atw 2018-02

inforum

atw Vol. 63 (2018) | Issue 2 ı February

a) Stress corrosion cracking in the heat

affected zone of a welding seam:

TSS Experiment at Asse salt mine

temperature: 180 °C

Duration: about 11 years.

| | Fig. 6.

Localized corrosion phenomena of steel 1.4306: Stress corrosion cracking along grain boundaries.

high depth variations. The immersion

tests were terminated after about 500

days, therefore an increase in the pit’s

depths as determined in the case of

Hastelloy was not observed. The

average pit depth of both steel was

found in the range of 30 to 40 µm.

The steels 1.4306 and 1.4833

showed significant stress corrosion

cracking at 150 °C (tests at 90 °C were

not performed). Figure 6 shows polished

micrographs of steel 1.4306

specimen in contact with dry rock salt

(a) and immersed in NaCl solution.

Localized corrosion was found in

both cases, even in the almost dry

con ditions established in the TSS

experiment performed in the Asse salt

mine [22]. The penetration depths

of the cracks were measured in

the mm range. Contact samples in

MgCl 2 solution showed even more

pronounced stress corrosion cracking

[2].

With Hastelloy C4 corrosion tests

under γ-irradiation of 10 Gy/h were

performed (fuel element storage

pool at Dido test reactor at the

Research Center Juelich). Different

types of samples were examined:

plane samples as delivered, plane

samples with removed oxide layer

on the surface, U-shaped welded

samples, crevice samples, and samples

| | Fig. 7.

Intergranular corrosion in a Ni-Resist D4

sample after 776 days in MgCl 2 solution

at 90 °C.

b) Stress corrosion cracking of a plane

specimen of steel 1.4306 after 422 days

in NaCl solution at 150°C.

with different welding procedures

such as tungsten inert gas welding

(TIG) or electron beam welding (EB).

Significant deviation of the observed

mass losses in comparison to test without

irradiation were not found.

Almost all Ni-Resist steel samples

showed intergranular corrosion effects

(Figure 7). These referred to samples

as delivered and to crevice samples.

Summary and conclusions

The results of the corrosion experiments

with Cr-Ni steels, Hastelloy and

the Ni-Resist materials revealed a

significantly lower general corrosion

rate (mass loss rate) in comparison to

the fine-grained steels. On the other

hand, these materials were subdued

to localized corrosion processes such

as pitting corrosion, crevice corrosion,

intergranular corrosion and stress

corrosion cracking. The local corrosion

processes were enhanced in

welded or in contact specimen. In

many cases, the localized corrosion

phenomena were found only after

certain incubation periods. Especially

in the case of Hastelloy, the incubation

period was about 9 months at 90 °C in

MgCl 2 solution and the pitting corrosion

rate was relatively high. Stress

corrosion cracking by intergranular

corrosion of the Cr-Ni steels penetrated

deep into the materials. Intergranular

corrosion was also found in

the Ni-Resist steels.

As a consequence of the occurrence

of localized corrosion processes

as well as the unpredictable incubation

times of these processes, one

might understand the decision to

apply uniformly corroding steels for

waste canisters, even if the general

corrosion rate would be by a factor up

to 1,000 higher.

The mass loss is proportional to the

hydrogen produced under reducing

conditions in a deep disposal. A

POLLUX cask has a surface area of

about 30 m 2 . Under extreme conditions,

15 kg of steel could be corroded

per year in NaCl solution, forming

360 mol H 2 per year (8 m 3 standard

conditions). Hydrogen keeps a reducing

environment, however, by

increasing pressure it acts as driving

force for gas, solution and contaminant

transport. Internationally efforts

are undertaken to reduce the potential

amount of hydrogen produced by

corrosion phenomena.

Based on the measurements

reported in this contribution, Cr-Ni

steels seem not to provide a reasonable

solution for a long-lived stable

waste package. Even, if the hydrogen

production is reduced, the long-term

sealing function of these steels is

unclear. Under the almost dry condition

of the in-situ experiment (TSS)

in the Asse mine, stress corrosion

cracking in the heat affected zone of

a welding seam of Cr-Ni steel was

observed after 11 years at 180 °C.

Acknowledgment

The corrosion studies of canister

materials for heat producing wastes

cover exclusively the research performed

by Dr. Emmanuel Smailos and

his working group. Until his retirement

in 2004, Dr. Smailos was responsible

for the corrosion studies of

various materials at the Institute for

Nuclear Waste Disposal (INE).

References

[1] H. Lahr, H.-O. Willax, and H. Spilker,

Conditioning of spent fuel for interim

and final storagein the pilote conditioning

plant (PKA) at Gorleben, in

International Symposium on Storage of

Spent Fuel from Power Reactors,

Vienna, Austria, 9-13 November 1998,

1998.

[2] E. Smailos and B. Fiehn, Korrosionsuntersuchungen

an der Werkstoffkombination

des POLLUX-Behaelters

zur direkten Endlagerung abgebrannter

Brennelemente in Steinsalz formationen,

Forschungszentrum Karlsruhe, KfK-

4552, 1989.

[3] Kommission Lagerung hoch radioaktiver

Abfallstoffe, ABSCHLUSSBERICHT:

Verantwortung für die Zukunft: Ein faires

und transparentes Verfahren für die

Auswahl eines nationalen Endlagerstandortes,

Geschäftsstelle der

Kommission Lagerung hoch radioaktiver

Abfallstoffe, K-Drs 268, 2016.

[4] Gesetz zur Fortentwicklung des

Gesetzes zur Suche und Auswahl eines

Standortes für ein Endlager für Wärme

entwickelnde radioaktive Abfälle und

anderer Gesetze, 2017.

[5] Uhligs corrosion handbook, 3 rd ed

( Online-Ausg.) ed. Hoboken, N.J: Wiley,

2011.

DECOMMISSIONING AND WASTE MANAGEMENT 109

Decommissioning and Waste Management

Corrosion Processes of Alloyed Steels in Salt Solutions ı Bernhard Kienzler

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