Nuclear Engineering and Design 238 (2008) 2529–2535

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Nuclear Engineering and Design

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Electroless nickel-plating for the PWSCC mitigation ofnickel-base alloys in nuclear power plants

Ji Hyun Kima,∗, Il Soon Hwangb

a HH Uhlig Corrosion Lab, Massachusetts Institute of Technology, 185 Albany Street, NW 22-123, Cambridge, MA 02139, USAb Department of Nuclear Engineering, Seoul National University, Gwanak 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea

a r t i c l e i n f o

Article history:Received 30 January 2008Received in revised form 17 March 2008Accepted 27 March 2008

a b s t r a c t

The feasibility study has been performed as an effort to apply the electroless nickel-plating method fora proposed countermeasure to mitigate primary water stress corrosion cracking (PWSCC) of nickel-basealloys in nuclear power plants. In order to understand the corrosion behavior of nickel-plating at high tem-perature water, the electrochemical properties of electroless nickel-plated alloy 600 specimens exposedto simulated pressurized water reactor (PWR) primary water were experimentally characterized in hightemperature and high pressure water condition. And, the resistance to the flow accelerated corrosion (FAC)test was investigated to check the durability of plated layers in high-velocity water-flowing environmentat high temperature. The plated surfaces were examined by using both scanning electron microscopy

(SEM) and energy dispersive spectroscopy (EDS) after exposures to the condition. From this study, it isfound that the corrosion resistance of electroless nickel-plated Alloy 600 is higher than that of electrolyticplating in 290 ◦C water.

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. Introduction

Primary water stress corrosion cracking (PWSCC) has been onef major degradation modes that occur in structural materials forteam generators tubes or control rod drive mechanism (CRDM)ozzles in nuclear power plants. Recently, the PWSCCs were found

n the inner diameter of penetration nozzles near the weldment ofRDM and reactor pressure vessel upper head in pressurized watereactor (PWR). The safety issue has been raised by crack initiation ofickel-base alloy by combination of high temperature, high residualtress and material properties (Gras, 1992).

In order to mitigate PWSCC in nickel-base alloys, severalechniques have been developed including mechanical surfacenhancement (MSE), environmental barriers or coatings, andhemical or electrochemical corrosion potential (ECP) control. MSEechniques represent processes that reduce surface tensile residualtresses or induce compressive surface stresses on a componenttem or weld. Examples of MSE techniques include shot peening

nd electropolishing. Environmental barrier or coating techniquesepresent processes that protect the material surface from anggressive environment. Nickel-plating and weld deposit overlaysre included in environmental barriers or coatings techniques.

∗ Corresponding author. Tel.: +1 617 452 3693; fax: +1 617 253 0807.E-mail address: kim@psfc.mit.edu (J.H. Kim).

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029-5493/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2008.03.019

© 2008 Elsevier B.V. All rights reserved.

hemical or ECP control techniques represent changes to the envi-onment that alter the corrosion process or produce corrosionotentials outside the critical range for PWSCC. Examples of chem-

cal or ECP control include zinc additions to the primary water andodified primary water chemistry (Fyfitch, 2003).Among these mitigation techniques, the electrolytic plating has

een successfully developed and applied to nuclear steam gener-tor tubing and pressurizer heater nozzles as a preventive and/ororrective measure against the PWSCC. In 1994, this technique haseen successfully applied for the repair and mitigation method ofuture degradation in steam generator tubes in Pickering Unit 5uclear power plant and pressurizer heater nozzles in Calvert Cliffsnit 1 nuclear power plant (Darling and Richards, 1994).

Nickel metal is intrinsically suitable for the formation of coatedlm on the surface of Ni-base alloys, because it can form a non-orous deposit layer with a good adhesion to Ni-base alloys by itsomposition and thermal expansion close to the base metal (noifferential thermal expansion) as well as its high corrosion- androsion-resistance in primary water condition. The formation ofickel metal on Ni-base alloys can be achieved either electrolyticlating or electroless one. In the electrolytic Ni-plating, separate

lectrode (anode) and external current supplies are needed for thelating process. Pure nickel solution is used and pure nickel layer

s formed on the substrate in electrolytic Ni-plating. On the otherand, electroless Ni-plating can be achieved by autocatalytic sur-

ace reaction without external current or separate electrode during

2530 J.H. Kim, I.S. Hwang / Nuclear Engineering and Design 238 (2008) 2529–2535

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Table 1Chemical composition and mechanical properties of Alloy 600 test materials at roomtemperature

