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143

ENSI Erfahrungs- und Forschungsbericht 2011

3.3.2. Evaluation of EN during SCC

Initiation by a Novel

Micro-Electrochemcial Approach

The main goal of this PhD thesis work is to achieve

a basic understanding of the EN of intergranular

(IG) SCC initiation in stainless steels on a micro-

structural level and to identify noise pattern on a

macroscopic level that can be correlated with the

onset of SCC. An unique, combined micro- and

macro-electrochemical experimental approach

was selected to identify and isolate the relevant

microscopic sources that contribute to the macro-

scopic EN signal during the SCC process. On one

side, the macroscopic noise signals during SCC

initiation and general corrosion on a macroscop-

ic heterogeneous surface are investigated at PSI.

On the other side single noise sources, which gen-

erate macroscopic EN are identified and electro-

chemically characterized by the use of micro-elec-

trochemical techniques at EMPA.

After a big experimental effort, an environment

system was finally found, which is sufficiently ag-

gressive to initiate IG SCC in micro-capillary tests

and mild enough to avoid pitting, intergranular

and crevice corrosion in the macroscopic experi-

ments. As a highlight, IG SCC on an individual sen-

sitized grain boundary was the first time detect-

ed by electrochemical micro-capillary tests world-

wide (Figure 6a) [11]. The observed current and

potential transients, which are related to individu-

al passive film rupture, anodic dissolution and re-

passivation events, confirm the intermittent crack

growth process by the slip dissolution mechanism

(Figure 6b). Based on these investigations, the

macroscopic EN of IG SCC can be explained in a

semi-quantitative manner.

P-6

Research Project KOR

-II

Figure 5:

Scratch electrode

facility for repassi-

vation investigations

in high-temperature

water (a) and exam-

ple of a current tran-

sient after oxide film

rupture in high-purity

water from pre-tests

(b).

Figure 6:

Current transients

during IG SCC on

an individual grain

boundary measured

by the micro-capillary

technique (a) and

schematic explana-

tion of the observed

shape by slip dissolu-

tion mechanism (b).

Research Project KORA-II

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