Prevention of SCC in the Expansion Transition Region of Steam Generator Tubing of PWRs by Ni-plating

Transcription

1 2 nd International Symposium on Nuclear Power Life Management, Shanghai, October 14-19, 2007 Prevention of SCC in the Expansion Transition Region of Steam Generator Tubing of PWRs by Ni-plating Joung Soo Kim, Myung Jin Kim, Dong Jin Kim, and Hong Pyo Kim. Nuclear Materials Research Center 1

2 Contents 1. Background 2. Purpose 3. Experimental 4. Results and Discussion 5. Summary 2

3 1. Background Nickel-base Cr-Fe alloys, especially, Inconel 600 having been used as steam generator tubing susceptible to PWSCC and ODSCC under the operating conditions of PWR nuclear plants. Even Inconel 690 tubing known to be susceptible to IGSCC in Pb-containing water solutions and the high concentration of caustic solutions such as NaOH from the laboratory experiences. Most SCC in steam generator tubing occurs in the expansion transition region, which is located around the top of tubesheet where sludge is piled up in the secondary side of steam generators. In the expansion transition region, SCC preferentially occurs due to - High residual stress developed during expansion process and/or during operation. - High stress possibly developed by denting. - High temperature and highly concentrated deteriorating chemicals in the crevice due to local boiling. Statistics of steam generators replaced and to be replaced in the world. Replaced 87 To be replaced 17 Remark As of 2007 Trend of dominant steam generator tube degradation mechanisms(2004) 3

4 1. Background Plated pure Ni and Ni alloys known to be immune to SCC in nuclear power plants. Pure Ni-plating been utilized to repair the cracked steam generator tubes in operating NPPs. But, this method not endowed the repaired tubes with structural integrity. Later on, nano-grain sized Ni-P electrodeposition method been applied to repair failed tubes and at the same time, ensured structural integrity to the repaired tubes. These existing Ni plating techniques developed for repairing damaged SG tubes, which is reactive but not proactive. In order to manage steam generators effectively, proactive measures should be devised and applied to prevent SG tubes from degradation by various corrosion mechanisms. In this study, Ni electrodeposition methods investigated for preventing SG tubes from corrosion damage as an proactive measure by applying Ni electrodeposit from both ends of SG tubes up to around the expansion transition region before steam generator manufacturing. Nickel plating Nickel plating for repairing cracked SG tube Nickel plating for preventing SG tube From SCC. 4

5 2. Purpose Investigation of a Ni-plating method for preventing steam generator tubes in nuclear power plants from damages occurring by various corrosion mechanisms 5

6 3. Experimental Procedures (1/3) 1. Ni-plating Used Material - Alloy 600 HTMA Tube, OD : mm, Wall Thickness : 1.07 mm - Chemical composition : Element C Si Mn P Cr Ni Fe Co Ti Cu Al B S N Wt% Ni-plating - Activation condition : 4 vol.% H 2 SO 4 at room temp. for 30 sec., Film thickness ~ 5 μm. - Strike condition : 1.6 mol/l NiCl mol/l H 3 BO 3, at 40ºC for 2.5 min. - Ni-plating condition : 1.39mol/l Ni(SO 3 NH 2 ) mol/l H 3 BO 3 at 60 C for 40 min. 100 ma/cm 2 direct current applied, Deposit thickness 50~80 μm. Tubesheets and Ni-plated tubes 6

7 3. Experimental Procedures (2/3) 2. Expansion processes - Tube material : Pure Ni plated Alloy 600 HTMA Tubes - Tubesheet material : SA 508 Carbon steel, Thickness 110 mm - Expansion method : Hydraulic expansion, Applied pressure 32,000 and 35,000 psi. - Expansion ratio after expansion : 1.23% for 32,000 psi, 1.67% for 35,000 psi Expansion ratio, R(%) = {1 [(H ID -I ID )/(D OD -D ID )]} x 100 where H ID I ID D OD D ID Inner diameter of tubesheet hole Inner diameter of tube after expansion Outer diameter of tube before expansion Inner diameter of tube before expansion Photographs showing (a) expanded tubes in tubesheets, and (b) expanded tubes in tubesheets and expanded tube after removal of tubesheet 7

8 3. Experimental Procedures (3/3) 3. Stress corrosion tests - Material : Ni-plated and afterwards expanded Alloy 600 HTMA Tubes Ø=5 - Specimens : C-ring and SSRT specimens - Test conditions : applied stress at apex 150% of Y.S. for C-ring L=12 Strain rate - 1.2x10-7 /sec. for SSRT 40% NaOH at 315ºC and 200mV above OCP for the secondary side stress corrosion test ppm B, 2.2 ppm Li, 5 ppb O 2 and 30 cc/kg H 2, at 330 C for the primary side stress corrosion test. 4. Microscopic examination - Optical and Scanning Electron Microscopy used to observe the morphologies of interface, SCC test specimens, and the microstructures of plated Ni before and after expansion. 8

9 4. Results and Discussion (1/10) Effect of DC density on film properties DC Density : 20~350mA/cm 2 96 Current Efficiency Hardness Current Efficiency / % Vickers Hardness / VHN Current Density / macm Current Density / macm -2 Current efficiency increase and hardness decrease with increasing DC density : Depletion of Ni ions near interface between plated layer/electrolyte ( Decrease in nucleation rate) Large grain formation Decrease in hardness. : Decrease in hydrogen evolution rate due to current density increase( Decrease in nucleation rate) Large grain formation Decrease in hardness, Increase in current efficiency. 9

