ALWC and MIC: looking for more from science



Jamieson Christie, Assistant Engineering Director at Aberdeen Harbour, Scotland


There are those who believe we’ve been ‘caught out’ by a distinct form of Microbiologically Influenced Corrosion (MIC) called Accelerated Low Water Corrosion (ALWC) in the ports and maritime civil engineering sectors. With many years of experience working on this subject, and with esteemed associates, I take a different view  believing we’ve done all we practicably could and that the time is right for scientists to take ownership of delivering answers to the riddles that ALWC still poses.

The general understanding is that ALWC occurs at low water and down to bed level in berths. Bacteria, mainly sulphate reducing (SRB) present in sea water colonise on steel pile faces for continuous immersion and maximum dissolved oxygen, creating conditions for rapid localised corrosion.

When coincident with cracks and pits, or once established, the corrosion process is exacerbated by the prevalence of anaerobic, acidic and corrosion cell conditions within the pitting. ALWC is character ised by br ight orange patches underlain with black/grey sludge and a bright, shiny, pitted steel surface, and disturbance releases the foul odour of hydrogen sulphide, a by-product of SRB. The consequences of unchecked ALWC include severe pitting and holing of steel berths and jetties that require, at best, corrosion arrestment and, at worst, removal from service and total reconstruction.

ALWC is peculiar to steel maritime structures, where it was not a common issue until the late 1980’s. At the same time MIC was known offshore by the oil and gas sector which had been protecting steel jackets and pipelines from all corrosion types, whereas shore-based maritime structures had been traditionally constructed as relatively durable ‘fit and forget’ foundation structures. Since then, a culture of inspection, maintenance and protection for maritime structures has evolved, but how did
ALWC become so invasive in the first place?

The emergence of ALWC

Civil engineers are aware of the structural and performance qualities of steel which, unprotected, corrodes freely in sea water. But steel is a flexible and cost effective material, and if adequately protected against corrosion and mechanical damage, is relatively durable and may even be so if allowed to corrode at predetermined corrosion rates. When designing steel piled berths, the process was to agree a design life, evaluate the loadings, select an adequate pile section, upgrade if required
for installation forces, check against published cor rosion rates (0.2mm per year maximum) and then coat the piles if considered necessary to avoid corrosion. 

Because of financial constraints, many ports’ new builds would be designed to the absolute most economical solutions while barely satisfying the minimum requirements of the design codes. Economics would be client driven, and reducing design life, keeping the use of steel to a minimum, and avoiding corrosion  protection, if not strictly necessary, could make great savings. There is nothing wrong with this approach provided all design parameters transpire as anticipated, and many such structures remain in operation even beyond their design working lives. But then along came ALWC with concentrated corrosion rates 5 to 10 times those published, occurring where it’s generally out of sight, masked by marine growth, difficult to access, where the structure is most highly stressed, expensive to address, most hazardous to work, and in an unpredictable fashion! MIC had not been an issue on coastal maritime structures and so under these circumstances it was not possible for designers to foresee such a risk at that time. In the meantime, the British Standard for Maritime Structures (BS6349) does not prescribe rates for concentrated corrosion, leaving the design engineer uninformed when selecting a pile section.

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