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Bacterial infection

John Martin Construction engineering director Mike Hodgson has probably seen more accelerated low water corrosion (ALWC) than anyone outside universities and research laboratories.

Recently Hodgson surveyed nearly 30 UK ports for traces of the bacterial corrosion which eats steel piles and can halve their lives. He found it in all but one, at Avonmouth. More cases came to light when Hodgson visited Europe, but even he was surprised when the distinctive orange on black blotches turned up in St Georges (News last week). On reflection, however, Hodgson says this is typical of the ALWC story.

We still dont know why it occurs and exactly how it works, even though it has been recognised as a potentially serious problem since the late 1980s. And were not much clearer on why it attacks different parts of different sheet piles, and why some areas in some ports are attacked while others seem to be immune.

The serious nature of the problem is underlined by the corrosion rates measured beneath the bacterial growths, up to 1mm a year. Affected piles would typically be perforated in 20 years or less and often the first sign of trouble the port operator will see is subsidence on the quay above as material is washed out through the holes in the sheet piles below.

In the longer term it is quite likely that steel producers will come up with an ALWC-resistant alloy with a 60 to 90 year life. In the meantime, however, limpet dams can be used to carry out a variety of remedial treatments to corroded piles, such as the welding on of sacrificial steel plates or the application of protective coatings. John Martin Construction has recently launched a free quay refurbishment survey service for ports, which will look for ALWC, and other defects, and has began work on compiling an international database which will log all reported incidents of ALWC.

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