AbstractStainless steels are widely used in the nuclear industry for their well-known behaviour,
favourable mechanical properties, and resistance to corrosion. However, under certain
conditions stainless steels are susceptible to localised corrosion. Forms of localised corrosion,
such as stress corrosion cracking (SCC), are particularly detrimental as they can occur without
any obvious visual cues. This can lead to sudden and unexpected failure. The development of
accurate prediction methods and technologies for SCC is important for the safe and economical
running of nuclear power plants, as well as in various other applications.
Atomic force microscopes (AFMs) are capable of high-resolution mapping of structures and
the measurement of mechanical properties at nanometre scales within gaseous and liquid
environments. The contact mode high-speed AFM (HS-AFM) invented at the University of Bristol
operates at speeds orders of magnitude faster than conventional AFMs and is capable of capturing
multiple frames per second. This allows for direct observation of dynamic events in real-time,
with nanometre lateral resolution and subatomic height resolution. The enhanced capabilities of
HS-AFM make it a viable tool for the in-situ imaging of nanoscale corrosion initiation events,
such as metastable pitting, grain boundary dissolution and short crack formation during SCC.
Observations of such events could give valuable insight into the processes that take place and
the mechanisms behind them.
SCC occurs due to the synergistic interactions of three factors: a susceptible material, a
corrosive environment, and sufficient stress. Within this project, SCC and the factors leading
to SCC, were analysed using HS-AFM in combination with electron and ion beam microscopy
Working in collaboration with the National Nuclear Laboratory, sensitised microstructure in
nuclear relevant stainless steels was analysed by correlative microscopy. HS-AFM measurements
of an irradiated sample of ex-service stainless steel revealed nanometre scale radiation-induced
voids identified as plastic voids, helium bubbles, or cavities. present across the sample, previously
only observed by transmission electron microscopy. The high throughput of the HS-AFM allowed
for statistical analysis across large areas of the sample. This work extended to comparisons
between the microstructure and grain boundary chemical compositions of thermally sensitised
and irradiated austenitic stainless steels. These comparisons are important when considering
thermally treated stainless steels as an analogue to irradiated samples. Further experimental
work revealed how such a surface responds to applied stress and corrosive environments.
HS-AFM was used to observe localised dissolution events and pitting corrosion in-situ, both
of which can lead to SCC. These measurements were performed by imaging within a custom
liquid cell with parallel potentiostatic control. The high resolution of the HS-AFM allowed
for observations to be performed at individual reaction sites and accurate measurements of
the dimensions of pits formed. Using these measurements, it was possible to calculate, and
subsequently model, the volumes of metal reacting with respect to time, and so the current
densities and ionic fluxes at work. In this manner, the local electrochemistry at nanoscale
reaction sites may be reconstructed.
Lastly, factors were brought together to study SCC using both in-situ and ex-situ techniques.
During in-situ SCC measurements by HS-AFM, uplift of grain boundaries before cracking was
observed, indicating a subsurface contribution to the cracking mechanism. Focussed ion beam
milling revealed a network of intergranular cracks below the surface lined with a thin oxide,
indicating that the SCC process is dominated by local stress at oxide-weakened boundaries.
Analysis by atom probe tomography of a crack tip showed a layered oxide composition at the
surface of the crack walls. The formation of this oxide is posited to be mechanistically linked to
grain boundary uplift. This study shows how in-situ HS-AFM observations in combination with
complementary techniques can give important insights into the mechanisms of SCC.
|Date of Award||24 Jun 2021|
|Sponsors||National Nuclear Laboratory, Gloucester|
|Supervisor||Tomas L Martin (Supervisor) & Oliver D Payton (Supervisor)|