In-situ nuclear steam generator repair using electrodeposited nanocrystalline nickel

NanoStruetured Materials. Vol. 9. pp. 737°746° 1997 Elsevier Science Ltd O 1997 Acla Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/97 $17.00 + .00

Pergamon

PII S0965-9773(97)00160-8

IN-SITU NUCLEAR STEAM GENERATOR REPAIR USING ELECTRODEPOSITED NANOCRYSTALLINE NICKEL G. Palumbo, F. Gonzalez, A.M. Brennenstuhl, U. Erb ~f, W. Shmayda and P.C. Lichtenberger Ontario Hydro Technologies, 800 Kipling Avenue, Toronto, ON M8Z 5S4 tDepartment of Materials and Metallurgical Engineering, Queen's University, Kingston, ON K7L 3N6

Abstract. Degradation of nuclear steam generator tubing via outside surface initiated - corrosion, -stress corrosion cracking, and -fatigue, often leads to costly forced outages and system de-rating (i.e. tube plugging). A commonly applied approach to system rehabilitation has been to repair the damaged areas of tubes by the application of tubular 'sleeves' which are either welded or mechanically bonded at their extremities to the host tubing; the applied sleeve restoring the mechanical integrity of the damaged region to its original state (i.e., pressure boundary). An alternative in-situ repair technology involving the electrodeposition of a continuously bonded, fully dense, integral sleeve of nanocrystalline nickel to the host tubing has recently been developed and successfully field-demonstrated in both CANDU and PWR systems. In this paper, the Electrosleeve rMprocess, which represents one of the first large scale industrial applications for nanostructured materials, is described. Emphasis is placed on the unique properties of the nanostructured sleeve material - i.e., microstructure, mechanical strength, corrosion resistance, thermal stability, etc. ©1997 Acta Metallurgica Inc. INTRODUCTION Degradation of nuclear steam generator tubing, primarily via localized corrosion and intergranular degradation processes, has been a major worldwide concern, often leading to costly unscheduled nuclear plant outages and system de-rating (i.e., plugging and removing of tubes from service). A commonly applied approach to system rehabilitation has been to repair the damaged areas of tubes by insertion of tubular 'sleeves' which are either welded or kinetically bonded at their axial extremities to the inside surface of the host tubing; the applied sleeve restoring the mechanical integrity of the degraded region to its original state (i.e., pressure boundary) and allowing the tube to remain in service. Although widely used in the nuclear industry, various concerns persist regarding the tong term reliability of such repairs due to potential deleterious effects on the host tubing, (e.g., residual stresses, microstructural alteration), arising from installation (welding, kinetic impact) and required post- installation heat treatments. In this paper, we report on a recently developed nuclear steam generator tube repair technology - ElectrosleeveTM [1]. Unlike previously used repair techniques, the 737

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G PALUMBO,F GONZALEZ,AM BRENNENSTUHL,U ERB,W SHMAYDAANDPC LICHTENBERGER

ElectrosleeveTM process, which involves the in-situ electrodeposition of nanocrystaUine Nickel microalloy sleeves, is non-intrusive to the parent tubing (i.e., no induced stresses, no high temperature exposure, no microstructural modification). This technology, which has been field implemented in both Canadian CANDU and U.S. PWR systems, represents one of the first large-scale industrial applications for structural nanocrystalline material.

MATERIAL REQUIREMENTS

AND SYNTHESIS

Commercially pure Ni has long been recognized as an ideal stress corrosion cracking (SCC) -resistant material for typical nuclear steam generator environments and it has been used extensively in Europe as a protective coating on the inside surface of steam generator tubes [2]; however, its relatively poor mechanical strength has precluded its use as a structural repair. Nanocrystalline Ni however, on the basis of previously determined Hall-Petch [3] behaviour [4], can indeed possess the mechanical strength required for pressure boundary applications. On the basis of previous pioneering efforts by Erb and coworkers [5] in the electrodeposition of fully dense nanocrystalline metals and alloys, an electrodeposition process was developed by Ontario Hydro [6] for the synthesis of nanostructured Ni sleeves. In this process, an electroplating cell is created between the host steam generator tube, and a positioned probe comprised of a rigid central tubular non-consumable anode, and sealing modules at both ends of the anode; the region between modules defining the length of the sleeved region. A polymeric conduit connected to the probe delivers both power and process chemicals from a remote chemical process station. Process chemicals are delivered at controlled flow rate and temperature through the annulus defined by the anode and the inside surface of the steam generator tube, and are circulated back through the central tubular member. Upon application of an electric field between the central electrode (anode) and the inside surface of the steam generator tube (ground), electrodeposition occurs on the inside surface of the tube; the desired thickness being accurately controlled by charge integration (current-time). Strong adhesion of the electroformed sleeve to the host tubing is ensured by the prior application of suitable surface activation and prefilming procedures. As previously outlined by Erb and co-workers [e.g., 7], grain size is controlled by appropriate selection of current density, solution ionic strength, temperature and current waveform. C H E M I C A L AND M I C R O S T R U C T U R A L