Chemical composition wt.%

C 0.049 ± 0.002

Yieldstrength = 380 MPa

Si 0.16Mn 0.21P 0.009S 0.001Cr 15.37Co 0.10Nb 0.01Ta 0.01Ti 0.26Al 0.16CFN

33

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pdcwrthat4ntLTrIaaht

2

Fig. 1. CRDM nozzle on the reactor pressure vessel head in PWR.

he process (Dennis and Such, 1993). Since phosphorous as a reduc-ng agent is required in the autocatalytic reaction of electrolesslating, phosphorous is contained in the plating layer and its prop-rties can be varied depending on the phosphorous contents in theayer.

While Ni-plating technique has been successfully applied asitigation or repair methods in nuclear power plants, the study

nd the application of Ni-plating technique have been limited tohe components which have relatively easy accesses by using elec-rolytic method that requires external current supplies and separatelectrodes. By electroless Ni-plating, however, the application cane further extended to the places or mechanical components whichannot be reached by electrode, e.g., narrow gap regions betweenRDM penetrations and thermal sleeves in reactor pressure ves-el heads as shown in Fig. 1, because external current or separatelectrode are not necessary in the process of electroless Ni-plating,s described previously. Despite of possible expanded applicationf electroless Ni-plating, there have been little experience andata, especially in corrosion properties at high temperature for theuclear power plant application.

Therefore, this paper is focused on the feasibility of elec-roless Ni-plating as a mitigation method for the cracking inRDM penetration nozzles, particularly understanding of corro-ion behavior at high temperature water. This paper presentsnitial results on the characterization of corrosion behavior oflectroless Ni-plated Alloy 600 in comparison with electrolytici-plated one in high temperature and high pressure aqueous con-ition.

. Experimental methods

.1. Nickel-plating procedure

Alloy 600 was used as substrate for nickel-plating in this study.he plate materials were produced for commercial applications.

hemical composition of Alloy 600 used in this study is given inable 1. Test specimens for potentiodynamic polarization mea-urement were prepared by two different plating techniques withwo different thickness of plating on the substrate; electrolytic Ni-lating with 15 �m thickness (designated as EC15 hereafter) and

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Young’smodulus = 218 GPa

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0 �m (EC30), and electroless Ni-plating with 15 �m (ES15) and0 �m (ES30).

For the electrolytic Ni-plating specimen, following processesere applied in steps; cleaning, surface deoxidation, surface acti-

ation, and nickel-plating. The cleaning process was achieved byolvent solution containing trichloroethylene, and deionized wateras used for rinse, and then electrolytic cleaning in NaCN (concen-

ration: 80 g/l) and NaOH (15 g/l) solution was applied to removexide layers on the substrate. This was followed by surface activa-ion of substrate by 3% hydrochloric acid solution which resultedn good initiation and adhesion of the coating to the substrate. Anlectrolyte bath of nickel sulfate (300 g/l), nickel(II) chloride (50 g/l)nd boric acid (45 g/l) solution was used for the plating, and thislectrolytic nickel process produced a high-purity plating of nickelayer upon Alloy 600. The current density of 0.03 A/cm2 was applieduring the plating process, and it took about 7 and 15 h to make thelating layer of 15 and 30 �m, respectively, in this study.

During the electroless nickel-plating process, four primaryrocesses were applied in steps; chemical cleaning, surface deoxi-ation, surface activation, and nickel-plating. The cleaning processonsisted of solvent pre-cleaning, water rinse, alkaline soak, andater rinse. This was followed by surface deoxidation which

emoved surface oxides in order to obtain good bonding of nickelo the substrate. Then, surface activation of substrate by 10%ydrochloric acid solution was applied to obtain good initiation anddhesion of the coating to the substrate. During the surface activa-ion, the temperature of activation solution was maintained from0 to 60 ◦C. Finally, nickel layer containing phosphorous depositedickel on the substrate surface by autocatalytic reaction in the elec-rolyte bath (ICP Nicoron GIB series, Okuno Chemical Industries Co.,td., Japan) containing disodium malate and hypophosphorus acid.he temperature of nickel-plating solution was maintained in theange of 60–80 ◦C by using immersion heaters in the solution bath.t took about 30 min for each step of cleaning, surface deoxidationnd surface activation. The plating of 15 and 30 �m thickness tookbout 50 and 120 min, respectively. All specimens after plating wereeat-treated at 200 ◦C according to ASTM Method B 689, in ordero remove hydrogen absorbed during plating process.

