FR9601486

CORROSION RESISTANCE OF HfB2_s COMPOUNDS Xavier DESCHANELS Commissariat a I'Energie Atomique - CE Saclay DMT/SEMJ/Laboratoire d'Etudes desMate'riaux Absorbants Bat.524, F-91191 Gif-sur-Yvette

Abstract : The corrosion characteristics of hafnium diboride is measured in high temperature water (P=15.5 MPa, T=345°C). The significant result obtained in this investigation reveals that metal-rich compositions of hafnium diboride provide the most corrosion resistant material. Keywords : corrosion, hafnium diboride, corrosion-cracking

Introduction Hafnium Diboride should be an attractive control rod materials for Pressurized Water Reactor (PWR), because of the high neutron absorption cross section of the hafnium and boron 10 isotope. Even more, this ceramic has a good refractarity (Tmemng = 3380±20°C) and a high thermal conductivity. For these reasons, studies have been in progress in our laboratory to define the limits of applicability of this ceramic as neutron absorber. This paper reports the results of the corrosion behaviour in high temperature conditions of hafnium diboride elaborated from two types of powders. A description of the synthesis and experimental procedures used to prepare and to characterize specimens is given. A continual surveillance of chemical composition of the materials before and after the fabrication and an evolution of density, phases constitution and microstructural features prior to and after corrosion exposure are described.

Bibliographic survey The results of studies concerning the corrosion behaviour of hafnium diboride compounds is presented in table 1. In these studies, the corrosion rate is expressed by means of weight change or by determination of boron dissolution. Hoyt [1] observes the formation of the monoclinic phase HfC>2 on the surface of the sample. He shows that hafnium diboride has a bad corrosion resistance. On the opposite, Arabei [2] measures a very low corrosion rate. As can be seen in the table 1, relative density and chemical composition of these two materials are different. For his part, Byron [3] determines a very bad corrosion resistance of all the diboride materials (HfB2, I, TiB2) in high temperature conditions.

Reference Composition Operating conditions Relative Weight change Boron Impurity density (%) (mg/dm^) dissolved (%) (%) Hoyt [1] HfB212 400°C - 187 bars 80 -12660 2.11 490 hours Arabei [2] HfB,.64 350 °C -168.8 bars >95 +100 1.5 300 hours Byron [3] Hffi2 360 °C - 187 bars 80 73.8 48 hours Table 1: Bibliographic review

Fabrication and characterization of the samples The commercial powders used to elaborate the pellets have been supplied by H.C. STARCK GmBh & Co (pi) and CERAC incorporated (p2). Prior to the fabrication of the pellets, the powders are characterized by X-Ray diffraction, particle size analysis (Coulter technique), BET specific surface. These powders are also characterized by quantitative chemical analysis for hafnium, boron, carbon and by quantitative emission-spectrographic analysis for trace impurities. The indicated stoichiometry is based on the atomic ratio of total boron to total hafnium. All these results are reported in the table 2. Powder pi has a boron rich composition and presents a higher level of impurities than p2. Powder p2 is nearly stoichiometric. In spite of the fact that powder pi has the lowest mean size, its grain-size distribution shows that 4 % of the volume of particles have a diameter larger than 20 urn. Material Chemical analysis (% wt) Physicals X-Rays characteristics characteristics Hf B C Metallic d50 SBET a (A) c(A) extraneous impurity (nm) (mr/g) phases HfB206(pl) 87.43 10.91 0.073 1.342 2.4 1.04 3.1429 3.4763

HfB199(p2) 89.4 10.8 0.198 0.321 4.41 0.38 3.1406 3.4737 HfCorHfB Hf(p3) 2.58 7.16 Table 2 : Characteristics of the powders

