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ALWC, a false problem

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Author(s): Acotec NV, Erembodegem, Belgium

ALWC should not exist, it is the result of neglecting structurally sound measures. One should not go to a doctor and take medicines for skin disease if he never takes a bath. ALWC is very simple to prevent for a quasi unlimited time. Protecting the piling in situ with a dry set device and a coating system, which disposes of a proven background is a fail-safe solution.

The Humidur® coating has been exclusively developed, in combination with a cofferdam for dry setting, to protect pilings against accelerated corrosion in aqueous environment. Humidur® disposes of a demonstrated resistance of 20 years in practice and on large scale, which proves its lasting properties.

The installation cost for the Humidur® protection averages £6.50 or €10.00 per square metre per year. It is generally accepted that all maritime steel, as well as construction steel, should be protected. And indeed, their surfaces are always carefully protected. There is however one single exception: static maritime structures such as quay walls.

The severest corrosion on earth

The irony is that exactly these precious structures, which should operationally never be out of use, are exposed to the severest corrosion on earth. Acotec have at their disposal thousands of photos illustrating this phenomenon, including a brochure which shows that at the Belgian seaports as well as on any inland waterway, accelerated corrosion is present. During Acotec’s daily activity the evidence is met of alarming steel loss by the frequent appearance of holing and crater-formation (pitting).

There are bacteria which can create an environment of up to about 10 per cent sulphuric acid. A published table of corrosion rates of steel under varying concentrations of sulphuric acid shows a peak at a concentration of 10 per cent, with a steel loss of 58mm per year. Such data is based on laboratory work under ideal conditions with only three components: water, sulphuric acid and steel, but in reality there are always disturbing factors, nevertheless it is a strong indication
for the corrosivity of steel in practice.

In addition Acotec have met several cases where plate thicknesses of at least 10mm showed holing after only 10 years of service (see Figure 2). Simple linear, and thus underestimated, calculating results then in an average rate of corrosion of one mm per year. Furthermore, from the scientific side, H.A. Videla brings to light the worrying seriousness of microbial corrosion and refers to J.A. Costello that thus the corrosion rates can be 1,000 to 100,000 times greater than the ones of abiotic corrosion.

Prof. Videla explains: “…biocorrosion cases where current densities increase dramatically in an exponential way due to sharp increases of anode/cathode relationships that are due to by microbial interactions within the biofilms attached to the metal surface.

That is why the damaging effects of biocorrosion are frequently unexpectedly serious and highly localised.”

Taking into account their economical importance of staying operational, it is obvious that expensive and risky structures, like quay walls, should not stay unprotected against the severest kind of corrosion: the exposure to bacteria and their metabolites. With this knowledge of biological corrosion in mind, it should be obvious that just welding reinforcement steel plates over holes and over areas of slight residual thickness is not a wise measure.

Besides the well known metallurgical risks for corrosion when protecting pilings with welded patches, biocorrosion, if it is the origin of the damage, is expected just to start up again and with renewed intensity, as all of its components were already present in the environment. It is just a new start without any hindrance of biomass. Figure 3 shows a detail of a berth consisting of Zprofiles with plate thickness of 10mm. 20 years after construction, holing had been stated.

The linear calculation of corrosion of 0.5mm per year was encouraging. In order to reach another 20 years, prefolded plates of 10mm were systematically welded, when holing came into sight. But within three to five years the same holes appeared at the same places.


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Edition 34

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