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Biocorrosion - time for the state to step in

Microbiologists and engineers are uniting in the fight against a new threat to the world's infrastructure.

Biocorrosion is a problem of national importance - the Government should be funding research into the phenomena.

That was the emphatic conclusion of last week's Great George Street conference on the microbial corrosion of steel marine structures. Delegates at the packed event heard that accelerated low water corrosion - as this form of bacterial attack is known - had been positively identified in the majority of UK ports as well as in many other parts of the world.

But no-one could explain why ALWC had suddenly become such a serious problem - or even identify the key factors which determine why one piece of steel in one port should be attacked while an overtly similar piece of steel somewhere else is unaffected.

Calls for urgent research into this particular form of biocorrosion will be reinforced by the recent revelation that in some circumstances, bacteria can vastly exacerbate sulphate attack on concrete foundations (NCE 8 October). Here the mechanism is better understood, in principle at least, but the factors which contribute are still unclear.

In one particular case described at an earlier London conference, in which an office building on the east coast of the US had to be demolished, fairly unusual ground conditions were involved - anoxic estuarine silts heavily contaminated with sewage. But such conditions are not necessarily the only place where bacteria will pose a threat to even the best quality concrete.

Many brownfield sites could offer conditions equally favourable to the biological depletion of the iron contained in the hardened cement matrix - which eventually reduces the concrete's resistance to sulphates in groundwater to almost zero.

At least in this form of biocorrosion there seem to be some obvious additional checks that can be made during a preliminary site investigation. Oxygen and free iron levels can be measured and bacterial activity established. The bigger question is what to do if the investigation suggests biocorrosion exists.

Almost the reverse situation exists with ALWC. There are a number of techniques which appear to be capable of reducing or eliminating the problem once it is discovered. Cathodic protection is one option, various forms of coating have been tested and appear to work. There was also strong support at last week's conference for adding sacrificial steel in the most threatened zone.

But, as several contributors to the discussions pointed out, the real problem for port engineers is that there is still no effective way of assessing the risk of ALWC occurring. And without an estimation of the risk it can be hard to justify the expense of preventative measures.

Identifying the key factors - which means developing a much better understanding of how the bacteria actually contribute to the corrosion - must now be the number one research priority.

Perhaps the most disturbing revelation at Great George Street last week was that something remarkably similar to ALWC has been found well away from the narrow band around lowest astronomical tide level where ALWC was thought to be confined. This served to emphasise how much more the industry needs to know about the distribution and characteristics of this form of biocorrosion - a need that led to calls for the establishment of a confidential ALWC reporting project to gather as much hard data as possible.

Confidentiality is essential because of the obvious commercial sensitivity of some of the information needed. Such an exercise could be the first step towards unlocking the ALWC mystery. A similar exercise on the biological acceleration of sulphate attack on concrete foundations is less practical - but the industry will need to know the risks as soon as possible.

Dave Parker

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