrosion beneath molten chloride is given in Fig. 1 [11].In that model corrosion starts with the dissolution of the metal at the substrate/saltmelt interface and the formation of iron chloride. Disregarding any chlorine in thegas phase, the chlorine source must be the molten chlorides. In this case chlorinecan be released by the oxidation of the chloride. The produced iron chloride can dif-fuse outward to the salt melt/gas atmosphere interface. At the outer region of the salta higher oxygen partial pressure is present, which allows the oxidation of FeCl 2 toFe 2 O 3 and Cl 2 due to its favoured thermodynamic stability.

The recreated chlorine diffuses back to the metal/salt melt interface and allows asubsequent formation of iron chloride. Equal to the corrosion process induced bysolid chlorides [5,6] chlorine is not consumed and thus this process is catalysed bychlorine.

Fe

Fe + Cl 2 FeCl 2 (dissolution)

Ar — O2

2FeCl 2 + – O 2 = Fe 2O3 + 2Cl 232

gas flow

melt

metal

p(O 2) p(Cl 2)

Fe

Fe + Cl 2 FeCl 2 (dissolution)

Ar — O2

2FeCl 2 + – O 2 = Fe 2O3 + 2Cl 23232

gas flow

melt

metal

p(O 2)p(O 2) p(Cl 2)p(Cl 2)

Fig. 1. Schematic model of chloride melt induced high temperature corrosion process [11], showing thetransport of gas species and chemical reactions forming iron oxide and iron chloride (as described in thetext).

A. Ruh, M. Spiegel / Corrosion Science 48 (2006) 679–695 681



2. Experimental

The corrosion kinetics of chloride melt-induced corrosion has been studied bythermogravimetric experiments. The samples were rst cut into specimens of about10 · 10 · $ 2.5–3 mm, then roughly polished using SiC grinding paper (Grit 1000)and nally polished to a mirror image using diamond paste. Subsequently the sam-ples were cleaned in an ultrasonic bath lled with acetone and dried by hot air.

Prior to carrying out the experiment the polished samples were covered with a50 mol% ZnCl 2 –50 mol% KCl salt deposit, a composition close to the lowest eutecticmelting point of about 250 ° C [2]. The amount of the salt deposit has been varied[0–30 mg/cm 2 ] throughout the experiments. The TG tests have been conducted at320 ° C in an atmosphere of Ar mixed with O 2 (0, 4, 8 and 16 vol%). The tests in pureargon have been done to study the evaporation of the chloride melt deposit. The owrate of the reaction gas was 2.5 ml/s. The heat up of the specimen was done in Ar andthe reaction gases were added as soon as the reaction temperature was reached andthe balance reached a stable state.

After the experiments the corrosion products have been examined using atomicabsorption spectroscopy (AAS), X-ray diffraction (XRD) and scanning electronmicroscopy (SEM).

3. Results

3.1. Experiments with iron

Hot corrosion of pure Fe covered with ZnCl 2 –KCl melt shows three kinetic steps,described by Ruh and Spiegel [11]. The stages are clearly seen in the TG curve ( Fig.2). At the beginning corrosion starts slowly (incubation phase). After that the corro-sion process accelerates. This is accompanied by a high mass gain. During this stagethe TG curve follows a linear rate law. Finally, the reaction slows down and the TGcurve follows a rate law that is either parabolic or logarithmic. In comparison to testswith uncovered samples ( Fig. 3 ) the corrosion is aggravated and enhanced mass gain

is observed.The corrosion products were examined by using SEM with EDX ( Fig. 4 ) andXRD. The formation of iron chloride is the major reaction during the incubationphase. Only little oxide formation occurs at this stage but increases rapidly duringthe linear stage. During the outset of corrosion the formation of Zn-bearing magne-tite was detected by XRD. After a longer corrosion time hematite is the predominantcorrosion product. In both cases the oxide is present as precipitate embedded insidethe molten KCl–ZnCl 2 –FeCl 2 mixture, as seen in metallographic cross-sections andidentied by EDX and XRD.

