Accelerated Low Water Corrosion

7/29/2019 Accelerated Low Water Corrosion

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ICE Briefing Sheet CONCENTRATED CORROSION ON

MARINE STEEL STRUCTURES

Briefing Sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken to ensure information is correct. However readers are advised to consult sourcedocuments for authoritative information. The Institution of Civil Engineers is a registered charity No 210252, 1 Great George Street, London SW1P 3AA.

This briefing sheet is a summary of British Standard

BS6349-1:2000 Clause 59 as published by, and available

from, the British Standards Institution. This clause dealswith the general considerations to be taken when structuralsteel and other metals are to be incorporated into maritime

structures and makes particular reference to steel piles andconcentrated corrosion.

BACKGROUND

It has come to the attention of the Maritime Board that thenumber of cases of concentrated corrosion and, in particular,accelerated low water corrosion (ALWC) on marine steelstructures are on the increase, both within the UK and

worldwide. There is concern that many authorities andengineers are still not aware of the problem or are unsurehow to deal with it. This briefing will therefore attempt to

redress this to some degree by highlighting the reasons for corrosion on weldable structural steels (mainly piles) used inmarine structures and the need to control it. The differenttypes of known corrosion mechanism, including ALWC,will be described along with perceived typical corrosionrates and measures to be taken against corrosion.

MARINE CORROSION

Metals are more prone to corrosion in a marine environment

than inland due primarily to:-

• Galvanic cell formation – caused by anodes andcathodes forming between different metals or on thesame metal but due to varying conditions e.g.

differential aeration, or by strong currents along with thesolution of salts in the water acting as the electrolyte.

• Erosion of corrosion products – removal of rust products by wave action, rubbing or cyclic deflectionscauses quicker regeneration of replacement products.

• Neglect – inadequacy of planned preventativeinspection and/or maintenance by owners.

Miles of piles – but they need taken care of

CORROSION RATES

Table 25 in the standard classifies exposure of an area of

steel in a marine environment into vertical zones in whichcorrosion requires separate consideration. Notional averageand upper limit values of corrosion for exposed, unprotected

structural steels in temperate climates in mm/side/year aregiven as a guide to what could be expected.

These are summarized below:-

Avg. U.L.

Atmospheric zone (in the dry) 0.04 0.10

Splash zone (above MHWS) 0.08 0.17

Tidal zone ( MLWS and MHWS) 0.04 0.10

Intertidal low water zone 0.08 0.17(0.5m below LAT to MLWS)

Continuous seawater immersion zone 0.04 0.13

Embedded or below seabed zone 0.015

(max)



Briefing Sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken to ensure information is correct. However readers are advised to consult source

documents for authoritative information. The Institution of Civil Engineers is a registered charity No 210252, 1 Great George Street, London SW1P 3AA.

CONCENTRATED CORROSION

In marine environments, however, corrosion rates in excessof those given in Table 25 can be caused locally and by

several mechanisms. These forms of accelerated corrosionare referred to as “concentrated corrosion” and aredescribed as follows:-

• Repeated removal of the protective corrosion

product layer by the means describes earlier causesthis area to become anodic to the unprotected areas.

• Steel that is electrically connected to metals havingnobler potentials, or where weld metals are

significantly less noble than the parent metal,causes bi-metallic corrosion.

• Microbiological activity in concentrations around

(and below) the low water zone will causeaccelerated corrosion. This type of concentration iscommonly referred to as ALWC. It is understood

that the microbiological activity creates localizedand relatively severe anodic/cathodic conditionsand the visual appearance is characterized by bright

orange patches overlying a black sludge and ashiny, pitted steel surface.

Corrosion rates for concentrated corrosion are given astypically 0.5mm/side/year averaged over time to the point of complete perforation of steel plate. Cases of

0.8mm/side/year are reported to have occurred in UK coastalwaters.

CONSEQUENCES OF CONCENTRATED

CORROSION

The British Standard warns that loss of material through a

holed sheet-pile wall can lead to collapse of adjacent

surfacing or structures supported on the backfill.Concentrated corrosion is, however, unlikely to cause

catastrophic global failure of a sheet-piled structure (due toits continuous nature) but it may be more critical on a king- pile or other structural member.

Concentrated corrosion on a Universal Bearing Pile

It is recommended that the hazard of corrosion along withthe safety and environmental risks posed as a consequenceshould be the subject of assessment by those responsible for

the structures concerned. Financial consequences are bestconsidered by way of cost benefit analyses of either thecorrosion control measures for the structure at design stage,

or based on a survey of the existing structure, as the casemay be. Generally, for existing structures, early interventionmakes economic sense.

GENERAL MEASURES AGAINST CORROSION

In situations where high rates of corrosion are expected, or

are likely, it is recommended that a specific solution bedeveloped using one or more of the following methods:-

• A thicker steel section, a higher grade of steel, or

the addition of steel plates.

• A high quality protective coating.

• An electrical bonding system and cathodic

protection.

• A design to ensure that high bending moments

occur in areas away from anticipated concentratedcorrosion sites.

This list is not considered exhaustive and other examplesinclude concrete facing, wrappings, impressed currentsystems and the like. Some of the measures are more suited



Briefing Sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken to ensure information is correct. However readers are advised to consult source

documents for authoritative information. The Institution of Civil Engineers is a registered charity No 210252, 1 Great George Street, London SW1P 3AA.

to maintenance and/or repair of existing structures where

access by limpet dams or divers may be necessary.

Structures should be inspected at regular intervals so that

corrosion activity can be identified at an early stage. Theadverse effect of removal of localized fouling and corrosion products if left unchecked should not be overlooked.

Plating repairs at low tide

COMMENTARY

The solution to concentrated corrosion on marine steel

structures is by and large an empirical subject. Eachmaritime site is unique and every quay or jetty installationdifferent in terms of environment and possibly design,construction and operation. Contemporary reports of

occurrences frequently confound previously perceived patterns and trends and are found in areas difficult to access both for inspection and repair.

As far as solutions are concerned, there appears to be no

panacea or absolute design formula so engineers must drawtheir own conclusions for each individual case. Informationavailable can only be taken as a guide as it is based on what

is currently known.

Other sources of information on the subject include clients,consultants, contractors and other specialists with knowledge

and experience of the issues involved. The challenges of this subject are not limited to civil engineers and corrosionspecialists and metallurgists can also have a contribution tomake.

The Maritime Board has participated in two recentconferences on ALWC at which papers from members andother practitioners with varying expertise were presented.

Further information may be obtained through Anne-Marie

Ferguson (Secretary, Maritime Board) at the ICE as

follows:-

Tel. +44(0)20 7665 2232

Fax +44(0)20 7799 1325E-mail [email protected]

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Accelerated Low Water Corrosion