New Corrosion Resistant Materials
for Next Generation Energy Processes
Dean Gambale
Tantaline
Waltham, Mass.
Material
performance
limitations
have been an
obstacle to the
adoption of
many new
renewable
power source
technologies.
Tantalum
surface alloys
open up new
opportunities.
T
he promise for renewable clean energy
is not new. In fact, many of the technologies considered today for the energy of
tomorrow such as hydrogen production, fuel
cells, solar panels, bio-fuels and others have existed for decades. But these exciting technologies have not been readily adopted. For
example, fuel cells were invented by German
scientist Christian Friedrich Schöbein in 1838,
and were first used commercially in 1959 by
NASA. In 1838, the first solar cell was built by
Charles Fritts. During World War II, a variety
of biofuels were used by the German and
British forces to supplement fuel shortages. To
a large extent, the limitations of materials
needed to make these technologies technically,
economically, and commercially feasible and
competitive with existing fossil-fuel infrastructure prevent the adoption of renewable clean
energy technologies.
To be efficient, the next generation of energy production technologies requires more aggressive process conditions, potent chemistries,
and pure/clean operating conditions that are
free from contamination. As a result, traditional materials have been pushed beyond their
Heat
H2SO4
830°C
½ O2 + SO2 + H2O
H2SO4, (H2O)
½ O2
SO2, O2, H2O
H2SO4 + 2HI
120°C
I2 + SO2 + 2H2O
H2O
Heat
2HI, (I2, H2O)
I2, (H2O)
Heat
2HI
320°C
I2 + H2
H2
Fig. 1 — Sulfur-iodine thermochemical process: sulfuric acid concentration
and decomposition (top); recycle and acid generation (middle); and
hydrogen iodide concentration and decomposition (bottom).
performance limits. The materials challenges
faced from corrosion, mechanical durability,
and economics limit the commercialization of
these technologies.
This paper will explore some of the challenges faced specifically with hydrogen production via sulfur-iodine (S-I) thermochemical
water splitting, and also will focus on the material problems that exist and some of the solutions that are being applied.
The sulfur-iodine thermochemical process,
developed by General Atomics, San Diego,
Calif., allows producing hydrogen gas without
the use of fossil fuels. The process has extreme
operating environments, and, therefore, creates
a variety of significant material challenges. As
conventional materials are pushed to their limits to achieve the next generation of chemical
processes, new materials are required to meet
those challenges.
This paper discusses the conditions and the
challenges faced in the sulfur-iodine thermochemical process and how tantalum surface alloys were used to meet these corrosive,
mechanical, and economic challenges where
virtually all other traditional corrosion-resistant materials like nickel, titanium zirconium
and tantalum metal alloys have failed.
Taking on an aggressive
process environment
Combustion of fossil fuels currently provides
about 86% of the world’s energy[1, 2]. To reduce
our dependence on fossil fuels and lessen the environmental impact, hydrogen fuel (from nonhydrocarbon feedstock) presents an attractive
alternative. The sulfur-iodine thermochemical
process requires heat and water as the only inputs, and oxygen and hydrogen are the only outputs. All of the reagents are self-contained
within the process and recycled, creating no
waste. When combining this technology with
solar and nuclear power as a heat source, it becomes a very attractive process for hydrogen
production and a viable alternative to fossil fuels.
The sulfur-iodine water-splitting cycle represents a leading candidate for thermochemical hydrogen production, consisting of three
chemical reactions that result in the dissociation of water as shown in Fig. 1[3].
The sulfur-iodine thermochemical process
ADVANCED MATERIALS & PROCESSES • JANUARY 2011
19
250
5 mpy (0.13 mm/y)
Temperature, °C
200
150
100
50
0
0
5
10
15
20
25
HCl concentration, %
30
35
40
Fig. 2 — HCl corrosion resistance by metal[4]
300
5 mpy (0.13 mm/y)
Temperature, °C
250
200
150
100
50
0
0
10
20
30
40
50
60
70
H2SO4 concentration, wt%
80
90
100
Fig. 3 — H2SO4 corrosion resistance by metal[4]
is aggressive both mechanically and
chemically. It is a hot process where the
hydrogen production efficiency is a function of the process temperature. Efficiencies as high as 55% are possible at a
process temperature of 900°C (1650°F).