CHARACTERISTICS

The electrodeposited nanostructured material is >99.5% Ni, containing microalloyed phosphorous (<3000ppm by wt.). The Electrosleeve TM material is generally free of macroscopic defects (e.g., voids), and possesses a microstructure not resolvable by optical microscopy. Figure 1 shows A brightfield transmission electron micrograph of the nanocrystalline Ni. An average grain size of approximately 50-100nm is evident. Consistent with findings from a previous study [8], the electrodeposited nanostructured Ni has been shown by optical, scanning electron microscopy, and atomic force microscopy to be free of porosity. X-ray diffraction measurements of the ElectrosleeveTM material show weak fibre textures associated with the major poles ([200; [220]; [111 ]), with texture intensities usually less than 2.0.

IN-SITUNUCLEARSTEAMGENERATORREPAIRUSINGELECTRODEPOSITEDNANOCRYSTALLINENICKEL

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Figure 1. Brightfield transmission electron micrograph of the nanocrystalline Ni ElectrosleeveTM.

THERMAL STABILITY Thermal stability of the ElectrosleeveTM material is a major concern since nuclear steam generator materials are subjected to long term (approx.40years) thermal exposure at operating temperatures as high as 343°C. Moreover, previous studies (e.g., [9]) have shown that nanocrystalline Ni can possess a driving force for grain growth which can be several orders of magnitude greater than that for conventional polycrystalline materials. Figure 2 shows the effect of annealing time at 343°C on the Vickers microhardness of nanocrystalline (1) pure Ni, and (2) Ni + 1500ppm P; both materials having an as-plated hardness of approximately 400VHN. As shown in this figure, the pure Ni shows a rapid decay in hardness to below 150VHN within the first few hours of annealing; this being representative of rapid grain growth leading to a resultant grain size consistent with that of conventional polycrystals (i.e., 1030ttm). The nanocrystalline Ni containing 1500ppm (by wt.) phosphorus shows no evidence of hardness decay within the total test period evaluated (i.e., approx. 10 months). The influence of minor solute additions on retarding grain growth in nanostructured Ni has been previously documented [9,10], and attributed to (1) solute drag effects on the grain boundaries, (2) reduction in grain boundary energy from solute segregation, and (3) Zener [11] drag effects associated with the possible formation of 'nan0-precipitates' [9,10]. In order to determine a margin of safety for the thermal stability of the Electrosleeve TM material, differential scanning calorimetry measurements were conducted. Figure 3 shows the results of this analysis in the form of a Kissinger [12] plot. Exothermic peaks were observed at approximately (Tp) 560-580°C over a range of heating rates (B) from 30-100 K/rain., yielding an apparent activation energy of 59kcal/mole. Mehta et al [13] have recently evaluated the thermal stability of a Ni-l.2wt%P alloy having an average grain size of 10rim by DSC. They found an exothermic peak at 360°C and an activation energy of approximately 52 kcaYmole,

740

G PALUMBO,F

GONZ/CEZ,A M

BRENNEN~rUHL,U ERa, W SHMAYOA AND P C LICHTENBERGER

which they attributed to the onset of rapid grain growth associated with a transition from solute drag as the primary stabilizing influence at lower temperatures to a weaker grain boundary pinning at the elevated temperatures by Ni3P precipitates. The higher values of activation energy and peak temperature noted herein can primarily be attributed to the larger grain size of the Electrosleeve TM material, leading to resultant reduced driving force for grain growth.

......

50(]

-I

......

4

......

I

- - :----I

......

I

......

-I

.......

