Hafnium corrosion behavior in high-temperature steam

Journal of Nuclear

HAFNIUM CORROSION BEHAVIOR IN HIGH-TEMPERATURE

Ryosho KUWAE, Tatsuya HATANAKA,

Materials 13Y ( 1986) 42247 North-Holland, Amsterdam

STEAM

Junko KAWASHIMA

Research and ~eueloFment Center, Tosbibo Corporation 1, Komukai ~o~bibu-rho, Suiwai-ku, K~w~s~k~210, Japan

and Seishi SHIMA h’uclear Enerp Received

Group, Tosfziba ~orForatio~ 8, Sbins~git~, Isogo-ku, ~ffkoh~rnff 235, Japan

9 October

1985; accepted

25 February

1986

Corrosion properties for three kinds of hafnium have been examined in 10.5 MPa steam at 773 K. Nuclear grade hafnium formed a shiny black film with a few white nodules. which were found to be monoclinic HfO,. Hydrogen pick-up fraction during the corrosion amounted to ca. 4.0% to produce hydrogen dissolved in the hafnium matrix with evolving the remaining 60% hydrogen as H, gas. Sponge and crystal bar hafnium were also examined and they showed superior and inferior corrosion resistance, respectively, to nuclear grade hafnium. The corrosion resistance for hafnium increased with increasing the iron content involved in hafnium for these three kinds of hafnium and, correspondingly related to the density of iron-containing second phase particles, which were characterized to be face centered cubic HfzFe. The corrosion mechanism, which was previously proposed for Zircaloy nodular corrosion, was adopted with making minor alterations, to explain the hafnium corrosion properties.

1. Introduction Nuclear and mechanical properties of hafnium (Hf) have been studied extensively and found to be relatively excellent and appropriate for neutron absorbers [l-6]. In addition, application of hafnium in water cooled reactors requires its high corrosion resistance in steam and/or water. Thus, out-reactor corrosion behavior of hafnium was examined in high pressure water or steam at 5899672 K to show that hafnium is well corrosion resistant under these conditions [7]. However, hafnium has hardly been studied in the high pressure steam around 770 K, where in-reactor corrosion for Zircaloy components can be nearly reproduced in a short time [S-13]. It is reasonable to research hafnium corrosion behavior in this steam condition, since both zirconium and hafnium belong to the IVa group in the periodic classification. Here, hafnium corrosion in 10.5 MPa steam at 773 K was studied. Difference in corrosion resistance and mode was observed for three kinds of hafnium and discussed in terms of a corrosion mechanism proposed. Second

0022-3115/86/$03.50 6 Elsevier Science Publishers (North-Holland Physics Publishing Division)

phase particles were identified corrosion mechanism.

in connection

with the

2. Experimental

2.1. Specimens Nuclear grade, sponge and crystal bar Hf were commercially available. They were melted, hot rolled, annealed at 1123 K for 2 h, and cold rolled to give hafnium plates used for corrosion tests and metallography. Their chemical compositions are shown in table 1. Corrosion specimens of the hafnium plates were machined to give the size 27 x 20 X 3 mi3, polished

with #600 paper, etched in an aqueous solution containing 5 vol.% HF and 50 vol.% HNO, to make the surface state uniform,

washed in ethanol,

and dried.

2.2. Corrosion test The specimens were suspended by stainless steel wire in an Inconel 600 autoclave for testing. The autoclave

B.V.

43

R. Kuwae et al. / Hafnium corrosion behavior Table 1 Chemical

compositions

for hafnium

A1

plates a

C

Cr

Fe

N

Ni

0

Si

Zr

Nuclear grade

150

<50

6

290

30

6

216


2.6

Sponge

68

<50

39

380

20

4

460

<5

3.0

<35

<50

3

71


1

<50

<5

3.7

Crystal

bar

a Unit is wt. ppm except wt% for Zn.

was equipped with a steam generator which supplied refreshed, deaerated (0, content < 0.1 ppm), and high purity steam to the specimen zone during testing. The specimens were exposed to 10.5 MPa steam at 773 K for 6, 12, 24, 48 and 96 h. The corrosion of the specimens was evaluated by weight gain, surface and cross sectional observation.

microscopy and energy dispersive X-ray spectrometry. In addition, thin foils were prepared by electropolishing of previously mechanically sliced sheets, in a solution of 10 vol.% HClO, and 90 vol.% CHJOOH at room temperature. Crystal structure for the second phase particles was identified by electron diffraction.

2.3. Analysis of second phase particles

3. Results and discussion

Non-corroded hafnium specimens were polished by alumina powder of ca. 0.05 pm diameter and etched in an aqueous solution containing 5 vol.% HF and 45 vol.% HNO,. Distribution and composition of second phase particles were examined by phase contrast optical

3. I. Characterization

Table 2 Corrosion

test results for nuclear

Corrosion of nuclear grade Hf proceeded as observed for nodular corrosion of Zircaloys [13]. In the initial corrosion stage, the surface of nuclear grade Hf

grade hafnium

Time/h

0

6

12

24

48

96

0

1.1

1.7

2.5

5.8

7.1

Oxygen content a wt. ppm

216

191

221

311

232

252

Hydrogen content a wt. ppm

0.8

4.4

7.2

9.3

20.3

24.6

43

43

Weight gain gmw2

Hydrogen pick - up fraction % a Matrix content.