Additionally the amount of the water soluble fraction, which corresponds to the

amount of iron chloride, has been determined by AAS after different reaction times(2–96 h). The amount increases during the rst 48 h of corrosion but decreases whenthe reaction time is longer than 48 h ( Fig. 5 ). This is caused by consumption of iron




chloride in the oxidation process. In general the slight mass changes of iron chlorideconrm to the slight mass gains observed in TG experiments after 24 h. It is impor-tant to note that the formation of iron chloride itself would not cause any mass gain

0

0.05

0.1

0.15

0.2

0 20 40 60 80 100 120

time [h]

m a s s g a

i n [ m g

/ c m

2 ]

A

B

AB

16 vol.% O 2

8 vol.% O 2

B

A

Fig. 3. TG curves for high temperature corrosion ( T = 320 ° C) of pure Fe without a salt deposit in an Ar– O 2 atmosphere (8 and 16 vol%). Mass gains are negligibly small in comparison to tests with the ZnCl 2 – KCl deposit (see Fig. 2 ).

0

5

10

15

0 20 40 60 80

m a s s g a

i n [ m g

/ c m

2 ]

100

0

5

10

15

02 4 6 8 10

time [h]

m a s s g a

i n [ m g

/ c m

2 ]

A

B

C

A

B

C

0

5

10

15

time [h]

0

5

10

15

0 4 6 8 10

time [h]

m a s s g a

i n [ m g

/ c m

2 ]

A

B

C

A

B

C

2

Fig. 2. TG curve for high temperature corrosion ( T = 320 ° C) of pure Fe covered with a 50 mol% KCl– 50 mol% ZnCl 2 deposit in an Ar + 4 vol% O 2 atmosphere. A, B and C are different kinetic stages. Thesmall plot gives a snap-shot of the rst 10 h of this experiment.




at all, because the chlorine is delivered by the melt itself and only slight amounts of

oxygen are picked up by the KCl–ZnCl 2 melt. If the reaction proceeds and as soon asthe iron chloride diffuses to the melt/gas interface, oxide is formed and mass in-creases rapidly, as seen in the linear stage.

0.0

0.5

1.0

1.5

2.0

0 20 40 60 80 100

time [h]

C r / N

i / F e

[ m g

/ c m

2 ]

Cr NiFe

Fig. 5. Amounts of Fe, Cr and Ni in the water soluble parts of the scale after 2–96 h of corrosion. Theiramounts correspond to the amount of their chlorides. The dissolution of chromium increases signicantlyafter 48 h of corrosion while the amount of iron chloride decreases slightly during these long corrosiontimes. The amount of nickel chloride shows a marginal increase after about 50 h.

Fig. 4. Left: SEM overview micrograph of pure Fe after 4.45 h of corrosion beneath molten ZnCl 2 –KCl

salt mixture (15 mg/cm2

) in Ar–8 vol% O 2 at 320 ° C. The upper scale consists of porous and non-protectiveiron oxide (hematite) having a needle-like structure and minor amounts of chlorine incorporated into it.Right: Metallographic cross-section of the same sample. Scale consists of two layers with ne precipitatedoxide embedded inside the molten chloride.




The incidence of the kinetic stages (average values) is strongly dependent on thegas atmosphere and the amount of salt deposit. Fig. 6 shows that the duration of theincubation phase decreases with increasing oxygen partial pressure ( p(O 2 )) andincreases with increasing amount of salt deposit. The linear rate constant of the sub-sequent stage is also dependent on both the amount (thickness) of the salt depositand p(O 2 ) (Fig. 7 ). The values given here are also average values.

While the inuence of the amount of oxygen on the total mass gain after a certaintime of corrosion is quite small, the thickness of the molten chloride deposit consid-erably inuences the total mass gain. Using 30 mg/cm 2 of the ZnCl 2 –KCl salt mix-ture the total mass gain approximately doubles in comparison to experiments with15 mg/cm 2 of the ZnCl 2 –KCl deposit.