.2. Potentiodynamic polarization measurement

To characterize the corrosion behaviors of Ni-plated Alloy 600 in

igh-temperature aqueous condition, electrochemical propertiesf nickel-plated Alloy 600 were investigated. For high tempera-ure corrosion test of nickel-plated Alloy 600, the specimens werexposed for 770 h to typical PWR primary water condition with000 ppm boron, 2 ppm lithium, and 2.68 ppm dissolved hydrogen

J.H. Kim, I.S. Hwang / Nuclear Engineering and Design 238 (2008) 2529–2535 2531

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ig. 2. Schematic of corrosion experimental facility in high temperature aqueousPTFE); PTEP, platinum electrode as a counter electrode; T/C, thermocouple.

as at a pressure of 18 MPa and a temperature of 290 ◦C. An experi-ental loop for the potentiodynamic polarization measurement in

igh temperature aqueous environment was constructed as shownn Fig. 2. EG&G Potentiostat/Galvanostat 273A was used as experi-

ental apparatus for potentiodynamic polarization measurement.uring the test, electrochemical corrosion potentials (ECPs) of all

pecimens were measured using Ag/AgCl electrode as a referencelectrode, 0.5 mm� Pt wire with platinization as a counter elec-

wwsd

Fig. 3. Schematic of flow accelerated corrosion experiment

onment. EREP, Ag/AgCl external reference electrode with polytetrafluoroethylene

rode. A custom-made Cu/Cu2O/Zr2O electrode which had beenrovided from General Electric R&D Center was used to acquirenother reference which could be compared with Ag/AgCl refer-nce electrode at high temperature water. Once ECP was stabilized,

hich usually took 24 h after each measurement, the electrodesere first polarized cathodically in order to reduce any oxidized

pecies on the surface. ECP was again stabilized, and potentio-ynamic polarization began at a rate of 0.5 mV/s ranging from

al facility in high temperature aqueous environment.

2532 J.H. Kim, I.S. Hwang / Nuclear Engineering and Design 238 (2008) 2529–2535

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ptacWec(cipmd3ccNstructure to amorphous structure depending on phosphorous con-tents in the plating layer (Allen and Vander Sande, 1982; Diegle etal., 1987). From the results in this study, the electroless Ni-platedsample with 15 �m thickness is supposed to possess the degree of

Table 2Electrochemical parameters from potentiodynamic polarization measurementsusing nickel-plated Alloy 600 specimens in high temperature water

Specimen ECPa (mV vs. SHE(T)) icorrb (�A/�m) ˇa

c (V/decade)

ES15 −734 35 0.235ES30 −726 35 0.21

ig. 4. (a) Photograph showing two locations of specimens installation in the flowccelerated corrosion experimental facility, and (b) drawing of specimen (left) andtandard high pressure fitting (right).

0.8 to 1 V versus the standard hydrogen electrode at temperatureSHE(T)).

.3. Flow accelerated corrosion (FAC) test

In addition to the potentiodynamic polarization measurementf Ni-plating layer on Ni-base alloys in high temperature water, thepecimens with electroless Ni-plating were exposed to the highow rate environment in high temperature water in order to inves-igate the durability of Ni-plating layer on substrate. While CRDMozzles which would be the potential application of electrolessi-plating are generally exposed to the fairly stagnant high tem-erature water condition, the high flow rate test would provides an accelerated picture of long-term exposure to the environ-ent. Test specimens were exposed for 480 h at 250 ◦C, 15 MPa and

he flow velocity of 2 m/s. The water chemistry was maintainedame as in the potentiodynamic polarization measurement. Fig. 3hows the schematic diagram of an experimental facility used forAC test in this study. The specimens were T-shaped nickel-platedlloy 600 specimens with 10 �m thickness, plugged into standard9.05 mm high pressure compression fitting plugs, and placed inhe elbow region of flow system, as shown in Fig. 4. After exposureso high flow rate aqueous condition, oxide films formed on the sur-aces of specimens were investigated using both scanning electron

icroscopy (SEM) and energy dispersive spectroscopy (EDS).

