THE BACTERIAL colonies that cause accelerated low water corrosion on steel marine structures are 'determined, tenacious and adaptable' according to specialist contractor John Martin Construction engineering director Mike Hodgson. Rarely growing beyond 'hand-size', they disappear for no apparent reason, only for a new colony to appear close by. And as long as they flourish, the steel below corrodes away at up to 5mm a year.
This description, given by Hodgson at last Thursday's London Association meeting entitled Dams, Limpet Dams and Statistics and ALWC, may have alarmed the Great George Street audience. But he went on to explain that the short life of the colonies and the random nature of their re-occurrence meant that the attack took the form of localised pitting. Average corrosion, he said, based on time to penetration, is more like 0.5mm per year, with the highest average recorded so far being close to 1mm annually.
The conclusions were based on hundreds of inspections and surveys of sheet pile and other steel structures at ports and harbours in the UK and Europe, Hodgson added. He emphasised that he was not a biochemist, a metallurgist nor a corrosion scientist, but was speaking as an engineer and recommending straightforward engineering solutions.
Most of these were based on the use of limpet dams, which have been developed by John Martin Construction over the last few years. These are effectively open steel boxes with one side missing which are sucked on to quay walls when powerful pumps drain out the water inside them.
John Martin claims to have developed a range of seals which allow quick set-up on a variety of walls and other structures, and Hodgson described the use of tailor-made limpet dams on harbour refurbishment projects, from the West Indies to Germany. Not all of these involved ALWC damage, he pointed out.
In a lively question and answer session that followed Hodgson's presentation, discussion was split between ALWC and the capabilities and limitations of limpet dam technology. Ove Arup & Partners associate Graham Gedge challenged Hodgson's use of the terms 'cathode and anode' in connection with the apparent concentration of bacterial corrosion in a narrow band only 500mm high around lowest astronomical tide level.
Hodgson said his surveys showed that corrosion on the underwater regions of piles above and below the ALWC zone was virtually zero, suggesting that the ALWC zone was acting like a sacrificial anode in some way. But Gedge said there was no hard scientific evidence to support the theory and that many other factors apart from biological action had to be considered.
Others asked if Hodgson had returned to sites where ALWC damage had been repaired to see if the attack continued. He reported that where uncoated steel plates had been used he often discovered new ALWC colonies in virtually the same locations as before the repairs. But he pointed out that by using a thickness of 25mm or more for the repair steel, the life of the structure concerned would be extended by about 50 years. Impressed current cathodic protection also seemed to work, he added, as did high performance paints and coatings.