T=343C 450

> 350

¢1 to

Io"1 •

25o

NI-P

._o :E

200

150

1011

......

4 10

....

~

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4

......

lO0

~ I

z)~n .....

1000

4

D

[1

o

.....

-4

......

104

105

4

......

106

107

Annealing Time, minutes

Figure 2. Vickers hardness as a function of annealing time at 343°C for nanocrystalline and ( 2 ) N i - 1 5 0 0 p p m P.

-8.5

....

I ....

I ....

I ....

(1) Ni

I ....

Q:-59 kcal/mole -9 T .5

E

%. -=

-9.5

-10

-10.5

....

0.00115

I .... 0.00116

I .... 0.00117

I .... 0.00118

I .... 0.00119

0.0012

1/Tp, K I

Figure 3. Kissinger [12] plot determined

from DSC measurements

material.

of

the Electrosleeve TM

IN-SITuNUCLEARSTEAMGENERATORREPAIRUSINGELECTRODE~0=TEONANOCRYSTAUJNENICKEL

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TENSILE AND FATIGUE PROPERTIES

Table I summarizes the mechanical properties of the nanostructured Ni ElectrosleeverU; the material demonstrating a desirable combination of strength and ductility at both room temperature and elevated temperature. The values of yield strength and tensile strength are shown to be significantly greater than that of conventional commercially pure Ni (e.g., Ni201) [14]. The modulus of elasticity however, is shown to be only marginally higher in the nanocrystalline material. This latter result provide~ further support for the findings of Krstic et al [15] which demonstrated that previously reported reductions in modulus with nanoprocessing (e.g., [16]) were likely the result of high residual porosity. The ElectrosleeveTM material displays room temperature elongations to failure in excess of 15% in tension; somewhat greater ductility (i.e., >25%) is observed in bending (1800 reverse U-bends), which is consistent with behaviour often observed with metallic glasses (e.g., [17]). TABLE 1 Summary of the Mechanical Properties of Electrosleeve TM Ni Conventional

Property

[14]

Yield Strength r MPa (25°C) Yield Strength, MPa ~350°C) Ultimate Tensile Strength r MPa /25°C) Ultimate Tensile Strength, MPa ~350°C) Elongation~ % ~25°C) Modulus of Elasticity, GPa (25°C)

103

403 50 207

Ni

Eiectrosleeve TM

690 620 1100 760 >15 214

The fatigue performance of the Ni ElectrosleeveTM material has been evaluated in air at both room temperature and elevated temperature (300°C) with tests performed in fully reversed bending (R=-I) at frequencies in the range 0.5-25Hz. Figure 4 summarizes the fatigue performance of the ElectrosleeveTM material at room temperature relative to that previously reported for conventional Ni. As shown in this figure, the ElectrosleeveTM fatigue performance is generally consistent with that of conventional Ni. Fatigue performance was not found to be compromised at elevated temperature. CREEP PERFORMANCE

As a result of required elevated temperature service, creep testing has been conducted on the ElectrosleeveTM material [21]. Creep rates at 343°C have been shown to be in general agreement with values reported for conventional polycrystalline Ni. Power law behaviour (stress exponent >5) is noted at stresses above 200 MPa; grain boundary mechanisms are likely to be operative at lower stresses where a stress exponent of approximately 3 has been observed; however, to date, no evidence of intergranular failure has been noted in any of the specimens examined, with completely ductile fracture features exclusively observed.

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G PALUMBO,F GONZALEZ,AM BRENNENSTUHL,U ERe, W SHMAYDAANDPC LICHTENBERGER

10 4

10 3

,o

10 =

0 • • • •

r

10 ~

10 2

Ni Electrosleeve A.~M Cold Drawn A~ld ~ n e a l e d S¢;Ool, ~ n e a l c d X u e l o l AnnentecJ ( R = 0 , x

i

I

I

I

I

103

104.

