Volume 6 Paper 21
Corrosion in Hot Gas Converters of Sulphuric Acid Plant
M.B. Ives, K.S. Coley and J. Rodda
Keywords: Sulphuric acid production, hot gas converter, sulphur dioxide, stainless steels, silicon effects
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JCSE Volume 6 Paper 21
Submitted 6th July 2003
Corrosion in Hot Gas Converters of Sulphuric Acid
M.B. Ives, K.S. Coley and J. Rodda
Walter W. Smeltzer Corrosion Laboratory, McMaster
University, Hamilton, Ont. L8S 4L7
§1 Weight loss measurements on a range of
austenitic stainless steels exposed for up to three years in the hotter
sections of SO2/SO3 converters of sulphuric acid plants
are compared with the behaviour of similar steels in laboratory-controlled
gravimetric experiments in simulated sulphur-containing atmospheres.� The plant
data indicate that the alloy currently in common use, S30409, exhibits accelerated
corrosion above 630°C.� However, the
microstructure is quite stable, with only some sensitization evident in the
periods under study. High silicon stainless steels are shown to have superior
corrosion resistance at all operating temperatures, but microstructural
variations, such as the precipitation of silicon-rich phases, suggest the need
for additional mechanical property analysis on the exposed alloys.
§2 Microstructural examination of the scales
formed on the alloys after plant exposure indicate that S30409 corrodes by the
formation of a sulphate phase which is not protective. The high-silicon alloys
demonstrate superior corrosion resistance which is attributed to a silicon-rich
phase at the oxide-metal interface.
Keywords: Sulphuric acid production, hot gas
converter, sulphur dioxide, stainless steels, silicon effects
§4 Figure 1.� Schematic of a Converter and Hot Heat Exchanger
§5 Many sulphuric acid plants in Canada use
the off-gas from metal smelting operations. Corrosion of the hot gas converter
heat exchangers, main body and catalyst support systems has been found to
produce significant quantity of scale in the gas stream, requiring regular
screening of the catalyst beds, sometimes at higher frequencies than regular
Converter and Heat Exchanger Configuration
§6 The hot gas converter system is designed to
optimize the reaction whereby SO2 is converted to SO3. It
is a system of vessels which consists of the converter, usually containing four
beds of V2O5 catalysts, with heat exchangers located
after each catalyst bed, which utilise the heat of the reaction to preheat the
incoming gas to the reaction temperature, and cool the reacted gas.�
§7 A schematic is shown in Figure
1.� For simplicity only the one of the heat exchangers is shown. This heat
exchanger is located in the gas stream directly after the 1st bed,
where the majority of the conversion takes place, the majority of heat is
generated, and thus is called the �hot� heat exchanger.� It is the only process
vessel between the 1st and 2nd catalyst beds, and therefore
the tubes of this heat exchanger, along with the 1st bed support
structure and the associated gas ducts, represent the sources of the scale
which foul the catalyst bed.
§9 Figure 2 Bottom vestibule of a hot
exchanger with some loose scale clearly visible in the tubes.
The photograph in Figure 2 shows the bottom
vestibule of the hot exchanger.
§10 In the plant where the 2nd and 3rd bed fouling is the
most serious, the materials of construction are AISI 304H for the converter
vessel walls, ducting and heat exchanger, while AISI 321 was
used as the support screen for the� catalyst bed.
The Alloy Coupon Programme
§11 Over the last few years, a
series of (stainless steel) alloys were mounted on racks and mounted within the
hot gas converters. The programme called for two coupon racks of alloys to be
inserted into each of five Canadian acid plants.� One coupon rack would be
placed at the exit of the first catalyst bed (the hottest region of the
converter), and one in the gas stream before it enters the second catalyst bed.
It was supposed that the gas content would be the same at these two locations,
with gas temperature the only variable. Additionally, in one plant a coupon
rack was placed below the 2nd pass to obtain an intermediate
§12 The access to the converter is
limited to the normal operating cycle of the� plant.� The converter is only
opened for catalyst screening, which is scheduled to coincide with the
maintenance shut-down of the smelter.� These shut-downs are not always scheduled
annually, but are taken when required. This meant that the coupons would be
installed at one shutdown and removed at the next, and the duration of the
exposure was fixed by the plant operations. A summary of the actual exposure
periods is provided in Figure 3.
§14 Figure 3, Coupon rack exposure program.
