Volume 6 Paper 13
Titanium Incorporation into the Oxides of Nuclear Reactor Materials
W.G. Cook, D.H. Lister, K. Ishigure and S. Ono
Keywords: High Temperature Corrosion, Reactor Materials, Titanium, Magnetite, Nickel Ferrite, Ilmenite
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JCSE Volume 6 Paper 13
Submitted 6th July 2003
Titanium Incorporation into the Oxides of Nuclear Reactor Materials
W.G. Cook*, D.H. Lister*, K. Ishigure�
and S. Ono��
* Department of Chemical Engineering,
of New Brunswick, Canada,
� Saitama Institute of Technology,
and Innovation, Tokyo,
§1 Titanium, in components such as autoclaves and tubing, constitutes a
significant fraction of the construction material used in many experimental
high-temperature water loops and autoclaves. At the
of New Brunswick, such a system has
been used for corrosion studies of reactor materials under pressurized water
reactor (PWR) conditions (0-1000 ppm boron, 0.5-2.5 ppm lithium and hydrogen saturated at STP). A
stainless steel section of the loop is used to condition coolant to reactor
inlet conditions. The coolant is further heated to reactor outlet temperatures
in the titanium sections, allowing minimal addition of corrosion products to
the coolant and in this respect simulating the reactor core. However, the
incorporation of titanium into the oxides of the test materials has indicated a
significant solubility of titanium under these conditions and led to the
suggestion that titanium dosing of coolant systems may be used for corrosion
§2 To examine the titanium transport properties, coupons fabricated of A106-B
carbon steel, 316L stainless steel, Alloy-600 and Alloy-800 were placed in a
titanium autoclave and exposed to simulated PWR coolant (600 ppm boron, 2.5 ppm lithium) at
290oC for a period of 1100 hours. A basket filled with
titanium turnings placed upstream of the coupons in the autoclave ensured
coolant saturation in titanium. After exposure, the coupons were examined
with several surface analysis techniques. Results showed significant
titanium incorporation in the oxide layers of all materials tested. The
oxide layers were generally duplex � precipitated crystals overlaying a compact
inner layer. The titanium tended to concentrate at the middle of the
oxide between the two layers. Since the inner layer grows inwards towards
the metal and the outer layer grows outwards towards the coolant, the
disposition of the titanium suggests that it was incorporated in the oxide
mostly at the start of the exposure.
§3 Keywords: High Temperature Corrosion, Reactor Materials,
Titanium, Magnetite, Nickel Ferrite, Ilmenite.
§4 Modern pressurized water reactors (PWRs) operate
under strict regimes with regard to primary system chemistry. Optimization of coolant chemistry conditions leads to
improved corrosion resistance of the primary system materials and reduces the
crud circulating through the system. Activity transport is therefore
reduced since coolant borne crud and dissolved corrosion products may become
activated while traversing the reactor core and subsequently deposit on
out-of-core components such as the steam generators.
§5 Much progress has been made in recent years to reduce the material transport
throughout the primary circuit. The coordinated chemistry program of the
1980s and 1990s  and its modifications, in conjunction
with removing, as much as possible, cobalt-containing materials such as Stellite significantly reduced the collective dose to plant
workers . Other programs, such as dosing
the primary circuit with zinc, have also been shown to reduce the out-of-core
radiation fields by displacing the cobalt from deposited oxides such that it
can subsequently be removed via the purification system .
§6 Recently, experiments have been ongoing at UNB to study the morphology of
oxides formed on 316 SS under primary circuit conditions, where the material
was subjected to a high velocity jet. The results of this work were put
into a model describing the material transport through the primary heat
transport system . The results, however, were
somewhat difficult to interpret due to interference of titanium by the
surface-grown oxide films. Significant portions of the loop used for this
work were constructed of titanium, which was apparently transported though a
dissolution-precipitation mechanism to the surfaces of the test coupons.
§7 The incorporation of titanium into corrosion films is not a new phenomenon,
in fact, titanium has been proposed as an oxide modifying agent to suppress
IGSCC (Intergrandular Stress Corrosion Cracking) and
boiler tube denting on the secondary side of steam generators [
- . Titanium dioxide (TiO2)
in the anatase form has also been proposed as an
additive to CANDU primary coolant to mitigate the accelerated corrosion of the
outlet feeder pipes by a flow-assisted corrosion mechanism .
