ML20247H081

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Metallurgical Failure Analysis of Leaking Canopy Seals
ML20247H081
Person / Time
Site: Salem  PSEG icon.png
Issue date: 01/31/1989
From: Mager T, Peze C
WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML18094A451 List:
References
WCAP-12088, NUDOCS 8905310154
Download: ML20247H081 (61)
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Text

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wasmonouse class a woo assionsens oisenveen WCAP-12088' METALLURGICAL FAILURE ANALYSIS OF LEAKING CANOPY SEALS C. W. Pezze January 1989 Approved by: Ib T. R. Magerff anager Metallurgicari & NDE Analysis Work Performed Under Shop Order MUHU-1088 Although information contained in this report is non proprietary, no distribution shall be made outside Westinghouse or its licensees without WOG's i approval. l

. WWESTINGHOUSE ELECTRIC CORPORATION

Nuclear and Advanced Technology Division P.O. Box 2729 Pittsburgh, Pennsylvania 15230-2728 i

8905310154 890522 mmveines:io DR g ADOCK OS%72

ABSTRACT This report describes the results from the failure analysis of canopy seal welds. The canopy seal welds evaluated were from five plants: Diablo Canyon Unit 1, Prairie Island Unit 2, Salem Unit 2, Turkey Point Unit 3 and Zion Unit

1. The evaluations performed included a review of welding procedures,

. nondestructive exaniinations, visual examination, metallography, scanning electron microscopy, electron probe analysis and chemistry evaluations. It is concluded that the failure mechanism is transgranular stress corrosion cracking caused by a combination of chloride and oxygen contamination.

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TABLE OF CONTENTS Section Title. Page

1.0 INTRODUCTION

1-1 l

2.0 WELD' PROCEDURE REVIEW 2-1 3.0 METALLURGICAL EVALUATIONS AND RESULTS. 3-1 3.1 Sectioning 3-1 3.2~ Nondestructive Examination . 3-3 3.3 ' Visual and Macroscopic Examinations 3-3 3.4 Meta 11ographic E;.e.inations - 3-4

'3.5 Scanning Electror. Microscope Examinations 3-4 3.6 Electron Microprobe Analysis 3-5 3.7 ' Chemistry Evaluations 3-5

.u; 4.0 DISCUSSION 4-1

5.0 CONCLUSION

S 5-1 6.0 RECOMMENDATIONS 6-1 0

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LIST OF FIGURES Figure Title Page 2-l' Schematic Illustration of Canopy Seal Weld Configuration. 2-2 3-1 Microphotograph of the Head Adapter and Plug Following 3-12 Removal of the Canopy Seal Weld. This is E-9 from Prairie Island Unit 2. Note Black Deposit on Head Adapter Threads.

s 3-2 Microphotograph of the Head Adapter and Plug Following 3-13 i Removal of the Canopy Seal Weld, This is L-9 from Diablo Canyon Unit 1.

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3-3 Microphotograph of the Head Adapter and Plug Following 3-14

, Removal of the Canopy Seal Weld. This is J-5 From Diablo Canyon Unit 1. Note the Black Deposit has been Removed from Head Adapter Thread.

3-4 Microphotograph of the Inside of the Head Adapter Plug 3-15 from J-5 Diablo Canyon Unit 1.

3-5 Schematic Illustration of Section Locations on Canopy 3-16 Seal Weld L-11 from Diablo Canyon Unit 1.

3-6 Schematic Illustration of Section Locations on Canopy 3-17 Seal Weld L-9 from Diablo Canyon Unit 1.

3-7 Schematic Illustra! :" of Section Locations on Canopy 3-18 .

Seal Weld E-9 fro = rre in Island Unit 2.

3-8 Schematic Illustration of Section Locations on Canopy 3-19 Seal Weld G-7 from Turkey Point Unit 3.

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d LIST OF FIGURES (cont.)

Figure Title Page 3-9 Schematic Illustration of Section Locations on the 3-20 Canopy Seal Weld from Salem Unit 2.

