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Low tide corrosion probe

It is my personal opinion that steel still remains an appropriate medium for maritime construction.

In new construction simple measures (sacrificial plates) can be taken to control the effects of ALWC. In existing structures there are effective repair techniques.

The letters (NCE 26 March), indicate a considerable disparity in the perception of the rates and incidence of ALWC.

Dr Peter Howsam (NCE 19 March) is quite right that we need to be more aware and have greater understanding of this type of corrosion.

My surveys have shown that:

ALWC occurs in virtually every port or quay that I have inspected. (A further two incidences were discovered on the good spring tides at the end of last week.)

ALWC is not restricted to sheet piles, and has been identified on tubular and box piles.

Maximum corrosion rates vary geographically between 0.3mm/yr and 1.0mm/yr.

I suggest that an inspection of sheet piled quays by port owners or operators is essential to verify the extent of ALWC.

The importance of undertaking these surveys at one of the lowest spring tides, such as those spanning 26 April, cannot be over stressed.

One feature of ALWC is that the effect of the very high rates of corrosion in the susceptible zone causes the rate of corrosion in the inter-tidal zone to be very substantially reduced. Surveying at higher tides therefore gives a false and misleading impression of the condition of the quay.

I suggest that if sufficient quays and jetties are properly surveyed, and this data used as the basis for a new survey, the incidence of ALWC will then be found to be very much greater than previous surveys.

Would the Institution be a suitable forum to debate the whole question of ALWC?

Mike Hodgson (M), The Airfield, Shipdham, Thetford, Norfolk, IP25 7SP.

Anchor answer

Some 10 or 15 years ago I was called on to investigate the failure of some lifting anchors cast into concrete shield blocks at an Atomic Research Establishment in Cheshire.

Some of the blocks had been used to form a shield wall round an experiment indoors in the lab, and on completion of the experiment they had been stockpiled outside along with other unused blocks.

Now after two years outside they were wanted for use again, but it was found that the anchors on the previously used blocks had corroded almost completely away. The corrosion was unlike anything I had seen before, in perhaps 40 years of experience, being bright orange nodules.

As far as I remember, only those blocks that had been subject to radiation showed this failing, unused blocks showing just the normal amount of ordinary rusting.

At the time I assumed that the radiation had something to do with this, but the nuclear scientists could come up with no logical explanation, and the cause of this rapid corrosion remained an unsolved mystery.

However, having now seen the picture of AWLC (NCE 5 March) I am convinced that what we saw was an early form of AWLC, although the site was well inland, and the corroding liquid was ordinary rainwater, so perhaps the name is wrong.

DS Mason (M), 25 Meadway Court, Meadway, Golders Green, London NW11 6PN.

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