In the 21st century, every country in the world now has many hundreds of thousands of tons of steelwork that is being protected by hot dip galvanising or paint coatings. These structures can be telecommunication towers, steel road and rail bridges, crash barriers along the highways, steel lighting poles, overhead road and rail gantries and many more steel structures.
Paints and hot dip galvanising (HDG) has done an excellent job of keeping all of these structures protected against corrosion for a very long time, and in fact one of the oldest HDG transmission pylons is in Los Angeles, California, and it is now 99 years old, which is an incredible service-life to have from a single layer of zinc.
HDG zinc has been in use for well over a hundred years now, and it is still very popular all over the world. Why is this system so popular?
With regards to the maintenance of these structures, hot dip galvanising (HDG) is far superior to any paint system as it will never peel, crack, blister, flake off or delaminate in layers. It simply depletes in thickness from the top of the zinc coating, and the depletion-rate is dependent on the geographical location of the structure.
For example, an electricity pylon situated in the mountains could lose as little as 0.1 µm per year, and galvanised steelwork inside a coastal oil refinery could lose up to 8.4 µm per year (based on figures from the International Corrosion Research Association).
As the depletion is always from the top of the zinc layer, the lower sections of this layer will always remain intact on the surface of the steel, providing both cathodic and barrier protection properties, and will carrying on doing so until the zinc has all gone, whereupon corrosion will begin to corrode.
While the coating still has some remaining thickness, the 15 mm ‘throw’ will still be in place, so that where any physical damage from a heavy impact or abrasion takes place, the ‘throw’ (or in scientific terms the ‘linear polarisation’) will still be able to protect any exposed steel surface within 15 mm of the zinc’s edge.
The big problems can arrive when the zinc layer is fully depleted on a structure like and overhead gantry or an electricity transmission pylon, where the dismantling of the structure for re-galvanising is never an option. It is not always possible to turn off the power to thousands of homes and businesses to replace pylon components, because it could several days. It is also very expensive to close down highways in order to dismantle overhead sign-gantries and other heavy steel structures without causing massive traffic disruptions and lane closures, which run into thousands per week in costs.
During the early 1970’s, this future problem had already been forecast by a chemist, and he produced a liquid zinc product that could actually re-galvanise these structures on site, or in-situ wherever they stood.
So the galvanising product ZINGA was born, and it was developed into a solution specifically for the repairs and the re-building of HDG. Why? Because this chemist realised, even back then, that at some point in the future there would never be enough time or money to dismantle so many millions of structures worldwide and replace them with new ones. So the concept of ‘on site galvanising’ was born.
Like hot dip galvanising, ZINGA has some excellent attributes.
Properties of Zinga film-galvanising system:
It is:
- a one component, and hence no application or over-coating windows have to be followed
- a liquid zinc coating with 96% pure zinc in the dried film
- galvanic, with almost exactly the same electrical potential as regular hot-dip galvanising (1118 mV)
- a coating that depletes in a similar manner to HDG
- fast drying, and cam normally be walked on after one hour or even less in the arm months
- a coating system that can be applied in the middle of winter, where required for urgent repairs
- easy to apply over worn, depleted or damaged HDG and will rejuvenate it
- a pure zinc and as such will form a patina like HDG does
- the same as HDG in that it prevents corrosion-creep from damaged areas.
- easily paintable on pylons that require the red and white banding colours near airports
- not a paint and hence will never crack or flake off onto fields where animals are grazing
- easy to ‘re-load’ without producing any fall-out whatsoever.
The two treatments of steel structures:
Damage repair:
Where a galvanised or painted structure or one of its major components has been damaged, regardless of how small or how large the damage area is, it can be reinstated with the use of Zinga. The two zincs, HDG and ZINGA, a film-galvanising system (FG)S, have equal electrical potentials, so when the FGS is used as a repair, or when bolting a component that has been newly-repaired with FGS onto an HDG component using spun-galvanised nuts and bolts, there will never be any form of galvanic reactions whatsoever. Once they have both fully carbonated over a period of several months, it will be very difficult to tell them apart.
Build-up of HDG coating:
There have been instances where a galvanised structure has been inspected, and found to be ‘sub-spec’ as the zinc layer only has a NFT of, for example, 67 µm. In such a situation, the entire surface is sweep-blasted and the ZINGA film galvanising is then applied, and accurately (using a wet-film gauge) and the HDG galvanising is brought up to specification. Normally within two or three hours.
Like HDG zinc, any steel components coated with the ZINGA zinc can be thrown onto the back of truck without any worries. It will never crack or flake, and simply behaves as a galvanising layer.It also has a ‘throw’ of up to 15 mm to protect any exposed steel surfaces.
Old structures:
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The turntable as it has been for many years[/caption]
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The turntable after being film galvanised[/caption]
There are a great many very old steel and iron structures going back to the 18th century, and many of these have been protected with the FGS for many years now.