10 Schematic Illustration.of Section Locations on Canopy 3-21 Seal Weld J-5 from Diablo Canyon Unit 1.

3-11 Microphotographs of the Outside and Inside Diameter of 3-22 Section F from Prairie Island E-9 Canopy Seal Weld.

3-12 Microphotographs of the Outside and Inside Diameter of 3-23 Section A from Diablo Canyon L-11 Canopy Seal Weld.

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This Section is in the Leak Location.

3-13 Microphotographs of the Outside and Inside Diameter of 3-24 Section A from Pr'airie Island E-9 Canopy Seal Weld.

This Section is in a Weld Repair Location.

3-14 Light Optical Meta 11ography Results Illustrating the 3-25 Weld Prcfile.of Section B from Diablo Canyon L-11 Canopy Seal Weld.

3-15 Light Optical Meta 11ography Results Illustrating the 3-25 Weld Profile and Base Metal Cracking of Section C from Diablo Canyon L-11 Canopy Seal Weld.

3-16 Light Optical Meta 11ography Results Illustrating the 3-26 Weld Profile and Base Metal Cracking of Section C from Prairie Island E-9 Canopy Seal Weld.

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LIST OF FIGURES (cont.)

Figure Title Page

< 3-17 Light Optical Meta 11ography Results Illustrating the 3-26 Weld Profile and Base Metal Cracking of Section F from Prairie Island E-9 Canopy Seal Weld.

3-18 Light Optical Metallography Results Illustrating the 3-27 Weld Profile, Weld Metal Cracking and Base Metal Cracking of Section D from Turkey Point G-7 Canopy Seal Weld.

3-19 Light Optical Meta 11ography Results Illustrating the 3-28 Weld Profile, Weld Metal' Corrosion, Weld Metal Cracking

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and Base Metal Cracking of Section A from Prairie Island E-9 Ct.nopy Seal Weld.

3-20 Light Optical Meta 11ography Results Illustrating the 3-29 Weld Profile, Weld Metal Cracking and Base Metal Cracking of Section F from Prairie Island E-9 Canopy Seal Weld.

3-21 Light Optical Meta 11ography Results Illustrating the 3-29 Weld Profile, Weld Metal Cracking and Base Metal Cracking of Section 'C from the Salem Canopy Seal Weld.

3-22 Light Optical Meta 11ography Results Illustrating the 3-30 Weld Profile, Weld Metal Cracking and Base Metal Crt: king of Section A from Diablo Canyon J-5 Canopy Seal Weld.

3-23 Light Optical Metallography Results Illustrating the 3-31 I Through-wall Leak on Diablo Canyon L-9 Canopy Seal Weld.

3-24 Light Optical Meta 11ography Results Illustrating the 3-32 Through-wall Leak on the Salem Canopy Seal Weld.

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I LIST OF FIGURES (cont.)

Figure Title page 3-25 Light Optical Meta 11ography Results Illustrating the 3-33 Through-wall Leak on Zion K-4 Canopy Seal Weld.

3-26 Light Optical Meta 11ography Results Illustrating the 3-34 I Electrolytic 0xalic Etched Microstructure of Section C from Diablo Canyon L-9 Seal Weld. l 3-27 Scanning Electron Micrographs Illustrating the Outside 3-35 Diameter Surface of the Leak on Diablo Canyon L-11 Canopy Seal Weld.

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3-28 Scanning Electron Micrographs Illustrating the Outside 3-36 Diameter Surface of the Leak on Diablo Canyon L-9

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Canopy Seal Weld.

3-29 EDS Results ef the Inside Diameter of Diablo Canyon L-9; 3-37 (a) Base Metal, (b) Weld Metal and (c) Deposit.

3-30 Scanning Electron Fractographs of the Endexed Fracture 3-38

. Face of the Leak on Diablo Canyon L-9 Canopy Seal Weld.

l 3-31 Scanning Electron Fractographs of the Endexed Fracture 3-39 Face of Base Metal Cracks on Prairie Island E-9 Canopy Seal Weld.