. Results and discussion

Fig. 5 shows the measured potentiodynamic polarization curvesf all specimens including Ni-plated and as-received Alloy 600pecimens exposed to typical PWR primary water condition at8 MPa and 290 ◦C. Table 2 summarizes the obtained electrochemi-

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ig. 5. Potentiodynamic polarization curves of Alloy 600 base metal and Ni-platedlloy 600 in typical PWR water condition.

al parameters from potentiodynamic polarization measurements.rom Table 2, it can be seen that the ECPs are nearly the same for allpecimens irrespective of plating method and thickness. But corro-ion currents of electroless nickel-plated Alloy 600 specimens areonsistently lower than those of electrolytically plated Alloy 600ase metal in this test condition.

The improved corrosion characteristic of electroless nickel-lating can be attributed to about 7.8% phosphorous contents inhe electroless plating. According to Lu and Zangari (2002), theylso showed the increase of ECP and the decrease of corrosionurrent with the increase of phosphorous contents of Ni-plating.

hile electrolytic Ni-plating leads to mostly crystalline Ni-layer,lectroless Ni-plating makes a form of Ni–P alloy as a mixture ofrystalline and amorphous binary, ternary and quaternary alloysPark and Lee, 1988). Generally, amorphous alloys exhibit higherorrosion resistance than crystalline Ni as the former is character-zed by extreme heterogeneity and low concentration of defects orreferential corrosion paths, such as grain boundaries in crystallineaterials (Sorensen and Diegle, 1987; Yu and Latanision, 1993). The

ifference of corrosion current observed between 15 �m (ES15) and0 �m (ES30) in the electroless Ni-plating samples appears to beaused by significant change in diffusion kinetics of corrosion pro-ess as thickness increases. It has been known that the electrolessi-plating layer shows various structures from close to a crystalline

C15 −725 53 0.36C30 −722 87 0.36

a Electrochemical potential.b Corrosion current density.c Anodic Tafel slope.

J.H. Kim, I.S. Hwang / Nuclear Engineering and Design 238 (2008) 2529–2535 2533

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ig. 6. (a) Scanning electron micrograph and (b) EDS measurements of nickel-platater with 250 ◦C, 15 MPa and 2 m/s for 480 h.

eterogeneity similar to that of crystalline structure in electrolytici-plating, but the inhomogeneity increases in the sample with0 �m thickness, which resulted in the significant change of ioninetics by increased diffusion barrier through the plating layernd the consequent decrease in the corrosion current at high tem-erature water. The difference observed between EC 15 and 30 wasnexpected since the larger thickness of crystalline layer still showsenerally higher resistance to corrosion at high temperature water.ore detail microstructural analyses including TEM needs to be

erformed in order to identify the structure of each plating layern the future. From the current result, electroless Ni-plating con-aining phosphorous can be considered to show higher corrosionesistance than electrolytic plating containing pure nickel in theespect of potentiodynamic polarization behavior at high temper-ture water.

Fig. 6(a) shows the scanning electron micrograph of cross-ectional area of Ni-plated Alloy 600 specimen exposed to highemperature, high pressure and high flow rate water with 250 ◦C,5 MPa and 2 m/s for 480 h. As seen in Fig. 6(a), there exist threeifferent layers in the specimen cross-section after test: (i) Alloy00 substrate layer, (ii) Ni-plating layer and (iii) oxide layer. The

ut layer of nickel oxide is considered to be formed by oxidationf nickel-plated layer exposed to high temperature water contain-ng small amount of oxygen. EDS analysis was performed to get thehemical composition of each layer and the results are representedn Fig. 6(b). From Fig. 6, it can be known that the Ni-plating layer

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loy 600 specimen exposed to high temperature, high pressure and high flow rate

till remains uniform and close to its original thickness after long-erm exposure to high temperature and high-flow velocity water.rom this result, electroless Ni-plating can play a role of protec-ive layer for structural parts in erosion and/or erosion–corrosionnvironment with high-velocity water.

In this study, remote process for electroless nickel-platingas been simulated in laboratory scale to apply the method toRDM nozzles. For the field application of nickel-plating to actualegraded components such as CRDM nozzles in nuclear powerlants, electroless nickel-plating technique has significant advan-age over electrolytic counterpart since electroless Ni-platingeeds no separate electrodes which are essential for electrolytici-plating. Such provisions cannot be readily installed within thearrow gap between the inner diameter of nozzle and thermalleeve. Schematic diagram of nickel-plating process developed inhis study is shown in Fig. 7. As shown in Fig. 7, all process wasontrolled by electopneumatic pumps and valves which makeirculation of each solution at each step. Fig. 8 shows a specimenesign which has 3-mm gap between stainless steel and alloy 600lates, and its actual set-up to simulate remote Ni-plating process

n the CRDM nozzle gap geometry with narrow gap. The same

rocedure as described in the preceding section was applied for thelectroless Ni-plating. The temperature of nickel-plating solutionas maintained in the range of 60–80 ◦C by using immersioneaters and controller in the solution bath. The target thickness oflating on the substrate is about 10 �m. It took about 30 min for