0

20

40

60

80

0 5 10 15 20

time [h]

k l i n

[ m g

/ c m

2 * h ]

30 mg/cm 2

Saltdeposit:

15 mg/cm 2

Fig. 7. Dependence of the linear rate constant on the amount of oxygen and salt deposit (average values).K lin increases with increasing amount of oxygen and decreases with increasing amount of salt deposit.

0

1

2

3

0 4 8 12 16 20

O 2 [vol.%]

I n c u

b a

t i o n

t i m e

[ h ]

15 mg/cm 2

30 mg/cm 2

Saltdeposit:

Fig. 6. Duration of the incubation time in dependence on the amount of oxygen and the amount of KCl– ZnCl 2 deposit (average values). Incubation time decreases with increasing amount of O 2 and increases withincreasing amount of KCl–ZnCl 2 deposit.




3.2. Experiments with nickel

The corrosion of Ni covered by molten chloride deposit is less severe than the cor-rosion of Fe ( Fig. 8 ). At 320 ° C the corrosion test on Ni covered by a 15 mg/cm

2

ZnCl 2 –KCl deposit in Ar–8 vol% O 2 even yields a mass loss. Comparison with theexperiments of Fe and Pt covered with a KCl–ZnCl 2 deposit in an inert Ar atmo-sphere demonstrates that the mass loss in this experiment is due to the evaporationof the molten salt deposit. Slight oxidation takes place nevertheless since the massloss in the case of Ni in Ar–8 vol% O 2 is lower than in the case of Fe and Pt heatedin Ar. An increase of the oxygen content to 16 vol% leads to a signicant higher oxi-dation rate as well as an increase in thickness of the chloride deposit to 30 mg/cm 2 .

The metallographic cross-section of corroded Ni samples shows scales, which arestill predominated by molten chloride ( Fig. 9 ). Formation of nickel chloride is evi-dent. Nickel oxide occurs as ne precipitations in small quantities. For a higheramount of the salt deposit a higher amount of nickel oxide is observed together withthe formation of nickel chloride ( Figs. 10 and 11 ).

As in the experiments with Fe in the case of Ni the water soluble fraction has alsobeen analysed chemically by AAS ( Fig. 5 ). Its amount, which corresponds to theamount of Ni chloride, increases very slightly with increasing reaction time.

3.3. Experiments with chromium

Corrosion tests with Cr show little mass gain. It increases slightly when the oxy-gen amount is increased from 8 vol% to 16 vol% ( Fig. 12 ). Application of 30 mg/cm 2

-3

-2

-1

0

1

2

3

0 20 40 60 80 100 120

time [h]

m a s s g a i n

[ m g

/ c m

2 ]

real ∆ m

1

2

3

4

5

1 Ni 16 vol.% O 215 mg/cm 2 ZnCl 2 /KCl

2 Ni 8 vol.% O 230 mg/cm 2 ZnCl 2 /KCl

3 Ni 8 vol.% O 2

15 mg/cm2 ZnCl 2 /KCl

4 Fe 0 vol.% O 230 mg/cm 2 ZnCl 2 /KCl

5 Pt 0 vol.% O 230 mg/cm 2 ZnCl 2 /KCl

1

2

3

4

5

Fig. 8. TG curves for high temperature corrosion ( T = 320 ° C) of pure Ni covered with different amountsof a 50 mol% KCl–50 mol% ZnCl 2 deposit in an Ar–O 2 atmosphere (8 and 16 vol%). Tests on Pt and Fecovered with ZnCl 2 –KCl in an inert Ar atmosphere are shown to illustrate the evaporation rate of thechloride deposit. Another test result on Fe with 15 mg/cm 2 ZnCl 2 –KCl in inert gas atmosphere is identicalto that with 30 mg/cm 2 ZnCl 2 –KCl.




of the ZnCl 2 –KCl mixture slightly enhances the mass gain in comparison to theexperiment where 15 mg/cm 2 ZnCl 2 –KCl was applied. It must be mentioned thatthe real mass gains may be higher because of evaporation of the chloride deposit sim-ilar to the Ni experiments.