4-1 The Effects of Oxygen and Chloride on the SCC of Austenitic 4-6 i Stainless Steel in High Temperature Water.

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LIST OF TABLES Table. Title Page 3-1 ~ Matrix of Test Techniques Employed on Canopy Seal 3-2 Welds from Specific Plants.

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3-2 Chemistry Evaluation Results of Head Adapter P1ug 3-6 Base Metal.

3-3 Chemistry Evaluation Results of Head Adapter Base 3-7 Metal.

, 3-4 Chemistry Evaluation Results of Canopy Seal Weld 3-8 i Metal.

3-5' Water Chemistry Results, Anion Analysis. 3-9 3-6 Chemistry Evaluation Results of Black Deposit Removed 3-11 from Head Adapter Threads.

i 4-1 Canopy Seal Welds from Specific Plants and Defects 4-4 Observed.

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ACKNOWLEDGEMENTS l

I* I appreciate the contributions to this study from Ian Wilson for his stress corrosion cracking and failure analysis expertise and Dave De1 Signore for his  ;

welding expertise. I would also like to recognize Ken Galbreath atd Bob Reese for their contributions in the >:Poratory. I appreciate their efforts in providing the technical suppoi .iecessary for the completion of this project.

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SECTION

1.0 INTRODUCTION

Every penetration attached to the reactor vossel closure head en a Westinghouse Pressurized Water Reactor (PWR) consists of a two (2) piece construction, an Inconel tube section welded to a stainless steel flange. The Inconel tube section is inserted in the opening of the reactor vessel closure heed and held in place by a partial penetration weld. The stainless steel flange has male ACME threads ~and is fabricated with a canopy lip. Every flange on a particular plant is designed the same.

Each attachment is threaded on to the flange. Since ASME Section III Code requirements state that the threaded joints in which threads provide the only seal shall not be used, a seal weld between the flange and attachment is utilized. The canopy seal weld is a circular attachment between the head adapter flange and the attachments. The weld is designed to retain the pressure of the reactor coolant while the strength of the. joint is maintained by the threaded joint. The types of attachments mated to the reactor vessel closure head penetrations include: full length Control Rod Drive Mechanisms (CRDMs), part length CRDMs,' capped latch housings (utilized for Plutonium Recycling Capabil-ity, the same component used for the pressure boundary of the full length CRDM), female flanges (utilized at core exit thermocouple locations for the penetration of the thermocouple columns through the pressure boundary), head adapter plugs (utilized at spare locations), and upper probe housings (utilized as the pressure boundary penetration when the utility has elected to install the Combustion Engineering heated junction thermocouple assembly as a reactor vessel level measuring system).

l In the past two years, laakage through the lower canopy seal region has been reported at several plants. The concern associated with the leak is corrosion of carbon steel (reactor vessel closure head and reactor vessel studs) in a boric acid environment.

The Westinghouse Owner's Group (WOG) funded a program tc determine the cause j of the canopy seal leakage and provide recommendations for corrective

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J actions. This report describes the technipes employed and the results obtained during the failure analysis of canopy seal welds removed from five

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(5) plants: Diablo Canyon Unit 1, Prairie Island Unit 2, Salem Unit 2, Turkey l ,

Point Unit 3, and Zion -Unit 1. The evaluations included a review of welding procedures, nondestructive examinations, visual examination, light optical microphotography, metallography, scanning electron microscopy, electron microprobe analysis ar.d chemistry evaluations. The final section of the report provides recommendations for the mitigation of the leaking canopy seal weld.

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SECTION 2.0 WELD PROCEDURE REVIEW The lower canopy seal welds are assembled utilizing an automatic gas tungsten arc weld (GTAW) process. This process produces coalescence of metals by heating them with an are between a tungsten (nonc>nsumable) electrode and the work piece.. The filler metal, when used, is not transferred across the are but melted by it. The arc area is protected from the atmosphere by an inert shielding gas.

