Synergy Effect of Simultaneous Zinc and Nickel Addition on Cobalt ...

Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 40, No. 3, p. 143–152 (March 2003)

Synergy Effect of Simultaneous Zinc and Nickel Addition on Cobalt Deposition onto Stainless Steel in Oxygenated High Temperature Water Hiromitsu INAGAKI1,* , Akira NISHIKAWA1 , Yuji SUGITA1 and Toshihide TSUJI2 1

Electric Power R&D Center, Chubu Electric Power Company, 20-1, Aza Kitasekiyama, Odaka-cho, Midori-ku, Nagoya 459-8522 2 School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi-machi, Nomi-gun, Ishikawa 923-1292 (Received October 18, 2002 and accepted December 27, 2002) Effect of zinc and/or nickel addition on cobalt deposition was studied under simulated normal water chemistry condition of boiling water reactor. Type 316L stainless steel coupon was exposed to high temperature water including cobalt ion, together with zinc and/or nickel ions up to 1,000 h using experimental recirculating loops. Either addition of zinc or nickel ions reduced amounts of cobalt deposition on stainless steel. Simultaneous addition of both zinc and nickel ions showed the synergy effect; less amounts of cobalt deposition were observed for simultaneous addition of zinc and nickel ions comparing to individual addition at equivalent concentration. Glow discharge spectrometry and conversion electron M¨ossbauer spectroscopy revealed that the different mechanism of reducing cobalt deposition prevails between zinc and nickel ions. KEYWORDS: BWR type reactors, water chemistry, cobalt deposition, oxide film, stainless steel, high temperature water, zinc addition, nickel addition, synergy effect, M¨ossbauer spectroscopy

I. Introduction Main source of radiation field in BWR is radioactive cobalt deposited on primary loop recirculating piping. In the past one policy to reduce the radioactivity buildup was to purify the reactor water, because the radioactivity buildup was proportional to the concentration of radioactive cobalt in water, directly.1, 2) On the contrary, active control by the metallic elements has been introduced to the BWR plant in the last decade.3, 4) Zinc addition to reactor water has achieved remarkable success in many US plants5) and some Japanese plants.6) Similarly, the addition of nickel ion in the water was found to reduce cobalt deposition7) and has been used as ultra low crud control8) at some Japanese plants.9) Some laboratory tests were performed regarding the effect of zinc and nickel ions on cobalt deposition and showed that zinc ion in the water makes the oxide film formed on stainless steel thinner and the cobalt concentration in the oxide film lower.10–12) On the other hand, it was found that nickel ion in the water compete with the cobalt ion for deposition on the oxide film.13) In the introduction of these techniques to the plant, the concentrations of additive ions are also preferred as low as possible due to the reducing running cost or anxieties of unexpected side effect. However, systematic study focused on these points has not been conducted yet. There are few reports that cobalt deposition is much suppressed by mixed addition of zinc, nickel, etc. than each addition experimentally, but the reducing mechanism of cobalt deposition is not known.13, 14) The main objective in this study is to investigate the simultaneous addition of zinc and nickel ions, and to compare them with individual addition. For this purpose, we intended to clarify the concentration dependence of additive ions on cobalt deposition, the distribution of deposited cobalt in oxide film by means of glow discharge lamp ∗

Corresponding author, Tel. +81-52-621-6101, Fax. +81-52-6235117, E-mail: [email protected]

spectrometry, the characteristics of the oxide film by conversion electron M¨ossbauer spectroscopy.

II. Experimental 1. Test Coupons and High Temperature Water Loops Test coupons of the size 50×20×2 mm were cut from commercially available one sheet of 316L stainless steel (SS) (in weight %: Fe 65.6, Cr 17.3, Ni 12.8, Mo 2.62, Mn 0.97, Si 0.69, C 0.016, P 0.043, S 0.003). Their surfaces of the test coupon were mechanically polished up to 800-grit with emery paper, then ultrasonically degreased in acetone and distilled water, and dried in vacuum. Surface roughness of the test coupon was 0.01 µm as arithmetic-mean roughness (Ra) and 0.18 µm as maximum height (Ry). Figure 1 shows a schematic diagram of the high temperature water loops (Toshin Kogyo). There were two loops (α and β loops) with each autoclave whose capacity was 1.3 l so that two different experimental condition tests could be done at the same time to decrease unexpected effects during the test. Loop materials used in high temperature water region were type 316L SS identical to the test coupon. Test coupons were set in the autoclave, and test water to the autoclave was fed at the flow rate of 0.1 l/min by high-pressure pump. Dissolved oxygen concentration and chemical additives (cobalt, zinc and/or nickel ions) in test water were controlled at the desired concentrations. 2. Experimental Condition of Water Chemistry Experiment was carried out under simulated NWC condition of BWR. Test coupons of 316L SS were exposed to high temperature and high pressure water added some metallic ions under oxygenated condition up to 1,000 h, in which one third of test coupons were taken out at 190 and 500 h. The weight of test coupons was measured by chemical balance as a function of the exposure time. Temperature and pressure in the