After the experiment the samples have been investigated using SEM. Part of apartially spalled scale shows some rod-shaped chromium oxide grown on the chlo-ride melt ( Fig. 13 , left). Metallographic cross-sections exhibit scales similar to those

exhibited by corroded Ni. They show predominantly the chloride melt ( Fig. 13 ,right). EDX analysis indicates the presence of chromium oxide and chromiumchloride.

Fig. 9. Metallographic cross-section of pure Ni after reaction (126 h) beneath 50 mol% ZnCl 2 –50 mol%KCl (15 mg/cm 2 ) in Ar–16 vol% O 2 showing a porous layer of molten chloride together with smallamounts of ne precipitated nickel oxide. EDX analysis indicates the presence of nickel chloride.

Fig. 10. Left: SEM micrograph of pure Ni after reaction (123 h) beneath molten ZnCl 2 –KCl salt mixture(30 mg/cm 2 ) in Ar–8 vol% O 2 at 320 ° C showing a K–Zn chloride melt (smooth surface) and K–Nichloride regions with minor amounts of Zn (1). Right: Salt melt poor part of the surface of the same Nisample showing nickel oxide as predominant phase.




In contrast to Ni and Fe the amount of the water soluble fraction of Cr increasesdramatically with increasing reaction time ( Fig. 5 ). This is due to the formation of

chromate, which is dissolved in the molten salt. The formation of oxychlorides hasbeen also considered by thermodynamic calculations. In the p(Cl 2 )– p(O 2 ) diagramsfor Cr they are found to be stable at very high p(O 2 )– p(Cl 2 ) () 1).

Fig. 11. Metallographic cross-section of pure Ni after 123 h of reaction beneath molten ZnCl 2 –KCldeposit (30 mg/cm 2 ) in Ar–8 vol% O 2 showing a NiO-rich layer (1), Ni-poor chloride melt layers (2+4) andsignicant precipitation of nickel oxide (3) within the molten chloride layer.

-3

-2

-1

0

1

2

3

4

0 50 100 150

time [h]

m a s s g a

i n [ m g / c m

2 ]

real ∆ m

1 Cr 8 vol.% O 2

30 mg/cm 2 ZnCl 2 /KCl

2Cr 16 vol.% O 2


3 Cr 8 vol.% O 2


4 Fe 0 vol.% O 2


5 Pt 0 vol.% O 230 mg/cm 2 ZnCl 2 /KCl

1

2

3

4

5

1

2

3

4

5

Fig. 12. TG curves for high temperature corrosion ( T = 320 ° C) of pure Cr covered with different amountsof a 50 mol% KCl–50 mol% ZnCl 2 deposit in an Ar–O 2 atmosphere (8 and 16 vol%). Tests on Pt and Fecovered with ZnCl 2 –KCl in an inert Ar atmosphere are shown to illustrate the evaporation rate of thechloride deposit. Another test result on Fe with 15 mg/cm 2 ZnCl 2 –KCl in inert gas atmosphere is identicalto that with 30 mg/cm 2 ZnCl 2 –KCl.




4. Discussion

TG tests were carried out on pure Fe, Ni and Cr while varying the amounts of chloride deposit and p(O 2 ). Both lead to increased mass gain. For Fe a higher p(O 2 ) yields a reduction of the incubation phase and a higher gradient during the lin-ear stage. A larger amount of chloride deposit causes a longer incubation time.

In comparison to pure oxidation the corrosion beneath molten chloride deposits isaggravated and accelerated. As this deposit acts as a chlorine source processes thatrelease chlorine should be discussed. Thermodynamic calculations show that atthe top of the salt melt layer ZnCl 2 will be oxidised at 320 ° C and 8 vol% O 2 withP total = 1 bar (reaction (1)) during the initial part of the corrosion process whenthe chlorine partial pressure is low ( Fig. 14 ).

