| Inspection
Inspection for ALWC is not always easy, this fact has
lead to the position we are in today with structures
under attack undiagnosed and still a lack of awareness
of the phenomenon (PIANC
and CIRIA). Too many structures have been constructed
in the last 30 years without protection, possibly because
the phenomenon is difficult to see, prove or measure.
LAT only occurring at low water springs, the maximum
exposure of the localised area of attack will be for
an hour or so on a few tides per month and possibly
outside working hours, when it is blowing, raining or
in the dark.
Tides, being affected by atmospheric pressure and winds,
will not always be low enough each month, nor the weather
conducive.
The Signs
The classic signs of ALWC can be distinguished from
normal splash zone corrosion. The latter has hard adherent
rust product many times thicker than the parent metal
which has been oxidised, while the ALWC is typified
by orange/brown patches overlaying a black paste See
images 1-2, with both of these corrosion products
being easily removed to reveal bright pitted steel below
See image 3.
Sometimes the ALWC corrosion products do not show above
marine growth or fouling. In these cases the presence
of a continuous barnacle layer and the random nature
of the initial attack seriously impedes the identification
of attack (until holes appear) See
image 4.
This effect has been described as the formation of
pustules See image 5.
See Corrosion
stages.
Where the bed level rises into the tidal zone attack
may rise from LAT with the bed See
image 6.
Recent discovery
Recent discovery of ALWC-like corrosion below LAT has
been found to extend in some cases to bed level. Now
accurate surveys of quay faces with this attack have
shown that the attack at the lower level may happen
without connection to LAT corrosion. This is a relatively
new and concerning finding See
image 7.
Depression
Unfortunately, the first signs of advanced ALWC attack
on sheet piled retaining walls have often been depressions
in surfacing behind the wall indicating loss of backfill
through holes in what was apparently a sound structure.
Such surface depressions could appear behind large Frodingham
piles after only 10 years, and are a substantial safety
concern See image 8.
See:
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