The specific GTAW setup used on the lower canopy seal welds initially requires the consumable insert, either a 'J' or a 'Y' shaped insert, to be tack welded to the attachment (e.g. CRDM, head . adapter plug, female flange, etc.). Figure 2-1 illustrates the canopy seal weld configuration. The filler material consumed in this process is a type 308L stainless steel insert. Following the tack weld process the attachment is threaded onto the head adapter flange. A purge needle is then inserted in the space between the ends of the insert.

Welding grade, '99.9% pure argon is then introduced into the cavity on the inside diameter of the weld. During the purging process, the welding head is assembled to the pipe and the torch positioned inte place. After a minimum ten minute purge, the arc is initiated and the welding commenced. GTAW is widely used for joining thin sections of stainless steel and usually produces welds of extremely high quality.

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ATTACHMENTS 1

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FILLER METAL INSERT '

("Y" OR "J" SHAPED)

HEAD ADAPTER f.

l1 Tigure 2-1. Schematic Illustration of Canopy Seal Weld Configuration 1

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l SECTION 3.0 {

I METALLURGICAL EVALUATIONS AND RESULTS This section describes the techniques employed in the failure analysis of the leaking canopy seal welds. The steps performed during the investigation are discussed and the results presented. Table 3-1 is a matrix of the techniques employed on.the canopy seal welds from the specific plants. The canopy seal welds from Zion Unit 1 were evaluated at Argonne National Laboratory located in Chicago Illinois and the results of the investigation made available to Westinghouse. In an attempt to provide the most comprehensive evaluation, the techniques employed with each canopy seal weld varied as data became available. A discussion of the results is presented in sect' ion 4.0 of this report.

3.1 Sectioning The as-received condition of the canopy seal welds submitted varied from plant to plant. The samples submitted from Diablo Canyon Unit 1 and Prairie Island Unit 2 were attached to sections of the head adapter plug and head adapter.

In order to isolate the canopy seal welds in their entirety the as-received pieces were sectioned. This was accomplished utilizing a lathe and a single point tool with no cutting fluid. Figures 3-1 through 3-4 are macrophoto-graphs illustrating the head adapter plug and head adapter pipe which remained following removal of the canopy seal weld. The procedure used for isolating the canopy seal welds was relayed to those utilities that performed the sectioning prior to submitting samples for failure analysis.

Following the evaluation of canopy seal welds E-9 from Prairie Island Unit 2 and L-9 and L-11 from Diablo Canyon Unit 1 a step was added to the sectioning procedure. The step involved drilling a hole into the canopy seal weld and draining the water trapped in the inside diameter cavity prior to removing the canopy seal weld. This was performed on the canopy seal welds from Salem Unit

'2, Zion Unit i and J-5 from Diablo Canyon Unit 1.

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3.2 Nondestructive Examinations i e In an attempt to identify the leak locations liquid penetrant inspection was performed on the outside diameter of a number of canopy seal welds submitted

- for evaluation. The leak locations, as identified utilizing macroscopic examination described in section 3.3, were not successfully identified with the liquid penetrant examination, i

i As a supplement to the visual and macroscopic examinations described in section 3.3, radiography was performed on the canopy seal welds from Salem Unit 2 and J-5 from Diablo Canyon Unit 1. The radiography successfully confirmed leak locations identified on the canopy seal weld from Salem Unit 1.

Liquid penetrant inspection was performed on the head adapter threads of the Diablo Canyon J-5 and Prairie Island E-9 samples. The inspection was performed following the removal of the black deposit as described in section c 3.7. There were no indications found during the inspection.

