Technical Presentations at the October 2011 Meeting
many engineering applications, pitting is the precursor to stress
corrosion cracking (SCC) as it provides the required combination of an
aggressive local solution chemistry and a stress concentrating feature
(the conventional perspective). The fundamental steps in the overall
process of crack development involve pit initiation, pit growth, the
transition from a pit to a crack, short crack growth and long crack
growth. A reliable prediction of the complete damage process in
engineering components would require a quantitative understanding of each
stage. A number of
deterministic (mechanistic-based) models has been developed to address SCC
initiation and propagation form pits. In these predictive schemes, the
pit-to-crack transition is based on the phenomenological requirements
proposed by Kondo for corrosion fatigue: that the pit depth must be
greater than a threshold depth and that the crack growth rate should
exceed the pit growth rate. Implicit also is the assumption that the pit
is actively growing. In simple terms, a stable crack will emerge from the
pit if the threshold stress requirement for crack development is exceeded
(the pit acting as a concentrator of stress and strain) and if locally the
crack growth is sufficient for the crack to outrun the dissolving pit
surface at that location. At the point of transition from a pit to a
crack, the crack is considered effectively to have the same depth as the
validity of this latter proposition has been examined in detail for a
steam turbine disc steel exposed to sustained upset conditions of aerated
1.5 ppm chloride (under normal operating conditions the steam turbine is
aerated only off-load and transiently as the system comes on-load; on-load
the condensate is oxygen free and the chloride concentration is closer to
300 ppb). Using the Skyscan X-ray tomography instrument at U. Birmingham,
remarkable 3‑D microtomographic images have been obtained that
demonstrate that cracks actually develop predominantly at the shoulder of
the pit, at or below the pit-surface interface, for specimens stressed to
50%, 70% and 90% of s0.2.
Thus the assumption of a crack depth equivalent to the pit depth at
the point of transition, implying the crack initiates at the pit base, has
no foundation (at least for this system).
explanation for initiation of SCC near the pit mouth based on
electrochemical differences between and pit mouth and base was not tenable
and a mechanics-based explanation was sought.
To explore this, finite element (FE) analysis was undertaken of
model pits with hemispherical and “bullet-shaped” geometries in a
cylindrical rod subjected to longitudinal applied stresses ranging from
At the high stresses relevant to service, the principal strain was
predicted to be a maximum in the material just below the pit mouth and
included a significant plastic component (reduced constraint to plastic
deformation near the surface). The principal stress was lowest in this
region. The localisation of plastic strain to just below the mouth would
in itself be argued as a basis for the location of stress corrosion crack
initiation. However, in another novel feature of this work, it was
recognised that if the pit is growing in a static stress field that
generates plastic strain then that plastic strain is dynamic.
Since slow strain rate testing is recognised as being the most
sever test condition for evaluation of stress corrosion cracking, here we
have a situation in which the material local to the pit is undergoing slow
plastic strain due to the pit growth. FE simulation of a growing pit in a
static stress field indicated corresponding plastic strain rates that were
commensurate with values associated with stress corrosion cracking. This
observation introduces a wholly new concept in understanding the evolution
of stress corrosion cracks from pits.
it may be suggested that the role of growing pits in initiating SCC is not
simply to produce a local aggressive environment and a localisation stress
and strain but most critically to induced dynamic straining of adjacent
L. Wright, L. Crocker and S.Zhou: NPL; A.D. Horner, B. Connolly: U.
presentation gave an outline of thermal metal spray as a long established
method of corrosion control, including common types and methods of
application. The morphology
of the thermal sprayed film was discussed; and due to the porous nature of
the film, it is common practice to apply a coat of paint in order to seal
sealer coat is not decorative and does not have an overlay on the surface
of the metal spray, therefore the concept of ‘duplex’ systems of metal
spray overcoated with high build coating specifications has been common
practice, both in order to enhance the longevity of the corrosion
protection, and also to give better aesthetic appearance.
systems of thermal srayed zinc (TSZ) have a long track record, however
duplex thermal sprayed aluminium systems have shown examples of failure in
service, with premature detachment of high build coatings from the TA, and
exhaustion of the aluminium coating.
mechanism for the failure of these duplex systems under high chloride
concentrations was discussed, whereby aluminium chloride is converted to
hydrochloric acid and the electrolytes are trapped within the TSA by the
high build paint system, leading to rapid degradation and detachment of
investigations on critical wall shear stresses for CuNi 90/10 in
artificial ASTM seawater revealed that CuNi 90/10 can endure much higher
wall shear stresses than is generally reported in the literature.
These results raised again the question on the validity of
corrosion results obtained in ASTM seawater for the prediction of
materials performance in natural seawater.
The major difference is the formation of biofilms known to play a
significant role. The
justifications for the previous experiments in ASTM seawater were that the
flow velocities applied were above the adherence limits of marine biofilms
and that the focus of the work was on the effect of disinfectants, in the
presence of which biofilms should not exist.
Nevertheless, a comparative study was launched to evaluate the
critical wall shear stresses of CuNi 90/10 and CuNi 70/30 in untreated and
chlorinated artificial ASTM seawater and natural sea water.
results confirmed that the CuNi alloys perform very similar in both
environments and that the flow resistance of these alloys is significantly
higher that reported in the literature.
The resistance to flow induced localized corrosion (FILC) depends
on the alloy composition, the nature of protective corrosion product
layers formed during different surface treatment procedures and the
presence and concentration of hypochlorite.
steel containers are used for interim storage and planned for final
disposal of intermediate level radioactive waste in a geological disposal
facility. Although stainless
steel is generally resistant to corrosion, there is concern that, during
interim storage above ground and in the operational phase of the facility,
pitting corrosion due to chloride salts deposited from aerosols may take
place. It is therefore
important to develop and validate models for the prediction of corrosion
damage over relatively long timescales. In order to predict the rate of
pit growth, it is necessary to understand the mechanism of pitting
corrosion, develop prediction models and validate them by comparison with
intense X-rays from a synchrotron are an ideal probe for in situ
investigation of the evolution of corrosion processes, since they are able
to penetrate water and even metal. The
corrosion behaviour of 2D pits growing in the edge of stainless steel foil
has been observed in situ using fast radiographic imaging, showing low
growth at the pit bottom, but higher rates of growth in the lateral
direction, undercutting the surface and producing a characteristic
“lacy” pit cover. A
similar morphology has been observed in 3D by X-ray microtomography.
local corrosion current density in the 2D pits can be determined from the
rate of advance of the pit into the metal. The local solution composition
can be backcalculated from the ions injected into the solution by
dissolution and their transport out of the pit.
The local potential distribution can also be determined.
Thus the local current density as a function of local metal ion
concentration and interfacial potential along pit boundary has been
calculated for 304 SS, and compared with the values previously published
for 1D artificial pits.
is shown that the shape of pits depends on the solution chemistry: in
dilute chloride concentrations, the pit cover provides a diffusion barrier
and maintains sufficiently aggressive chemistry inside the pit cavity for
stable growth. It is also shown that pits do not necessarily maintain
their hemispherical/dish-shape during growth and more tunnel-like
morphologies may develop under current-limited
conditions. In these cases, the actively corroding area of the pit does
not increase with depth, suggesting that pits can regulate their growth to
fit the supplied current. A cathodic current limitation, therefore, would
not necessarily support an upper bound on the pit depth, although the
electrochemical potential drop owing to current transport in deeper pits
may do so.
results from the in situ real time synchrotron experiments have been used
to refine an existing model of pit propagation in stainless steel.