Bridges are a significant investment and can span roads, railways or rivers, making maintenance of the steel structure costly in both time and money. For this reason, a high performance and proven corrosion protection coating is often required and specified. Metal sprayed coatings are commonly applied by flame spray or arc spray methods onto both new and existing bridge structures.
Image Provided Courtesy of LUMEN.
As bridge steelwork (e.g. girders, bridge beams) is very large, metal spraying is an ideal anti-corrosion solution. There is no limit to the size of job and work can be completed on-site without the need to dismantle existing structures.
Images Provided Courtesy of LUMEN.
Around the world, thermal sprayed zinc (TSZ), thermal sprayed aluminium (TSA) or thermal spray zinc/aluminium (TSZA) coatings are commonly specified and applied to give a life to first maintenance of the coating in excess of 20 years. The choice of material often depends on the environmental conditions expected for the bridge structure or sometimes local familiarity with a given material.
Image Provided Courtesy of LUMEN.
The flame and arc spray process has been used to protect bridges from the elements and road salts for decades, minimising maintenance and increasing the life of the road and rail network.
Thermal spray coatings are not only applied to structural steel bridges, they have also been applied onto concrete bridges to give corrosion protection to the reinforcing bars within the concrete structure. This is a common repair method used to extend the life of a bridge suffering from rebar corrosion. This process has been used to protect bridges for over 100 years.
1947 - Bridge Beams Zinc Sprayed with the MK16 Flame Spray System (superseded model)
Forth Road Bridge
Most bridges are prone to corrosion from the weather and from salt used to ‘grit’ roads in winter. Bridges such as the Forth Road Bridge face the additional threat from the harsh sea salt environment in which it is situated. Metal spraying is the best protection from corrosion in this environment.
When the Forth Road Bridge was opened in 1964, by her Majesty the Queen, it was the longest single span bridge outside the United States. The road bridge, not to be confused by its neighbour, the Forth Rail Bridge, is 2.5km in length, including the approach viaducts. Metallisation was chosen to metal spray the steel structure, which makes up the bridge, to protect it from corrosion. The contract to protect the bridge was split between Metallisation and Merseyside Metal Sprayers Ltd. The latter company was commissioned to treat the 1200 balustrades and vehicle grillage panels.
The project took around two years to complete. The process started with the delivery of the prefabricated steel sections to the Metallisation plant in Drem, near Edinburgh. Each of these giant sections was then treated individually, starting with the cleaning process using grit blasting. Grit blasting is the most effective way to remove scale and rust from steel to provide a clean, prepared surface for the zinc to adhere to. The second stage was to spray the steel surface with zinc – more – using the Metallisation Flame spray process, which is still used today, with the Metallisation MK33 system. The zinc was supplied at the time by Charles Clifford, manufacturers of zinc. Charles Clifford subsequently became an integrated part of Metallisation.
In the Metallisation Flame spray process, the raw material in the form of a single wire, cord or powder, is melted in an oxygen-fuel gas flame. This molten material is atomised by a cone of compressed air and propelled towards the work piece. The molten spray solidifies on the component surface to form a dense, strongly adherent coating suitable for corrosion protection. Major advantages of the Flame spray process are that the coatings are available for almost instant use with no drying or curing times and there is no risk of damaging the component through heat distortion.
The modern day version of the MK33 is the Metallisation Mark 73, which represents a breakthrough in anti-corrosion spraying. With a new choice of continuous or stop/start nozzles, throughputs have been raised by 33%. With its predecessor already one of the fastest guns around, this new development puts the Mark 73 way ahead of the field. The combination of the new head with an improved valve and pilot assembly and a high power air motor drive must make this the fastest and most reliable system in existence.
The metal spraying process, coupled with the equipment used in protecting the Forth Road Bridge, was very innovative for its time. Although, now watching the video of the work being carried out, it all seems incredibly antiquated and labour intensive. When the large prefabricated panels were being sprayed a rather unique process was devised in order to control the pistol heads of the MK33.
Using brass plates with holes punched in strategic places, the pistols were controlled and moved by a flow of air being pushed against the plates. Whenever the air hit one of the strategically placed holes it caused the pistol head to change direction. Thankfully, the controlling of the pistol heads is now far simpler.
The third and final phase in the metal spraying process was to apply the etch primer and then the surface was painted. Even this stage in the protection of the steel – more – structure was researched and evaluated, to ensure the longevity of the life of the bridge. Sample steel panels were taken to a laboratory and exposed to a continuous atmosphere of salt spray, much stronger and harsher than the reality that the bridge would face.
