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.