Accelerated low water corrosion (ALWC) affects sheet-piled retaining walls and steel structures in marine environments. A three-stage protection technique (LATreat), which involves a ‘naturally’ occuring coating, could mitigate the increasing problem on existing structures.

Professor Stuart Lyon, one of a group of researchers investigating the process at the University of Manchester, UK, says, ‘This type of corrosion [ALWC] is a concern because the rates can be at least a factor of 10 times faster [than average low level corrosion]. Resulting in a potential failure in less than 10 years.

‘Under these circumstances, it would be advisable for new installations to employ cathodic protection (and this happens). However, it would be enormously expensive to demolish and rebuild all marine structures. A method to mitigate corrosion damage to existing structures would be of considerable benefit.’

The first stage of the three-step process performs surface cleaning and removal of deposits via an applied voltage to the structure. Through this step, ‘hydrogen is evolved from the surface – this helps to dislodge and remove the corrosion produce and associated biofilm’, explains Lyon.

The second phase involves  a voltage reversal. Where chlorine is directly generated on the surface structure by salt water electrolysis. Once the chlorine has reached a significant concentration (a few parts per million), it kills local bacteria and other microbial agents by sterilisation. ‘This is a similar process to the chlorination of swimming pools – i.e. direct electrolysis of salt solution to produce chlorine’, he adds.

In stage three, the applied voltage polarity is reversed again, reverting to the initial phase. The current flow, however, is controlled to a much lower value and is pulsed on and off. Instead of hydrogen, water is reduced directly by electrolysis and produces alkali locally at the structure surface.

Lyon outlines, ‘Seawater is a dilute solution of CO2 in the form of calcium bicarbonate. If the pH is altered and raised (such as by the production of alkali), the solubility of calcium bicarbonate reduces and a film of calcium carbonate is deposited. The coating, consisting of calcium carbonate and magnesium hydroxide is deposited by a similar mechanism, as it is naturally layered by the pulsed nature of the current’.

The team further claims the film has environmental credentials as it is derived from seawater. Furthermore, as the coating is not ‘applied’ using paint or spray it requires no human intervention.

The durability period of the film is yet to be confirmed. In addition, Lyon explains, that while the calcareous films formed during conventional cathodic protection are maintained by the continuous passage of current, in the LATreat process, the current is stopped once a significant film has formed. Seawater might therefore be expected to slowly dissolve the film, yet researchers claim the film can be easily re-established by repeat treatment.

Professor Julian Wharton, a Surface Engineering expert at Southampton University, UK, comments, ‘Exploitation of this innovative technology has a clear potential to provide cost effective protection of semi-immersed and tidal zone areas of marine structures affected by ALWC’.

The research, which has been carried out as an extension of CIRA’s commisioned study, Management of ALWC in Steel Maritime Structures, is now being commercialised by Mott MacDonald.