(click on the image for a larger copy)
Stainless Steels Studied
§15 Table 1 lists the alloys used in this project, along with the detailed compositions for each alloy from the
mill specifications provided by the steel supplier.
§16 TABLE 1, Composition of Alloys
AISI - Trade & Heat No.
Al, 0.0005B, 0.01 Nb
§17 In addition some experimental
surface-alloyed samples were produced using a pack cementation technique, which
diffused aluminum and silicon into the surfaces of 304H samples.
Plant Operating Data
§18 Operating data was collected from the plants in
hourly averages for the full exposure periods. This included the temperatures,
above and below the catalyst beds where the racks were exposed, and the %SO2
and %O2 gas contents entering the plant.
§19 This data provided information
on the number of shutdowns and temperature excursions occurring during each
exposure period. Histograms of the data were prepared to obtain averages for
the temperatures and observe the spread in temperature.
§20 Since the exposure durations
were quite long,� much of the scale had spalled by the time of sample removal.
Therefore the samples were cleaned of all retained scale using a glass bead
blast, and the corrosion rate was calculated from the weight loss data.
Corrosion Rates Measured
§21 The �1 year� corrosion rates
as a function of temperature are summarized below for all the samples in
plants, for S30409 (Figure 4a), alloys with higher chromium and nickel content
(Figure 4b), and for the �high silicon� alloys (Figure 5).
§23 Figure 4a, Corrosion of S30409 as a function of
temperature, 1 year exposures
§25 Figure 4b. Corrosion of tested alloys high in
chromium and nickel.
§26 It is clear that all the alloys included in
Figures 4a and 4b exhibit increased corrosion rates above 600°C, and that increasing the chromium and
nickel contents is not effective at these higher operating temperatures.
§27 The corrosion rates of the
alloys containing more than 1% silicon are substantially better at the higher
temperatures, as seen in Figure 5.� The influence of the amount of alloyed
silicon in the alloys is shown in Figure 6.
§29 Figure 5, Corrosion of high Silicon alloys, 1
§31 Figure 6.� Effect of alloyed
silicon on corrosion rates at 635°C,�(2-year exposures)�
§32 It is clear that increasing silicon
increases the corrosion resistance. However, for effective application it is
important to appreciate the mechanical behaviour of the Si-containing alloys,
since silicon is frequently considered an embrittling species in iron-base
§34 Figure 7, multiple years exposure for
S30409 at 420°C, above
§35 In order to clarify the time dependence of
corrosion rates, it is instructive to superimpose the rate measurements from 1,
2, and 3 year exposures in a given plant.� These are shown graphically in
Figures 7 (for samples placed at the lower temperature, 420°C) and Figure 8 (for samples at 620°C).
§37 Figure 8, Multiple years analysis for S30409 at 620°C, below bed 1
Conclusions from the Coupon
§38 The results summarized above
indicate that the corrosion rate of the alloy currently the most commonly used
in converters,� S30409, and other stainless steels with higher content of
chromium and nickel, experience significant corrosion above about 625°C (Figure 4), suggesting a prime cause of
bed fouling when converters are operated above this temperature.� The
silicon-containing alloys exhibit much lower corrosion rates at these
temperatures (Figure 5), with even better performance as the Si alloy level is
increased (Figure 6), suggesting good alloy candidates for future plant,
assuming their mechanical properties are acceptable.� In this regard the creep
strength of S30600 alloy, containing over 4% Si,� has been determined
to be quite similar to S30409.
§39 The multiple-year comparisons
suggest that at the lower temperatures, corrosion rates decrease with time over
a 3-year period such that superpositioning the rates for the individual years
sum to much less than the measured rate of the 2- and 3-year samples (Figure
7).�� However, at� the higher temperature (Figure 8) the superpositions
indicate a better correlation among the multi-year samples. One conclusion from
this observation is that spalling of the scales occurs within one year at the
higher temperature so that each sample is regularly reduced back to �bare�
metal. However,� at the lower temperature (Figure 7), the scale is adherent for
a longer time � more than one year � and consequently the rates measured for
one year are low, since breakaway has not yet occurred.� The topic of scale
spalling will be discussed further below.
§40 Samples removed from the
exposed racks, after cleaning of the scale, were metallographically prepared
to determine microstructural modification occurring during exposure. The
steel samples were etched� by swabbing with a modified �Glyceregia� etchant (mixed
in order: 3 drops of glycerol,
10 mls Acetic Acid, 15 mls HCl, 10 mls HNO3).� The following
originally obtained at 320X magnification.� Figures 9-12 suggest some grain
boundary �sensitization� over the 3-year period, but no changes which might
suggest significant� degradation of mechanical performance.