One that was done in 2018 was the old railway turntable at their Irish Rail works in Inchicore Dublin. It was totally blast-cleaned and given the FGS treatment, which means that future generations of people will be able to visit and see an important part of their history.
Another prime example is the cast-iron fountains in Peoples Park that were cast in Glasgow in 1851.
They were coated with the Film Galvanising System Zinga back in 2008, and the fine artitsic detail on the castings was fully preserved and with no loss of detail anywhere. So once again, future generations of families will be able to enjoy the skills and craftmanship from a bygone era.
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The cast-iron fountains in Peoples Park that were cast in Glasgow in 1851[/caption]
Modern buildings
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Zinganised box beams fully carbonated[/caption]
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Fully carbonated HDG flange[/caption]
Assessment of pylons:
When carrying out an assessment on a pylon, there can be times when the upper levels are more corroded than the lower levels, so for example a 70:30 tower will be one where 70% of the tower requires a sweep-blast, and the remaining 30% requires a full-blast back to bare steel.
The quantity surveyor will get the coating-rates for each blast-type from the contractor, and can then calculate how many hours will be spent on each tower.
Sweep-blasting will use a lot less abrasive/m
2, roughly 70 - 80% less, so again these figures can be factored into the assessment.
The final section of the assessment is with regards to the tower type. There are the normal towers that support the power-lines, and then there are straining towers, positioned at each place where the line changes direction. These towers are often larger than normal towers, as they have to take a loading of over a hundred tons. Normal towers have different classifications for the different electrical loading that they carry, and this means that they also have different heights.
Electricity transmission pylons are amongst the tallest free-standing structures in most countries around the world, and for this very reason they can be problematic when it comes to carrying out their maintenance regimes. They have an average height of 50 metres, comprising of a lattice structure, which means there are many hundreds of nuts and bolts on each pylon and a great many crevice-joints that have to be treated to prevent the ingress of water, salts and atmospheric contaminants.
Applications:
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Vapour-blasting a tagged-out pylon[/caption]
Different contractors have different methods of working, but a it also depends on the tower owners/operators, because the demand for power is so high in modern times that a tower can very seldom be totally closed down, and usually one side of the pylon will be ‘tagged out’, where the contractors can only work on one side of the tower on a certain day. Under these circumstances, a list of demarcated towers will be ‘tagged out’ so that the contractors can work through them systematically.
All of the towers will then have their one side blast-cleaned and re-galvanised during the specified ‘down’ period.
The blast-cleaning commences at the top of the tower structure, and the operators often work in pairs or in groups of three, The blaster will be up the tower, the number two person will help feed the blasting-hose upwards through the lattice-work and the third person is the ‘pot-man’ who keeps the blast-pot filled with abrasives. In the case of vapour-blasting, the pot-man will also refill the pot with water.
After the blasting has been completed, the tower components are brushed down to remove any dust and debris that has collected, and in the case of vapour blasting, the tower components are washed off with the vapour-blast machine which has been switched over to ‘rinse’ mode for a fresh water rinse.
After this phase is complete, two painters will scale the tower, where they have small pulleys hooked onto the cross-bars, and have climbing ropes fed through them and back down to the ground. The pot-man has already diluted the Zinga and has given it a good mix with a small portable power-mixer, and hooks the container onto the rope and pulls it up to the height where the painters are working.
The two painters each have a special harness to hold 2 x 5 litres containers for the Zinga, and while they are painting the pot-man opens two more containers for dilution and mixing. Many teams have a system whereby a full container of Zinga is always hanging near them, so they do not have to wait for a container to arrive.
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Zinga in Costa Rica[/caption]
As soon as a painter has finished his container, he simply hooks it onto the rope and takes the full one off the rope and carries on painting. The circular industrial brushes are the most commonly used types, as they are fast to get into corners, around nuts and bolt-heads, and for working the zinc into bolted crevice-joints. As they paint, they gradually climb down the structure, so that they never have to climb or step onto the freshly-applied zinc layer.
In the summer and autumn months the zinc layer is normally quite safe to walk on after 30 minutes, although it is preferred that they wait for an hour before doing so. After this time, the second layer of Zinga can be applied.
Some towers have their feet buried underground, encased in concrete footings, and others have their feet encased in concrete muffs above ground level. The buried type of tower-feet can be susceptible to stray-current corrosion where the power-lines follow a railway track, and the tower-feet mounted in concrete muffs above ground are very safe from such intrusive damage.
The above-ground tower-feet require a coating on top of the zinc layer to prevent the accumulation of atmospheric debris in the splash-back zone, which is normally the first 1000 – 1200 mm above the muff.
This coating also prevents splash-back from rain-drops bouncing back off the concrete and up onto the zinc surface. By also coating the entire upper surface of the muff, it prevents any water seepage into the concrete surface from above.
Zinga film galvanising system is distributed in Ireland by Igoe International Ltd, 135 Slaney Road, Dublin Industrial Estate, Glasnevin, Dublin D11 AW6D.
Website:
www.zinga.ie
Tel: 01 830 22 50