10 4. Results and Discussion (2/10) Effect of DC density on film properties True Stress, MPa / P/A 800 Current Density DC Density : 20~350mA/cm 2 20mA/cm 2 50mA/cm 2 100mA/cm 2 350mA/cm 2 Internal Stress / MPa Engineering Strain / (L-L 0 )/L o Current Density / ma/cm 2 Increase in ductility and internal residual stress, and decrease in strength with increasing direct current density : Increase in current efficiency (due to decrease in hydrogen evolution) : Increase in internal residual stress 10

11 4. Results and Discussion (3/10) Effect of electrolyte temperature Current Efficiency 190 Hardness DC density : 100mA/cm 2 Current Efficiency / % Temperature / o C Vickers Hardness / VHN Temperature / o C Increase in current efficiency and hardness with increasing electrolyte temperature : Increase in limit current density with increasing electrolyte temperature Effect of temperature on limit current density favorable more than that on hydrogen evolution. Increase in current efficiency nfdc i L= δ B : Increase in limit current density Increase in nucleation rate Hardness increase 11

12 4. Results and Discussion (4/10) Thermal stability of plated Ni layer mA/cm 2 Vickers Hardness / VHN Duty cycle : 0.3(Avg. 111mA/cm 2 ) Additive(Saccharin, Coumarin, SLS) 100mA/cm 2 Weak thermal stability of Ni layer around 350 ºC Initial hardness can be increased by adding additives or changing current duty cycles But hardness decreased sharply around 350 ºC Recrystallization occurred around 400 o C Heat Treatment Temperature / o C 12

13 4. Results and Discussion (5/10) TEM microstructure of Ni layer Plated on Alloy 600 Tube surface 1.39mol/l Ni(SO 3 NH 2 ) mol/l H 3 BO 3 at 60 C for 40 min. at 100 ma/cm 2 DC applied. Grain size : > 500 μm X50,000 X20,000 13

14 4. Results and Discussion (6/10) Adhesion force between plated Ni layer and Alloy 600 surface 35 Adhesion layer was endurable at this peak load point 30 Tension test 25 Electroplating layer was ruptured Plated Ni layer Ni deposit/substrate overlapped region Load / kg o C, 1.6mol/L NiCl mol/L H 3 BO 3 + 5% HCl, 5µm Thickness Displacement / mm Overlapped area between Ni layer/strike layer/alloy 600 surface = 0.5mmx4mm = 2mm 2 No detachment at the interface until Ni layer ruptured Adhesion force > Max. load 30.75kg, which means that Adhesion force > 30.75x9.8/2 = 150MPa 14

15 SEM morphologies of plated Ni layer surface 4. Results and Discussion (7/10) (a) (b) SEM micrographs showing the surface morphologies of the plated Ni layer (a) before, and (b) after hydraulic expansion at a pressures of 35,000 psi. and removal of TS The plated Ni layer surface observed to have been squashed by the tubesheet surface during expansion. 15

16 4. Results and Discussion (8/10) Microstructures of interface between plated Ni layer/alloy 600 Tube (a) As-expanded tube with tubesheet SEM micrograph showing the interface between Ni layer/alloy 600 before expansion No appreciable (micro)structural change in plated Ni layer before and after hydraulic expansion SEM micrographs showing the interface between the plated Ni layer and the Alloy 600 specimen after expansion by hydraulic pressures at (a) 32,000 psi and (b) 35,000 psi. (b) 16

17 4. Results and Discussion (9/10) SCC test results SEM micrograph showing the cross-section morphology of the C-ring specimen tested in a 40% NaOH solution at 315 C and 200 mv above OCP for 60 days. (Specimens tested after expansion, stresses outwardly) Optical micrographs showing cracks formed in the C-ring specimens tested in a 40% NaOH solution at 315 C and 200 mv above OCP for 7 days and arrested at the interface of the Ni layer and the tube surface. (Specimens tested before expansion) No SCC occurred on Ni-plated Alloy 600 HTMA 17

18 4. Results and Discussion (10/10) (a) (b) About 10% strain Rupture SEM micrographs showing the surface morphologies of (a) the bare Alloy 600 HTAM and (b) the Ni-plated Alloy 600 HTMA specimens tested in a 40% NaOH solution at 315 C under a strain rate of 1.2x10-7/sec.. 18

19 5. Summary From this study, It can be concluded that the Ni-plating technique on steam generator tubing surfaces could be applied to prevent the tubes from damages due to various corrosion occurring in operating nuclear power plants, as a proactive method. - Ni plating should be done before tube expansion during S/G manufacturing. - Ni plating should be carried out from both ends of a S/G tube up to well above the expansion transition regions, i.e. up to well above the top of tubesheet. (up to the region where sludge would be expected to be piled up) - One problem to be expected with applying this technique is ECT which can not be used to detect defects or damages possibly occurring in the Ni-plated regions of the S/G tubes. New UT technique(s) under developing to increase detection and analysis speed. New ECT which can be applied to the ferromagnetic materials being developed till almost final stage at in Korea. 19