It is a form of Microbiologically Induced Corrosion (MIC) produced by a consortium of micro-organisms, including Sulfate-Reducing Bacteria (SRB), Sulfur Oxidising Bacteria (SOB) and Acid Producing Bacteria (APB). Improvements in water quality over the years have led to increases in these micro-organisms and as knowledge of the phenomenon has increased so has the number of reported cases around the world. ALWC has a characteristic appearance with layers of poorly adherent, bright orange oxides of iron which cover a “sludgy” black layer of iron sulfide (FeS). This layer and marine growth will usually be detached easily when rubbed by hand, exposing a characteristically clean, shiny heavily pitted steel surface.
Consequences and impact of ALWC
The corrosion rate beneath ALWC patches is far worse than in other regions of the immersed zone – typically 0.3mm to 2mm/wetted side/year compared with 0.08mm to 0.17mm for the inertial low water zone. ALWC can cause premature perforation of unprotected steel, resulting in unexpected and often expensive remedial works. If left untreated, thinning of steel can occur to such an extent that it can lead to premature failure of a structure. The Institution of Civil Engineers Maritime Board [1] has conservatively estimated that current repair costs resulting from ALWC on port-related infrastructure in the UK alone would be in excess of £250 million in order to save assets that are worth billions of pounds. Port facilities around the world are likely to have similarly large costs and, since most imports and exports pass through ports, any disruption to their activities would have significant consequences for ports and their countries economies.
Conventional methods for dealing with ALWC
Given these significant consequences there has been a great deal of work done to investigate the methods of dealing with ALWC. Most of the published work to date, concerned with the treatment of and protection against ALWC, describes the use of conventional systems, some with a track record in related or in other applications. Details of the range of repair and protection measures currently in use are given in CIRIA Report “Management of Accelerated Low Water Corrosion in Steel Maritime Structures” [2]. The corrosion protection strategies that are applicable to ALWC are those based on conventional methods, primarily Cathodic Protection (CP), coatings of various types and concrete cladding or a combination of these (most commonly CP in combination with coatings). For new build or replacement structures conventional methods of protection would typically be the application of a suitable high performance coating system to the atmospheric, splash and tidal zone areas in combination with a well designed, installed and commissioned CP system, most commonly using galvanic anodes. Ongoing monitoring and maintenance of the corrosion protection system will be necessary throughout the life of the structure. The need for more regular inspection and monitoring will increase as the overall level of corrosion protection is reduced. For existing structures, where ALWC has already developed then conventional approaches include repairs to deteriorated elements, including steel renewal at critical areas, and the use of coatings and CP to arrest further deterioration by ALWC.
However, these methods have several disadvantages, including:
– they are generally expensive and difficult to apply
– some techniques can introduce new corrosion cells
– they can disrupt port operations
– installation/treatment is generally weather dependent
– they can introduce a risk of contamination
– CP systems require ongoing maintenance and costs throughout its life.
These techniques, particularly CP, were developed to deal with protecting steelwork against the common forms of corrosion and do this by protecting the whole structure rather than treating ALWC in a more focused way.
LATreat™ – an environmentally-friendly and sustainable solution
LATreat™ – a patented technology developed by Mott MacDonald and BAC Corrosion Control – is a “magic bullet” treatment that uses the components of seawater to sterilise affected steel and then deposit an environmentally-friendly protective calcareous coating to ALWC affected areas only. One of the drivers for the development of LATreat™ was the very stringent legislation regarding contamination of seawater and estuarine waters and concerns over the leaching of zinc into water where CP systems have been installed and the possible adverse affect that chemical coatings could have on the natural environment. Compliance with legislation is essential for a commercially viable technology and LATreat™ is an environmentally-friendly process, where all active agents are derived from seawater. The process also greatly reduces the requirements for new material resources and minimises waste, thereby enhancing sustainability of the infrastructure. The three optimised stages of LATreat™ are carried out sequentially using remote controlled equipment. The initial cleaning stage results in the release of the ALWC products, such as bacterial species, iron oxides and sulfides into the seawater. However, unlike CP, the treatment is specific to the areas where ALWC occurs and, therefore, the quantity of ALWC products removed is minimal. This is followed by sterilisation of the surface using short-term, controlled shock dose treatments to kill bacterial infestations. The final stage is a rapid in-situ application of a protective alkaline coating deposited from natural salts dissolved in seawater which provides local protection whilst normal marine deposits reform. Disruption to the working facility is minimised as application only takes 5 to 7 days and the design of the application equipment ensures that there are no requirements for berth closures. All of the equipment and cabling used in the treatment process is removed on completion with no requirement for any permanent installations and maintenance of expensive control equipment.
Development of the technology
Initial trials of LATreat™ were undertaken on ALWC affected steel sheet piles at 2 UK ports over 10 years ago. In 2001 LATreat™ trials were also carried out at 2 locations at Ipswich Port and in 2004 a patent for the method and apparatus wasgranted. A major step change in the development of LATreat™ occurred in 2006, when the then Department of Industry (now Technology Strategy Board (TSB)) awarded Mott MacDonald and its partners a research and development project to develop the process into a cost-effective treatment for ALWC. Funding of over £500,000 for the project has been provided by the TSB, Engineering and Physical Sciences Research Council (EPSRC) and the industrial consortium comprising, Mott MacDonald, BAC Corrosion Control Ltd, Aberdeen Harbour Board, Port of London Authority, Shoreham Port Authority and Hutchison Ports (UK) Ltd. The University of Manchester were the academic partner and have undertaken fundamental research aimed at understanding and optimising the LATreat™ process and assessing and improving the coating’s composition, application rate, longevity and durability. We now have a better understanding of how the calcareous coating forms and how to optimise the deposition phase of the process to obtain a durable, long lasting coating.
BAC has also developed and manufactured new apparatus which is robust and reliable under operational conditions. The new equipment is more compact allowing for easier on-site operation and provides more accurate control during the various stages of treatment. A key part of the work has been on-site validation to demonstrate to end users that the process is commercially viable. Over the last 3 years site trials have been carried out at several UK ports, including Aberdeen, Harwich International, London and Shoreham. These have been used to demonstrate the effectiveness of
LATreat™ in dealing with ALWC in operating port facilities. On-going work is being carried out to assess the durability of the coating but in 2007 we re-visited Ipswich Port and were able to see the LATreat™ coating applied some six years earlier. The coating was still in place with no visible signs of ALWC. The project is due to be completed at the end of the year and has already demonstrated that LATreat™ can effectively deal with ALWC and produces a sustainable and durable coating treatment when compared with other techniques. Over the next few months the focus will be on comparing the whole-life costs of LATreat™ with these other solutions, but we expect LATreat™ to be commercially available in mid-2010.
References
“Concentrated Corrosion on marine steel structures”, ICE Maritime Board, 2000
“Management of Accelerated Low Water Corrosion in Steel Maritime Structures”, Report C634, 2005, Construction Industry Research and Information Association.