stress corrosion cracking susceptibility of...

Joint EC-IAEA Topical Meeting on “Development of New Structural Materials for Advanced Fission and Fusion Reactor Systems”5 – 9 October 2009, Fusion for Energy

Stress Corrosion Cracking Susceptibility of Austenitic Stainless Steels in Supercritical Water Conditions

R. Novotny1), P. Hähner1), J. Siegl2), S. Ripplinger1), Sami Penttilä3), Aki Toivonen3)

1) JRC-IE, Petten, Westerduinveg, 1755 LE Petten, the Netherlands2) Czech Technical University in Prague, Zikova 4, 166 36 Prague 6, Czech Republic1) Materials and Building, Technical Research Centre of Finland, Espoo, Finland

http://ie.jrc.ec.europa.eu/http://www.jrc.ec.europa.eu/


FP6 project: HPLWR Phase 2

HPLWR 2: High Performance Light Water Reactor Phase 2

Start: Sept. 1st, 2006

Duration: 42 months

Partners: 12

WP4 on SCWR Materials and water chemistry

European Contribution to GIF



Light Water Reactor with supercritical coolant (25MPa) and more than 500°C core exit temperature

Advantages:

Direct steam cycle like BWR

No main coolant pump in PL

No recirculation pumps

No steam separators in RPV

40% higher turbine power

44% net plant efficiency

Major cost reductions envisaged


HPLWR Plant target data


HPLWR-2 Objectives

Working on critical scientific issues to assess the feasibility of a HPLWR concept to determine its future potential in the electricity market.

Critical Scientific Issues:

• Elaborate the nuclear island and balance of plant

• Design and analysis of a core and reactor internals

• Assess the safety of the HPLWR concept

• Find a selection of materials for in-vessel components

Model the relevant heat transfer phenomena


HPLWR-2 Objectives


WP4-Materials

Objective:

Investigate materials behavior in supercritical water and to select optimal in-core and out-of-core materials with respect to:

• Stress Corrosion Cracking (SCC) resistance

• Oxidation resistance

• Creep resistance

• Irradiation resistance

Tasks:

Autoclave experiments:

• Oxidation mechanisms of ferritic/martensitic and austenitic steels, Ni-based alloys

• Combined mechanism of creep and oxidation

• Stress corrosion cracking tests

Materials Data Base and Models for uniform corrosion, stress corrosion cracking, etc.

Construction of Supercritical Water Loop for in-pile materials testing


WP4-Materials Corrosion Test Facilities

• VTT autoclaves 2 x (695°C / 35 MPa) with option for mechanical testing

• JRC-IE autoclaves 2 x (650°C / 35 MPa) with different loading systems

• On-line corrosion monitoring (electrochemical potential, el.chem. noise,

contact electrical impedance, acoustic emission)

• Reference electrode Ag/AgCl development (VTT)

• In-pile SCWL development at Rez (NRI)

• Parallel corrosion tests at CEA using tubular specimens and furnace for

SCC testing >> reporting to GIF


WP4-Corrosion tests


Summary of oxidation tests (VTT, JRC)


Summary of oxidation tests (VTT, JRC)

Oxide thicknesses on the studied alloys after 1 year exposure to

SCW (125 ppb O2

) at 400, 500 and 650°C -

linear extrapolation from 600 or 300 h results

* = no measurements at lower temperatures- = too thin to measure

Alloy 400°C (mm/year) 500°C (mm/year) 600°C (mm/year) 650°C (mm/year)P92 0.058 0.215 1.745P91 0.058 0.236 1.365

ODS(1) 0.044 0.241 0.377ODS(2) 0.051 0.180 0.219PM2000 - - - 0.022316NG 0.009 0.029 0.7 1.4091.4970 * * 0.3 0.840800H - - 0.03 0.022BGA4 - - 0.02 0.015625 - - 0.01 0.015

-F/M steels: high oxide growth rate, ~1.5 mm/year -1.4970, AISI 316, AISI 347: >0.2 mm/year oxide growth rate

-9%Cr ODS: 0.2-0.3 mm/year oxide growth Low corrosion resistance for these components

Problems with assembly box, moderator box, fuel cladding: thickness 0.2-0.5 mm,T = 600-650oC


SCC Susceptibility –

SSRT VTT



SSRT VTT

All strength values have decreased considerably as the test temperature has been increased from 500°C to 650oC (strain rate was the same, 3x10-7 s-1, in both cases). Remarkable decrease has taken place in the yield stress of PM2000, on which the yield stress decreased to ~1/3 of the value at 500oC



