This article was published in the October 2010 issue of World Port Development. To receive a pdf of the article in its original format including charts and pictures please send an email to archive@worldportdevelopment.com
What lies beneath
Back in 1985 the first order for protection of quay walls against severe corrosion was performed by Acotec’s mother company General Coatings nv at quay walls north of the Belgian town Ghent. The Port of Ghent is connected with the North Sea via the 19-mile long Ghent – Terneuzen canal and the mouth of the river Scheldt. A sea lock system at Terneuzen provides passage between the canal and the Scheldt estuary. The berth walls which were completed in 1968 were steel pilings. The Z-profiles had copper added to the iron and were coated with coal tar at the steel mill. Severe corrosion of the steel piles must have started somewhere in the 70’s, less then 10 years after the construction and exposure of the bulkheads, as subsidences at the bank were noticed in 1978, indicating pile holing below water. The canal water was seriously polluted by industrial waste water in that period. The composition of the bulk water was at the edge of fresh and slightly brackish water, containing around 1000 ppm (parts per million) of salts. In 1978 underwater corrosion was incidentally observed. Acotec’s mother company started to investigate and discovered for the first time a strange form of severe corrosion, which was unknown at that time by the Authorities and was considered as an anomaly, an unusual but local phenomenon of pustule formation inside the biofilm covering the piles below water, and supposed to be caused by water pollution. Investigations were based on observations by divers, steel coupons cut out of the piles and samples of the biological film and tubercles that were covering the underwater part of the pilings, as well as research by the Brussels Pourbaix laboratories CEBELCOR. In that period corrosion speeds were considered to be of the order of magnitude of 10 microns per year, and thus hardly significant for strength calculations in comparison to plate thicknesses of 10 mm. Corrosion below water was not considered as a harmful phenomenon. It became clear that the severe corrosion with linear speeds of corrosion of about 1mm per year (10 mm of steel had been perforated over 10 years at these bulkheads) revealed a new and extraordinary form of corrosion. Pourbaix’s institute CEBELCOR discovered that the origin was bacterial and that it was MIC or Microbially Induced Corrosion.
The photo to the left shows, after local dry setting of the bulkhead, the biofilm covering the steel profiles below water. The bulge of a tubercle is visible in the centre of the photo and reaches a height of 5cm (2 inches). The tubercles are built up by iron oxides from the destructive activity of micro-organisms or their by-products, decaying the iron at an unusual speed (photo below shows the build-up of an example of a tubercle). Professor Hector Videl, in his Manual of Bio-corrosion published in 1996 calculated that “..by the presence of micro-organisms, (and) corrosion rates can be 1,000-100,000 times greater than in their absence.” The photo to the right is taken after removing biofilm and tubercle by high pressure water jet and shows the imprint of the bacterial tubercle, the craterlike corrosion pattern, where a steel loss of 4,5 mm (180 mils) has been measured.
Pioneering work
Acotec became the pioneer in bacterial corrosion research in the marine environment. Due to the seriousness of the damage and the very high speed of corrosion, the Authorities issued a Request for Proposal in 1978 to stop the severe corrosion. No working solution was however available at that time. Acotec continued research and developed a method to stop permanently the biological corrosion, based on 2 decisive inventions. The method consists on a purposeful patented Dry Setting Installation DZITM and a proprietary chemical coating system HUMIDUR® specially developed for the target of lifetime protection of steel pilings against severe corrosion. The photo above left shows the fully operational DZI set, put together on a barge at a busy port at low tide. The 5 floors DZI is hanging at the hoisting equipment, ready for dewatering a new section of the quay wall. The wall to the left of the DZI has been protected down to 7.5 m (25 feet) below the mean high water level (MHWL). To the right of the DZI, is the not yet treated zone which is marked by the splash zone between the brown atmospheric zone and the dark fouling to high water level.
The photo to the right shows a view within the DZI. The cofferdam has been dewatered and is in operation.
A global threat
Over the years it became obvious that the first sites protected by the HUMIDUR® system, were not isolated cases of Microbially Induced Corrosion but just local infections forming part of a global threat for steel structures in marine environment. This global problem of ALWC happens also to steel sheet pilings exposed to seawater as to fresh water. The formation of tubercles is a phenomenon that does not appear on steel surfaces exposed to tidal activity. Due to shear stresses of the waves the build-up of tubercles is inhibited or impossible. Corrosion holes are tightened and insufficient residual steel thickness is repaired by means of welding patch plates or pre-folded reinforcement plates. The HUMIDUR® system proved its excellence in permanently stopping corrosion.
Proven technology
In these last 25 years Acotec has gained extensive knowledge and experience in fighting marine corrosion and structural repair techniques. As a pioneer in this industry, Acotec continues to work in Europe and have expanded their activities worldwide. Testimony to their success in this field is that more than 25 years later, exactly the same corrosion pattern of the piles at Terneuzen can be seen at the coating surface. It is evident that not one single micron of steel has been lost thus demonstrating the quality and durability of the Humidur® coating system.