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Letters to the editor

Corrosion clarified

You have recently published two articles (NCE 5 and 12 March) reporting the alleged incidence of accelerated low water corrosion (ALWC) on sheet piling in St Georges Harbour, Grenada. Both articles fail to make clear a number of important points regarding ALWC, and the reader could easily misinterpret the risks of ALWC occurring.

No-one denies that ALWC does occur, nor that it is a potentially serious problem for the owner of an affected structure. However, it is important to keep the incidence of ALWC in perspective.

This alleged incidence of ALWC in Grenada is the first outside northern European waters. It is thus incorrect to state that the problem has spread worldwide.

The majority of UK sheet pile installations, in marine applications, have not been reported as suffering from ALWC.

ALWC has only been found, to date, on sheet piling and does not appear to affect other steel members.

The microbial colonies associated with ALWC do not eat steel. They generate environments that can be corrosive to steel, either by producing highly acidic conditions within the colony or by generating films on the steel surface that are strongly cathodic to steel.

It is far from certain that the corrosion shown in the photographs of St. Georges Harbour, is in fact an incidence of ALWC. Visual, or photographic, evidence is an unsatisfactory method of assessing corrosion phenomena, objective judgments can only be made by the interpretation of corrosion rate data collected from a given site.

Graham Gedge, Ove Arup & Partners, Research & Development, 13 Fitzroy Street, London W1P 6BQ.

Meeting criteria

The case of ALWC in Grenada is interesting because the excessive corrosion is very limited and scattered over the wall in discrete patches. The overall design of such marine walls and quays/jetties does not seem to be compromised by these isolated higher corrosion 'cells', - after all the walls are still serviceable and performing their function after 40 years of service.

From the evidence presented so far we are most impressed by steel sheet pile walls remaining serviceable for 40 years before first major maintenance because tidal seawaters are the most aggressive environmental exposure for civil engineering structures. An upper corrosion rate of 0.2mm/face/year is quoted in BS6349 for the inter- tidal zone and is considered to be prudent and sufficiently conservative for overall marine wall design in the UK. For a 16mm thick SSP this would give an expected design life of 40 years and this is as found at Grenada even with the ALWC occurring and in the tropics where we would expect corrosion rates for unprotected steel to be even higher.

We therefore conclude that current design advice in our British Standards is sound and realistic and the maintenance inspections for marine walls should start to be carried out after say half design life, ie 20 years. This seems to be applicable elsewhere in the world as well.

Tony Biddle (M), manager, civil engineering,The Steel Construction Institute, Silwood Park, Ascot, Berks SL5 7QN.

Practical advice

The role of microbial activity in the corrosion of steel in the low-water zone is something we have been studying for some time. It is certainly not a new phenomenon.

The Japanese have also studied accelerated low water corrosion and have published their experiences, (for example, 26th International Navigation Congress, June 1985). In view of these experiences and other publications I have not subscribed to the view that ALWC is confined to temperate zones. Neither have I actually seen any reliable evidence to support the claim in the article that the incidence of ALWC is the result of a disease-like transmission process.

British Steel published a document in 1996 dealing with various aspects of ALWC including appropriate maintenance regimes and the repair of any areas affected. We are currently involved in an European Coal and Steel Communities (ECSC) research project investigating the phenomenon. Findings from the second phase of this research are not due to be released for another two or three months but I can safely say that they will not be endorsing any of the claims, implied or otherwise, in your articles suggesting that we are facing any form of, in your words, 'corrosion bug epidemic'. Rather it will be providing an objective assessment of the risks from this limited mode of corrosion and provide sound, practical advice on preventative measures.

E Marsh, senior corrosion technologist, Surface Science & Engineering Department

British Steel, Moorgate, Rotherham S60 3AR.

Exaggerated claim

Your articles on accelerated low water corrosion indicate that of 30 UK ports visited, all but one were affected. This figure does not accord with the survey carried out by the ECSC research project team or the evidence from subsequent visits to port and harbour operators carried out by British Steel throughout the UK and independent Japanese research which indicates that the level of occurrence is in the order of 10% of the figure claimed in your article.

David Rowbottom (M), manager, piling technical services, British Steel, Scunthorpe.

Technical editor Dave Parker comments:

What I saw in St George's were patches of a soft, spongy and obviously organic orange growth that could be scraped off with fingers alone. Under this was a thinner, slimy black layer. This could be wiped away with a wet rag to reveal a shiny metal surface with extensive pitting.

These 'biofilms' matched every description I have been given by ALWC experts and looked very similar to photographs I have seen of ALWC. I was also assured by those present that it closely resembled other instances of ALWC they had seen in the UK.

The photo published on p16 of NCE 12 March mainly shows an area above the ALWC zone, where corrosion products were typically hard and scaly. As the feature made clear, the primary cause of the sheet pile deterioration at St George's was normal corrosion. It was my personal impression, given the apparent rate of attack by the biofilms I saw, that ALWC must be of recent origin or damage would have been much more severe.

It is worth noting that St George's is currently suffering from scour problems caused by the increasing size and power of the side thrusters fitted to large modern cruise liners, and that the biofilm patches were mainly to be found in the area where such liners dock and undock.

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