105

106

107

10 8

~IurvJDCf Of CyCler5 ~.0 FO'hJre

Figure 4. Fatigue behaviour of ElectrosleeveT M Ni (fully reversed bending) in comparison to the behaviour of conventional polycrystalline Ni: ASM cold drawn [18]; ASM annealed [18]; Seidal annealed [19]; Xu et al annealed [20]. Arrows denote non-failed specimens. Intergranular -sliding, -cavitation, and -fracture are the principal causes of premature creep failure of normally ductile engineering materials (e.g., [22]). Premature creep failure is manifested as a general reduction in the total elongation to failure with (1) decreasing applied stress, (2) decreasing overall creep rate, and commensurately, (3) increasing time to rupture. These failures typically arise as a result of the preferential accumulation of damage at intergranular sites. Although nanostructured materials, which possess a large 'grain boundary' component [23], would be expected to have their load-bearing applications restricted by creep and creep-cracking concerns, creep tests conducted on ElectrosleeveTM material show no evidence of premature intergranular creep failure, with elongation to failure being independent of rupture time (to greater than 6000h) and in the order of 35-45% [21]. Although creep rates are generally expected to increase with decreasing grain size, there exists some evidence [24] that decreasing grain size in conventional materials promotes the appearance of intergranular voids rather than cracks. In order to examine this phenomenon a general geometric model for grain size effects on premature intergranular creep failure in normally ductile materials, has been formulated [25]. A creep specimen having a rectangular gauge section of thickness t, width, w, and length L where t
IN-SITUNUCLEARSTEAMGENERATORREPAIRUSINGELECTRODEPOSITEDNANOCRYSTALUNENICKEL

O" fd = 1 - - GUTS

743

[11

where o is the applied stress and oUTS is the ultimate tensile strength of the material at test temperature. Assuming microstructural isotropy, an equivalent frequency of damaged grain boundaries will occur along the gauge length of the specimen, as occurs through the minimum gauge thickness t. The total number of damaged grain boundaries N d contained along the gauge length can be approximated by, L Nd = fd • - [2]. d Considering that each damaged grain boundary results in a unit displacement A, the total elongation AL arising from intergranular displacement can be approximated by, AL = Nd • A

[3].

The intergranular creep strain to failure can then be shown to be given by,

1 AL E---L

-A s [4]. d

Equation 4 shows that the intergranular creep strain to failure is strongly dependent on grain size for any value of A. As would be expected, increasing applied stress generally results in decreasing allowable intergranular creep strain; at the limit where the applied stress approaches the ultimate tensile strength, the allowable intergranular creep strain approaches zero. The maximum attainable intergranular creep strain is achieved when the applied stress approaches zero. Figure 5 shows the effect of grain size on the maximum allowable intergranular creep strain predicted from eqn.4 at the limit of zero applied stress for several values of mean intercrystalline displacement. The range o f values of A have been chosen such that lnm is considered to be a minimum since this value is co .mmensurate with the width of an intact grain boundary (i.e., 2-3 atomic diameters)[26]; the value of ll.tm (1000rim) can be considered a nearmaximum value corresponding to the often observed size of intergranular creep damage at grain boundaries. As shown in this figure, both the grain size and mean intererystalline displacement have a large effect on the total allowable intergranular strain. At a conventional average grain size of 30~tm, the total allowable intergranular creep strain can be as low as <0.01% and possibly achieve a maximum value of 3%. In contrast, at a nanocrystalline grain size of 10nm, the predicted allowable intergranular strain can never be lower than 10% (i.e., A=lnm limit). For nanocrystalline materials, the allowable intergranular creep strain would generally always be of the order of, or in excess of, the time-independent elongation to failure. Thus, according to this model, nanocrystalline materials are not likely to ever display failure by intergranular creep cracking. Moreover, the tendency for most conventional materials to display

744

G PALUMBO,F GONZALEZ,AM BRENNENSTUHL,U ERa,W SHMAYDAANDPC LICHTENBERGER

decreasing creep strain to failure with decreasing stress and increasing 'time to rupture' (i.e., due to increased relative grain boundary contribution) would not be expected with materials having ultra-fine grained structure. The results of this geometric analysis indicate that although nanocrystalline materials may exhibit higher steady state creep rates in the grain boundary sliding and Coble [27] creep regimes than conventional materials, this is likely to be compensated for by an expected significantly enhanced resistance to premature intergranular failure; this behaviour likely being manifested as (1) the exclusive appearance of ductile fracture features in failed creep specimens, (2) time-independent strain to failure, and (3) extended creep life despite possibly enhanced steady-state creep rates. The results of this analysis surprisingly indicate that nanocrystalline processing may lead to superior materials reliability for extended service load-bearing applications. 1 '? "~

"<.S

......~.-. .......I.......

....... I

........

