BACTERIAL CORROSION of steel sheet piles has now spread globally, putting hundreds of ports previously thought to be safe at risk from undetected corrosion.
Discovery of this aggressive form of corrosion in the southern Caribbean will leave port operators worldwide having to check sheet piling and could cost many millions of pounds to put right.
The confirmation comes as attacks from the bacteria recorded in the UK grew to more than 30 ports 10 times the number identified when the problem was first highlighted in September 1996 (NCE 19 September 1996). Of these around a dozen, particularly Lowestoft and Southampton, have needed major repairs.
Outbreaks of the bacteria which can erode sheet piles by 1mm a year and halve their expected life have also been recorded in northern Europe, the US, Canada and Japan. But until now experts thought it only occurred in temperate zones, with no reliable reports of accelerated low water corrosion (ALWC) in the tropics.
Professor Frank Walsh of Portsmouth Universitys applied electrochemistry group, which has been studying bacterial corrosion said: The problem is that finding ALWC is usually quite difficult and dangerous in busy ports, so we only see the worst cases. It is probably much more common than we suppose.
The distinctive signs of ALWC were spotted last month by Norwich-based marine contractor John Martin Construction on sheet piles in the port of St Georges, Grenada. The company is renovating the 40 year old sheet piling to the ports main quay using its patented limpet dam technology, and had expected to find no more than the typical corrosion associated with tropical marine conditions.
The discovery was identified by project manager Adrian MacDonald, who also supervised a 2M renovation contract in Southampton in 1996, where ALWC had hit 2,000m of Larssen 6 piles. I recognised it immediately, even though it isnt quite as advanced as at Southampton, explained MacDonald.
Despite years of research by British Steel and others, ALWC is still a little understood phenomenon. The first sign is a soft bright orange growth on isolated patches of the outer faces of the piles in a narrow zone no more than 500mm deep centred 300m above the lowest astronomical tide level.
Beneath this growth is a thinner slimy black layer with a distinctive sulphurous smell. This is in direct contact with the steel, and when scraped off reveals a bright shiny steel surface with extensive pitting.
Researchers believe the effect is produced by two separate bacteria acting together, although many other factors are thought to be involved. Different combinations of bacteria have been found at different ports and some suggest the bacteria are spread from port to port in the ballast tanks of ships.
Professor Walsh said there was no direct evidence that ships were spreading the bacteria, but accepted: There is a wide variety of bacteria in ballast tanks, and some of them do contribute to internal corrosion.
And while some experts have also associated growth of the problem with the change to a cleaner grade of steel with fewer impurities in the 1970s, Walsh said that this seemed unlikely, adding: Ive seen ALWC on much older piles.
British Steels recent research has concentrated on ways to resist attack on both new and old sheet piles. Piling products technical manager David Rowbottom said: Since 1996 weve tested
25 different paint formulations in the laboratory and on site. Most seem to work well, but glass flake products are definitely the best.
British Steel has also developed a new grade of steel now under test which, said Rowbottom, seems to be good at inhibiting ALWC.
Dave Parker in Grenada