Unheard of in the UK until around 15 years ago, the phenomenon of concentrated corrosion on marine sheet piles known as Accelerated Low Water Corrosion (ALWC) is posing port executives and facilities managers with unexpected engineering and financial challenges. A particularly virulent form of microbially induced corrosion, ALWC is characterised by localised areas of soft orange corrosion products, overlaying a black organic sludge containing colonies of several types of bacteria. Most incidents occur within a 0.5m band of the Lowest Astronomical Tide (LAT) making detection at a sufficiently early stage very difficult. The problem is not confined to salt water, and sheet piles exposed to fresh water can exhibit similar and sometimes more aggressive concentrated corrosion.
Corrosion rates for steel in a wet environment are normally predictable at around 0.1mm per year and are therefore manageable by increasing the thickness of “sacrificial” steel to give the required design life. However, concentrated corrosion rates of up to 1mm per year have been reported with the first signs of distress only becoming evident when it is too late, backfill has been washed-out and quays undermined. Detection is also hampered by the random nature of the corrosion and whilst it can preferentially corrode some parts of the steel section, the microbial attack is rarely uniform.
From this it is clear that the most economical solution is to provide protection from accelerated corrosion at an early stage, thus maximising the design life and avoiding the risk of catastrophic, sudden failure with all the associated expense of repairs, lost business and risks to health and safety. It is now widely accepted that some form of preventative action is required to stop or at least delay the onset of corrosion and coatings play an important part in the defence strategies available.
The ability of cement to protect steel from corroding has been utilised in reinforced concrete construction for hundreds of years. The high alkalinity of cement, resulting from the formation of calcium hydroxide as a by-product of its hydration, enables the formation of a passive layer of gamma ferric oxide which stifles the corrosion process. We have all seen heavily corroded steel reinforcement being surrounded and protected by the alkaline environment afforded by concrete, and provided this alkalinity is maintained, corrosion rates will remain at an acceptably low level. The use of this concept is not new in the protection of steel in marine environments and in the 1920’s cement slurries modified with a natural rubber latex were used to protect ships’ hulls. The cement matrix consists of a silica gel structure, which under microscopic examination reveals a surprisingly porous sponge-like appearance. Coastal structures are particularly vulnerable to chloride ions which readily diffuse through the pores and capillaries resulting in premature breakdown of the passivating layer to initiate corrosion. Whilst chloride ions are far smaller in diameter than the pores in a cementitious matrix, technological advances have equipped us with the techniques to densify a cement-based formulation, giving far better levels of protection than can reasonably be expected from organic coatings.
For the last 20 years, Flexcrete has led the development of advanced cementitious coatings with its unique blend of polymers, fibres and reactive fillers designed to minimise porosity and permeability, thereby maximising physical characteristics and durability. Highly reactive pozzolanic ingredients are used both to physically block pores and to further densify the silicate hydrate gel structure by reacting with the hydrated lime from the cement hydration. One such super-pozzolan, microsilica, is a waste product from ferro-silicon steel production, and is 100 times finer than Portland cement and almost 5 times more reactive. Research has shown that not only does microsilica refine the pore structure on the macro-scale, it also has excellent chloride ion binding capacity. An ongoing independent test at Taywood Engineering Laboratories demonstrates that a 2mm Flexcrete cementitious coating has resisted chloride ion penetration in a pure diffusion cell for over 15 years, consistently outperforming resin coatings.
The inclusion of a thermoplastic resin in the form of an advanced styrene acrylic copolymer, physically blocks pore and capillaries whilst imparting greatly enhanced flexibility, adhesion and resistance to abrasion and chemical attack. However these effects are magnified by the incorporation of a thermosetting epoxy resin, which also improves adhesion and gives greater tolerance to lower forms of surface preparation.
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