SSRT VTT

1.4970

BGA4



SSRT JRC

Autoclavebody

Autoclavelid

Case

Heater

Insulation

Insulation

AE

316 SS

Pt316 SS

Ceramic holdersPt + S1 + S2

Preheater/CoolerInlet/Outlet

Nuts/Screws

Pull rod

Slow Strain Rate Test (SSRT)

in SCW Autoclave

Material: 316L austenitic stainless steel


Results –

SSRT JRC –

Strain Rate

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16Strain (%)

Stre

ss (M

Pa)

316-2316-308-0308-04


Results –

SSRT JRC –

Strain Rate


Results – SSRT JRC – Oxygen Content

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14Strain (%)

Stre

ss (M

Pa)

316-36-01


Results –

SSRT JRC –

Oxygen Content

no significant influence on fracture micromorphology in areas corresponding to the stress corrosion cracking

c) 6-01

The main features -

ductile dimples. An occurrence of intergranular facets was found sporadically in the central part of fracture surface.


Results –

SSRT JRC –

Temp. Difference


Results –

SSRT –

Temp. Difference

Failure of the specimen 316 – 2 was initiated by stress corrosion cracks propagated from specimen surfaces.

specimen 8 - 01 stress corrosion cracks were found neither on the fracture surface nor on the surface of specimen.Fracture morphology corresponds to the static rupture.


Conclusions Corrosion and SSRT’s

Corrosion:

For the thin-walled components in the design of an SCWR, corrosion, stress corrosion cracking and creep resistance are anticipated to be important degradation modes that need to be understood and controlled.

The oxidation rates have to be lower than what is acceptable for materials in supercritical fossil power plants because of smaller wall thicknesses in the SCWR core designs.

• the oxidation rate of F/M steels is too high for SCWR core components even at the temperatures below 500oC.• austenitic stainless steels have a good enough oxidation resistance up to 500 -

550oC• 20% Cr ODS steel was selected for the fuel cladding because of its excellent oxidation resistance even up to 650oC, its SCC resistance and its good creep specifications

Stress Corrosion Cracking:

No clear SCC was observed on the fracture surfaces, but on side surfaces there were small cracks of which morphology, however, could not be identified except in the case of 316NG (which had both inter-

and transgranular cracks, IGSCC and TGSCC). On the other hand, the experimental creep resistant steel BGA4 specimen contained IGSCC

both on the fracture surface and side surfaces. At 500oC, PM2000 did not show any susceptibility to SCC at all.


Conclusions Corrosion and SSRT’s

Fractographic

findings confirmed that failure processes are combination of transgranular stress corrosion cracking and transgranular ductile fracture.

The proportion of SSC and ductile fracture on the failure process of individual specimens is predetermined by the parameters of slow strain rate test (i.e., oxygen content in test water solution, strain rate and test temperature).

Influence of individual parameters of SSR tests on stress corrosion cracking was estimated.

The SCC occurrence is favored by high oxygen content and slow strain rate. With decreasing test temperature, higher oxygen content and/or slower strain rate should be used to induce SCC. The repeatability of SCC occurrence for given SSR test parameters should be verified by subsequent experiments.


FM CGR Tests –

Bellows based loading system

Application for Crack Growth Rate SCC tests


FM CGR Tests –

Bellows based loading system

Bellows based Loading System –

Pressure Adjusting Loop


FM CGR Tests –

Materials and Environment

Ti-stabilized austenitic stainless steels was tested:

08Cr18Ni10Ti

SEN(B) specimens

pre-cracked in air, a/W = 0.5

T-L Orientation

Simulated BWR, SCWR water

Material C Si Mn S P Cr Ni Ti Mo

08Cr18Ni10Ti 0.085 0.45 1.07 0.015 0.011 18.0 10.0 0.64 ≤0.1

Temperature [oC] 288; 550 Pressure [bar] 88; 230

Inlet Conductivity [S.cm-1] 0.09 Outlet Conductivity [S.cm-1] 0.15 - 0.2

Inlet Dissolved O2 [ppb] 180 – 220; 2000 Outlet Dissolved O2 [ppb] 160 - 210


FM CGR Tests –

Materials and Environment

31.8.2009 –

four tests carried out:

1.specimen AC125

(t = 550degC, p = 230 bar, Diss. Oxygen = 2000 ppb, ultra-pure water)

260 280 300 320 340 360 380 400250

300

350

400

Load

(N)

Time (h)

250 300 350 400-0.00005

0.00000

0.00005

0.00010

0.00015

0.00020

DC

PD (V

)

Time (h)

stress corrosion cracking susceptibility of...

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