Chromium-free Nickel Alloys for Hot Sulfuric and Sulfur Environments
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Transcript of Chromium-free Nickel Alloys for Hot Sulfuric and Sulfur Environments
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Article history:
Received 21 December 2009
Received in revised form
19 May 2010
acid is decomposed at temperatures up to 850 C to form SO2,
advantages, including a high thermodynamic efficiency and
the ability to produce hydrogen directly from water. One
potential drawback is the extreme corrosion conditions which
limit the possible materials for construction.
pressures leave almost no materials available for use.
factory corrosion resistance over the entire concentration
range indicated. However, the high Si content and the D03structure make it extremely brittle. The precious metals, like
Pt, have great corrosion resistance, but the price is too high to
* Corresponding author. Tel.: 1 573 341 4725; fax: 1 574 341 6934.
Avai lab le at www.sc iencedi rect .com
w.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4E-mail address: [email protected] (J.W. Newkirk).which is combined with iodine and water to form HI mole-
cules and reform sulfuric acid. The HI molecules decompose
into iodine and hydrogen. The iodine and the sulfuric acid are
re-used in this closed loop cycle. This cycle has a number of
Fig. 1 and Table 1 show corrosion rates of several
commercial corrosion resistance alloys in different concen-
trations of boiling sulfuric acid at atmospheric and high
pressure conditions. Duriron (Fe-14.5 wt% Si) has the satis-1. Introduction
The SulfureIodine thermochemical cycle has been proposed
as a method for producing large quantities of hydrogen gas
from water using the waste heat of a nuclear reactor. Sulfuric
The sulfuric acid decomposition loop, which includes a re-
concentration of the sulfuric acid after the reaction to formHI,
is a particularly difficult area to find suitable construction
materials. The high temperatures combined with the highly
corrosive nature of sulfuric acid at these concentrations andAccepted 4 June 2010
Available online 23 February 2011
Keywords:
NieSieNb
G-phase (Ni16Si7Nb6)
Corrosion
Cold rolling0360-3199/$ e see front matter Copyright doi:10.1016/j.ijhydene.2010.06.007There are few adequate materials available for severe corrosion conditions, like those of
the SeI thermochemical cycle. High Si, Ni-alloys have excellent corrosion resistance,
especially in mineral acids, but have typically been limited by poor mechanical properties
or difficult fabrication issues. The ductility of nickel silicide, Ni3Si, can be improved
through a combination of micro- and macro-alloying. Nb and other minor alloying
elements yield a cast alloy with excellent corrosion resistance to sulfuric acid and good
mechanical properties. In this paper, efforts to optimize the alloys performance are pre-
sented along with progress toward the development of a wrought version of the material. It
was found that an appropriate heat treatment provides the largest improvement in the cast
NieSi alloy microstructure. Trials have resulted in more than a 50% reduction by the cold
rolling process. This process not only increases homogenization but also results in a more
uniform distribution of G-phase particles, which is beneficial for the improvements in
ductility and corrosion resistance. These alloys have great potential for use in future
hydrogen production as well as fossil energy combustion.
Copyright 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.a r t i c l e i n f o a b s t r a c tChromium-free nickel alloys fenvironments
Joseph W. Newkirk a,*, JenHsien Hsu a, RichaaMaterials Science & Engineering, Missouri University of Science anbMaterial Sciences, Idaho National Laboratory, P.O. Box 1625, Idah
journa l homepage : ww2010, Hydrogen Energy Phot sulfuric and sulfur
K. Brow a, Thomas Lillo b
echnology, 223 McNutt Hall, Rolla, MO 65409-0340, United States
lls, ID 3415-2218, United States
e lsev ie r . com/ loca te /heublications, LLC. Published by Elsevier Ltd. All rights reserved.
-
would be a single-phase material consisting of b-phase exclu-
sively. In order to produce an optimized single b-phase micro-
structure, three major directions were studied in the present
work: 1. A homogenization treatment could result in the elim-
ination of the high temperature phases. 2. Control the Nb
content to maximize its effect on ductility while reducing the
likelihoodof formingG-phase. 3. Coldworking thealloyprior to
homogenization should increase the rate and also break up the
G-phase particles, resulting in a finer, more uniform distribu-1
1.5
2
2.5ro
sion
rate
(mm/
yr.)