Two series of pellets (lenght = 12 mm, diameter = 7 mm) are fabricated by vacuum hot-pressing of powders into graphite die. A first one is prepared from the starting powders pi or p2. A second one is elaborated by adding hafnium Sintering conditions powders to the diboride powders pi and p2. Powders N° T At P Annealing The blending of the powders is carried out by (°C) (min) (MPa) conditions ultrasonic dispersion in alcohol. This blending Pi Sn 1800 60 22.5 is dried and sieved before sintering. After Sl2 1800 60 45 sintering, the material is annealed 24 hours at Sl3 1800 60 90 1500°C for homogenizing the microstructure. Sl4 1950 60 45 2000 60 45 Typical fabricating conditions for each material are presented in the table 3. P2 s21 1900 60 60 s?? 2100 60 60 For a same final density (see table 4), samples pl+p3 Cn 1900 60 60 24h-1500°C S2X are elaborated at much higher temperature C\7 1200 15 70 24h - 1500°C than samples Slx. The highest level of p2+p3 C? 1900 60 60 24h - 1500°C impurities and the highest BET surface are Table 3 : Fabricating conditions for probably the reasons of the better sinterability preparation of corrosion samples of the powder pi. We also observe that hafnium addition decreased the sintering N° B/Hf Metallic Density Porosity (%) temperature of the materials C^, C^, Cj impurity Physical and chemical characteristics of 3 (%wt) (g/cm ) (%) Open Closed samples after sintering are reported in table 4. Sn 8.5 76.1 21.72 2.1 The density is obtained by geometrical S1? 9.4 84.15 13.25 2.6 measurements and weighging. The theorical Sn 2.06 1.22 9.7 86.8 9.27 3.93 density of hafnium diboride is equal to 11.17 Sl4 2.07 1.26 10.4 93.1 0.28 6.62 g/cm3 [4]. Helium pyenometric measurements S 2.12 1.33 10.9 97.6 0.36 2.06 15 are used to estimate open and closed porosity. 2.07 0.458 8.29 74.2 24.4 1.4 S?l Closed porosity of the composite could not be s?? 2.02 0.325 10.7 95.79 0.36 3.85 Cn 1.9 1.46 11.19 * 100 0.01 a0 calculated because we do not know the theorical density of these materials. However, C12 1.64 1.28 9.54 «85 5 «15 the micrographs lc and Id show that porosity C? 1.9 0.57 11.13 «100 0.01 «0 Table 4 : Physical and chemical of the sample C L 2 and C2 are very low, so we

characteristics of corrosion samples can estimate the closed porosity of these samples equal to zero. Prior to the corrosion test, electron-microprobe, X-ray analysis and metallographic examination are performed on the samples. By combining these results, it is possible to characterize their microstructure as follows : a) Samples S^x : Chemical analysis indicate that these samples are boron rich materials. This result is confirmed by electron microprobe analysis which show some boron rich phase segregated in the grain boundary. The observation of the microstructure reveals an heterogeneous grain size (see figure la) which may result from the heterogenous grain-size distribution of the powder pi. b) Samples S22: This sample is single phase with a homogeneous microstructure composed of a grain size lower than 20 urn (fig lb). The microstructure of this sample is quite different from Six one. c) Samples Cjx : Materials fabricated from powder pi with hafnium addition are multiphase specimens. Electron microprobe analysis detects four phases. The matrix phase which has the stoichiometry HfB2. A second phase, HfO2 which is grey on optical micrography (fig lc ). X-Rays analysis indicate this phase represents quantitatively 1 or 2 % of the mass of the sample. The third phase which looks like HfB2 phase on optical micrography, has the stoichiometry near HfB with an oxygen contamination. This phase has been identified by Glaser [5] as a cubic phase. X-rays and electron microprobe analysis also indicate the presence of hexagonal hafnium. The quantity of free hafnium is higher for sample Q2 than for C\\. From a microstructural point of view, this sample is composed of heterogeneous grain-size like the Six sample (fig lc). A typical evolution of X-rays pattern composite material C] ] is presented on figure 2. d) Sample C2 : These pellets also present a multiphase microstructure composed of hexagonal HfB2, cubic HfB with oxygen contamination, hexagonal Hf and monoclinic HfO2. The proportion of these phases is quite similar with sample Cn We show on figure Id that the grain-size of this sample is homogenous. This is the main difference between this sample and C\x materials.

a) b)

c) d) Figure I: Hafnium diboride samples: a) S}$ sample, b) SJJ sample, c) Q; sample, d) Cj sample. Etchant: HN03 1; HF 1; Lactic acid 3 20.00 25.00 30.00 35.00 -10.00 45.00 50.00 55.00 2«TH£TA a)

a a HfB2 0 HfO2 b Hf a c HfB i i a a I A c A 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 2»THETA b) Figure 2 : X-Rays diffraction patterns a) powder pj+p3, b) CJJ sample

Corrosion results Three pellets of each material, weighing approximatively 20 grams, were heated in 7 liters of boiling degassed water at 345°C under 155 bars. This water is representative of a PWR water chemistry (LiOH and H3BO3 additions). During a test, the pellets are put in a stainless steel container. The corrosion rate of the pellets is followed by opening the autoclave and weighing the samples. Moreover, the concentration of boron in the water is measured. We define two parameters, A and B, which quantify the corrosion of the pellets. These parameters are expressed as follows :

lOOxVx(Ci-Cn) lOOx(mi-mn) A (%) = B %) =

Mg : Molar weight of boron : Autoclave volume M : Molar weight of HfB 2.5 : Boron concentration before test mo : Pellets mass before test : Boron concentration after test mj : Pellets mass after test