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Fig. 1 A schematic diagram of the experimental loop

autoclave were kept at 553 K and 8.0 MPa, respectively. All additive elements of cobalt, zinc and nickel were dissolved as nitrate. Concentration of cobalt ion was fixed as 1.0 ppb, which was several ten times higher than that in actual plant, because of the detection limit of glow discharge lamp spectrometry (GDS). Concentrations of either zinc or nickel ions were controlled in 10, 20 or 50 ppb to examine the concentration dependence of individual additive ion. Furthermore, the test of simultaneous addition of both zinc 10 ppb and nickel 10 ppb was done in order to know synergy effect.

phasizes the information on the chemical state of the oxide film rather than the substrate, because of the short penetration depth of conversion electron, compared to usual transmission γ -ray M¨ossbauer spectroscopy. The source of 57 Co diffused in rhodium matrix and He/CH4 flow 2π proportional counter were used for CEMS (Topologic Systems).18, 19) The relative velocity was referred to metallic iron. The measured spectra were fitted by assuming singlet, doublet or sextet of Lorentzian shaped peaks using MossWinn 3.0 computer program.20)

3. Analysis and Characterization of Test Coupon After exposing to high temperature water, oxide film is grown on 316L SS. The amounts of oxide film (W F ) and total corroded metal (WC ) were determined by weighing the test coupons before exposure (W0 ), after exposure (W1 ), and after removal of the oxide film by AP-AC method (W2 )15) as follows:

III. Experimental Results

W F = W1 − W2

(1)

WC = W0 − W2 .

(2)

Amounts of metal released to water (W R ) were estimated from W F and WC by using the following equations16) W R = WC − R · W F ,

(3)

where R means average weight ratio of total metallic element to oxide film. For example, when the oxide film is formed only by iron, R equals to 0.69 for Fe2 O3 and 0.72 for Fe3 O4 . The values of W R are not always correct because of unnegligible amounts of deposition from water in case of additive condition. Morphology and crystal structure of the oxide film were analyzed by scanning electron microscope (SEM) (JEOL; JSM6330F) and X-ray diffraction (XRD) (Rigaku; RINT-2000) methods, respectively. Depth profile of metallic elements (Co, Fe, Cr, Ni, Zn, Mo, Mn) in oxide film was measured by GDS (Shimadzu; GDLS-5017).17) Conversion electron M¨ossbauer spectroscopy (CEMS) em-

1. Physical Properties and Growth of Oxide Film Figure 2 shows SEM photographs of test coupons exposed for 1,000 h without and with additive ions. The surface of the sample exposed to water without additive ions in Fig. 2(a) is covered by crystalline particles of approximately 0.5 to 1.5 µm in size. The surface of the sample exposed to zinc 10 ppb ions added water in Fig. 2(b) is densely covered by smaller particles. The surface of the sample exposed to nickel 10 ppb ions added water in Fig. 2(d) is covered by larger particles, which is similar to the surface of the sample exposed to both zinc and nickel ions added water in Fig. 2(f). In higher concentration of 50 ppb of either zinc or nickel ions addition, two kinds of particles covered the surface: larger one sparsely and smaller one densely. The oxide film formed on the steel exposed to high temperature water is known to consist of two layers:21, 22) the outer layer (diameter of oxide particles: 0.5 to 1.0 µm) and the inner layer (diameter of oxide particles: 0.1 to 0.3 µm) under oxidizing condition.23) Judging from the particle size, outer and inner layer oxide particles were observed in Figs. 2(c) and (e), but inner one was not seen in Figs. 2(a), (b), (d) and (f). X-ray diffraction spectra demonstrate two patterns. Oxide film without additive ions consisted of the mixture of corundum- and spinel-type oxides, whereas that with additive ions was only spinel-type one. It is clear that zinc (or nickel) ions change the crystal structure from the corundumJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

Synergy Effect of Simultaneous Zinc and Nickel Addition on Cobalt Deposition

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Fig. 4 Amounts of total corroded metal (WC ) of test coupon against exposure time The dashed lines are drawn by assuming that WC follow a logarithmic law.