. 3.3 Visual and Macroscopic Examinations Once the canopy seal welds were removed from the head adapter plug and head '

adapter pipe a complete visual examination of the inside and outside diameter surface was performed. As additional documentation, the visual examination of the canopy seal weld from Salem Unit 2 and J-5 from Diablo Canyon Unit I was recorded on VHS tape. Identification of the leak locations was accomplished through macroscopic examinations (3X magnification). Following the examinations, the canopy seal welds were sectioned for more detailed i

analysis. Figures 3-5 through 3-10 are schematic illustrations of the section locations on each of the canopy seal welds. Each section location was again examined visually and by low power microscopy. Figures 3-11 through 3-13 are

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microphotographs illustrating the inside and outside diameter weld surface of' some of the sections from the canopy seal welds. The surface conditions I

. illustrated are representative of all the welds examined.

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3.4 Meta 11ographic Examinations Metallographic examinations were conducted on sections identified in figures 3-5 through 3-10. The samples were mounted, polished and etched in a solution

- comprised of glycerol, acetone, nitric acid and hydrochloric acid. The samples were examined under the light microscope in both the as polished and etched condition. Figures 3-14 through 3-22 illustrate the results of the metallographic examinations of sections away from leak locations. The through-wall leak locations were mounted and successively ground, polished and etched at measured increments so as to assure metallographic examination of the defect causing the leak. Figures 3-23 and 3-25 illustrate through-wall leak locations. The results illustrated are representative of the metallurgical conditions observed during the examinations.

Following the examinations, one sample from each canopy seal weld examined was polished and subjected to an electrolytic oxalic etch in accordance with ASTM-A-262 practice A. The purpose of this was to establish the degree of sensitization. Figure 3-26 illustrates representative results of this

. examination.

3.5 Scannino Electron Microscopic Examinations The inside and outside diameter weld surface of sections in the leak locations were examined on the scanning electron microscope. Figures 3-27 and 3-28 illustrate the results of the examinations performed on canopy seal weld L-9 and L-11 from Diablo Canyon Unit 1.

Prior to performing metallography on sections taken away from leak locations, a number of samples were examined on the scanning electron microscope. An energy dispersive X-ray spectrographic (EDS) analysis was performed at various

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inside and outside diameter locations in an attempt to identify contaminating species which may have contributed to the failure. Figure 3-29 illustrates

. representative results. .

., A number of defect locations were opened and fractographic examinations were l- performed on the freshly opened fracture face by scanning electron microscopy J n n.mu neae 34 I

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techniques. The examinations were conducted on the as-opened and endoxedl structure. The purpose of the examination was to identify the mechanism of cracking, fracture. morpSology and contaminating species which may have contributed to the failure. Figures 3-30 and 3-31 illustrate the representative results of this examination.

3.6 Electron Microprobe Analysis An electron microprobe analyzer was used in an attempt to identify contaminating species which may have contributed to the failure. The elements  !

analyzed included oxygen, chlorine, boron, and sulphur. The results indicated oxygen in all cracks examined.

3.7 Chemistry Evaluations Chemical analysis techniques were performed on the weld, head adapter and head

,. adapter plug metal of the canopy seal welds evaluated. The purpose of the ,

examinations was to confirm the materials of construction. The techniques

- included: energy dispersive X-ray spectrographic (EDS) analysis, inductive coupled plasma spectrometry, carbon analysis utilizing the LECO carbon analyzer and sulphur analysis utilizing combustion / titration techniques. The results are listed in Tables 3-2 through 3-4.

J Chemical analysis was performed on a water sample removed by Public Service Electric and Gas Company personnel from the cavity behind the Salem Unit 2 canopy seal weld. The purpose of the examinations was to identify contaminating species which may have contributed to the failure. The techniques utilized included: emission spectrography, liquid ion )

chromatography and inductive coupled plasma spectrometry. The metals detected in significant amounts (greater than one weight percent) included: iron, chromium, nickel, silicon, titanium, and zirconium. These metals are most likely corrosion products of the system. The results of liquid ion

chromatography analysis are. listed in table 3-5. Table 3-5 also contains the

,, 1. A. Madeyski, Hydrogen Reduction of Oxide on a Metal Fracture Prior to Fractography, Praktische Metallography, Volume 17, 1980, pp. 598-607.  !

