Volume 6 Preprint 77
Degradation performance of Al-containing alloys and intermetallics by molten ZnCl2/KCl
Y.S. Li and M. Spiegel
Keywords: hot corrosion; molten chloride salt; Fe-Al; Fe-Cr; NiAl
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Volume 6 Paper H022
Degradation performance of Al-containing
alloys and intermetallics by molten ZnCl2/KCl
Y.S. Li, M. Spiegel
Max-Planck-Institut für Eisenforschung GmbH. Max-Planck-Straße 1,40237
Spiegel@mpie.de ; firstname.lastname@example.org
The hot corrosion behaviour of Fe-Al and NiAl alloys was studied in
comparison with Fe-Cr alloys beneath ZnCl2-KCl melt in air
atmosphere. All the materials experienced enhanced corrosion and
particularly, Cr exhibited a detrimental effect on the corrosion
resistance of Fe-Cr alloys. However, the corrosion resistance of both
Fe-Al and NiAl was significantly improved with higher Al
concentration. The degradation mechanism of these materials was
discussed and the different microstructure evolution properties
between FeAl and NiAl systems were compared.
Keywords: hot corrosion; molten chloride salt; Fe-Al; Fe-Cr; NiAl
Rapid corrosion of the thermal components has been frequently
encountered in waste incinerators or other advanced biomass-fired
plants when the metal temperature is higher than 300 °C. Such kind of
corrosion is usually caused by the complicated chemical reactions
between tube materials with gaseous species and especially with low
melting point eutectic salts of chlorides and sulfates, Grabke [#ref1],
Spiegel [#ref2]. Conventional carbon steels, low alloy steels and
stainless steels normally exhibit enhanced degradation and there are
examples of superheater tubes operating at 484-538 °C in MSW
applications which had to be replaced every few months. In order to
reduce the amount of superheater materials consumption, it is
necessary to develop more cost-efficient protective coatings for such
components in the hostile combustion environments.
Al-bearing alloys exhibit candidate coating materials for such
applications considering the reduced cost and other useful properties,
such as low density, high strength and good wear resistance. In
particular, iron and nickel aluminides have considerable potentials in
hostile environment for their excellent corrsion resistance by
developing a protective alumina scale, Tortorelli [#ref3]. For example,
these alloys are more likely to make an impact as a corrosion resistant
cladding, through the processes such as high velocity oxy-fuel (HVOF)
spraying, friction surfacing with powders and plasma transferred arc
cladding. However, it is of great importance to evaluate the feasibility
of applying such materials as a surfacing medium in corresponding
ZnCl2-KCl eutectic salt is identified to be one of the major corrosive
species in the ash deposits from waste incinerators and plays a
detrimental role on the rapid tube thinning. Recently, the corrosion
behaviour of some Fe-base and Ni-base alloys has been examined by
Spiegel [#ref2] and Li [#ref4] in presence of molten ZnCl2-KCl salt. In
this project, the aggressiveness of such chloride melt on the Al-
alloyed materials will be further investigated. Some Fe-Cr alloys are
also included for comparison purpose.
The materials used in this study contain one NiAl intermetallics, three
Fe-Al alloys and four Fe-Cr based alloys. The nominal chemical
composition of the above materials is listed in table 1. All the
materials were machined into specimens with dimensions of about 10
x 15 x 1.5 mm, then ground to 600# SiC paper and subsequently
cleaned in a supersonic bath of acetone. After drying the samples were
coated with a ZnCl2-KCl mixture (48: 52, weight ratio) of 60mg/cm2
on the surfaces. Exposure experiments were performed in static air at
400°C for 340 h and also at 450 °C for 96 h, using a horizontal furnace
equipped with a quartz working tube.