§42 Figure 9. S30409, not exposed in the plant
§43 Figure 10. S30409, 1 year at 635°C
§44 Figure 11. S30409, 2 years at 635°C
§46 Figure 12. S30409, 3 years exposure at 635°C
§47 Figure 13. S30601, unexposed
§48 Figure 14. S30601, 1 year at 635°C
§49 Similar micrographs for the 5%
Si alloy S30601 after shown in Figures 13 to 16. The high silicon content
appears to modify the microstructure significantly after 3-years exposure on
the same racks as the S31609 coupons shown above. Figure 16 indicates a
profound change in the alloy, presumably by the precipitation of a Si-rich
phase which is readily etched. This alloy also exhibits a distinctive subscale
below the exposed surface.
§51 Figure 15.� S 30601, 2 years at 635°C
§53 Figure 16. S30601, 3 years at 635°C
§54 The development of new phases
in the high Si alloys is also demonstrated in Figures 17 and 18, for the 4%
alloy S30600. A distinct Widmanst�tten structure has developed in this alloy,
in addition to the subscale.
§56 Figure 17. S30600, 1 year exposure at 625°C.
§57 Figure 18. S30600, 3 year exposure at 625°C.
§58 Pottable samples for scale
§60 Figure 19. Rack of �metallographic� samples after
1 year above Bed 2 (high temperature) in Plant A
§61 In an effort to retain the
scale on the alloys at the end of a campaign, a set of small samples were
installed on the rack shown in Figure 19.� Immediately on removal from the
converter the samples were potted in curable resin to permit sectioning and
metallographic examination back in the laboratory.� As is seen from the Figure,
significant scaling still occurred, although some of the scale seen above
originated from the holder plate.
§62 Despite difficulties in
retaining the scale, conditions within the wet acidic atmosphere within the
potting, and in moisture pickup on transfer to the scanning electron
microscope, some complete scale structures were observable.� Typical scales are
seen in Figure 20, for S30409 and Figure 21, for S30601.�
§64 Figure 20. Scale structure on S30409 after 1 year at
625°C (marker is 200�m)
§66 Figure 21. Scale structure on S30601 after 1 year at
625°C (marker is 200�m)
§67 Recalling the significant
difference in corrosion rate for these two alloys at this temperature, the
scale on the high silicon alloy (Figure 21) is compact, probably with an
unresolved barrier layer close to the metal. The scale on the low alloy sample
(Figure 20) is much thicker and shows a clear gap at the scale/alloy interface,
suggesting spalling � probably during the cool-down period.
§68 X-ray diffraction analysis of
these scales suggested they comprise both oxides and sulphates, but no
sulfides.� Table 2 provides a simple breakdown of the phases detected in the
two alloys. after plant exposure to a gas containing 11% SO3 at 635°C for 26,000h.
§69 Table 2: X-ray diffraction analysis of
scales on S30409 and S30601
Scaling Kinetics Determined in the Laboratory
§70 In order to better understand
the oxidation processes occurring in the converters, a parallel study has been
made in the laboratory, using a gravimetric balance to continuously monitor
sample mass, and thereby determine corrosion kinetics.� SO2 was
added to the gas stream and a gold wire was installed near the sample in order
to aid the conversion to SO3.��
§72 Figure 22 Parabolic plot of the weight gain for
S30409 at four temperatures in 7% SO2
§73 Figure 22 summarizes the weight
gain data for S30409 at four temperatures, plotted on scales to demonstrate
parabolic kinetics.� The largest oxidation was observed at 620°C. At 720°C the rate has been reduced.
§75 Figure 23 Kinetic data in 7% SO2
at 620°C for the two alloys of interest
§76 Figure 23 shows the kinetic data in 7% SO2 at 620°C for the two alloys of interest. The
S30409 alloy shows an initially linear rate followed by an increase
(�breakaway�) after 150h. The S30601 alloy exhibits a significantly lower
reaction rate, consistent with the observations of scale structure.