NiMoFeZr
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4 4589be an engineering material. The Ni(Si,Nb) alloy has excellent
corrosion resistance in different concentrations of sulfuric
acid. It easily meets the target of 2.54 e
NiSi20Nb3B0.5, Ni Base 0.07 0.10
a Tested on 37.4 atm.The compression test was done with a Gleeble system.
Rockwell C hardness testingwas donewith an InstronWilson/
pressure boiling sulfuric acid [1,2].
80 wt% 96 wt%
1 atm 1 atm 13.8 atm
e 2.03 8.00
>2.54 0.76 3.00
w0 e 0.10ation. The effects of these three approaches on the microstruc-
ture and properties of several NieSieNb alloys are described.
2. Experimental procedures
The alloys were melted in an induction furnace under an
Argon atmosphere. Themelt was initially heated to 1500 C. Itwas held for about 15 min to homogenize the melt. The
temperature was then lowered to 1350 C and the melt washeld for about 5 min at this temperature before pouring. The
melt was poured into one of two graphite molds. One mold
was a rectangular mold which held 15 lbs of metal. The other
was a cylindrical bottlemoldwhich held 20 lbs ofmetal. The Si
plus Nb contents were kept close to a combined 23 atomic
percent to increase the chances of a stable b-phase. Table 2
shows the composition, hardness and ultimate strength of
alloys prepared for this study.
An Electro-Discharge Machine (EDM) was used to cut
15 mm long cylindrical compression specimens with a 10 mm
diameter and rolling specimens with dimensions of
30 10 3mm from the homogenized (950 C 4 days in argon)ingots. The compression specimen surfaces were abraded by
SiC paper before the test. The tests were conducted in an
argon atmosphere in the temperature range between room
temperature to 950 C. The strain rate used was 3.3 104 s1.For the cold work process, the samples were rolled either at
room temperature or at 300 C with intermediate anneal of950 C for 5 h. The rolling reduction per pass was set to 10%.The hardness and reduced dimensions were recorded at each
step and the microstructures were evaluated after various
steps.
For corrosion tests, rectangular samples were cut to
10 7 3mmand ground by #1200 SiC paper. The corrosiontests were done by immersing the samples in 70 wt% H2SO4 at
the boiling temperature (165 C). After each immersion,
-
Rockwell hardness tester. Microstructure was observed by
optical microscopy and Hitachi S570 SEM. Differential
Thermal Analyses (DTA) was done using a PerkineElmer
DTA7 under argon at a heating rate of 10 C/min.
3. Results and discussion
22 and 23 at%, so the stable structure of those alloys should be
single b-phase. So, a homogenization heat treatment could be
used.
Fig. 4 shows the DTA results indicating the phase trans-
formation temperatures in each alloy. After comparing these
results to the NieSi phase diagram, 950 C was chosen as anappropriate homogenization temperature for these alloys.
a b eutectic, see Fig. 6. The changes in microstructure arerelated to the improvement in corrosion resistance, see Table
Table 2 e Mechanical properties of alloys [3].
Alloy Hardness (RC) UTS (Mpa) Elogation (%)
As cast 900 C 1 Day 950 C 4 Day
NiSi18Nb5B0.5 e 45.0 1.4 39.7 2.4 e 0.7bNiSi20Nb2B0.5 46.4 2.7 43.3 1.5 42.5 1.5 e eNiSi20Nb3B0.5 e 44.4 1.9 41.2 1.2 876 17a 3.6 1.6aNiSi21Nb1.6B0.5 e e 39.6 1.3 810 33a 1.9 0.6a
a Heat treatment: 950 C 4 Day.b Heat treatment: 900 C 1 Day.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 44590The goal of this work is to determine howmicrostructure and
alloy composition affect the ductility and corrosion properties
of NieSi alloys that could be used for components of the SeI
thermochemical cycle for hydrogen production. The effects of
heat treatments, alloy modifications and cold working in
these properties are discussed.