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  • TABLE 3-5 WATER CHEMISTRY RESULTS ANION ANALYSIS ANIONS, micrograms per ml Fluoride Chloride Nitrate Sulfate Phosphate Salem Sample <0.053 0.356 0.186 0.679 0.050
  • Zion Sample #1 <0.09 <0.1 <0.25 0.31 -
  • Zion Sample #2 <0.09 <0.1 <0.25 0.87 -
  • Zion Sample #3 <0.09 0.27 0.25 0.81 -
  • Note: Results provided by Argonne National Laboratr,ry, Chicago, Illinois.

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results obtained by Argonne National Laboratory in the analysis of three water samples removed from the cavity behind Zion Unit 1 canopy seal welds.

The black deposit observed on the head adapter threads, illustrated in figures 3-1 and 3-2, was removed and analyzed for anion and metal content. The purpose of the examinations was to identify contaminating species which may have contributed to the failure. The analysis was conducted on the deposit from the Prairie Island E-9 and Diablo Canyon J-5 samples. The black deposit was removed using a nylon scrub brush and demineralized water (figure 3-3 is a microphotograph illustrating the threads after the deposit was removed). The collected samples were filtered and the aqueous solution subjected to liquid ion chromatography for anion content, the results are listed in table 3-6.

The fine black powder samples were subjected to X-ray and spectrographic analysis for metals content. The metals analysis revealed primarily iron, chromium and nickel on both samples analyzed.

, Figure 3-4 is a microphotograph of the inside of the head adapter plug from the Diablo Canyon Unit 1 J-5 sample. A sample of the deposit was removed and

- subjected to emission spectrography for metals content. The metals analysis revealed primarily iron, chromium, nickel, aluminum and zirconium. These are most likely corrosion products of the system, i

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TABLE 3-6 CHEMISTRY EVALUATION RESULTS OF BLACK DEPOSIT REMOVED FROM HEAD ' ADAPTER THREADS -

ANIONS, micrograms Fluoride Chloride Nitrate ' Phosphate Sulfate

  • Diablo Canyon J-5 <1 232 23 <5 41
    • Prairie Island E <15 244 36 <5 458
  • Note: Total sample size was 0.0259 grams.
    • Note: Total sample size was 0.1123 grams.

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, Figure 3-9 Schematic Illustration of Section Locations on the Canopy Seal Weld from Salem Unit 2.

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50X Figure 3-17 Light Optical Metallography Results Illustrating the Weld Profile and Base Metal Cracking of Section F from Prairie Island E-9 Canopy Seal Weld.

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Figure 3-31 Scanning Electron Fractographs of the Endoxed Fracture Face of Base Metal Cracks on Prairie Island E-9 Canopy Seal Weld. ,

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SECTION 4.0  ;

DISCUSSION

.. j The plants included in this evaluation are as follows: Diablo Canyon Unit 1, l Prairie Island Unit 2, Salem Unit 2, Zion Unit 1 and Turkey Point Unit 3. l Some of the results discussed are from work performed by persons other than the author of this document. The canopy seal welds from Zion Unit I were evaluated at Argonne National Laboratory and the detailed analysis is the subject of an independent report to be published at a later date, i Westinghouse performed the failure analysis of the canopy seal welds from f Turkey Point Unit 3 in 1987 and the results are documented in WCAP-11590. As l the samples' from Turkey Point Unit 3 were available, some additional metallography was performed during this evaluation.

Visual examination of the as received condition of the canopy seal welds revealed poor penetration, incompletely fused insert, and oxidation on the i

,. inside diameter of the welds. These defects, in varying severity, existed on all canopy seal welds examined with the exception of G-7 and G-9 from Turkey

. Point Unit 3. The canopy seal welds from Turkey Point Unit 3 contained good fusion in the areas other than the leak locations. The visual examination of outside diameter of the canopy seal welds revealed no indication of weld i defects except in the leak and weld repair locations. The typical weld conditions observed are illustrated in figures 3-11 through 3-13.

