Runcorn: A Project First for Europe

Transcription

1 Page1 Runcorn: A Project First for Europe A Combination of Film-Galvanising and Hot-Dip Galvanising This is an interesting look at the first project in Europe to use both film-galvanising and hot-dip galvanising to address certain project requirements. Background development: The Zinga film-galvanising process has been around globally for nearly 40 years now, and it has been used on a great many projects throughout the UK and Ireland for almost 20 years during this period. The system of film-galvanising was introduced during the 1970 s, when it was found that hot-dip galvanising was awkward to repair under certain circumstance, as to do the job correctly involves using a stick of zinc alloy and a gas torch. Not all damaged surfaces were easily accessible to carry out such work. According to the repair standard ASTM A780, the surface of the damage-area on steelwork must first be cleaned of with an angle-grinder or lightly blast-cleaned, and then pre-heated to a minimum of 315 o C for application of the repair material from a stick or using alloy powder. Sometimes pre-fluxing of the damage-area is also required, in order to get the steel surface clean enough to allow the alloy to bond properly with the steel surface. This often entails wire-brushing the affected area while it is being heated up to 315 o C, and care has to be exercised to avoid burning the existing adjacent zinc coating. The chemists who developed the Zinga film-galvanising system wanted to create a material that could be applied at ambient temperatures, so as to remove the health and safety aspects of working with high temperatures and to give more control over the application. Especially when the galvanised steelwork was high off the ground or where it was situated in a confined area. So contractors began repairing small areas of damaged hot-dip galvanising (HDG) using the filmgalvanising system of blast-cleaning and then either spraying or brushing the affected area/s with a liquid zinc coating at ambient temperatures. Many of these repair-areas would have been blast-cleaned anyway, so no additional costs were incurred during this process, and because the heat element had been removed from the process it rapidly gained favour with contractors.

2 Page2 Soon contractors realised that the while the film-galvanising process was excellent for doing repairwork, it could also be used in place of HDG where there were obstacles such as trying to galvanise tubular or closed-in box sections. Designers did not want their carefully-designed structures drilled full of de-gassing holes to avoid explosions in the dip-tank, and they were also concerned at the risk of distortion. This meant that items like certain types of beams or welded frames could not always be dipped into a tank of molten zinc at 450 o C. This new system was offered to members of the European Galvanisers Association, who initially rejected it as not being proper galvanising, or else they perceived it as an opposition process, even though it would be them who were to going apply it!! Over time, a few of them tried it out and discovered that whilst the film-galvanising did not form alloys like the hot-dip version, its molecular bond with the steelwork did in fact invoke a galvanic charge that was identical to that of the HDG ie 1.04 volts and offered the same levels of protection. Zinga film-galvanising is now used in more than 50 countries for on-site galvanising of steelwork in nuclear power-stations, on bridges, telecommunication towers and on many other large structures. Film-galvanising has one other totally unique property: it is used in over 15 countries for the re-loading of old HDG on power-transmission pylons. The original hot-dip galvanising is restored to its original zinc-thickness by being steam-cleaned and then coated with Zinga. New project: The designers of the 180 million Runcorn Thermal Power Station near Liverpool are building a structure which is comprised of a mixture of normal RSJ s and massive box-beams. The box-beams are 50 feet in length and weigh 20 tons each and are totally film-galvanised. If they were drilled for de-gassing for the HDG process, the amount of molten zinc potentially passing into the interior of even one beam could be quite staggering, plus the potential distortion factor over such a long length could be high, considering that there are twelve weld-seams on each beam plus the end-flanges are drilled with rows of mounting holes. So the obvious choice was to have a mixture of film-galvanised and HDG beams, having the RSJ s done with HDG and the box-beams done with film-galvanising. This method of galvanising will totally avoid any risk of distortion, whilst giving the beams the same protection as they would have had from HDG.