The same panels were also exposed to the extreme pressures of an accelerated weathering machine, to see just how strong and protective the Metallised surfaces were. After 18 months of testing both zinc sprayed surfaces and the paint surfaces, the Metallisation process had shown no sign of any breakdown. This included a test area that had been deliberately scratched through to the metal itself.
The Forth Road Bridge is still going strong today. This is just one project of many that is testament to the metal spraying process in protecting steel structures from corrosion.
Ross Swing Bridge Refurbishment
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Image Attribution: Jowaninpensans / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
Reason for use: Refurbishment and corrosion protection.
The historical Ross Swing Bridge by the Isles of Scilly ferry port on Penzance sea front was subject to a major refurbishment to improve its appearance, reliability and reduce the costs of future disruption and maintenance. The works were carried out by Cornwall County Council’s contractor Cormac and part of the refurbishment included the thermal spraying of the bridge with aluminium to protect it from corrosion.
Bridge Corrosion Protection
Metallisation’s customer, J Kirkaldy Limited, a preservation and painting specialist, won the competitive contract to protect the bridge from corrosion, using Metallisation’s MK73 Flame spray equipment.
The original swing bridge was built in 1881 from an old railway turntable to give access to the Abbey Basin and dry dock. This bridge was replaced in 1980, constructed by a local Cornish company called Visick’s Foundry. The new bridge was painted with a multi-coat paint system, which over the years broke down, resulting in significant visible corrosion, which if left, would have resulted in potential structural damage. The total refurbishment of the bridge took about 12 weeks to complete and included the renewal of the hydraulic actuating mechanism, repair and repainting of the steel structure and the replacement of the timber bridge deck.
The bridge, which opens to allow boats into the dry dock approximately 20 times per year, is not only subjected to the harsh sea environment it’s located in, but also the wear and tear of the traffic it carries into the Penzance seafront and harbour areas.
Metal Spraying
In 2008, while the Planning Transport and Estates Department considered how to protect the bridge long term, Cornwall County Council consulted with Metallisation and TSSEA. The aim of the discussion was to explore and evaluate thermal spraying as an option to protect the bridge from corrosion, instead of a standard paint system.
Scott Perry, Civil Engineer, Cornwall County Council, says: “Following consultation with Metallisation and TSSEA and considerable research into corrosion protection, we opted for the Highways Agency specification for thermal spraying due to its proven durability and long term corrosion protection. We were also very impressed with Metallisation and J Kirkaldy, as both companies inspired confidence and were extremely knowledgeable during our discussions. This was reassuring for us, as it’s our first experience of using thermal spray as corrosion protection for refurbishment.”
The option to use thermal spray is not only promoted within the industry but is also backed up by its inclusion in independent International Standards (EN ISO 14713), as a system to provide unrivalled corrosion protection in a number of environments. This is why it appears in specifications for key civil and marine applications, as the Highways Agency and Network Rail coating specifications, as well as many offshore oil industry companies.
Before work could start on the major overhaul, the bridge was lifted one metre off the ground to allow access to all areas. The turning mechanism was then dismantled and sent away for refurbishment. Once the wooden deck was removed, which was replaced by a lightweight aggregate concrete deck (weighing the same as the timber deck), the steel structure was covered with a large industrial shrink wrap enclosure, under which all of the thermal spraying took place.
The tent was fitted with an extraction system, de-humidifiers and heaters to create the optimum environment for surface preparation, metal spraying and painting of the bridge. The enclosure also ensured that dust generated during the surface preparation and coating process was contained and didn’t contaminate the local environment.
To meet the recommended Highways Agency specifications, surfaces were first grit blasted to SA 3 cleanliness, using garnet, and then sprayed with 100µm minimum of aluminium using Metallisation’s MK73 Flame Spray system. A single coat of epoxy sealer was then applied at a spreading rate of 15 – 20 metres² per litre to seal the Aluminium metal spray the same day, followed by three coats of paint. The inclusion of an aluminium coating should guarantee a protective, maintenance free surface well in excess of 20 years, which is important considering the harsh environment the bridge is located.
The bridge steel frame is 6.8m wide, 27.5m long and is 686mm high. In total 627m² of steel structure was metal sprayed including barriers and handrails. Over time the refurbished bridge will require some cosmetic enhancements to the paint system, but this compared to the previous high level full maintenance, will mean the bridge is closed for less time, which will result in less disruption for local traffic and businesses during the important tourist season. This will also see a significant reduction in maintenance costs.