The parameter A represents the ratio of the boron mass contained in the pellets before the test by the boron quantity dissolved in the water during the autoclave test. We have checked that the hafnium was not dissolved during the test. Arabei [2] observes a same result. We have also checked that the acidity of the water does not change during the test. 100 - Test duration = lOOh

80 ~ 1 60-

C3

1) 40-

D •

70 80 90 100 relative density (%) a) b) Figure 3 : Corrosion parameters versus density Figure 4 : Autoclaving corrosion aspect after for Sjx and S21 samples. A : percentage of boron dissolution 100 hours a) high density sample S]4 b) low B: weight change of the pellets during the test density sample Sj2

Figure 3 shows the influence of the density on the corrosion parameters A and B for the samples S jx. As it can be seen, the lower the pellet density, the higher the boron dissolution and the weight change of the samples during the test. The parameter A presents a quite good correlation with the percentage of open porosity of the sample (see table 4). We also observed that the shape of the high density samples (>90%) remains the same after 100 hours autoclaving (fig 4a), we just noticed some small chips of diboride on the botton of the container. On the opposite, we observe an important swelling and a fragmentation of the samples Sn, S12, S13, S2i and C12 which present a high percentage of open porosity (fig 4b). The sample S^ is completely reduced in a white powder after the test. X-Rays diffraction pattern (figure 7) indicate that HfO2 monoclinic phase is the main constituant of this powder. Figure 5 shows the evolution of A versus test duration for high density pellets. After the test, only the pellets C2 conserve their integry, the others (Cll5 S14, S22) beeing fragmented (see figure 6). The pellet S14 is the most fragmented. A comparative analyse of the results presented on the figure 5 indicate that the metal rich compounds have a better corrosion resistance than the samples (S14, S22) elaborated from the starting powders pi and p2.

0 200 400 600 800 1000 test duration (h)

Figure 5 : Percentage of boron dissolution for Figure 6 : Autoclaving corrosion aspect after high density pellets 980 hours a) sample C}] b) sample C2 o o HfO2 A a HfB2 11 °

o o AJJ \J\jJ\ o Xl 0 / —I 1 1 1 1 1 1 1 1 1 V —1 ' 21.00 24.00 27.00 30.00 33.00 38.00 39.00 42.00 15.00 43.00 51.00 2»THETA Figure 7 : X-Rays diffraction pattern of the powder obtained after the corrosion test of sample SJJ

Discussion We have mentionned that X-rays analysis of the powder obtained after the autoclave test of the pellets S|i, indicated that monoclinic HfO2 is the main constituant of this powder. On the basis of this observation, we can write the reaction between hafhium diboride and high temperature water as follows :

HfB2 (s) + 8H20o HfO2 (s) + 2 H3BO3 (s) + 5 H2 (g) (1)

Another reaction could occur in boron rich samples like Six. This reaction may be expressed as follows:

B (s) + 3 H20 <-> H3BO3 (s) + 3/2 H2 (g) (2)

AG61SK, 155b kJ^1"0' H,0(vap) H,0 (liq) (1) -277.96 -142.16 .X (2) -80.69 -29.77 Table 5 : Variation of the free enthaplyfor reaction (1) and (2) Am/m (%) AV/V (%)

