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Biocorrosion threat to UK coastal defence

HUNDREDS OF kilometres of UK sea and flood defences and North Sea oil platforms are threatened by the aggressive biocorrosion of steel, a London conference on accelerated low water corrosion was warned this week.

Delegates heard that ALWC, thought to be mainly a problem for steel sheet piling in commercial ports, had now been positively identified on most types of steel construction in the marine environment. These include offshore structures and the near 200km of steel sheet piles in sea and flood defences around British coast and estuaries.

Port of Felixstowe civil engineering manager Tom Shelley, one of the keynote speakers, said: 'I have seen ALWC cause the total progressive collapse of a flood wall defence. Flood walls are not inspected as regularly as walls in busy ports. ALWC could turn the proposed managed retreat on the east coast into a rout.'

Outbreaks of ALWC have recently been discovered in sites as far apart as Iceland and Israel, the meeting was told. All types of steel piles have been attacked, as well as dock gates, pontoons and other marine structures. And there was general dismay when the audience heard several reports of the biocorrosion being found outside the normal narrow low tide zone, even stretching down to the seabed.

Maximum rates of corrosion reported to the meeting were close to 2mm per annum, though there was general agreement that typical figures for UK waters were 0.5mm to 0.8mm annually.

'Civil engineers should count themselves lucky to experience this relatively low rate of biocorrosion,' said AEA Technology senior corrosion consultant Dr Andrew Pritchard. 'Inside oil tankers rates of 15mm per year have been recorded.'

Pritchard speculated that less-polluted estuaries and changes to anti- fouling paints on shipping - which meant less toxic heavy metals in coastal waters - might have encouraged greater bacterial activity in harbours. But Dr Iwona Beech of Portsmouth University said the exact biological mechanism of ALWC is still a long way from being understood.

'Surface science could well hold the key,' she added.

'These aggressive biofilms can be as thin as 2 microns - yet they are capable of eating away even stainless steel.'

Another speaker, John Martin Construction engineering director Mike Hodgson, described a port in Scotland which had been extended at roughly 10 year intervals for the last 50 years, using a very similar design of sheet pile structure each time. 'The last four extensions all show similar signs of ALWC attack,' he said. 'But the section which was built during the last war is still unaffected.'

Dave Parker

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