However, titanium autoclaves and components have long been used for corrosion
testing equipment at high temperatures and pressures because it was thought to
be �inert� under most conditions so it would not affect the test results.
Evidence is mounting that this is not the case for the conditions encountered
in the primary heat transport system of nuclear reactors.
§8 The experiments undertaken here were initiated to examine the pick-up of
titanium into the oxide layers of typical reactor materials under PWR primary
system conditions. A collaborative program was established between UNB
and Dr. Ishigure, then of the
currently at the Saitama Institute of Technology in Saitama, and Mr. Ono of the
and Innovation in Tokyo.
§9 PWR primary system conditions were simulated in the high-temperature loop
shown schematically in Figure 1. Simulated primary system coolant is
circulated via the positive displacement pump through an interchanger and heater
and passed into a two-litre autoclave. All components in the system to
this point are Type 316 SS. The coolant is then passed through the three
one-litre titanium autoclaves through the valve arrays (all components in this
section are Grade 2 titanium) and is returned to the reservoir via the
interchanger, cooler and back-pressure regulating valve (BPRV). The system also
has full flow purification via an ion exchange column equilibrated to the
desired chemistry condition. Typically, experiments consisted of heating the
coolant to reactor inlet temperatures (260oC � 285oC) in
the stainless steel section and increasing to reactor outlet temperatures (300oC
� 315oC) in the titanium section, thus simulating the heat pick-up
in a virtually ferrous-free reactor core.
§11 Figure 1.
Diagram of high-temperature loop.
§12 Four materials of interest were fabricated into rectangular coupons (20 x 18
mm) that were between 1 mm and 2 mm thick. The four materials were A106-B
carbon steel, 304L stainless steel, Alloy-600 and Alloy-800. The
available stock of these materials was all in the form of tubing, except for
the carbon steel, so the tubing was sectioned and rolled flat to fabricate the
coupons. Prior to insertion into the loop for exposure, all the coupons
were polished to a 600 grit finish, degreased with acetone and weighed.
§13 For this test, duplicate coupons of each material were hung from a sample
tree using stainless steel wire and placed in the third titanium autoclave
(Ti-3). Fresh titanium turnings were put into a stainless steel basket
that was suspended from the autoclave head (Figure 2). The turnings were
used to ensure that the coolant in the autoclave would be saturated in titanium
at the test conditions.
§15 Figure 2.Configuration of Ti-3 autoclave.
§16 Following exposure, the coupons were removed from the autoclave, dried with
nitrogen and weighed. One set of coupons was examined at UNB using
scanning electron microscopy (SEM) and energy dispersive
X-ray (EDX) analysis while the other set was sent to the Department of Quantum
Engineering and Systems Science at the University of Tokyo for analysis by glow
discharge spectroscopy (GDS) and X-ray photoelectron spectroscopy (XPS), which
was carried out under the collaboration between the University of Tokyo and the
Institute of Research and Innovation. The GDS measurement was performed
with a constant electric current of 20 mA loaded to
each coupon and quantitative analysis of the Fe, Cr, Ni and Mn
contents was made from calibration curves prepared using austenitic stainless
steels and Alloy-600 as standard samples. Calibration for Ti was
similarly performed using Type 321 stainless steel (0.7% Ti by weight) and an Fe-Ti alloy (37.15% Ti by weight) specially manufactured
for this purpose. The weight fraction of each metal element was represented as
that of the relevant element to the total weight of the metal elements
measured; oxygen was excluded since its quantitative analysis by GDS is rather
§17 The operating parameters for the experiment are shown in Table 1. The
boric acid and lithium hydroxide concentrations were chosen to simulate a PWR
chemistry near the conditions at the beginning of the operating cycle; ie. a pH300�C of around
6.9. Hydrogen was continuously bubbled through the sealed reservoir
ensuring low oxygen concentration, which was measured using Chemets colourimetric indicators.
Table 1.Experimental conditions.