38

46

42

of corrosion products

became covered with shiny black film. Small off-white nodules were dotted in the film. Cross sectional observation indicated the nodules to have a lentitular shape. Nodule size increased with corrosion time, However. the black film remained even after 96 h. The off-white nodules were confirmed to be monoclinic HfO, (a = 0.512 nm, h = 0.518 nm, c = 0.525 nm, @ = 98”) [14] by X-ray diffraction. The crystal structure of the black film on nuclear grade Hf has not been identified owing to interference by the coexistent nodules. The structure, however, was also estimated to be monoclinic HfO, on the analogy of the monoclinic HfO, structure assigned to black film forming on sponge Hf mentioned later. Oxygen and hydrogen contents in the hafnium matrices were analyzed by gas analyzers, after eliminating the superficial oxide of nuclear grade Hf specimens. Analytical results are listed in table 2. Oxygen content is 216 wt. ppm for a non-corroded specimen and stays almost constant during corrosion for up to 96 h. On the other hand, hydrogen content in the matrix increases significantly with time. This agrees with the changes in corrosion weight gain listed in table 2. Microscopic examination showed hydride present in the matrix. which was identified to be face centered cubic HfH,.,,_,.,O (a = 0.471 nm) [l5] by the X-ray diffraction. Hydrogen solubility in hafnium is suggested to be ea. 20 wt. ppm at 773 K 1161. Thus, the hydride starts to be formed and equilibrated with dissolved hydrogen at around 48 h. The hydrogen pick-up fraction, which is the ratio of the hydrogen amount picked up by the hafnium matrix to the total hydrogen amount produced by water dissociation during corrosion, was about 40% (see table 2). Mass spectral examination for a vapor phase after corrosion indicated the delivery of the remaining hydrogen as H, gas into the corrosion ambience. Consequently, the hafnium corrosion in high pressure steam at 773 K gives HfO,, dissolved hydrogen (in equilibrium with HfHi.53 - r.,a f and Hz. 3.2. Mechanistic consideration Sponge Hf showed good corrosion resistance to produce shiny black film without nodules even after 96 h. On the contrary, all the surface of crystal bar Hf was coated with an off-white layer of monoclinic HfO, in the earlier corrosion stage. The layer was thickened with time, until it became apt to peel off from the hafnium matrix. Accordingly, from the viewpoint of the surface appearance. sponge Hf is the most corrosion resistant and crystal bar Hf is the least corrosion resistant among

50

f

20

-

IO

-

5

-

2

c

N

‘E 0 \

grade

1 0.5 0.2’

~ 4

I 10

1

I

20

50

I

1

100

Time/h Fig. 1. Change in corrosion weight gain with time lapse.

the three kinds of hafnium used. Nuclear grade Hf shows medium corrosion resistance. Fig. 1 shows the change in weight gain for hafnium specimens as a function of time, both on logarithmic coordinates. The order of corrosion resistance for hafnium, in terms of the weight gain, is consistent with that determined by the surface observation mentioned above. In addition, it is clear that these hafnium specimens show an almost linear increase in log (weight gain) with log (time). n values. which stand for slopes of the linear part of the lines, are approximately i, f and 1 for sponge, nuclear grade and crystal bar Hf, respectively. Crystal bar Hf reveals deviation from this linearity after 48 h. This deviation may be ascribed to the increase in effective surface area with time, because the matrix surface presumabiy becomes coarser with corrosion progress. Corrosion rate is said to be determined by reaction (fast) of a Zircaloy matrix with steam, when n = 1. or by diffusion (slow) when n = 4 [17]. The latter unusual cubic rate law has been attributed to a self-generated stress in surface oxide due to atomic mismatch between the oxide and the matrix; the stress suppresses the rate-determining oxygen ion migration [lg]. These corrosion modes can be valid for hafnium corrosion, since hafnium and zirconium are classified into the group IVa. That is, rate-determining steps in corrosion of sponge and crystal bar Hf are diffusion and reaction, respectively. Nuclear grade Hf is suggested to corrode

45

R. Kuwae et al. / Hafnium corrosion behavior

Fig. 2. Observation

of second

phase particles

in non-corroded

by the intermediate process where both reaction and diffusion paths can concurrently participate. Microstructure of non-corroded specimens was examined by phase contrast optical microscopy. The results are shown in fig. 2. It is obvious that second phase particles appear and that the density of the particles corresponds to the degree of corrosion resistance; i.e., crystal bar Hf, which is the least corrosion resistant, has the least particle density. In sponge Hf which shows the most corrosion resistance appear the greatest number of the particles. Energy dispersive X-ray spectrometric analyses of these particles showed that the particles contain iron. It is natural that the particle density should increase with increasing the iron amount in hafnium. Particle appearance and its typical electron diffrac-

hafnium

using an optical microscope. Etched bulk was examined.