l The metallography performed on sections away from the leak areas revealed various weld profiles and cracking. Figures 3-14 through 3-22 are micrographs illustrating structures observed. The cracks observed initiated at weld fusion areas (see figures 3-15, 3-16, 3-18 and 3-19), lack of penetration areas (see figures 3-20, 3-21 and 3-22), weld inside diameter areas (see figures 3-18 and 3-19) cnd at base metal inside diameter areas (see figure

3-17). The cracking observed is transgranular and propogates from the inside l diameter. It is also imprtant to note that many weld defect locations, such as lack of penetration, contained no cracking (see figure 3-14). These weld defects form crevises which can act as initiation sites for cracks. The ,

cracking observed during this evaluation occurred independent of the weld defects.

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There is no data that indicates that the weld defects caused the cracking, however, the number of weld defects observed warrants explanation. The weld

.. profiles observed may have resulted from insufficient heat during welding and/or sulphur mismatch in the base metals. Insufficient heat can be caused by the current being set too low or the travel speed being set too high.

l Exper'...e m have shown that irregular weld penetration may occur when welding low sulphur stainless steel to higher sulphur stainless steel. The weld shifts markedly towards the low sulphur side. Oxidation of the backside of these welds is the result of inadequate back purge. This condition can be caused by insufficient purge time' prior to initiation of welding operation, contaminated purge gas or a defective purge hose which may have introduced air into the inside diameter cavity.

The metallography results from leak' locations are illustrated in figures 3-23 thrnugh 3-25. The through-wall cracks initiated at base metal inside riiameter locations (see figures 3-23 and 3-25) and weld defect locations (see figure 3-24). Figure 3-23 is a micrograph of a through-wall crack that initiated and propogated through the base metal. Figure 3-24 is a micrograph of a

, through-wall crack that initiated in the crevice formed by poorly penetrated weld and propogated through the weld. Figure 3-25 is a micrograph of a through-wall crack the initiated at a base metal inside diameter location and propogated thrcqh the base and weld cetal. Each of the cracks are transgranular witn branching.

The oxalic acid etch of two sections from ecch plant revealed a nonsensitized stainless ' steel structure. Figure 3-26 illust ates the annealed austenitic stainless steel structure observed.

Examinations of defect locations on the scanning electron microscope (SEM) revealed corrosion produ:t in the center of the defects. Figures 3-27 and 3-28 illustrate scanning electron micrographs of two defect locations. Energy dispersive X-ray spectrographic (EDS) enalysis was performed on the corrosion I M. J. Tinkler, I. Grant, G. Mizuno and C. Gluck, " Welding 304L Stainless

l. Steel Tubing Having Variable Penetration Characteristics," International

- Conference on the Effects of Residual, Impurity and Micro-Alloying Elements on Weldability and Weld Properties, London, England, November 15-17, 1983.

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product evident in the defect locations as well as the corrosion products on the. weld inside diameter surface of a number of sections. The EDS results.

indicated that the corrosion product, in all cases, is predominantly comprised

'ofiron(seefigure3-29(c)).

Fractography was performed on some leak, weld repair.and base metal cracking locations. The freshly opened defects were examined on the SEM in the as-opened and endexed condition. Figure 3-30 shows scanning electron fractographs of a through-wall leak located entirely in the weld. The initiation at a lack of penetration and the fracture morphology at the through-wall leak area are illustrated. Figure 3-31 shows scanning electron fractographs of a base metal crack that was not through-wall. The cleavage-like cracking observed is typical of transgranular stress corrosion crack'ing (SCC) in austenitic stainless' steel. EDS analysis of the oxide on the fracture faces revealed results similar to those observed during the outside and inside diameter f.' aminations, i.e., the corrosion product is

,. predominantly comprised of iron.
  • The material of construction specified for the head adapter and head adapter plug is an austenitic type 304 stainless steel. The weld metal is a specified AWS type 308L. Given the experimental error of the techniques employed

(+ 5%), the results listed in tables 3-2 through 3-4 indicate that each component is within the specified chamical composition.