I

........

i

....

o.1 0.01

0.001

. 0.0001 10.~

o

--

,*-=10nm

-'"

olooonm t

= .......

10

........ 0.01

......................................

i 0.1

........

i ld,

........ tam

i 10

........

100

=-'it 1000

Figure 5. Maximum allowable intergranular creep strain as a function of grain size as predicted from eqn 4 for mean intercrystalline displacements in the range 1 - 1000nm, and at an applied stress approaching zero.

CRACK ARREST CAPABILITY One of the most prevalent degradation modes afflicting nuclear steam generators, and the most likely impetus for application of the Electrosleeve repair is intergranular stress corrosion cracking (SCC). As previously discussed, Ni is considered to be highly resistant to such attack under nuclear steam generator conditions. Moreover, a previously developed geometric model has shown that decreasing grain size should yield enhanced resistance to intergranular cracking [28]. However, from a purely mechanical standpoint the Electrosleeve is required to arrest tight cracks propagating from the host tubing towards the bi-material interface. Sugimura et al [29] have shown that for bi-material systems of similar elastic modulus, but different plastic properties, cracks propagating toward the bi-material interface from the material of lower yield strength will always tend to blunt upon encountering the interface. Such a condition is expected upon application of the ElectrosleeveTM to common nuclear steam generator tubing alloys (Alloy 400, Alloy 600, Alloy 690, Alloy 800). In order to evaluate the cracking resistance of nanostructured Ni sleeves, C-ring testing was conducted in autoclave in a 10% NaOH solution at 350°C; this environment being commonly applied to evaluate the SCC performance of nuclear steam generator tubing alloys (e.g., Alloy 600, 690, 800). C-rings, having a thinned outer layer of host Alloy 600 tubing, were statically stressed (2% outer surface tensile strain), and exposed to the environment for up to 3000h. Figure 6 shows a cross sectional optical micrograph of a C-ring following 3000h of TM

TM

IN-SI'ru NUCLEARSTEAMGENERATORREPAIRUSINGELECTRODE~OSaTEO NANOCRYSTALUNENICKEl.

745

exposure. As shown in this figure, intergranular cracking which is highly prevalent in the Alloy 600 tubing, is completely arrested upon encountering the nanostructured Ni sleeve. SCC was not observed to propagate into the ElectrosleeveTM in any of the specimens evaluated. Despite the relatively lengthy time the cracks were likely to have remained 'blunted', under stress (as confirmed by post-test dimensional analysis of unloaded C-rings) at the ElectrosleeveTM interface (i.e., greater than 1500h ), no evidence of either (1) sleeve detachment, or (2) crack propagation along the interface, was noted in any of the test specimens.

Figure 6. Cross-sectional optical micrograph of a stressed (2% tensile strain) electrosleeved Alloy 600 C-ring following 3000h exposure to 10% NaOH at 350°C. SUMMARY

A non-intrusive nuclear steam generator tube repair process has been developed based upon the electrodeposition of continuously bonded, fully dense nanocrystalline Ni. The nanocrystalline material is shown to possess the thermal stability and mechanical properties required for such a structural repair (strength, ductility, fatigue resistalace, creep resistance), and promises to offer unique crack arrest and intergranular creep-cracking resistance not typically observed in conventional polycrystalline materials. This repair process, which has been field demonstrated in both Canadian CANDU and U.S. PWR systems, and has been in nuclear steam generator service since March of 1994 (Ontario Hydro Pickering Nuclear Generating Station, Unit 5) represents one of the first large-scale industrial applications for structural nanocrystalline material. ACKNOWLEDGEMENTS The authors would like to acknowledge the contributions of R. Barton, A. Robertson, J. Cocuzzi, and S. Corazza of Ontario Hydro Technologies, D.M. Doyle, and E. Renaud of Babcock and Wilcox Canada Ltd., J. Galford, R. Schaefer, M. Mastilovic, and S. Cook of Framatome Technologies, V. Krstic, T. Turi, G. Panagiotopoulos and P. Nolan of Queen's University, K.T. Aust, D.D. Perovic and S. Boccia of the University of Toronto, W.T. Thompson of the Royal Military College, Canada, and J.A. Szpunar of McGill University.

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G PALUMBO,F GONZALEZ,AM BRENNENSTUHL,U ERa,W SHMAYDAANDPC LICHTENBERGER

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