水化学部会第18回全体会議

Contribution of Cathodic Reaction inside Crevice
to development of Crevice Corrosion in 304L SS
2020年3月6日
舘和希*, 渡邉豊*, 深谷祐一*** 東北大学大学院工学研究科** 東京電力ホールディングス株式会社

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1. Seawater contamination in Fukushima SFP
Unit 2-4: Seawater was injected into SFP just after the accident for emergency
fuel cooling.
Temperature & [Cl-] raised up to 92℃ and 1944ppm.
It was crevice corrosion possible environment1 for 304 SS.
Ø Localized corrosion, especially crevice corrosion may have occured.
Environment was purified & deoxygenated.
Low possibility of another localized corrosion.
Whether localized corrosion once initiated andpropagated in high temp. & [Cl-] repassivate
when environment is improved is not confirmed.
Study on crevice corrosion propagation
continuity of when the environment is improved:
cathodic reaction on the surface outside no
more contribute to propagation of the corrosion.

[1: M. Akashi, et al., CORROSION ’98, 158, (1998)]

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2. Crevice corrosion initiation and propagation[2: M. G. Fontana, et al., Corrosion Engineering (New York, NY: McGraw-Hill, 1967), p. 28]• Oxygen reduced in/outside. → Oxygen diffusion into occluded crevice is blocked.
→ Oxygen deplete inside. → Cathodic reaction occur only outside crevice.
• Hydrolisys of dissolved metal cation → Acidification• Condence of cation inside crevice → Cl- condensation}

Aggressive solution
inside crevice

Metal surface depassivation,Crevice corrosion initiation

Ø Anodic reaction inside crevice is supported
by cathodic reaction outside crevice.3-8[3: O. B. Ellis, et al., CORROSION 7,(1951): p. 362]
[4: Z. Y. Chen, et al., J. Electrochem.
Soc. 155, 7 (2008): p. C360][5: N. Sridhar, et al., CORROSION 50,11 (1994): p. 857]
[6: F. Bocher, et al., CORROSION 71, 9(2015): p. 1051][7: J. R. Galvele, J. Electrochem. Soc.
123, 4 (1976): p. 464]
[8: S. Tsujikawa, et al., Corrosion
Chemistry within Pits, Crevices andCracks, paper 10 (London, Oct. 1-3,1984), p. 171]

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3. Cathodic reaction inside crevice[9: Q. Hu, et al., Corros. Sci. 53, 12 (2011): p. 4065]

u Generally, anodic reaction mainly occur inside crevice and cathodic reaction couldbe ignored.
u Gas generation inside crevice have been observed
during crevice corrosion propagation underopen circuit immersion as below.
2H++2e-→H2↑

13Cr SS, NaCl solution

Ø Contribution of cathodic reaction inside crevice topropagation of the corrosion could not be ignored actually.

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4. Objective of this study & Way of research• Clarify contribution of cathodic reaction inside and outside crevice
on propagation of crevice corrosion.
With outer cathode

Without outer cathode(a) Anodic dissolution(b) Oxygen reduction @outside(c) Cl- migration(d) H+ generation by hydrolysis(e) Hydrogen gas generation(a) Anodic dissolution(b) Oxygen reduction @mouth(c) Cl- migration(d) H+ generation by hydrolysis(e) Hydrogen gas generation

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5. Experimental setup
wt%Specimen (P500 polished):• Full crevice specimen: 304L (Φ15-6.0=1.48mm3) - acrylic (Φ20, P2400)• Outer cathode: 316L SS (H65×W50×T3.0=71.9mm3)
Solution・Temperature・Injected gas:
• NaCl (Cl-=10000ppm), 50℃, Artificial air (N2: 80%, O2: 20%)

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6. Passive potential: with/without outer cathode• To determine whether the corrosion repassivated or keep propagating,measured the potential of passive state of full crevice specimen with/withoutouter cathode and coupling current.

passive potential

Surface after immersion
With outer cathode: above 0VSCE, under 0.1μA
Without outer cathode: about -60mVSCE

When the corrosion propagation
terminated and repassivated.

87. Experimental procedure
Step ①
Electrochemical operation(polarization)

Step ②
Open circuit immersion
Outer cathode connected

potentio/
galvanostat

potentio/
galvanostat

Initiate and propagate
crevice corrosion onfull crevice specimen.

zero shunt
ammeter

Propagate the corrosion.
Measure potential andcoupling current.

Step ③
Open circuit immersion
Outer cathode disconnected
potentio/
galvanostat

Measure potential.
Consider propagating orrepassivated.

98. Propagation with outer cathode
Step ① to Step ② under artificial air aerated.
passive potential

u Corr. vol. converted from coupling current
was 93.1% of actual corrosion volume.
Ø Cathodic reaction outside crevice was dominant of cathodic reaction which
support anodic dissolution inside crevice. Cathodic reaction inside was limited.
Ø The potential may be too high for H2 generation to occur inside crevice.

109. Propagation without outer cathode
Step ① to Step ③ under artificial air aerated.

passive potential

redox potential of H+at pH=0

u The potential was lower than passive state.
Ø Crevice corrosion continued propagating by only cathodic reaction inside crevice.
u The potential was almost equal or higher than redox potential of H2 generation.
Ø Not only H2 generation but also O2 reduction is required.
Ø It may be difficult to continue to propagate when DO outside crevice is low enough.

1110. Summary• Clarify contribution of cathodic reaction inside and outside crevice
on propagation of crevice corrosion.
• Outer cathode contributes mainly when it is available.
• Crevice corrosion is able to propagate with only cathodic reaction inside crevice,
but the corrosion rate will be more than one magnitude smaller.
• Not only hydrogen but also oxygen reduction may play an important role.
With outer cathode

Without outer cathodeØ It may be difficult to continue to propagate when DO outside crevice is low enough.