Fig. 2 SEM photographs of oxides formed on test coupons exposed for 1,000 h

type hematite, α-Fe2 O3 to the spinel-type ferrite, ZnFe2 O4 (or NiFe2 O4 ). Figures 3 and 4 show amounts of oxide films (W F ) and amounts of total corroded metal (WC ) as a function of the exposure time, respectively. In these figures, the dashed lines are drawn by assuming that W F and WC follow a logarithmic law;24, 25) W F (or WC ) = A log(Bt + 1),

(4)

where A and B are constant. The values of W F and WC decrease with increasing zinc concentrations as seen in Figs. 3(a) and 4(a), respectively. On the other hand, as seen in Figs. 3(b) and 4(b), nickel ions of 10 ppb increase W F , but decrease WC , though both W F and WC decrease at more than

Fig. 3 Amounts of oxide film (W F ) formed on test coupon against exposure time The dashed lines are drawn by assuming that W F follow a logarishmic law.

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20 ppb. This difference between nickel and zinc ions is also related to their released amounts of metal, W R , calculated by Eq. (3). The values of W R exposed for 1,000 h in various additive conditions are shown in Fig. 5. Negative W R value in additive ions means that metallic element incorporated to oxide film from water is larger than that released by corrosion. Moreover, absolute W R value of nickel 10 ppb addition is larger than that of zinc 10 ppb addition and either WC value of zinc 10 ppb or nickel 10 ppb addition is smaller than that of non-addition as seen in Figs. 4(a) and (b). Therefore, added zinc ions decrease the oxide film by suppressing corrosion, whereas added nickel ions increase the oxide film by forming nickel ferrite in spite of suppressing corrosion. 2. Depth Profiles of Metallic Element and Cobalt Distributions in Oxide Film The depth profiles of metallic elements of SS and those of added elements (Fe, Cr, Ni, Zn) in oxide film obtained by GDS are shown as a function of discharge time corresponding to depth in Fig. 6, where the depth profile of oxygen is excluded in order to compare them with initial metallic composition of 316L SS. The depth profile of the oxide film without additive ions in Fig. 6(a) demonstrates duplex oxide layers consisting of a chromium-rich inner layer, where atomic fraction of chromium is higher than that of a base metal, and an iron-rich outer layer, where atomic fraction of iron is higher

Fig. 5 Calculated values of released methal (W R ) to high temperature water from test coupons exposed for 1,000 h

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Fig. 6 GDS depth profiles of Fe, Cr, Ni and Zn ions in atomic fraction of test coupons exposed for 1,000 h

than that of a base metal.15, 17) These depth profiles may be caused by selective corrosion of iron in stainless steel in high temperature water.26) Some parts of iron in stainless steel dissolve into water and precipitate on the surface of the outer layer. Thus the atomic fraction of chromium increases in the inner layer. However, the depth profile of oxide film with additive ions cannot be divided into two layers in the same way because of lower atomic fraction of iron than that of base metal due to incorporation of added metal. Therefore, the boundary between outer and inner layers is defined as the

