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New
structures
New structures if unprotected by CP or paint will soon
gain a heavy oxide (rust) layer in the splash zone,
which although heavy is unlikely to be significant in
most situations and is to a great extent passivating
against the initial unprotected rapid corrosion rate.
There will be a transition between this and the marine
fouling which will grow in the tidal and sub tidal zones.
From the experience of new repairs, ALWC has re-colonised
clean steel within 6 months of a repair. It is therefore
reasonable to assume that the same would happen to a
new structure where the potential for ALWC exists.
Where the potential exists, and the Incidence
Maps strongly suggest that it may be expected anywhere,
new structures should be treated with Cathodic
Protection. Be very sure that there are no stray
currents being received from adjacent installations
or that floating rubber tyre fenders are not creating
Concentrated Corrosion forms apart from ALWC See
image 1.
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Young
Structures
As the ALWC attack proceeds a cathodic protective effect
occurs. The ALWC LAT corroding zone becomes anodic to
the remainder of the immersed pile. This is evidenced
by many quays exhibiting very low corrosion (in fact
near perfect ‘as-rolled’ condition) in the
tidal zone See image 1.
This has given a misleading impression of the condition
of the structure because the LAT (active corrosion)
zone is so rarely visible, masking the worst corrosion
See image 2.
This is a time when thorough inspection should be carried
out by experienced professionals to ascertain whether
there is Concentrated Corrosion or not. Bear in mind
that inspection of this kind is not ‘early’,
corrosion rates of 0.8mm per year occurring on the thickest
Frodingham pile webs of perhaps 12mm would cause holes
in 15 years, let alone heavy corrosion before then;
on thinner sections it may be too late See
image 3.
It is important to avoid holes at all costs, the flow
of water and materials through them will accelerate
matters, cause consequential settlement and damage and
complicate repair.
Planned inspection should be set up at this stage.
If necessary CP could be retrofitted.
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Middle
aged structures
In design terms a piled structure of 45 years is probably
considered ‘middle aged’ (imagining a design
life of 60, or perhaps 90, years). A traditional corrosion
allowance (calculated at 0.125 mm per year for normal
marine corrosion) may have been lost, but Concentrated
Corrosion rates will almost certainly have holed the
structure within the last 15 years. Even on large structures
using heavy Larssen piles, outpans of 28 mm thickness
of essential material will be three-quarters corroded
at a rate of 0.5m per year, and at 0.8 mm will have
been holed for the last 10 years.
All structures need to be thoroughly inspected, and
if ALWC is found, regular inspections must be set up
to assess the current rate of corrosion. It may not
be sufficient to assume a linear corrosion rate from
installation.
Assessment of corrosion and corrosion rate is extremely
difficult (see recognition and inspection), but if the
need for repairs of worst cases have already been identified,
then further inspection can be economically arranged
with the repair.
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Old
Structures
The bad news can be seen in the photo on the right.
However:
Pre-war structures have been found which do not suffer
from ALWC even though younger adjacent structures do.
This is heartening, not fully explained, and frankly
should not be used to gain a false sense of security
regarding the condition of any structure of whatever
age.
It has been suggested that the older piles may have
trace elements (intentionally or merely in the ‘dirtier’
steel) which are toxic to the bacteria. Alternatively
the piles may have become thoroughly passivated before
the bacteria consortium was imported. Or again perhaps
the estuary has been cleaned to meet EU legislation
and toxic elements have been removed from the water,
or possibly the nitrate level in the estuary has increased
with modern farming methods. For most purposes it doesn’t
really matter why, we advise ignoring the theory and
taking practical inspection and repair steps. On top
of this, for new structures, it must make sense to establish
scientifically if ALWC is possible regardless of good
local examples to the contrary, that is the bad news
of the heartening earlier example .
Apart from any localised repairs which may be required
on old structures, the good news is that CP should still
be effective in stopping further attack. Galvanic anodes
may take a while to counteract the corrosion cell, but
if the design current density and potential are sufficient
it will stop it. The orange disappearing probably within
6 months or earlier. See Cathodic
protection.
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Also see: Case
Histories
For more detail see:
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