Table 4-1 is a list of canopy seal welds from specific plants and the type of degradation observed., Through-wall leaks were not confirmed in every sample, but every canopy seal weld examined contained some degree of cracking. The type of cracking observed during the investigation is indicative of trans-granular SCC. Transgranular SCC susceptibility in austenitic stainless steels is influenced by the environment, stress and microstructure. The most commonly reported cause of transgranular SCC in annealed austenitic stainless steels is a chloride environment at elevated temperatures (>140 degrees F).

1 Experiments1 have shown that typo 304 stainless steels exposed to low 1

. P. A. Andresen and D. J. Duquett, "The Effects of Dissolved Oxygen, Chlor-ide Ion and Applied Potential on the SCC Behavior of Type 304 Stainless Steel in 290 C Water," NACE, Volume 36, No. 8, August 1980, pp. 409-415.

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concentrations of chloride ions (100 ppm) in aerated or deaerated water will experience transgranular. SCC. However, the water chemistry results (see table 3-5) indicate much lower concentrations of chloride ions present in the water

, samples. The relationship between chloride concentration and cracking a susceptibility is not simple. Numerous studies have been performed on the SCC propensities of austenitic stainless steels. In high temperature environments, it is linked with oxygen concentration at illustrated in figure 4-1.1 In reviewing figure 4-1 it is evident that transgranular SCC can initiate on annealed austenitic stainless steel at low concentrations of chloride ions given a sufficient level of oxygen.

Westinghouse details operating PWR primary coolant chemistry in Information Document 5-1, " Chemistry Criteria and Specification Manual Revision 4". The specification for chloride and oxygen concentration is 0.15 ppm and less than 0.005 ppm respectively. Water is introduced into the canopy seal weld inside diameter cavity during the required hydrostatic test performed following

, welding. This stagnant water cannot be drained. The water chemistry data obtained during this investigation indicates that the water drained from the cavity is slightly contaminated. The data indicates that the trapped water may not have been subjected to the controls implemented during PWR operation.

This is likely as the water is in this ' dead end' cavity behind the canopy seal weld. During start up it is likely that air can enter the cavity and the trapped water behind the canopy seal weld. Once in solution the air will increase the oxygen content of the water. Without tha influence of PWR chemistry controls, this increased level of oxygen will remain in the trapped water. The low levels of chloride contamination coupled with suspected increased levels of oxygen would promote transgranular SCC on the annealed 304 stainless steel canopy seal weld. Although the actual source of the contamination has not been identified, the leaks can be fixed and prevented which is discussed in detail in section 6.0.

1

, B. M. Gordon, "The Effects of Chloride and Oxygen on the Stress Corrosion Cracking of Stainless Steels: Review of Literature," NACE, Volume 36, No. 8, April 1980, p. 32.

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., -SECTION

5.0 CONCLUSION

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=: Based on the results of the evaluations performed it is concluded that the leaking canopy seals resulted from transgranular stress corrosion cracking.

The data indicates that the cause of the cracking observed on the annealed 304 stainless steel cancpy seal is a combination of small levels of chloride and oxygen contamination that are present in the " dead end cavity" that is formed by the canopy seal. The source of the contamination was not identified.

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SECTION 6.0 RECOMMENDATIONS Stress corrosion cracking is the non-ductile fracture of a material under

'g static load caused by the combined action of environment'and tensile stresses (residual or applied). Essentially the three' factors which contribute to stress corrosion cracking are a susceptible material, an environment and stress. To mitigate stress corrosion cracking one of the three factors must be eliminated.

Mitigation of the leaking canopy seals in operating plants can be accomplished by either the elimination of the environment or using an alternate material that is not susceptible to chloride induced stress corrosion cracking. A hole introduced through the head adapter pipe will eliminate the " dead end cavity" and assure the 304 stainless steel canopy is exposed to a PWR primary coolant thus eliminating the environment. Materials which could be used to replace the 304 stainless steel are Inconel alloys 625 or 690.

m n a. m m eio 6-1

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