2534 J.H. Kim, I.S. Hwang / Nuclear Engineering and Design 238 (2008) 2529–2535

electro

eedpo

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Fig. 7. Schematic of remote

ach step and the whole process was sequentially carried out. Afterlectroless nickel-plating was completed, the specimens wereisassembled and cut to small section in order to examine thelating quality as well as thickness. Fig. 9 shows the cross-sectionf a nickel-plated Alloy 600 specimen cut from the specimen

aptna

ig. 8. A schematic design (a) and actual set-up (b) of a plate specimen for remote electenetration and thermal sleeve.

less nickel-plating process.

fter remote electroless nickel-plating. It can be seen that thelating layer is very uniform and the thickness is very close to thearget thickness. From this result, it is shown that the electrolessickel-plating can be applied by the remote process on a lab scale,t least, to the very narrow gap of CRDM nozzle geometry.

roless nickel-plating process to simulate the narrow gap (∼3 mm) between CRDM

J.H. Kim, I.S. Hwang / Nuclear Engineering

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ig. 9. Scanning electron micrograph of cross-section of plate specimen after remotelectroless Ni-plating.

. Summary

In order to develop a preventive and corrective measure toitigate environment assisted cracking of Ni-base structural alloy

omponents in nuclear power plants, electroless Ni-plating tech-ique has been explored in this paper. Corrosion behavior oflectroless Ni-plated Alloy 600 has been investigated in high tem-erature aqueous condition. From this study, following conclusionsre made:

1) Electroless Ni-plating containing phosphorous has shownhigher corrosion resistance than electrolytic plating contain-ing pure nickel in the respect of potentiodynamic polarizationbehavior at high temperature water.

S

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and Design 238 (2008) 2529–2535 2535

2) Electroless Ni-plating can play a role of protective layer forstructural parts in erosion and/or erosion–corrosion environ-ment with high-velocity in high temperature water.

3) Electroless Ni-plating can be as effective as electrolytic plat-ing for PWSCC mitigation, provided mechanical properties areacceptable.

As future work, mechanical properties of Ni-plated materialseed to be measured so that the mechanical integrity of the layeran be assessed. The measurement should include tensile, fatiguend fracture toughness behaviors with nickel-plating on Alloy 600pecimens. Also, stress corrosion cracking test including slow strainate test and crack growth test need to be done in order to verifyts effectiveness in the field application. In addition, special endeals for the application to upright annulus of CRDM need to beesigned in detail for the plating process between nozzle inner andleeve outer surfaces.

eferences

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arling, D.B., Richards, J.A., 1994. Nickel plating of pressurizer heater nozzles toprevent PWSCC. Nucl. Plant J. 12 (7), 34–39.

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iegle, R.B., Clayton, C.R., Lu, Y., Sorensen, N.R., 1987. Evidence of chemical passivityin amorphous Ni–20P alloy. J. Electrochem. Soc. 134, 138–139.

yfitch, S., 2003. Alloy 600 PWSCC mitigation: past, present and future. In: Proceed-ings of Conference on Vessel Head Penetration Inspection, Cracking and Repairs,US NRC, Gaithersburg, MD, September.

ras, J.M., 1992. Stress corrosion cracking of steam generator tubingmaterials—review and assessment. In: Parkins Symposium on FundamentalsAspects of Stress Corrosion Cracking, TMS, pp. 411–432.

u, G., Zangari, G., 2002. Corrosion resistance of ternary Ni/P based alloys in sulfuricacid solutions. Electrochim. Acta 47, 2969–2979.

ark, S.H., Lee, D.N., 1988. A study on the microstructure and phase transformation

of electroless nickel deposits. J. Mater. Sci. 23, 1643–1654.

orensen, N.R., Diegle, R.B., 1987. Corrosion of amorphous metals. In: Metals Hand-book, Corrosion, vol. 13, 9th ed. American Society for Metals, Metals Park, OH,pp. 864–870.

u, Z., Latanision, R.M., 1993. The pitting corrosion of amorphous Ni63Cr12.5-Fe4Si8B12.5 alloy in chloride solution. Corros. Sci. 34, 1697–1706.