ALWC:  Where are we now?

09 Feb 2011 - By Jamieson B. Christie - Industry Blog

It is now 20 years since “Accelerated Low Water Corrosion” (ALWC) was first recognised and it has since become a global steel maritime structure corrosion phenomenon.  As a corrosion mechanism its nothing new, but had never before become as prevalent as in the last two decades.

ALWC is a form of microbially influenced corrosion that can cause severe pitting and holing of the steel piles and other steel components, these form berths and jetties in both sea-water and freshwater ports.  Deterioration is rapid and occurs generally around the low-tide level in seaports but can also occur down to bed-level.  The consequences require, at best, corrosion arrestment measures and at worst total reconstruction of the facility!

Because its occurrence is random and the reasons remain unknown, it is not yet possible to obviate the risk of ALWC “at source”, although prediction models have been attempted.  However, 20 years of practice by a cross-section of professionals has resulted in a range of evidently sound, tried and tested pro-active mitigation measures and re-active steel-pile repair and protection techniques, equally applicable to any aqueous corrosion mechanism.   These are presently detailed in two dedicated publications, viz.  'Management of Accelerated Low Water Corrosion in Steel Maritime Structures' by CIRIA ref. C634, and 'Accelerated Low Water Corrosion' by PIANC /AIPCN MarCom Working Group 44 and are summarised as follows:

• Application of corrosion protection (cathodic protection, coatings, sacrificial steel) from the outset
• Retro-fitting corrosion protection systems on unprotected steel piles
• Regular inspection of steel piles above and below low water level, more frequent if un-protected
• Repair of serviceable corroded piles and installation of protection at the earliest opportunity
• Monitoring and maintenance of corrosion protection systems and piles

In addition Port Technology International has published a number of ALWC articles since its inception, conveying the experiences and views of those of various backgrounds challenged with the task of dealing with ALWC-related issues.

As port engineers become more aware of the consequences of all forms of steel corrosion, bespoke asset management systems are being developed to manage the thousands of steel piles that literally form the foundations of the maritime infrastructure, the Port of Rotterdam and Aberdeen Harbour being two known examples.

Also, the latest development in protection involves the use of the components of seawater to sterilise the affected steel and then deposit an environmentally friendly protective calcareous coating to the affected areas, by use of a variable pulsed current from a temporary anode.  Site trials at various UK ports to optimise the system have proven successful and the method is now coming on-stream as commercially available.

So it would appear that ALWC is now common knowledge among port engineers and that, given adequate physical and financial resources, it can be controlled.  It is a fact that an early investment in corrosion control significantly reduces the costs to manage corrosion in the lifetime of that asset. However, questions concerning the propagation, growth and spread of ALWC do still remain and for that there could be an argument for further research to obviate its risk “at source”.

Enquiries:

Aberdeen Harbour Board
16 Regent Quay
Aberdeen AB11 5SS
Tel. 01224 597052
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