However, the materials of construction
for the process components, including
valves, fittings, vessels, and instrumentation, are pushed beyond their limits in an
environment consisting of high temperatures and pressures, as well as concentrated acids. This leads to corrosion
failures, creating an unstable, unsafe
process environment, which can often
lead to higher operating costs making the
process economically unfeasible. Therefore, new materials are needed to meet
the challenges of the sulfur-iodine thermochemical process and to make it efficient, economical, reliable, and safe.
Material choices are limited due to the
aggressive process environment.
Corrosion is a problem because the
process contains mixtures of sulfuric
acid (H2SO4) and hydroiodic acid (HI) at
temperatures greater than 300°C (570°F)
and pressures between 20 and 30 atmospheres. It is important to note that while
hydroiodic acid is not a common acid, it
is one of the strongest acids in the halide
group compared with HCl and HBr
(Figs. 2 and 3). This makes containing
the hydroiodic solution (HIx – hydroiodic acid + water) very difficult.
A solution of sulfuric acid, hydroiodic acid, and water at temperatures
to 280°C (535°F) and a pressure of 150 psi
present the most corrosive conditions
that the materials face. This environ-
Ta 1. run
Ta 2. run
80
Tantalum layer
60
0.5 μm
Alloy zone
40
20
Substrate
0
-800
-600
-400
Interface
Tantalum concentration, wt%
100
Stainless steel
Tantalum surface
-200
0
200
400
Distance from interface, nm
Fig. 4 — Concentration profile of tantalum alloy in 316 stainless steel.
20 ADVANCED MATERIALS & PROCESSES • JANUARY 2011
600
800
10 μm
Fig. 5 — Cross section of tantalum surface
alloy.
ment is so corrosive that even specialty materials like Hastelloy Alloy C276 (Haynes International), a Ni-Cr-Mo-W alloy,
can only survive up to 50 hours[5]. Because of the high temperatures and pressures, polymeric materials are not a consideration. Although glass would fare well corrosively, because of the
high process pressures and glass’s brittleness, it also is not feasible option in a production environment, leaving metals as the
only practical option.
A variety of specialty metals were considered including
nickel-base alloys like Monel (Ni-Cu) alloys, Hastelloy Alloy B
(Ni-Mo) and C (Ni-Cr-Mo-W) grades, zirconium, titanium,
tantalum, and gold. Of these materials, only tantalum demonstrated the corrosion resistance needed to survive in this corrosive environment, and was selected as the material of choice
to deal with the hydroiodic acid solutions.
While tantalum metal is known as the most corrosion-resistant material commercially available, there are many problems associated with its practical use. First and foremost is the
price, which is about 50 times more than stainless steel. Another problem is its unavailability in the form of usable products. It is possible to obtain tantalum in the form of ingots,
rods, tubes, and sheets, but it is difficult, if not impossible, to
obtain tantalum in the form of common process equipment
like valves, fittings, pumps, and instrumentation due to the
price of the metal and poor machinability and weldability. Furthermore, custom fabrication of solid tantalum is not easily
performed, and, therefore, it is typically carried out by specialized, highly skilled fabricators.
Although tantalum metal from a purely corrosion point of
view is ideal, designing a process out of solid tantalum metal
has some serious practical limitations, engineering difficulties,
and economical flaws. Because tantalum is the ideal material for
the process environment, the feasibility of designing the sulfuriodine thermochemcial system was dependent on finding an alternative to solid tantalum metal without sacrificing the
performance.
Overcoming materials performance challenges
The main driver for deciding on a particular material solution was based on having the ability to use commercially available products having a corrosion resistance similar to that of
solid tantalum at an affordable price. This led the end users to
select Tantaline’s tantalum surface alloy as the material best
suited to meet the challenges of the S-I process.
Tantalum surface alloys are created by chemically reacting
and vaporizing tantalum at high temperatures. The tantalum
metal in the gaseous tantalum atmosphere diffuses into and
continues to grow on the surface of the substrate (Figs. 4 and
5). Because the process occurs at an atomic level and at high
temperatures, an alloy zone is created in the substrate material, which is typically stainless steel. Because this is a chemical/metallurgical bond as opposed to a mechanical bond, the
tantalum surface is extremely rugged and durable and not susceptible to chipping, spalling, and delamination associated
with coatings.
Once the alloy zone is formed, the process continues to produce a pure tantalum metal surface having all of the chemical
properties of commercially pure tantalum metal as specified by
www.asminternational.org/access
ADVANCED MATERIALS & PROCESSES • JANUARY 2011
21
Fig. 6 — Deformation and photomicrograph of tantalum surface alloy after deformation.