Calculation 5.18 16.29 Measurement: 50 100 150 200 sample S11 7.99 test duration (h) sample S?i 5.5 92 Table 6 : Variation of the volume and the Figure 8 : Corrosion kinetics of low density mass of the samples during the reaction (1) samples SJJ and Thermodynamic computations have been made to establish the possibility of a reaction between the diboride or boron and water considered both as gas or liquid. In the two cases, these calculations yielded negative values for free enthalpy (table 5), consequently the two reactions are possible. In a first approximation, we suppose that only the reaction (1) is operating in the mechanism of corrosion. The calculation of the weight change of the pellets during this reaction is given in table 6. This calculation is in good agrement with the measurement of the parameter B, which is equal to 7.99 % for the sample Sn and 5.5% for S2i during a test of 100 hours. We have also calculated pellets swelling due to the variation of the density between hafhium diboride (p = 11.17 g/cm3) and hafhium dioxide (p = 10.1 g/cm3). If the reaction is total, this swelling is equal to 16.29%. We have measured a higher swelling, equal to 92% for sample S2i. The swelling is evaluated by density measurement of the pellets in mercury. The difference between the calculated and the measured value is due to the formation of cracks in the sample (fig 4b). For low density pellets, we think the swelling is at the origin of the formation of the cracks observed on the macrography of figure 4b. The swelling induces stresses in the bulk of the material which break the weak sintering necks of this low density material. In the case of sample SJJ, this mechanism is accelarated by the high level of open porosity and leads to a complete disintegration of the sample. This mechanism is also operating for low density materials with a metal rich composition. We show on figure 8 that for a same density, the corrosion rate of the composite material C^ (Hf/B=1.64, p » 85%) is nearly the same as for sample S13 (Hf7B=2.07, p = 87.6%). The bulk swelling observed in low density materials can not be called for high density pellets, because these samples do not present any open porosity. In this case, the corrosion starts by fragmentation of the pellets. The fresh created surface, increases the boron dissolution and consequently the corrosion. We observe on the surface of these samples a white oxide layer of HfO2, which is relatively adherent to the diboride. The difference of density between HfB2 and HfC>2 induces some microcracks as it can be seen on the micrograph of figure 9. We have performed metallographic examinations on polished sections of the fragments collected after the test carried out on sample C^ which shows at the surface of the fragments a fine layer of microcracks parallel to the surface (fig 10). Some microcracks also exist on sample Ci, but there are rather less numerous. In addition, we observe some macroscopic cracks (fig 11) in the bulk of the sample Cn which are the cause of the fragmentation of these pellets. These macroscopic cracks do not exist on sample C2. Around these cracks, the electron microprobe analysis indicates the presence of an oxide phase. We have not clearly established whether this phase is at the origin of the formation of the cracks, or it has been formed after the opening of these cracks by high temperature water infiltration. In the first case, preferential corrosion channels would be the main mechanism at the origin of the fragmentation, in the second case the cracks formation would have another origin which is not fully understood. A significant improvement of the corrosion resistance is obtained by elaboration of metal rich compounds (Cn, C2). For these materials, the fragmentation is delayed and sometimes this phenomena has not been observed. For example, the shape of the pellets C2 remains the same after 980 hours testing. We try to understand why the metal rich material has a better corrosion resistance than the others. The results reported on figure 8 show that hafnium addition is not a sufficent protection against chemical corrosion of diboride, because, in the case of low density sample, the corrosion rate of metal rich compounds is nearly the same as in the boron rich one. We suggest that the reason of the limitation of the fragmentation for the metal rich compounds is the enhancement of the mechanical properties (Young modulus and fracture toughness) of these materials. A study is in progress to measure the value of these parameters for the differents samples. This fact is coherent regard of the bad corrosion resistance of pellets elaborated from powders pi, because these samples present grain size heterogeneity. This heterogeneity is defavorable for the mechanical properties. The high level of impurities of the powder p 1 could also be a second reason for the bad corrosion behaviour of the pellet elaborated from this powder.

Conclusion Hafnium diboride reacts with high temperature water to form the monoclinic oxide HfG>2, so that the densification of the pellets is a means to increase their corrosion resistance. Nevertheless, for long time autoclave test (980 h) of high density pellets, we observe a fragmentation of pellets which induces an enhancement of the corrosion phenomenom. A significant improvement of the corrosion resistance has been obtained by elaboration of metal rich compounds. One of these samples keeps its integrity after 980 hours testing. From a technological point of view, this result is very interesting and promising, if we consider the hafnium diboride as a candidate to be used as a control rod absorber material in a new generation of Pressurized Water Reactors. w r:

Figure 9 : Surface of the sample 5"? ? Figure 10 : Microcracks observed on a polished section of the sample CJJ

Figure II : Macroscopic cracks observed on a polished section of the sample C 11

References : [1] E. W. Hoyt. J. Chorne. W.V. Cummings. GEAP-354S. (1960) [2] B.G. Arabei, V.A. Glukov, Yu. M. Markov, Zashch. Met., Vol 12, N°6, 742-744, (1976) [3] E.S. Byron, J.F. Thompson, S.VV. Porembka, Bettis technical review. WAPD-BT-6, (1958) [4] Rudy, tech. Report AFML-TR-65-2, Part V, (1969) [5] F.W. Glaser, D. Moskowitz, B.W. Post, J. Metals 5, 1119, (1953)