3.1. Heat treatment
For the NieSieNb alloys studied in this paper, the as-cast
microstructure usually includes a b eutectic, b, G and g-phase, see Fig. 2. G and g-phase are brittle, and the a beutectic corrodes rapidly in strong acids [3]. Further, the
galvanic cell created between phases accelerates corrosion.
Fig. 3 shows the corrosion attack seems to have been focused
on the interdendritic regions. So, a single b-phase micro-
structure should be ideal. There is no complete NieSieNb
phase diagram. An assumption that small additions of Nb and
B do not significantly change the NieSi diagram was made.
The SiNb content of those alloys listed on Table 2 is betweenFig. 2 e Microstructure of as-cast NiSi20Nb3B0.5. It contains
needle-like g-phase, aDb eutectic (black), b-matrix and
G-phase (bright) [3].3, and reduction in hardness as shown in Table 2.
3.2. Decreasing Nb content
After the homogenization heat treatments, the G-phase was
still prevalent in the microstructure. G-phase is apparently
a stable phase for these compositions. As G-phase is created
when the Nb content exceeds the solubility of Nb in b-phase,
decreasing the Nb content could eliminate or reduce G-phase
in the microstructure. However, Nb was added to improveThe peritectoid transformation, a b2/ b1, may happen forsome alloys at temperatures over about 1000 C. This trans-formation should be avoided because it separates b-phase into
two phases. Fig. 5 shows themicrostructure for 1 day at 950 Cand it is evident that the alloy was not completely homoge-
nized; a high Si area produced from the dissolved g-phase is
still obvious. Homogenization for four days at 950 C wasfound to be necessary to dissolve the g-phase and most ofFig. 3 e Optical Micrograph of NiSi20Nb3B0.5 heat treated at
900C for 1 day, after 352 hrs of exposure in boiling 96%sulfuric acid at 37.4 atm. The attack seems to have been
focused on the interdendritic regions.
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850 900 950 1000 1050 1100 1150Temperature (C)
En
do
th
erm
ic
E
xth
erm
ic
1012C
1092C 1111C
1097C1015C
1092C1112C
1022C 1101C1137C
NiSi20Nb3B0.5
NiSi18Nb5B0.5
NiSi20Nb2B0.5
NiSi21Nb1.6B0.5
Fig. 4 e Phase transformation temperatures of Ni-Si alloys
from DTA [3]. All transformation temperatures in these
Ni-Si alloys are greater than 1000 C, so heat treatment at950C was chosen for homogenization.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4 4591ductility by increasing Ni3Si(b) matrix grain boundary
strength. In order to produce a single-phase alloy, the total
SiNb appears to be limited to the range 22e24.5 at%. In orderto maximize ductility, adding enough Nb to just meet the
solubility limit seems prudent. Additionally, keeping Si levels
high should maintain the stability of the silica film.
Fig. 7 shows the area fraction of G-phase of Ni3(Si,Nb) alloys
with different Nb contents after homogenization at 950 C forfour days, including alloys not reported on here. There is an
apparent linear relationship between Nb content and G-phase
area fraction. Extrapolating from this data, the Nb solubility in
the Ni3Si(b) matrix might bew1.2 at%.
3.3. Rolling process
The heat treatment alone was not able to remove the G-phase
from alloys which exceeded the Nb solubility limit. Even theFig. 5 e Backscattered micrograph of NiSi20Nb2B0.5 heat
treated at 950C for 1 day. The alloy didnt completelyhomogenize; the high Si area where the needle-like
g-phase dissolved is still obvious.alloy whose Nb content was close to the extrapolated solu-
bility of the matrix still contained G-phase. Local variations in
the chemistry due to segregation could lead to the presence of
G-phase, and b-phase with less than themaximum amount of
Nb. Cold working should produce a more complete homoge-
nization process, more uniform microstructure, and finer b-
grain size. The rolling annealing process should break upthe G-phase particles and distribute the resulting smaller
particles more uniformly. Mechanical properties are expected
to improve due to the shorter crack paths associated with the
small brittle particles and smaller grain size. Additionally, the
fabricability of the material can be evaluated.