ZnCl 2 þ 0.5O 2 ¼ ZnO þ Cl 2 ð1Þ

But as the reaction takes place a p(Cl 2 ) of about 2 · 10À4 bar will be established at

these conditions. This in turn prevents the KCl compound from oxidation. But at

the metal/salt melt interface the established p(Cl 2 ) allows the formation of iron chlo-ride (reaction (2)) because the p(O 2 ) is very low at inner parts of the chloride melt.

Fe þ Cl 2 ¼ FeCl 2 ð2Þ

The formation of iron chloride could be conrmed by SEM observations. When ironchloride diffuses outward to areas ruled by a higher p(O 2 ) iron chloride may be oxi-dised to hematite according to reaction (3):

2FeCl 2 þ 1.5O 2 ¼ Fe 2O3 þ 2Cl 2 ð3Þ

Cl 2 may diffuse back to the substrate/salt melt interface acting as new chlorine source

for subsequent iron chloride formation.A catalytic cycle is accomplished since no chlorine will be consumed. The stability

elds of iron chloride and iron oxide are illustrated in the phase diagram ( Fig. 15 ).

Fig. 13. Left: SEM micrograph of pure Cr after reaction (120 h) beneath molten ZnCl 2 –KCl salt mixture

(15 mg/cm2

) in Ar–16 vol% O 2 at 320°C showing chromium oxide crystals grown on the chloride melt.The image was taken from the inner part of the corrosion layer. Right: Metallographic cross-section of the

same sample showing chromium oxide and chromium chloride inside the chloride melt.




K2O(s2) K

2 O

2 ( s )

KCl(s)

ZnO(s)ZnCl 2(s)

Zn(s)

log 10(p(O 2)) (bar)

l o g

1 0

( p ( C l 2 ) ) ( b a r

)

-60 -40 -20 0-60

-40

-20

0

2

1

3

Fig. 14. Superimposed phase diagram of the system K–Cl 2 –O2 and Zn–Cl 2 –O2 at T = 320 ° C. The oxygenpartial pressure of the atmosphere (here corresponding to 8 vol% O 2 ) is indicated with ‘‘1’’. At the saltmelt/gas atmosphere interface ZnCl 2 may be oxidised to ZnO releasing chlorine. The established p(Cl 2 ) isabout 2 · 10

À4 bar (line ‘‘2’’). The p(Cl 2 ) is high enough to prevent KCl from oxidation. Calculated withFactSage [12].

Fe 2O 3(s3) F e 3

O 4

( s )

Fe(s3)

FeCl 2(s)

FeCl 3(liq)


l o g 1

0 ( p ( C l 2 ) ) ( b a r )

-60 -40 -20 0-60

-40

-20

0

1

Fig. 15. Phase diagram of the system Fe–Cl 2 –O2 at T = 320 ° C. The chlorine partial pressure establishedby reaction (1) at 8 vol% O 2 is indicated with ‘‘1’’. Close to the metal/salt melt interface iron chloride isstable. Close to the salt melt/gas atmosphere interface hematite is stable. Calculated with FactSage [12].




0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

0.10.20.30.40.50.60.70.80.9

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

FeCl 2

KCl ZnCl 2 mole fraction

LIQUID + FeCl 2

LIQUIDZnK 2Cl 4 + KCl + FeK 2Cl 4

L I Q U I D

+ F e

C l2

+ F e K

C l3

LIQUID + FeK 2Cl 4 + Fe KCl 3

LIQUID + ZnK 2Cl 4 + FeK 2Cl 4

Fig. 18. Phase diagram of the system FeCl 2 –KCl–ZnCl 2 at T = 320 ° C. It shows that about 10–15 mol%FeCl 2 can be dissolved in a liquid KCl–ZnCl 2 melt near the eutectic composition. Calculated withFactSage [12].

