Coastal
Engineering - Steel Structures
The
hostile salt-laden coastal environment poses a greater risk
of corrosion to steel and the many challenges surrounding
the application of a coating system are exacerbated when
working in tidal zones. Traditional resin coatings used
to protect steel structures require high levels of surface
preparation including removal of all contaminants and corrosion
by-products back to bright metal, and the absence of residual
chlorides and surface moisture. This generally cannot be
achieved during a tidal window necessitating the use of
costly temporary coffer or limpet dams, and even then space
restrictions inside these refuges and the more hostile environmental
conditions around the coast make it a less than ideal solution,
and premature failure can be a consequence.
Cementitious coatings offer a range of benefits to overcome
these challenges. They can be applied to a damp substrate,
achieving bond when only surface rusting has been removed,
and they are tolerant to residual chlorides due to their
excellent resistance to rust creep. Offering high resistance
to wash-out they withstand immersion as little as 60 minutes
after placing. The high build application and rapid cure
makes them less susceptible to damage, especially during
their early life. Furthermore, as water-based systems, the
environmental credentials attached to this technology are
attractive, posing minimal risk in application with all
equipment being washed in water after use.
Of course, any coating system can be undermined by defects
or holidays which leave preferential corrosion sites. However,
cementitious coatings have excellent resistance to rust
creep, and should a discontinuity occur, the area can be
easily reinstated by localised wire brushing and a further
application of material.
Accelerated Low Water Corrosion
Unheard
of in the UK until around 15 years ago, the phenomenon of
concentrated corrosion on marine sheet piles known as Accelerated
Low Water Corrosion is posing port executives and facilities
managers with unexpected engineering and financial challenges.
A particularly virulent form of microbially induced corrosion,
ALWC is characterised by localised areas of soft orange
corrosion products, overlaying a black organic sludge containing
colonies of several types of bacteria. Most incidents occur
within a 0.5m band of the Lowest Astronomical Tide (LAT)
making detection at a sufficiently early stage very difficult.
It is now becoming clear that the problem is not confined
to salt water, and sheet piles exposed to fresh water can
exhibit similar and sometimes more aggressive concentrated
corrosion.
Corrosion rates for steel in a wet environment are normally
predictable at around 0.1mm per year and are therefore manageable
by increasing the thickness of “sacrificial” steel to give
the required design life. However, concentrated corrosion
rates of up to 1mm per year have been reported with the
first signs of distress only becoming evident when it is
too late, backfill has been washed-out and quays undermined.
Detection is also hampered
by the random nature of the corrosion and whilst it can
preferentially corrode some parts of the steel section,
the microbial attack is rarely uniform.
From this it is clear that the most economical solution
is to provide protection from accelerated corrosion at an
early stage, thus maximising the design life and avoiding
the risk of catastrophic, sudden failure with all the associated
expense of repairs, lost business and risks to health and
safety. It is now widely accepted that some form of preventative
action is required to stop or at least delay the onset of
corrosion, and although solutions to ALWC tend to be case
specific, coatings play an important part in the defence
strategies available.
|