3.3.1. Rolling reductionsNiSi18Nb5B0.5, NiSi20Nb2B0.5 and NiSi21Nb1.6B0.5 alloys were
deformed more than 50% without cracks, using multiple
Fig. 6 e Microstructure of NiSi21Nb1.6B0.5 heat treated at
950C for 4 days. g-phase and most aDb eutectic havedissolved, but G-phase remains.passes. The reduction per roll pass is about 8% for these three
alloys when rolled at 300 C, using a 10% reduction setting.The compression test results, Fig. 8, show behavior consistent
with L12 crystal structure intermetallic compounds. The yield
stress increases with temperature from RT up to a peak
strength, before dropping off at higher temperatures. The
alloy still retains significant strength (>200 MPa) up to 900 C.Based on these characteristics, rolling effectiveness may be
increased by lowering the temperature to less than 300 C,which many studies recommended [6e9]. However, using the
same (10%) reduction setting, the reduction achieved per roll
pass is smaller when samples were rolled at room tempera-
ture, shown on Fig. 9. On the other hand, the alloys appear to
be able to endure higher pressure without cracking at room
temperature. In the case of NiSi21Nb1.6B0.5, when reduction
was set at 12%, samples rolled at 300 C cracked but onesrolled at room temperature survived. The reason might be
Ni3(Si,Nb) alloys have higher ductility at room temperature.
3.3.2. Area fraction and distribution of G-phaseFor the highest Nb alloy, NiSi18Nb5B0.5, comparing Fig. 10 and
Fig. 11, it is obvious that the microstructure of the alloy was
extended along the rolling direction. After 10 roll anneal
-
passes, the average size of the G-phase particles decreased
and the number increased, see Table 3, indicating that the
G-phase was being fractured by the process. However, the
relative area of the phase, a b eutectic, did not changesignificantly. For the lowest Nb alloy, NiSi21Nb1.6B0.5, seen in
Figs. 6 and 12, after 10 roll anneal passes, the a b eutectic
recovered. However, hardness did not decrease much after
a more severe corrosion environment than a high concen-
tration of sulfuric acid [1].
Fig. 14 shows a comparison of the normalized weight loss
with sulfuric acid exposure for Ni3(Si,Nb) alloys in various
conditions. After a period of time, the weight loss rate
decreased significantly for all samples under different condi-
tions. Two important data, average corrosion rate and
passivation time, were obtained from the figure. In this paper,
Table 3 e Corrosion rate and microstructure analysis of alloys.
NiSi18Nb5B0.5 NiSi20Nb2B0.5 NiSi21Nb1.6B0.5
HT Rolled HT Rolled HT Rolled
Corrosion rate (mm/yr.) 1.14 0.64 0.05 0.05 0.18 0.08
Passivation time (min) 370 700 300 300 450 90
G phase Size (um) 8.3 4.4 5.3 3.7 4.3 3.6 4.2 3.4 5.5 3.8 2.3 2Counts/mm2 2500 6100 4700 5200 1330 4700
Area fraction 16 1.5 14 2 5 0.5 4 1 1.4 0.5 2 0.5Eutectic area fraction 27 3 25 2 1 w0 1 w0
0
1000
1200
-300 0 300 600 900 1200Temperature (C)
NiSi20.4Nb3B0.5
NiSi18Nb5B0.5
NiSi22
Fig. 8 e Compressive yield stress of the Ni3(Si,Nb) alloys as
a function of test temperature. Tested in Argon
atmosphere at strain rate of 3310-4 s-1 [5].
0.12
0.15RT roll; set 10%300C roll; set 10%
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 44592several rolling and annealing passes. The reason might be G-
phase did not dissolve in matrix after several rolling pass,
although G-phase distribution became more uniform, see
Table 3.