tion pattern are shown in fig. 3. Particle size is 1 pm, on the average. All the spots in the diffraction pattern can be indexed by assuming the particle to have crystal structure of face centered cubic Hf,Fe (a = 1.206 nm) [19], with an incident electron beam vertical to its (233) plane. Iron has been found to exist as a Zr(Cr, Fe), type particle in Zircaloy and to possibly improve nodular corrosion resistance of Zircaloy [13]. By analogy, the iron-containing particles in hafnium will improve the hafnium corrosion resistance. Moreover, clarification of the role played by H, gas in Zircaloy nodular corrosion has provided a corrosion mechainism [13]. In the mechanism, protective ZrO, film is broken down to initiate nodular corrosion, when the pressure of the H, gas accumulating in the ZrO,-Zircaloy interface exceeds the pressure which the ZrO, film can withstand. The

.

Fig. 3. Electron diffraction analyses of second Appearance observed by a transmission electron cubic Hf, Fe.

phase particles. A thin foil sample of nuclear microscope. (b) An electron diffraction pattern.

grade hafnium was examined. (a) (c) The (233) pole of face centered

R. Kwoe

46

et ul. /

Hafnium

corrosion

hehacror

I-ifO, Film HfO, Film .4eHf

4.

Hf

Particle

I

P&tick

Fig. 4. Mechanism for sponge hafnium corrosion

second phase particles promote H, gas formation on the surface of the ZrO, film to prevent nodular corrosion from occurring. Likewise, a corrosion mechanism can be proposed for hafnium, as follows. Sponge hafnium (fig. 4) 02- is formed from H,O at the surface of initial HfO, film leaving H+ (1). 02-, diffusing in the HfO, film toward the Hf matrix (2) reacts with Hf at the HfO,-Hf interface to produce HfO, and electrons (3). Most electrons diffuse through the Hf,Fe particles (4) to react with protons producing hydrogen molecules at the surface (5) and the remaining electrons combine with protons to be dissolved into the Hf matrix (6-8). Thus, the HfO, film is not destroyed and keeps its protective property. In this mechanism, step (2) where the 02- diffusion takes place, may be the slowest and be rate-determining. This step will obey the cubic rate law (n = l/3), in agreement with the experimental result. Crystal bar hafnium (fig. 5) Because of the deficiency

of the Hf,Fe

Fig. 5. Mechanism sion.

for initiation

particles,

wt.

Sponge Nuclear grade Crysta bar

Particle

Corrosion

Surface

Rate -determining

density

resistance

product

process

380

highest

highest

290

medium

medium

71

lowest

lowest

Fe content

I

ppm

bar hafnium

corro-

electrons can hardly diffuse through the particles. Instead, protons diffuse through the HfO, film (4) to react with electrons producing hydrogen molecules (5). Thus, some hydrogen molecules become accumulative at the HfO,-Hf interface. Pressure of the accumulative H, gas gradually increases. The HfO, film is damaged when the H, pressure exceeds the pressure which the film can withstand. Once the protective black film breaks, it is difficult to repair, probably due to the coarse fresh surface of the matrix. Therefore, direct reaction of H,O with the matrix occurs and continues to form a non-protective oxide on the matrix surface. All the surface was observed to get immediately covered

Table 3 Corrosion properties and relevant phenomena

Hafnium

of crystal

black

film

nodules

porous

oxide

diffusion diffusion and

reaction

reaction

R. Kuwae er al. / Hafnium corrosion behavior

with the non-protective oxide layer as mentioned above. In general, oxidation through a non-protective oxide layer follows the linear rate law (n = l), consistent with the present work result. Nuclear grade hafnium This hafnium shows medium particle density. Therefore, both diffusion controlled process (n = l/3) and reaction controlled process (,n = 1) will happen simultaneously to give n = f . This mechanism is compatible with the fact that some nodules appear in the black film on the surface of nuclear grade Hf. Corrosion behaviors summarized in table 3.

and

relevant

phenomena

are

4. Conclusions On the basis of the observed corrosion features for hafnium in 10.5 MPa steam at 773 K, the corrosion mechanism was proposed. New findings obtained here are summarized: (1) Nuclear grade Hf suffered from nodular corrosion to form hafnium dioxide, dissolved hydrogen (or hafnium hydride) and hydrogen gas. The corrosion of nuclear grade Hf obeys a parabolic rate law. (2) Sponge and crystal bar Hf showed superior and inferior corrosion resistance, respectively, to nuclear grade Hf, and produced black oxide film and porous oxide layer, respectively, on their surface. They follow cubic and linear rate law, suggesting diffusion and reaction-controlled process to be dominant, respectively. (3) The corrosion behaviors were correlated with the amount of iron (an impurity element), which was found to exist as Hf,Fe type second phase particles. The corrosion resistance is increased with increasing the iron amount. of the roles played by the H, gas and (4) Clarification the second phase particles has provided a corrosion mechanism. The mechanism can explain the dif-

ference in the corrosion resistance mode of the three kinds of hafnium.

41

and corrosion

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