Flame Spray Process
During the wire flame process (mainly used for anti-corrosion coatings) a wire is fed by a driven roller system through the centre of an oxygen-propane flame where it is melted. An annular air nozzle then applies a jet of high-pressure air, which atomises and projects the molten material onto the work piece. The driving of the wire is typically via an air motor and gearbox that forms part of the pistol. Wire is typically dispensed from coils or production packs (drums). Major advantages of the Flame spray process are that the coatings are available for almost instant use with no drying or curing times. There is very minimal heat transferred to the component being spray so damage from distortion, which can be seen with galvanising of thin structures, is not experienced. As the coatings are actual metals (typically pure aluminium or zinc for corrosion protection), they are very durable and hard wearing compared to many paint systems.
“Thermal spraying the Ross Swing Bridge was the most obvious solution as far as we were concerned. It has been proven time and again that it provides excellent, long term protection against corrosion, particularly for large, exposed steel structures. We are also very proud that we have managed to undertake a project such as this in the heart of winter 2009, one of the coldest we have had for many years. This is a huge advantage of the Metallisation Flame Spray process, as there is no drying or curing time needed. Due to the importance of completing this job on time, we have just ordered another MK73 pistol from Metallisation to ensure we can meet the strict deadlines and ensure no further disruption is caused to the local community.”
New Zealand Bridges
Many Bridges worldwide have been metal sprayed for corrosion protection. New Zealand is at the forefront of using the Metal Spray technology with the production of various guides:
- AS/NZS 2312:2002(1) “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings”.
- NZS 3404.1:2009(2) “Steel structures Standard – Part 1: Materials, fabrication, and construction”.
- HERA Report R4-133:2011(3) “New Zealand Steelwork Corrosion and Coatings Guide”.
New Zealand's Transport Agency published "Protective Coatings for Steel Bridges" (2014), a guide for Bridge and Maintenance Engineers, stating that metal coatings on bridges can give up to 40 years of protection until first maintenance.
New Zealand has many bridge applications including the Auckland Harbour Bridge, NZ (1958) and the Taranaki Footbridge, NZ (above). Other bridges that have been coated around the world include, but are not limited to, The Pierre-Laporte Suspension Bridge across the St Lawrence near Quebec, Canada (where from 1977-79 some 165,000m2 was coated after failure of the original paint system over a six year period), Humber Bridge, UK, The Bosphorus Bridge, Turkey, Tsing Ma Bridge, Hong Kong and The Clifton Suspension Bridge, UK.
Cathodic Protection of Concrete Bridges
The cost to apply metal sprayed coatings to large concrete structures is not insignificant, particularly when many structures are difficult to access, such as bridges.
However, the long-term benefits can make the process extremely commercially attractive. If performed correctly and depending on the coating applied, the process can offer corrosion protection for up to 20 years before the next significant maintenance is required. The protection offered can greatly prolong the life of the structure and also prevent costly accidents from cracked sections falling from the structure. Once applied, the coating requires minimal maintenance. If required for aesthetic purposes, Metal Sprayed coatings can also be painted or powder coated.
Al-Zn-In Application of San Luis Pass Bridge, USA
Image Attribution: Larry D. Moore, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
A recently developed alloy of aluminium, zinc and indium has been used in a small number of applications. This material is more active than zinc and it is claimed to not require an impressed current to provide adequate levels of corrosion protection. A significant application of the Al-Zn-In alloy in the US is the San Luis Pass Bridge near Galveston, Texas. More than 30,000m² of concrete beams and caps are protected with this alloy, installed using Metallisation ARC 700 units by Corrosion Restoration Technologies of Jupiter, Florida.
Cape Creek Bridge, Oregon (USA)
Oregon Department of Transportation (ODOT) demonstrates another success story for cathodic protection on concrete. In a bid to reduce the high costs of bridge reconstruction, ODOT has applied a system of metallised zinc anodes and impressed current cathodic protection. This process has been used to protect its Cape Creek Bridge from corrosion and subsequent reconstruction. The bridge is exposed to a coastal environment and is subject to attack by chloride from the salty air.
Prior to the cathodic protection project on the bridge, it had suffered substantial concrete spalling on its columns and under deck. By selecting to protect the bridge in this way ODOT saved over $13 million by not having to reconstruct the bridge. The cost of cathodic protection is quite expensive. This is due to the requirement of a movable work platform, which is enclosed to contain the abrasive blasting and zinc spraying residues. These measures are critical when spraying zinc to protect the environment. However, when compared to the cost of reconstructing a bridge the size of Cape Creek Bridge – the savings are phenomenal.