4. Corrosion test
After the cold rolling process, the distribution of G-phase was
more uniform for all alloys, see Table 3. Both the heat treated
and rolled alloys were tested in 70% boiling sulfuric acid to
investigate the effect of rolling on the corrosion behavior. For
Ni3(Si,Nb) alloys, medium concentration sulfuric acid is
12
16
20
ac
tio
n(%
)dissolved into the matrix and the distribution of G-phase was
more uniform. The quantitative microstructure analysis of all
alloys is given in Table 3.
3.3.3. HardnessFig. 13 is an example of how the alloys hardness changes
during the rolling annealing process. Annealing successfullyremoved strain hardening and the hardness of alloys was0
4
8
0 1 2 3 4 5 6Nb (at%)
G p
ha
se
a
re
a fr
Fig. 7 e Linear relationship between Nb addition and area
fraction of G-phase in heat treated (950C 4 d) alloys,including alloys not reported on here. Extrapolating from
this data, the Nb solubility in the Ni3Si(b) matrix is about
1.2 at%.200
400
600
800
Yield
stress (M
Pa)0
0.03
0.06
0.09
0 3 6 9 12
Rolling pass
Red
uctio
n
Fig. 9 e The reduction achieved on NiSi21Nb1.6B0.5 versus
number of rolling passes is higher at 300C than at roomtemperature.
-
Fig. 10 e G-phase, aDb eutectic and b-phase in
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 4 4593the passivation was defined as the alloys corrosion rate
decreased significantly. When the normalized weight loss
with exposure time was demonstrated in the natural loga-
rithmic plot, the change of corrosion rate, which relates to the
slope, can be clearly recognized. And the passivation timewas
defined as the exposure time that the alloys begin to passivate.
The corrosion rate, was calculated on the basis of weight
losses (Dm, g), testing time (t, year), density (r, g/cm3), and the
original surface area (S, cm2), after alloys passivated, using
following equation:
Corrosion rate mm=year; 39:37 mpy Dm=10 S r t
microstructure of NiSi18Nb5B0.5 heat treated at 950C4 days.Table 3 compares the corrosion rate and passivation timewith
microstructure analysis of alloys. Several important observa-
tions can be made: (a) after rolling, the corrosion rate
improved and all alloys meet the corrosion resistance target
Fig. 11 e The microstructure of NiSi18Nb5B0.5 sectioned
parallel to rolling direction, after 10 roll(300C)Dannealpasses, total reduction was 52%.(
-
tion and G-phase uniformity which are positive to corro-
Acknowledgements
This work was supported by NERI-DOE project, DE-FC07-
06ID14753. The authors would like to thank Dr. Lillo, Idaho
National Lab, for the compression tests and high pressure
corrosion tests.
r e f e r e n c e s
[1] Davies Michael. Materials selection for sulfuric acid. 2nd ed.Materials Technology Institute; 2005.
[2] ThomasLillo M, KarenDelezene-Briggs M. Commercial alloysfor sulfuric acid vaporization in thermochemical hydrogencycles. AIChE Annu Meet; 2005.
[3] SanHong Zhang. The development of nickel silicide based
0
0.005
0.01
0.015
0.02
0.025
0.03
0 2000 4000 6000 8000 10000
Time (min)
Wei
ght l
oss
/ Are
a(g/cm
2 )
HT44% reduction61% reduction
Fig. 14 e The normalized weight loss with boiling 70%
sulfuric acid exposure for NiSi21Nb1.6B0.5 for homogenized
and rolled conditions.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 4 5 8 8e4 5 9 44594sion resistance.
5. The eutectic constituent is detrimental to corrosion
resistance.
6. The solubility limit of Nb in Ni3Si at 950 C is approximatelypassivation time and the microstructure analyses of alloys is
not clear just from the limited data. It needs more study to
understand the relationship.
5. Conclusions
1. After appropriate heat treatment, 950 C for 4 days, mostunstable and detrimental phases were eliminated in the
microstructure of the Ni3(Si,Nb) cast alloys.
2. The homogenized alloys have lower hardness and can be
deformed more than 50% by multiple cold rolling passes.
3. All homogenized and rolled Ni3(Si,Nb) alloys in this paper,
except the low Si alloy, meet the corrosion resistance target
(