§19 Boron concentration
§20 600 ppm
§21 Lithium concentration
§22 2.2 ppm
§25 Temperature in SS section
§26 260 oC
§27 Temperature in autoclave Ti-3
§28 290 � 1 oC
§29 Hydrogen concentration
§30 18 cc/kg
§31 Oxygen concentration
§32 < 20 ppb
§33 Flow rate
§34 300 ml/min
§35 Experiment duration
§36 1100 hrs
§37 The temperature in the third titanium autoclave was maintained around 290oC
for the duration of the experiment; this is significantly less than the 325oC
normally seen at the outlet of a PWR reactor core and was due to limited
heating capacity available in the third autoclave at the time of the
§38 Following the 1100-hour exposure time, the heating in the loop was shut down
and the system was allowed to cool. Once workable temperatures were
obtained, the pump was shut off, and the coupons were removed from the
autoclave, dried with nitrogen and weighed. Table 2 presents the weight
loss/gain data and apparent corrosion rate for each coupon exposed. The
values are low, as expected for this environment, but cannot be used for
accurate indications of corrosion rate without oxide stripping.
§39 As described above, one set of coupons was packaged and shipped to the
Department of Quantum Engineering and Systems Science at the
for analysis by GDS and XPS, whilst the remaining coupons were examined at UNB
by SEM and EDX analysis. Typical SEM micrographs for each material are
shown in Figures 3 � 6. As expected, the oxide layers formed on the
materials appeared quite different under the given chemistry conditions.
The carbon steel coupon shows a thick outer oxide layer composed of octahedral
crystals, approximately 1 µm in size, typical of magnetite (Fe3O4).
The stainless steel coupon shows a more-or-less exposed inner oxide layer with
a covering of octahedral crystals, again typical for magnetite, nickel ferrite
(NiFe2O4) or a nickel-substituted magnetite (NixFe3−xO4).
The Alloy-600 and Alloy-800 surfaces were virtually bare of large oxide
crystals with the grooves from the polishing performed before the experiment
2.Weight loss/gain measurements and apparent
§42 Coupon No.
§43 Weight Loss (Gain) (mg)
§44 Apparent Corrosion Rate (μm/a)
§52 304L SS
§74 ������������ Figure 3.SEM photo of the A106-B coupon following exposure.
§76 �������������Figure 4.SEM photo of the 304LSS coupon following exposure.
§78 ����������������Figure 5.SEM photo of the Alloy-600 coupon following exposure.
§80 �����������������Figure 6.SEM photo of the Alloy-800 coupon following exposure.
§81 Spherical or more-rounded crystals were also apparent on all the
materials. These were analysed by EDX to be composed of iron and titanium
in an approximately one-to-one ratio, indicating that they are probably ilmenite, FeTiO3. The EDX analysis results
from the surfaces of all the materials are show in Table 3.
Table 3.EDX analysis results.
§83 Coupon No. and Material
§85 Elemental Composition (%)�
§93 #15 - A106-B
§110 #4 - 304L SS
§135 #7 - Alloy-600
§152 #10 - Alloy-800
§169 � Balance is Oxygen.
§170 The GDS depth profiles obtained from the duplicate set of coupons are shown
in Figures 7 - 10. Titanium is apparent in each of the films formed, but
to varying degrees and depths. The disposition of titanium in the film
seems to be between the inner and outer oxide layers for cases where duplex
films were formed (on the carbon steel and stainless steel) and peaks at
approximately 7 � 8% by weight. Titanium appears on the surface of the
nickel alloys at approximately 10% by weight. This seems to correspond to
the SEM photos for these coupons since the oxide formed was very thin with a
scattering of crystallites on the surface. Even in this case the films
may be duplex, with the outer layer much sparser than those on the ferrous
§171 The depth profiles taken from the XPS all showed similar trends to the GDS
profiles. Titanium was apparent on each material, concentrating near the
surface and in between the duplex films for the ferrous materials and on the
surface of the nickel alloys. The XPS was also used to obtain the binding
energy of titanium in the films in conjunction with sputtering to obtain the
depth profiles. Figure 11 depicts the apparent binding energy of titanium
in the oxide grown on the A106-B coupon. The binding energy of 458.8 eV is assigned to Ti(IV), which is typically the most
stable valence for titanium and is consistent with our assumption that ilmenite (Fe(II)Ti(IV)O3) is the primary phase,
however, this could also be consistent with formation of titanium dioxide
§173 ������������ Figure 7.