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midpoint between local maximum and minimum of atomic fraction of iron in the oxide film. Since atomic fraction of iron reaches constant at shorter discharge time than the oxygen does, the boundary between inner layer and substrate is delineated by the oxygen profile instead of atomic fraction of iron. The added zinc makes the oxide film thinner, followed by incorporation of zinc ions into outer and inner layers as shown in Figs. 6(b) and (c). On the other hand, a little amounts of added nickel (10 ppb) is found to enlarge outer layer due to the formation of oxide film with higher atomic fraction of nickel as shown in Fig. 6(d), of which tendency is consistent with that of W F in Fig. 3(b) as mentioned before. As seen in Fig. 6(g), both zinc and nickel addition shows the oxide film composed of expanded outer layer with higher atomic fraction of nickel and thinner inner layer with relatively higher atomic fraction of zinc. The cobalt depth profile calculated from the intensity of GDS is shown in Fig. 7 in order to compare the cobalt concentrations of inner and outer layers in oxide film. These cobalt depth profile curves are calculated as follows: the cobalt intensity curve measured by GDS is subtracted from the background curve, the shape of which is assumed to be a sigmoid function, to eliminate the influence of the impurity of cobalt in 316L SS, the difference of the sputtering rate and/or efficiency of cobalt in oxides and substrates. This treatment of cobalt intensity data was confirmed by GDS measurement of the sample prepared in cobalt free high temperature water as blank exposure test. In the figure, the interface between outer and inner layers and the interface between inner layer and substrate are taken from Fig. 6. Cobalt distribution curves in Fig. 7 have two peaks. One is at near the surface of outer layer and the other is in the interface between outer and inner layers, because both outer surface and the interface between outer and inner layers are considered to contact with water. These wider peaks penetrating into another layer are caused by wider analytical area of GDS (about 80 mm2 ), comparing to the surface roughness of the sample. By fitting the cobalt distribution peaks as two asymmetrical Gaussian functions, integrating it across the oxide film and dividing by the width of the oxide film, the weight o ) and that in infraction of deposited cobalt in outer layer (FCo i ner layer (FCo ) are calculated. As shown in Fig. 7(b), cobalt deposition of 10 ppb zinc o i , but to decrease FCo addition is found to increase the FCo abruptly, comparing to that of non-addition. The increase of o is supposed to be related to the change of crystal structure FCo of oxide film from corundum to spinel, having high affinity of cobalt up-take, in case of zinc addition. On the other hand, i a little, 10ppb nickel addition shown in Fig. 7(d) reduce FCo o but increase the FCo , although the oxide form also change to spinel. This is interpreted that the change of crystal structure to spinel cause up-take of nickel in place of cobalt due to the larger affinity of nickel than that of cobalt in ferrite, as major oxide in outer layer. Similar explanation can be considered in inner layer, because affinity of zinc for chromite, as major oxide in inner layer, is larger than that of nickel. In case of o simultaneous addition of both zinc and nickel ions, both FCo i and FCo are lowered as seen in Fig. 7(g). In higher concentraJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

Synergy Effect of Simultaneous Zinc and Nickel Addition on Cobalt Deposition o i + WCo . WCo = WCo

147 (7)

Figure 8 shows the amounts of total cobalt deposition, WCo , on test coupon as a function of the exposure time. A linear relationship between the amounts of cobalt deposition and the exposure time without additive ions leads to the result that kinetic mechanism is different from that of corrosion such as W F in Fig. 3 and WC in Fig. 4. As seen in Fig. 8(a), added zinc ions are confirmed to reduce cobalt deposition. On the other hand, a little amount of added nickel ions of 10 ppb increase it as seen in Fig. 8(b), but in further increasing nickel concentrations up to 50 ppb, WCo is kept at low level sufficiently. Figure 9 shows WCo exposed up to 1,000 h as a function of the concentration of additive ions in high temperature water. The synergy effect of reduction on cobalt deposition is seen for the simultaneous addition of 10 ppb zinc and 10,ppb nickel, compared to either addition of zinc or nickel of 20 ppb corresponding to equivalent concentration. 4. Conversion Electron M¨ossbauer Spectroscopy Room temperature CEMS spectra of 316L SS coupon exposed for 1,000 h to high temperature water with additive ions are shown in Fig. 10, in which solid and dotted lines are the fitting results of the total spectrum and constituent sub-spectra, respectively. M¨ossbauer parameters obtained by computer fitting and the identified or supposed phases in oxide film are summarized in Table 1. All spectra in Fig. 10 contain the singlet characteristic of austenite stainless steel, which is identical to that of unex-

Fig. 8 Amounts of cobalt deposition (WCo ) on test coupons against exposure time Fig. 7 Cobalt concentration curve calculated from GDS depth profile of test coupons exposed for 1,000 h

tion of 50 ppb, the deposited cobalt distributions are very low level especially in case of nickel as seen in Figs. 7(c) and (f). 3. Amounts of Cobalt Deposition o ), in inner Amounts of cobalt deposition in outer layer (WCo i layer (WCo ) and in total oxide film (WCo ) were calculated from o i W F , FCo and FCo as follows: o o WCo = W F · FCo

(5)

= WF ·

(6)

i WCo

i FCo

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Fig. 9 Amounts of cobalt deposition (WCo ) against concentrations of additive ions

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Fig. 10 Room temperature CEMS spectra of 316L SS coupon exposed to high temperature water with additive ions for 1,000 h Oxide phases identified or supposed in CEMS spectra α: α-Fe2 O3 , β: Fe3+ (A) in Nix Fe3−x O4 , γ : Fe3+ /Fe2+ (B) in Nix Fe3−x O4 (x