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ALWC research gives half an answer

UNPUBLISHED RESEARCH into 'steel-eating bug' attacks on sheet piles in ports and harbours claims to show how to fight them - but fails to identify what triggers the attacks in the first place.

Results of the three year research programme, revealed last week, are said to prove that both high performance coatings and cathodic protection can resist accelerated low water corrosion - ALWC - which is caused by bacterial action in a narrow zone above the lowest astronomical tide level.

But the study, funded by the European Coal and Steel Community and carried out for European producers on ten ALWC sites throughout Europe, could only suggest that pollution levels might be one key factor.

British Steel Sections, Plates and Commercial Steels market development manager Jim Wilson commented: 'I think it's true to say we know how to stop it but we don't know why it starts. And there is no explanation of the anomalies often observed.'

Although the research programme was completed at the end of last year it is still not clear when it will be published. An earlier ECSC program of research into ALWC, completed in 1993, remained unpublished for nearly five years.

ALWC has baffled experts since the first incidents were discovered in the late 1980s. Dozens of ports worldwide have had to carry out expensive repairs to tackle the problem. In some ports, colonies of several types of common bacteria, including at least one capable of reducing sulphates in seawater to acidic sulphides, start to form on specific areas of piles - on U-shaped piles the centre of the outpans are attacked, but on Z-shaped piles it is the corners.

Attacks are characterised by a bright orange layer of soft corrosion products overlying a black organic sludge layer. Below this layer the steel is bright, shiny, and distinctively pitted. It is claimed that corrosion rates of up to 1mm a year have been observed

In the latest research low alloy steels containing known biocides such as copper and chromium were tested for resistance to ALWC but showed no more resistance than standard mild steels. British Steel now claims that another, still-secret, alloy - previously under development for improved resistance to normal low water corrosion - has also turned out to be highly resistant to ALWC.

Dubbed Harbour Master, the new alloy is now in production trial and should be available later this year. Cost is expected to be much higher than mild steel but cheaper than the pre-coated alternative.

Dave Parker

(see Commentary page 8)

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