Background

In 1999 a meeting was held with the Irish Department of Marine and Natural Resources to discuss the current state and proposed corrosion-protection of the 312 steel pier legs beneath the main pier at Killybegs, which had a fish-processing factory build on top of it, and also the pier legs of the Blackrock pier situated across the harbour. When Zinga was proposed as the corrosion-protection system, they were not convinced that a layer of zinc being applied onto steel pier-legs in a tidal zone would last more than a few weeks. The original planning was to achieve a 50-year life for the whole structure, and the currently available information back in those days stated that pier legs with a 40mm wall-thickness would last the full 50 years. However, at the time of the construction, it was not known that a bacterium within the ‘desulfo’ group which was first discovered in 1931 was spreading rapidly around the coastlines of several countries. Within a few years, it had reached the shores of Donegal. This anaerobic bacteria, which colonises behind biofilms on steel pilings, can ‘eat’ its way through 2mm of mild steel per year. Hence when the Killybegs pier-legs were measured by X-ray in 1999, which was 25 years after they had been installed, they had a reduced wall-thickness that had depleted from 40mm down to 16mm, and the Department of Marine and Natural Resources wanted another 25 years of service life for the structure. This was offered in the form of a Zinga coating on all 312 of the pier legs.

Preparation

The legs were coated with a mixture of shellfish, barnacles, metallic scale and other contaminants which all had to be removed before any blast-cleaning could be carried out. So in 1999 the carbon-steel piles of both the main fish-landing pier and those on the Blackrock Pier on the other side of Killybegs harbour were cleaned-off using an ultra-high pressure (UHP) water-jetting lance operating at 2700 bars, in order to remove embedded chlorides as well as chloride-ions inside the surface of the steel. This jetting-pressure also removes barnacles, scale and all corrosion products, leaving the steel surface very smooth and totally free of debris. After the UHP-cleaning phase was completed, the pier-legs were then blast-cleaned to a SA2.5 cleanliness according to ISO 8501 using a garnet abrasive, as blast-cleaning over open bodies of water falls under the Environmental Protection Agency (EPA). The specified blast-profile was in the range of Rz50-70μm, and due to the extent of the pitting that had taken place over the years, accurate measurements were very difficult to take. Some of the pits across the surface were up to 4mm in depth (4000 µm) and this made any profile measurements inside the pits virtually impossible.

Tidal

As with many marine projects, the tides played a major part in the overall planning of the project’s pre-preparation, and this was done several months in advance of the project start-date. The contractors had to be on site and fully set-up at least three or four hours before the tide went out, because the operators doing the UHP blasting had to ‘follow the tide’ and began water-jetting at the top of the pile, working downwards as the water-level receded. Once the water-level had dropped to the low-water mark, work began on the next pile and during the one-hour 'dwell time' where the tide never moved, the operators would clean as many piles as they could, removing all shellfish, scale and debris, leaving the piles clean and much smoother. On the following day the same system was employed, but this time it was the blasting team who “followed the tide down” and blast-cleaning the metal surfaces as soon as they were dry. On such projects it is not uncommon to use a large compressor (750 cfm) and run two blast-pots simultaneously in order to coat two pier legs each day. Timing is everything on these projects, and a late start or a delay caused by problems with a compressor etc could easily lose an entire day of production, causing project over-runs. Another factor to consider was that this project was started in late November and carried on throughout the winter months, so the working conditions were severe at times. However, the winds blowing onshore tended to dry-off the pier legs quite quickly and so was seen as a positive factor.

Application

The piles were then coated with four coats of Zinga @ 45 µm DFT each, and each layer of the coating system had to be applied at full low tide. This meant that the contractors had to be on site at least three to four hours before full low tide, as they had to start water-jetting the relevant pier-legs as the tide went out, and they had to ‘follow’ the water-level as it dropped. At full low tide, they had one hour of ‘dwell time’ in which the water never moved, and this was when the blast-cleaning was done, followed rapidly by the application of the zinc layer by brush. The EPA does not allow the spraying of coatings near any open bodies of water, so brushing was the only allowable option. The painters had to work from the bottom of the pier-legs upwards, in order to stay ahead of the rising water once the tide had started coming in again, so timing was everything. Tidal coating applications are always carried out on an extremely strict schedule, because any delays can cause them to miss that tide, which means a 12-hour wait for the next tide. The maximum thickness applied at each low tide was 50 µm DFT, in order to let the zinc layer dry thoroughly between tides. This also gave some time for the painting operative to spray fresh water onto the dried zinc surfaces in order for them to seal themselves with zinc salts. At each low tide the process was repeated with the applications, so the wet zinc layer was water-blasted at 150 bars pressure to remove any surface chlorides, left to dry, and immediately coated with the next layer of zinc. Although the full specified thickness was around 180μm DFT, many of the piles had a zinc layer well in excess of this in some areas, but primarily within the deeper areas of pitting, with some piles showing film-thickness readings over 350 μm DFT. Due to both the highly pitted surfaces of the steel piles as well as the depth of the pitting in these piles, it was difficult to obtain really precise readings during inspections. Follow-up inspections revealed that the annual loss of steel-thickness was reduced from 2mm per year (2000 µm) down to zero. This is because the class of bacterium responsible for the metal loss are classified as ‘electron receptors’, and the flow of electrons from the zinc layer into the steel was absorbed by the bacteria, upsetting their feeding and breeding cycles, and with the eventual breakdown of the bacterium colony itself.

Further information:

The Zinga Film Galvanising system is available through Igoe International Ltd, 135 Slaney Road, Dublin Industrial Estate, Glasnevin D11 AW6D. Tel (01) 830 22 50. Website: www.zinga.ie Contact: Brendan Igoe CPD presentations available on request.