After corrosion, metallographical cross-sections were prepared by dry
grinding of the samples in order to prevent the dissolution of the
chloride products from the scale. The morphological and composition
analysis were carried out using Scanning Electron Microscopy (SEM)
with Energy-dispersive analysis (EDX), X-Ray Diffraction (XRD) and
Electron Probe Microanalysis (EPMA). The mass change of the samples
before the tests and after removal of the corrosion products by
chemical etching in an alkaline KMnO4 solution at 80 °C and inhibited
HCl solution, was measured to determine the corrosion resistance.
Table 1. Nominal chemical composition of the materials tested in at.%
except Fe-Cr alloys in wt.%.
3. Corrosion loss and scale structure
Figure 1 shows the mass loss of the materials after exposure beneath
the molten ZnCl2-KCl deposit at 450 °C for 96 h. NiAl suffers from the
least metal loss of all the materials, while an increased Al content in
the Fe-Al alloys normally results in a smaller metal loss. In contrast, an
inverse effect of Cr is observed since the corrosion resistance of Fe-Cr
alloys becomes much worse with increased Cr addition.
The two low Al-content Fe-Al alloys, Fe-10Al and Fe-20Al, exhibit
similar corrosion products, mainly a very thick and porous mixture of
oxides, chlorides (Fig.2). The external scales consist of Fe2O3 and
some KCl, while those light particles in this region correspond to
metallic zinc. A continuous grey layer rich in Al, Cl, O and some K, is
present as an intermediate layer. Finally, a multi-layered iron oxide
forms as the innermost region of the scale in contact with the alloy
substrate. The general corrosion products present on Fe-45Al are
almost the same as the two low-Al alloys. However, no obvious
alternating scale of iron oxide is formed at the innermost zone
(Fig.3a). Instead, a boundary layer very rich in chlorine while somewhat
depleted in aluminium, is detected near the alloy/scale interfaces.
Fig. 1. Mass loss of the materials exposed beneath ZnCl2-KCl in air for
96 h at 450 °C.
The corrosion products formed on NiAl are quite different from those
Fe-Al alloys. The outermost layer from NiAl is mainly composed of
metallic zinc particles, KCl and a complex mixture of oxides and
chlorides of aluminium, yet no nickel oxide is detected. In addition, an
internal formation of alumina precipitates inside a layer of metallic
nickel has formed in contact with the alloy matrix (Fig.4).
Fig. 2. Micrograph of Fe-10Al corroded beneath ZnCl2-KCl in air for 96
h at 450 °C.
Fig. 3. Micrograph of Fe-45Al corroded beneath ZnCl2-KCl in air for
340 h at 400 °C.
Fig. 4. Micrograph of NiAl corroded beneath ZnCl2-KCl in air for 96 h
at 450°C. (a): General view; (b): expanded view of the corrosion
In agreement with the results reported before on Fe-base alloys by
Spiegel [#ref3], ZnO or zinc-rich spinel oxide in combination with iron
oxide, is largely developed on the surface scales of Fe-Cr alloys, then
followed by a mixture layer of the oxides of iron and chrominum
(Fig.5a). However, the microstructure changes after aluminium
alloying. For example, in the case of Fe-15Cr-5Al, metallic zinc,
rather than oxidized zinc, is formed at the outermost layer, which is
quite similar with those Fe-Al alloys (Fig.5b).
Mixture of oxide/chloride
Fig. 5. Micrographs of Fe-27Cr (a) and Fe-15Cr-5Al (b) corroded
beneath ZnCl2-KCl melt for 96 h at 450oC.
4.1 General remark
On consideration that the ZnCl2-KCl salt used in this study exhibits a
molten state at reaction temperatures, the attack occurs actually by
the chloride melt. As discussed above, Al and Cr exhibit quite different
effects on the corrosion behaviours of the examined materials, where
Al seems to be beneficial to improve the corrosion resistance whereas
Cr shows a detrimental behaviour. In addition, the microstructural
properties between NiAl, Fe-Al and Fe-Cr systems also differ a lot
after corrosion. Generally, fine-grained Zn oxide or spinel particles
were largely precipitated at the scale/gas interface on Fe-Cr alloys,
while it is quite not the same case for the Al-containing materials.
