Its life Jim, but not as we know it

SARS, Asian bird flu, hospital superbugs? what next? ALWC perhaps? The much-talked about Accelerated Low Water Corrosion certainly qualifies as a nasty as David Foxwell reports on in this issue. But at least its effects are limited to steel structures and not humanity at large.

In some ports and harbours, says CIRIA, the Construction Industry Research & Information Association, there are particular conditions which can cause localised, accelerated loss of metal on unprotected steel structures. This rapid form of attack – up to 4mm a year – results in extensive and costly remedial work at an unexpectedly early stage in the life of a structure.

CIRIA has developed a questionnaire and says it will produce a comprehensive, user friendly report, to be published in summer 2005, addressing all aspects of ALWC – what it is, how to recognise it, how to treat it and how to prevent it. The questionnaire is available from www. ciria. org. uk Areas affected by ALWC corrosion are identifiable by their characteristic appearance: a soft, spongy, bright orange/brown outer layer, a thinner, gelatinous, loosely adherent inner black liquid/paste layer beneath which the steel is bright, clean and shiny with extensive pitting. Charming!

But what really causes it?

The answer is microbiological corrosion and the villains, it seems, are sulphide oxidising bacteria (SOB) and sulphate reducing bacteria (SRB).

SRB are anaerobes that are sustained by organic nutrients, the scientists tell us. Generally they require a complete absence of oxygen and a highly reduced environment to function efficiently.

Nonetheless, they circulate in aerated waters, until they find an “ideal” environment supporting their metabolism and multiplication.