Dave Wixson, Metallisation distributor in the US says: “Cathodic protection is a cost effective way to stop rebar corrosion in existing structurally sound structures. Rebars in dry alkaline concrete are protected by a passive ferric oxide film, however, when the rebar is hit with 250 ppm chloride solution, generally from salt, the protection breaks down. The protective ferric oxide film is converted to red rust and corrosion begins.
Concrete thickness >4cm (>1.5 in), will prevent chloride penetration. For exposed rebar and thin concrete, where there is chloride concentration in excess of about 250 ppm, rebar corrosion will be initiated with the red rust spalling adjacent concrete. Protecting the rebar with a barrier using an impressed or passive cathodic protection system, counters the corrosion.”
Many thanks to Palmer Consulting of France, TMS of the USA and Corrosion Restoration Technologies Inc (now part of Structural Group, Inc.) of the USA for information supplied.
Footbridge Protection
Reason for use: Corrosion protection to Network Rail N1 specification.
Equipment: Metallisation MK73 Flame Spray System.
Solent Protective Coatings won a contract to metal spray a footbridge which was installed over a railway track at Hilsea in Portsmouth. All components of the footbridge were metal sprayed including two 12 metre stair sections, the 20 metre long main bridge deck, weighing 10 tons, and the support columns. The surface of the bridge sections were grit blasted to SA 3 before being thermal sprayed with aluminium to 100 microns, using the Metallisation MK73 Flame Spray system. An epoxy sealer was then applied to a maximum of 25 microns dry film thickness. A final primer and topcoat were then applied to complete the project.
Solent Protective Coatings Limited
Solent Protective Coatings Limited, based in Southampton, is a well established company specialising in surface preparation and protective coatings. Key services include blasting, steel preparation, protective coatings and UHP water blasting. Metallisation Limited is the global leader in metal spraying equipment and consumables. Metallisation also provides specialist training for its customers, ensuring maximum effectiveness of the metal spraying equipment and consumables.David Skeates, Managing Director at Solent Protective Coatings, says: “We have a great relationship with the Metallisation Team. At Solent we are committed to building strong working relationships with our customers and partners. Metallisation shares these beliefs and provides an excellent service to us. I wouldn’t hesitate in recommending them to any company needing metal spraying equipment, consumables or training.”
Rail Footbridge - Zinc Sprayed
Showing a considerable development from the traditional railway footbridge, the accompanying photograph illustrates the steel footbridge erected over the railway line. The protective treatment selected for this structure was grit blasting and metal spraying with zinc to a thickness of 150 microns (0.003”) followed by a paint system for decorative surfaces.
Cycle Bridges
The Grafton Gully Cycle Bridge in New Zealand was coated with Zinc using a Metal Spray system.
Niagara Falls International Rainbow Bridge
The Niagara Falls International Rainbow Bridge was subjected to the Metal Spray process. The steel bridge itself, an almost 42,000m2 structure, was sprayed using 85/15 wire (85% Zinc, 15% Aluminium).
Charles-de-Gaulle Bridge
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The box girders of the Charles de Gaulle bridge in Montreal, Canada were subjected to the Metal Spray process for anti-corrosive protection.
Norwegian Bridges
The Norwegian Public Roads Administration (NPRA) has specified TSZ duplex coatings for protection of coastal steel bridges since 1965. These bridges are exposed to harsh corrosive saltwater environments and road de-icing salts. Some of these Metal Sprayed coatings have lifetimes of over 40 years to first maintenance. The Metal Sprayed coating is topcoated with paint to provide the duplex coating to give both barrier and sacrifical corrosion protection, as well as aesthetic appeal.
The Norwegian Public Roads Administration (NPRA) introduced thermally sprayed zinc (TSZ) duplex coatings for coastal steel bridges in 1965 and for all bridges in 1977, replacing red lead coatings. Click Here to find out more.
The Rombak Bridge was coated in 1970 with a duplex coating of TSZ coating of 100 μm and two layers of alkyd paint at 100 μm each. It is a 750m suspension bridge with a bolted truss work under the bridgeway. The first preventative coating maintenance was completed in 2012 by washing and applying of the paint top coat which was subject to partial flaking, the TSZ coating however was still in perfect condition.
Ridge Avenue Bridge, PA
The Ridge Avenue Bridge in Philadelphia, US was subjected to Metal Sprayed Zinc in 1938. The bridge was last inspected for coating imperfections in 1984.
If you have any questions regarding your particular application or for more information on our equipment or consumables, call us on 07 3823 1004 or email us via our contact form.