Instead, a large number of metallic zinc particles were detected on the
surface of the oxide scale after Al addition. The chemical state
changes of zinc from ZnCl2 before and after corrosion should
approximately correspond to the different consumption process of
ZnCl2, which actually acts as an important clue to clarify the dominant
degradation process of these materials.
The enhanced corrosion of Fe-Cr alloys beneath ZnCl2-KCl melt
produced rather porous surface layer of oxide particles composed of
iron and zinc or their spinel, which implies that a fluxing mechanism
could be responsible for the accelerated corrosion rate of these alloys,
Rapp [#ref5]. The most possible degradation model has been recently
proposed by Spiegel [#ref3] to describe the corrosion process.
Normally, an oxidation reaction between the ZnCl2 melt with the gas
ZnCl2 (l) + ½ O2 = ZnO + Cl2(diss.)
2 ZnCl2 (l) + 2 Fe2O3 + O2 = 2 ZnFe2O4 + 2 Cl2(diss.)
or the reaction between oxides in the scale and the ZnCl2 in the melt:
Through the above reactions, ZnCl2 is consumed and free chlorine is
released, acting as an oxidant for iron (eqn. (3)). Due to the low
oxygen partial pressure at the salt/matrix interface, iron from the alloy
is dissolved in the chloride melt by forming soluble FeCl3, according to
the reaction equation:
Fe + 3 Cl2(diss.) = 2 FeCl3 (diss.)
The dissolved FeCl3 diffuses outwards through the molten salt to the
salt/gas interface, where Fe2O3 will precipitate again:
2 FeCl3(diss.) + 3/2 O2 = Fe2O3 +3Cl2
As the oxide scale formed in this way is rather porous and less
adherent, it can hardly provide any effective protection and enhanced
materials degradation occurs.
It has been reported and discussed recently that the higher Cr content
on Fe-Cr alloys usually leads to an adverse effect on the corrosion
protection property beneath chloride melt, or even in the presence of
HCl gases, Zahs [#ref6]. An important factor of such phenomena is
that Cr2O3 exhibits a high solubility in chloride melt, Ishitsuka [#ref7],
thus Cr2O3 films dissolve easily in the molten chlorides by forming
hexavalent Cr ions according to the reaction:
2Cr2O3 + 3 O2 + 4 O2- = 4 CrO42-
which has been confirmed by chemical analysis of the corroded melts.
In addition, the formation of highly volatile species CrO2Cl2 may also
explain the occurrence of a self-sustaining, accelerated hot corrosion
for Cr2O3-forming steels in such environment.
Since metallic zinc particles are present on the surface of these Albearing materials after corrosion, it is presumed that at the very
beginning of corrosion, a displacement reaction takes place between
ZnCl2 in the melt and aluminium from the alloys:
2 Al (matrix) + 3 ZnCl2(l) = 3 Zn(l) + 2 AlCl3(l)
This is thermodynamically favoured since the chloride of Al is much
more reactive than that of Zn, as seen from Fig. 6. From
electrochemical point of view, Zn2+ (from ZnCl2) is the oxidant for the
metallic Al. This is in contrast to those other metals like iron, where a
dissolved gas (O2 or Cl2) is the oxidising agent.
Fig. 6. Phase stability diagram of Zn-Al-O-Cl system at 450oC (unit:
Subsequent to reaction (6), the AlCl3 can be oxidised to Al2O3
2AlCl3 (l,g) + 3/2O2 = Al2O3 + 3Cl2
Most of the AlCl3 may escape into the gas atmosphere due to its high
volatility (melting point at 178 °C). In fact, this will be a reasonable
explanation for the presence of only a small amount of aluminium in
the mixed region after corrosion. However, part of aluminium is still
kept beneath the oxide layer, in form of oxide and/or chloride. This
reaction model is supported further by immersion test of pure Al in a
ZnCl2-KCl melt under the same experimental environment, Li [#ref8].