3 Page3 Note: This is exactly the kind of use of film-galvanising that has been promoted globally for a great many years. There has never been any doubt that whilst film-galvanising complements HDG extremely well, it cannot and will not ever replace it. Each form of galvanising has its own market, and film-galvanising has been well marketed globally as a complimentary form of galvanising to HDG, being used in areas where HDG can never be used, such as on in-situ galvanising of bridge structures, large silos, telecom towers or marine pilings etc. By combining the two different forms of galvanising on this current project and using galvanised fixing bolts, the entire structure is fully balanced out electrically, whereas all of the steel components are totally and wholly galvanised, with no anodic or cathodic areas anywhere. So no areas of zinc will deplete preferentially. A view of one of the 20-ton box-beams that were built by Fisher Engineering in Northern Ireland being examined in the paint-shop of Jack Tighe in Retford. The length of the beam is quite apparent in this photograph. One of the box-beams being inspected for the thickness of the zinc layer. Film-galvanised layers can be measured with a normal paint-thickness gauge, which makes life a lot easier for inspectors. Having 22 mounting holes on the flanges, the slightest bit of distortion on these long beams could prove to be disastrous, and a very costly nightmare to try and rectify.

4 Page4 Where film-galvanised steelwork is put out into the rain soon after the zinc has been applied (around 30 minutes) the zinc s reaction to the water rapidly produces surface salts of zinc oxides and zinc carbonates. The carbonates form the darker surface-salts, and in some cases the film-galvanising takes on the appearance of weathered HDG! This is prevalent near the coast. Two film-galvanised beams have been bolted together, and the holes that have been drilled and tapped for the next beam to go into position can be clearly seen here. This view from the top of the structure show its proximity to the River Mersey. The large film-galvanised beams that form the upper section of the structure can be clearly seen in this photograph.

5 Page5 This is a perfect example of how film-galvanised and HDG beams can be bolted together, illustrating the total compatibility and harmony in which they can work together in a saline environment to solve potential corrosion problems. Unique qualities: One other quality that is totally unique worldwide to the Zinga film-galvanising system is the fact that it is fully weldable, with minimal burn-back. On 3mm plate section, the burn-back is around 2mm on both sides of the weld-seam. On 20mm plate sections, the burn-back is never more than 5mm on either side of the weld-seam. Secondly, the zinc coating does not always have to be mechanically ground off before welding takes place. Where the zinc layer-thickness is below 80 µm DFT, it only requires manual wire-brushing on the two mating faces to remove any loose oxides and carbonates before welding is carried out as normal. Where the zinc coating is quite heavy, the excess zinc thickness can be removed using a sanding disc (as opposed to a grinding disc) and this means that (a) the zinc does not have to be completely removed from the steel as it is easier to control the removal and (b) the danger of removing any steel from the surface is minimal. Above: a bracket modification is added in-situ after welding, the new bracket is in place.

6 Page6 Some of the beams required modification after installation, and needed some heavy 25kg brackets welded on the top surfaces. The beams were cleaned off and the brackets welded on. In the photograph above right, the engineers have mechanically removed the zinc. As blast-cleaning was not allowed on site, the welds were bristle-blasted to a surface-profile of Rz 70µm and then the liquid zinc was applied by brush. Where the newly-applied zinc overlaps the existing zinc layer, it partially re-liquefies this existing zinc and simply melts into it, forming a new single homogenous zinc layer with no breaks or deformities. The first four large film-galvanised box-beams are in place. A view of the front section of the structure, showing some of the huge amount of galvanised steelwork used on this project Conclusion: This contract is the first of its kind in Europe to use the combination of film-galvanising and hot-dip galvanising, and it clearly demonstrates that engineers and architects can use the film-galvanising system to solve potential problems during the pre-construction phase of any project. It also shows that both galvanising systems are identical in their protection values, with both systems self-generating a protective galvanic voltage of 1.04 volts, and that they can work together in perfect harmony.

7 Page7 Furthermore, this demonstrates that both systems can also be used in a wide range of geographical and climatic locations to offer the maximum corrosion-protection to both steelwork and ironwork, where the film-galvanising is also used to fully protect cast-iron components as well. The use of these systems obviates the requirement of having to apply heavy paint coatings. The filmgalvanising was applied at a nominal thickness of 140µm DFT, whereas an epoxy paint system would have required a minimum film-build of 400µm DFT to offer a similar corrosion protection in a saline environment.