GDS depth profile for A106-B carbon steel.
§175 ��������� Figure 8.
GDS depth profile for 304L SS.
§177 ������������� Figure 9.GDS depth profile for Alloy-600.
§179 �������������� Figure 10.GDS depth profile for Alloy-800.
§181 Figure 11.
XPS spectra of binding energy for titanium in the oxide film formed on A106-B
§182 The deposition of titanium onto coupons of typical nuclear reactor materials
and its incorporation into the oxide layers demonstrates that the perceived
immobility of titanium ions in reactor coolants is not valid. It appears
that when fresh titanium metal is exposed to simulated reactor coolant and
operating conditions, its solubility is significant enough such that aqueous
titanium may be transported to surfaces away from the source. The
solubility of titanium in reactor coolants has been examined before in other
research programs at UNB . In those experiments, powdered samples
of rutile (TiO2) and ilmenite
(FeTiO3) were heated in an autoclave containing simulated reactor
coolants (pH adjusted with lithium) and samples were drawn off, concentrated on
ion exchange beds, eluted with concentrated nitric acid and the titanium
content measured through ICP-MS. Although there was considerable scatter in the
data, titanium was measured at the 0.1 � 0.5 ppb level, depending on pH.
§183 For verification of aqueous titanium in the autoclave used for the present
experiments, an additional test was run with the same chemistry and conditions
given in Table 1, only with a sample line inserted in place of the
coupons. Fresh titanium turnings were placed in the stainless steel
basket and the loop was operated for a period of one week before samples were
taken. The samples were digested in the same manner described above and
titanium was measured to be between 0.1 � 0.2 ppb.
§184 The highest concentration of titanium measured with GDS on the ferrous
materials appears to be between the inner and outer oxide layer. This
would be consistent with an initial large release of titanium from the freshly
machined surfaces, which would quickly become passivated
as the TiO2 film built up. Additional titanium would be
incorporated on the surfaces of the reactor materials as exposure time
progressed through dissolution of the TiO2 surface films on the
source material, albeit at a much slower rate. The titanium deposited on
the nickel alloys appears to be completely on the outer surface of the oxide,
presumably because it is incorporated into the sparse scattering of small crystallites
that cannot be distinguished as a separate layer.
§185 The presence of the rounded ilmenite crystals on
all the coupons suggests that, although titanium may be incorporated to some
extent into the spinel oxides formed on the coupons,
particularly in the early stages of oxide formation, much of the titanium
transported to the surface precipitates as a separate phase. Also of
interest is the manganese content in the ilmenite
crystals � it is present at about one percent by element as measured by
EDX. This is further evidence that these crystals are truly ilmenite, not rutile producing a
signal adulterated from the substrate, since in minerals containing ilmenite it is frequently seen that manganese will displace
some of the ferrous ions from the lattice structure producing ore rich in
§186 It has been demonstrated that, when titanium is applied as a construction
material in corrosion test loops used to simulate the primary heat transport
system of nuclear reactors, there is a measurable transport of aqueous titanium
species from the construction material to test surfaces within the
system. The titanium content in oxides formed on typical reactor
materials tended to concentrate near the interface between the inner and outer
layers, which is consistent with an initial high dissolution rate of a fresh
titanium surface that would significantly diminish as its own oxide film
developed. In such instances, the titanium transported to the surfaces of
the reactor materials will precipitate as a separate phase, ilmenite.
§187 These results point to the possibility of using titanium or titanium oxides
as an inhibitor for corrosion in the primary heat transport system of nuclear
reactors or in similar coolant systems. Corrosion protection may result
from the incorporation of titanium into native
oxide layers, probably by making them less soluble in aqueous environments.
§188 The authors would like to thank Norm Arbeau for
his assistance in operating the test loop and analysis of data, Dr. Joy Gray for further surface analysis and helpful discussions
and Dr. Haydn Starkie of
the HSE-NII in the UK
for allowing this work to be performed in parallel with contract work.
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