Al shows a rapid dissolution and after corrosion, there existed a rather
thick periphery rich in zinc on the surface of the residual Al substrate,
corresponding to an Al-Zn eutectic layer.
The replaced zinc by the above materials of pure Al and M-Al, mostly
remains metal state and is scarcely oxidized to form single oxide or
4.4 Comparison between FeAl and NiAl
It is also observed that iron experienced alternative re-oxidation in the
corrosion frontier of Fe-Al alloys while nickel remained stable in NiAl.
As discussed above, aluminium can be preferentially removed by the
displacement reaction at the initial stage of corrosion. Thus, a base
metal-rich subsurface zone is developed in the alloy surface. The free
chlorine liberated for example, by reaction (1), may penetrate inwards
as dissolved or gaseous species, readily through the pores and
concentrate near the scale/matrix interface. Due to the increased iron
activity and relatively high chlorine partial pressure in local regions,
iron is no longer stable and its reaction with chlorine is favored:
Fe + Cl2 = FeCl2(s)
2 FeCl2 (g, s) + 3/2 O2 = Fe2O3 + 2 Cl2
The decomposition of FeCl2 will leave a microporous alloy surface layer
allowing further deep penetration of oxidants. As a combined
consequence, a multi-layered iron oxide scale is gradually established.
The overall corrosion mechanism is described by the three key stages:
(1) aluminum depletion by the displacement reaction with ZnCl2.
(2) increased iron activity in the subsurface zone favors the
formation of dissolved FeCl3.
(3) oxidation of iron-chlorides leads to the porous iron oxide scale.
The early stage corrosion of NiAl is very similar to that of the FeAl
alloys. However, in comparison with iron and aluminum, nickel is more
resistant to oxygen and chlorine attack. Thus, it stays in the metal
state in contact with the matrix. As a direct consequence, aluminium is
selectively removed from the alloy as chlorides by inward diffusion of
chlorine, for example in reaction (10).
2 Al (matrix) + 3 Cl2 = 2 AlCl3(g)
The AlCl3 will be transferred into aluminum oxides afterwards during
its outward diffusion. As this oxidation reaction occurs at relatively low
oxygen pressures, alumina can form within the pores of the metallic
nickel. From this point of view, the presence of a continuous aluminum
oxide precipitation zone in NiAl would be able to slow down the
This paper compared the hot corrosion behaviour of FeAl, NiAl alloys
with Fe-Cr alloys under ZnCl2-KCl melt at 400-450 °C. The corrosion
resistance of the materials could be improved by high Al addition while
Cr shows an inverse effect. The enhanced corrosion of Fe-Cr was
described by a fluxing mechanism. However, a displacement reaction
became dominant in those Al-containing materials, which leads to the
preferential removal of aluminium by ZnCl2. Due to the different
thermodynamic stability of Fe and Ni, various microstructure evolution
properties were also observed in the corrosion frontier between the
The authors would like to thank Mrs. M. Nellessen, Mrs E. Strauch and
I. Parezanovic for preparing the metallographical cross-sections and
SEM analysis. Thanks are also given to A. Ruh for his assistance in
drawing the phase stability diagram.
!ref1 ‘The effects of chlorides, hydrogen chloride, and sulfur dioxide in
Spiegel, Corrosion Science, 37, 7, pp1023-1043, 1995.
!ref2 ‘Salt melt induced corrosion of metallic materials in waste
!ref3 ‘Critical factors affecting the high-temperature corrosion
Materials Science and Engineering A, 258, 31, pp115-125, 1998.
!ref4 ‘Accelerated corrosion of pure Fe, Ni, Cr and several Fe-based
Li, Y. Niu and W. T. Wu,