§77 Because of the large difference in time
scale, direct comparison of the short laboratory runs with the much longer
field exposures is not possible. However, the trends of the two observations
§78 §79 It is also probable that
thermal cycling in the plants, caused by both routine and unexpected shutdowns,
exacerbates the corrosion situation by producing additional opportunities for spalling at the relatively weak scale/metal interfaces.� The detailed frequency
of major temperature changes has been recorded for all plants, and there is
some relationship between these events and larger corrosion rates.� Such
temperature cycling is probably more significant than plant to plant variations
in gas stream SO2 contents.�� This has recently been demonstrated in
§81 Figure 24 Effect of thermal
cycling on oxidation kinetics S30409 alloy in 7% SO2 and gold
§82 Figure 24 shows oxidation
kinetics obtained from three samples of S30409 alloy placed in a tube furnace
with 7% SO2 and a gold catalyst.� One sample was removed and weighed
after 120h, and another after a total exposure time of 520h. The third sample
was removed from the furnace every 24h, cooled to room temperature and mass
determined before being replaced into the furnace.�� The first two samples
provide the �isothermal oxidation� kinetics, and the other (eight) measurements
provide the �cyclic oxidation� curve after successive removals and reheats.
§83 It is clear that temperature
interruptions significantly increase the weight changes, supporting the importance
of plant down times on the corrosion behaviour of S30409.
§85 Figure 25 Comparison of 24h
thermal cycling in air and sulphur-containing atmosphere
The observations from another
cycling experiment are shown in Figure 25. All weight changes� in this plot are
the result of 24h thermal cycling between 620°C and ambient.
§86 These data show that SO2
is necessary for the spalling effect. Cycling in air for 200h without
sulphur-containing species causes less than 2% of the weight gain when SO2
§87 The plant and laboratory data
show that in conditions typical of sulphuric acid converters the corrosion of
alloy S30406 (or 304H) proceeds by the formation of a duplex scale, the outer
layer consisting of a mixed iron-chromium-sulphate and the inner layer mainly
iron chromium oxide with either sulphide or sulphate dispersed through the
scale. The X-ray diffraction data suggests that the sulphur in the inner scale
arises from sulphate. Clearly, the experimental evidence of a sudden drop in
reaction rate above 620°C,
when the temperature exceeds that for the stability of iron and chromium
sulphate, emphasizes the importance of sulphate in the corrosion mechanism.
This importance has also been demonstrated by others,, but it was also
found that the kinetics were only rapid if a mixed oxide-sulphide layer existed
below the outer sulphate. There was no direct evidence for such a layer in the
current work. However the presence of sulphide was indicated by the evolution
of hydrogen sulphide when the scale was dissolved in nitric acid.
§88 The reaction kinetics follow an
essentially linear relationship in the plant and are approximately parabolic in
short term laboratory experiments. There is some consistency between the
laboratory and plant data if one considers that breakaway corrosion occurs
after about 500h at 620°C.
This would lead to approximately linear behaviour over 26,000h. If we assume
that the onset of breakaway is much later at 420°C, probably after one year, we would expect to observe
an increase in rate over a three year period at this temperature. Also if the
corrosion is on the cusp between breakaway and protectiveness at 420°C, we would expect to find a certain amount
of scatter in the plant data depending on minor plant to plant variations.
Indeed this scatter is observed in Figure 4.
comment(89);In sulphuric acid converters, the stainless steel S30409 corrodes via
iron-chromium-sulphate formation. The kinetics are parabolic in the early
stages but undergo breakaway to a linear rate law over longer periods.� Operating
hot gas converters at temperatures constantly in excess of 620°C will
require higher alloys to reduce the corrosion rates.
silicon is shown to decrease the corrosion rate by up to an order of magnitude.
Examination of the scales on the high-silicon alloy, S30601, indicate that
this decrease in rate may be due to a thin silicon rich oxide formed at the
§91 This research was funded by the
Natural Sciences and Engineering Research Council of Canada under a Cooperative
Research and Development Grant, and contributed to by a consortium comprising
Cecebe Technologies Inc, Teck-Cominco Ltd,� Falconbridge Ltd,� Inco Ltd, Kubota
Metal Corporation, NORAM Engineering and Constructors Ltd, and Noranda Inc.
The authors also wish to thank
Avesta Polarit and Thyssen Marathon for supplying the alloy samples; Sladjana
Zdero for much of the sample preparation and metallography; Khaled Draou and
Weiguo Yang for the laboratory kinetic data.
 J. Magnan, Kubota Metal
Corporation, unpublished research.
 Weiguo Yang, M.Eng. thesis,
McMaster University, 2003.
 K.P. Lillerud, B. Haflan and P.
Kofstad, Oxidation of Metals, 21, p.119 (1984)
 A. Andersen and P. Kofstad,
Corros. Sci. 24, p.731 (1984)