Eating into the Bottom Line

ALWC occurs on submerged steel surfaces. It can be treated with cathodic protection, or coatings such as this cementitious coating.
ALWC is characterised by the presence of: soft orange coloured (sulphurous) deposits; layers of black slime (iron sulphide); bright shiny pits or through wall penetration is observed under this layer.
Industry Database

Accelerated low water corrosion is becoming more widespread in sheet piling. David Foxwell looks at the latest thinking on how to tackle port nasties

Despite recent publicity and studies carried out by bodies such as the Construction Industry Research Association, anecdotal evidence suggests that Accelerated Low Water Corrosion (ALWC) is becoming an ever-greater problem in sheet piling in ports and harbours, and that ports remain unsure about potential solutions.A recent British survey revealed that up to 90% of ports have noted ALWC activity, and other evidence indicates that ALWC is a worldwide corrosion problem.

As Grant Alexander, a partner at Arch Henderson, a British company that specialises in offering independent advice on harbour works, explains, expertise on - and solutions to - ALWC continue to be in demand.

Arch Henderson first worked on ALWC in some old Frodingham piles at Lerwick Harbour in Scotland several years ago, and has seen the problem become much more widespread in recent years. The solution the company employed at Lerwick was a kind of shutter arrangement with poured concrete and anodes fitted over an extensive area, but the best solution for ALWC tends to be case-specific, said Mr  Alexander. 

Asked whether certain types of piles are more susceptible to ALWC than other, Mr Alexander says that, in his experience, older types of Larsen piles tend to withstand the onslaught of accelerated corrosion better than others, but that this is probably a reflection on their thickness compared with more recent types of piles.

"All kinds of ideas have been put forward about what kinds of conditions encourage ALWC to take hold, ranging from water temperature to exposure to sunlight to bow thruster motion from ships operating alongside piles, to stray currents from ships' generators," he explains, "but we have encountered it in all kinds of situations."

There are various solutions to ALWC, some highlighted previously in Port Strategy, but first and foremost, Mr Alexander recommends a good quality coating system for new piles - such as a high quality flake glass 'bitumastic', and the use of cathodic protection of one sort or another.

Where ALWC is believed to be present already, a survey by a diver is usually undertaken and ultrasonic thickness readings obtained, although there are alternatives (see box). "From thickness readings and calculations based on those the best solution to ALWC can be determined," Mr Alexander explains.

Roddy James, a director at corrosion specialist iicorr Ltd, agrees that solutions to ALWC need to be evaluated against agreed criteria with the client to satisfy the financial, environmental, safety issues relating to the chosen project. In a recent paper on the subject, he highlighted the fact that the European Commission recommended cathodic protection systems; change of material; and the use of coating systems; to combat ALWC.

iicorr also recommends what it calls an "integrated inspection programme" that prioritises inspections so that inspection and maintenance work can be minimised. This kind of prioritised programme can be applied to all types of 'water wetted' steel structural elements that could be subject to ALWC,including steel piles, ladders, plates, and bracing.

The aim is to ensure that steel piles are adequately located and identified in each berth and are assessed and monitored to ensure fitness-for-purpose in a cost-effective manner and an 'asset register' established, thus providing a costeffective risk-based solution to inspection and maintenance which is suited to the harbour environment.

There are, however, some potential pitfalls to be borne in mind with regard to coatings, as Chris Lloyd, a director of Flexcrete Technologies in Preston in the UK explained. These include the fact that it is sometimes difficult to achieve a finish to Sa2.5 standard, potential problems with residual chlorides and or residual moisture on surfaces, and with early immersion, susceptibility to damage in early life, and use in exposed locations subject to weather extremes.

There are, believes Mr Lloyd, also potential problems involved with the use of cofferdams of the type sometimes use to gain access to immersed surfaces, not least cost,due to their specialist nature,and the fact that they cannot always be used. Ensuring a fully effective seal is another potential problem,he claims,as is limited space within a cofferdam for surface preparation, the potential health hazards associated with applying with solvent-based coatings, and the relatively slow progress of work undertaken within a cofferdam.

There are, says Mr Lloyd, potential problems with coal tar epoxy coatings too - abrasive blast cleaning is essential,prior to application,generally to SA2.5 or SA3; dampness adversely affects adhesion; coal tar pitch is a carcinogen; drying times of up to 16 hours may be required, which is inconvenient - and difficult to provide - in the port environment; and such coatings are susceptible to mechanical damage for up to seven days.

Flexcrete's director also explains that these kinds of coatings provide what he calls "barrier protection" only. A primer coat (such as zinc phosphate) is also normally required, and a thin film, often only 400Ìm, is produced.Moreover, early immersion can be difficult. With the type of cementitious coating that Flexcrete recommends, however, the coatings will tolerate minimal surface preparation;are damp tolerant;solvent free and COSSH safe;the coatings are water-based;cure rapidly, in as little as 4-8 hours; are extremely durable after 24 hours; tolerate early immersion; and allow for a high-build coating of some 2,000Ìm.

The UK Department of Trade & Industry (DTI) is certainly taking the problem of ALWC seriously and recently committed to a £170,000 project that will see one particularly promising treatment technology tested in situ at several ports.

The three-year project will see 'LATreat', an innovative technology marketed by Mott MacDonald and BAC Corrosion Control, tested at seven to eight ports that have joined the DTI-funded project. Mott MacDonald and BAC Corrosion Control claim the technology is a cost-effective solution for ALWC and say it used 'micro-' and 'nanotechnology' to arrest the problem and provide long-term protection against future attack.

Mott MacDonald describes LATreat as a "magic bullet" treatment, utilising cathodically-generated molecular hydrogen to, firstly, clean the affected area, then sterilise the surface using anodically-generated chlorine.Finally,using pulse current techniques,ionised salts from the seawater are deposited on the surface to provide an alkaline coating. Being an electrochemical treatment, LATreat has the benefit of using seawater and electricity to generate all the necessary active agents.

It is applied using modular mesh electrode units that allow variable lengths of affected piling to be treated as required. The modular units can be easily removed and moved between locations.

According to literature provided by the companies, LATreat comprises three optimised stages which are typically carried out sequentially using control equipment located remote from the structure. The sterilisation process uses a 'shock dose' treatment of anodically-generated chlorine, a known biocide used to kill bacterial infestations.The final stage is a rapid insitu application of a protective alkaline coating deposited from seawater dissolved salts, which provides local protection while normal marine deposits reform.

One of the principal benefits of LATreat is that it is a one-off treatment and can be implemented in a relatively short space of time, typically five to seven days.There is minimum disruption to harbour activities while the treatment is being implemented, and the treatment is environmentally friendly. Chlorine generation is short-term, and controlled, and no external agents are used in the process.

BAC Corrosion Control says it believes that LATreat is "significantly cheaper" than possible alternatives, particularly if life cycle costs are taken into account, and says all equipment and cabling used in the treatment is removed on completion with no requirement for permanent installation and maintenance of expensive control equipment.

Under the DTI-funded project, LATreat will be tested in-situ at ports that are participating in the project along with Mott MacDonald and BAC Corrosion Control and corrosion experts at the University of Manchester.

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