ASTM-B364. The tantalum surface is typically 50 μm
(0.002 in.) thick, which has been shown to be the optimal
thickness for most applications, although surface alloy layers as thick as 200 μm (0.008 in.) have been successfully
produced.
The ruggedness and durability of tantalum surface alloys was demonstrated in a study by Dr. Hira Ahluwalia[6]
in an attempt to deform tantalum surface alloys locally and
uniformly over the surface. This was achieved by trying to
puncture the surface using a Rockwell C cone-shaped
hardness indenter and bending the samples 180 degrees
over a 0.5 in. diameter mandrel. The integrity of the surface
was verified by boiling the parts for 24 hours in concentrated HCl at 80°C (175°F).
In addition to tantalum surface alloy’s corrosion resistance, availability also was a key factor in its selection. Using
the material for the sulfur-iodine thermochemical process
was feasible because the technology could be used on standard commercially available stainless steel parts like valves,
fittings, pumps, and heat exchangers. Also, the economics
of tantalum surface alloys make it possible to get the performance of tantalum metal at a price that is competitive
with other specialty metals and alloys such as C276, titanium, and zirconium.
For the sulfur-iodine thermochemical application,
components including valves, Swagelok fittings, reactors,
thermowells, pumps, and heat exchangers were treated to
produce a 50 μm thick tantalum surface alloy.
Performance results
In the extreme corrosive conditions created in the sulfur-iodine thermochemical process, parts made of gold, titanium, Monel, and Hastelloy C276 did not survive longer
than 50 hours. Therefore, tantalum surface alloys were
adopted for all surface areas of the system that were exposed to the corrosive conditions, which amounted to
more than 90% of the surface area of the process valves, fittings, instrumentation, and custom parts.
Since August 2009, more than 1,000 parts have been
installed at General Atomics and the Korean Institute of
Energy Research, or KIER (Daejeon)[7]. With several years
of history and thousands of hours of operation, temperature cycling, mechanical abuse, and process spikes, the tantalum surface alloys have proven to be orders of magnitude
better in corrosion resistance, mechanically rugged, and
economically attractive compared with specialty alloys,
22
ADVANCED MATERIALS & PROCESSES • JANUARY 2011
with no failures to date.
In addition, it was learned in this process that tantalum
surface alloys are resistant to hydrogen embrittlement. Because the sulfur-iodine thermochemical process generates
hydrogen, there are relatively high concentrations of free
hydrogen in the environment, which has led to hydrogen
embrittlement and premature failure of solid tantalum
components. In the same environment, tantalum surface
alloyed parts resist the effects of hydrogen embrittlement,
surviving much longer than solid tantalum.
Conclusions
Tantalum surface alloys allow providing the required
performance properties of solid tantalum on stainless
steels at a price similar to that of nickel alloys. This not only
makes the process feasible both technically and economically, but also provides a level of safety that could not be
realized with other materials.
Since the adoption of tantalum surface alloys in the
General Atomics application, KIER built a similar sulfuriodine thermochemical process and selected tantalum surface alloys as the material of choice for its corrosion
resistant needs.
References
1. International Energy Outlook 2000: DOE/EIA-0484(2000).
2. Annual Energy Outlook 2000 with projections to 2020:
DOE/EIA-0383 (2000).
3. P.M. Mathias and L.C. Brown, Thermodynamics of the Sulfur-Iodine Cycle for Thermochemical Hydrogen Production,
68 Annual Mtg. for Soc. Chem. Engrs., p 1-3, March 2003.
4. Corrosion Engineering Handbook, 1996.
5. B. Russ, program manager, Energy Process Group, General
Atomics.
6. H. Ahluwalia, Materials Selection Resources.
7. Korean Institute of Energy Research.
Monel is a registered trademark of Special Metals Corp.,
Huntington, W.Va.; Hastelloy is a registered trademark of
Haynes International, Kokomo, Ind.; and Swagelok is a registered trademark of Swagelok Corp., Solon, Ohio.
For more information: Dean Gambale, president, Tantaline,
1050 Winter St. Suite 1000, Waltham MA 02451; tel: 781/2090208; fax: 888/292-9243; email; dgambale@tantaline.com;
Web site: www.tantaline.com.