Cr 2O 3(s)

CrCl 3(s)

CrCl 2(s)

C r ( s

)


l o g

1 0

( p ( C l 2 ) ) ( b a r

)

-60 -40 -20 0-60

-40

-20

0

1

Fig. 17. Phase diagram of the system Cr–Cl 2 –O2 at T = 320 ° C. The chlorine partial pressure establishedby reaction (1) at 8 vol% O 2 is indicated with ‘‘1’’. Calculated with FactSage [12].




0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

0.10.20.30.40.50.60.70.80.9

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

NiCl 2


LIQUID + NiCl 2

ZnK 2Cl 4 + NiKCl 3 + KCl

N i C l 2 +

Z n

K 2

C l 4 + N i K C l 3

L I Q U I D + N i C l 2 + Z n

K 2

C l 4

LIQUID + ZnK 2Cl 4

Fig. 19. Phase diagram of the system NiCl 2 –KCl–ZnCl 2 at T = 320 ° C. Almost no NiCl 2 can be dissolvedin a liquid KCl–ZnCl 2 melt near the eutectic composition. Calculated with FactSage [12].

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

0.10.20.30.40.50.60.70.80.9

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

CrCl 3


C r C l3 + Z n K 2 C l4

+K C l

L I Q U I D + C r C l3 + Z n K

2 C l4

LIQUID + CrCl 3

Fig. 20. Phase diagram of the system CrCl 3 –KCl–ZnCl 2 at T = 320 ° C. Almost no CrCl 3 can be dissolvedin a liquid KCl–ZnCl 2 melt near the eutectic composition. Calculated with FactSage [12].




rate of oxide formation in the case of iron and the lower one in the case of Cr and Ni,because less NiCl 2 and CrCl 3 may diffuse outward to the outer part of the melt,where oxidation to oxides is possible due to higher p(O 2 ).

Therefore the solubility of the metal chloride formed during corrosion of samplescovered by chloride melts is responsible for enhanced corrosion.

5. Conclusions

Corrosion tests have been carried out on pure Fe, Cr and Ni covered by a50 mol% ZnCl 2 –50 mol% KCl deposit. Results taken from TG tests yield enhancedmass gain at a relatively low temperature (320 ° C), especially when the amount of oxygen or salt deposit is increased. The formation of protective oxide layers can

be excluded.Corrosion of Fe results in higher mass gain than corrosion of Ni and Cr. The

three different corrosion stages that could be observed on Fe [11] (incubation phase,a linear stage, logarithmic or parabolic stage) are not discernible on TG tests carriedout on Ni and Cr.

Thermodynamic calculations show that iron chloride is soluble in molten KCl– ZnCl 2 near the eutectic compositions, while chromium chloride and nickel chloridehave a very limited solubility in molten KCl–ZnCl 2 . Thus the outward diffusion of the NiCl 2 and CrCl 3 components is more restricted while FeCl 2 diffuses easily toouter parts of the salt melt layer and will be oxidised there due to higher p(O 2 ). Thuswe conclude that the solubility of metal chloride in molten KCl–ZnCl 2 leads to ahigher diffusion rate resulting in a higher oxidation rate. Therefore the solubilityof metal chloride in the molten salt inuences the extent of corrosion of each metal.

In conclusion, more sophisticated solubility studies will be carried out in thefuture to understand the effect of gases like HCl and H 2 O on the metal solubility.The studies have shown that even chromium undergoes rapid solubility as chromatein the melt and that alloy/coating development for water wall and superheater appli-cation should try to focus on low chromium materials and using other protective ele-ments like Al and Si instead.

Acknowledgment

This work is part of the OPTICORR project, which is supported by the EuropeanCommission.

References

[1] M. Spiegel, Molten Salt Forum 7 (2003) 253.[2] G. Sorrell, Mater. High Temp. 14 (3) (1997) 207.[3] P. Kofstad, High Temperature Corrosion, Elsevier Applied Science Publishers Ltd., London, New

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thermodynamic and kinetic consideration on the corrosion of fe, ni and cr beneath a molten...

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