Metal Spraying of Zinc and Aluminum

Metal Spraying of Zinc and Aluminum
in the United Kingdom
J.C. Bailey (Consultant, UK)
F.C. Porter (Consultant, UK)
M. Round (Metalization Services Ltd, UK)

 

 

SUMMARY

A wide range of industries in the United Kingdom have used zinc and aluminum spraying for the corrosion protection of either mass produced articles or individual structures. Those reported include many which can still be seen as case histories demonstrating the long life possible. Still better technical and economic performance is possible with metal spray + sealer, which has been introduced widely in the past 15 years.

Zinc spraying of plastics, impregnated asbestos, and non-metallic substrates (such as paper in electrical capacitors) is widely used to provide electrical conductivity or electromagnetic screening properties of reflective surfaces. Meanwhile, zinc or tin-zinc spraying is used in the production of moulds and mould tools for the plastics industry. All these aspects will be dealt with in the verbal presentation.


INTRODUCTION - OLD ESTABLISHED BRIDGES

In the past 65 years or so, since metal spraying was introduced to the United Kingdom (its country of origin), the techniques for its application (including arc-spraying) have advanced and its application has grown both in quantity and in the variety of applications. Just over 50 years ago, the steel links in the Menai Straits bridge in Wales were metal sprayed and the success of that over the years led to metal spraying being specified for the new Menai Straits rails bridge erected in the 1970s.

The construction of bridges is one of the oldest pieces of engineering known to man and has provided one of the best documented applications throughout the years. The first part of this paper will deal with bridges ranging from the Forth and Severn Road bridges over sea estuaries, down to a vast number of small girder road bridges and footbridges over road and railways. These have been metal sprayed as standard mass-production practice. All these have provided a good basis for the much wider range of current uses.

The Birmingham Post on Friday, 29 September, 1964 stated that "It takes 24 men three years to paint the old Forth Rail Bridge in Scotland - and then they start all over again. But there will be no ‘jobs for life' available on the new Forth Road Bridge; at most, it will need a top coat of paint, largely for decorative purposes, once every ten years." This has been proved in practice and, after an initial period of 16 years when no maintenance was necessary or possible, a maintenance cycle of 10 years is quite sufficient for the upper structure.


FORTH ROAD BRIDGE, Queensferry, Scotland: one and a quarter miles long, it has a main span of 1,000 m and is in a windy marine environment.

Erected in 1960/64, the grit blasting of the steelwork and zinc spraying was done to the high standard required by British Standard 2569 in a mechanized plant specially built near the site. An etch primer and zinc chromate primer were applied at works and two coats of micaveous iron oxide on site after erection.

In 1975, after 13 years without maintenance, the protective scheme was still sound. The upper structure was in perfect condition: the main trusses below the road deck had not been easily accessible but, despite not being repainted, they were free of rust and only the paint coats had been dissipated. The Deputy Bridgemaster said at that time: "It is certainly easier to give maintenance painting to a zinc sprayed/painted surface than to one which has only been painted, as the latter involves more surface preparation to remove rust down to clean metal."

The General Manager commented in 1986 that the sprayed zinc had been invaluable in protecting the bridge for 15 years, while a system of walkways giving good access for maintenance and a water main (for regular washing of the structure) were being planned and installed. The zinc had been specially valuable where damage had occurred, and under the asphalt road surface where the zinc coating on the steel had prevented effectively any penetration of rust along the steel/asphalt interface where the latter had cracked or broken away.

Rusting had been observed only on some down-facing horizontal surfaces where, in some parts, the final cost of the initially specified paint system had not been applied. After about 16 years, the surface was lightly blasted with non-metallic grit and painted with zinc chromate primer, zinc phosphate epoxy ester and micaceous iron oxide/chlorinated rubber. An 8 year cycle of painting is now envisaged and further blasting is expected to be unnecessary.

Major UK bridges now provide more than 25 years experience of zinc-spray protection and this led to many smaller bridges being given similar treatment. A useful list is given by Bailey and Porter(1); the following notes cover several inspected recently.

TAMAR BRIDGE: Plymouth, Devonshire: Over 2,000 feet long. Girder construction. Marine/industrial environment. Erected 1961. Initially sprayed at works with 75 microns minimum zinc and painted with wash primer and two coats of white lead.

Maintenance painting is done every seven years with two coats of epoxy paint and two coats of micaceous iron oxide. Inspection in 1986 showed the protective scheme to be in good condition with only slight zinc corrosion on a few bolt heads. Bolts which had not been zinc sprayed initially had required blast cleaning even at the first seven year maintenance.

The consulting engineers responsible for maintenance comment in the light of their experience with many large structures that sprayed zinc is, in their view, an effective treatment for structures located in severe environments and especially where access to parts of the bridge for maintenance is difficult or impossible, which all too frequently has been the case.

QUEENHILL BRIDGE: Queenhill, Gloucestershire: Approximately 300 m long, taking M50 motorway over the River Severn. Four plate girders under the main span, concrete deck above. Rural environment. Erected 1959. Initial treatment 75 microns minimum sprayed zinc with wash primer, zinc chromate/red oxide primer applied at works, and two coats of micaceous iron oxide on site.

In 1967, after 8 years, zinc-rich epoxy primer was applied to a few corroded areas near joints where little or no zinc remained, and the bridge was given two coats micaceous iron oxide overall. In 1978, a major repainting was given with three coats of zinc phosphate, chlorinated rubber and alkyd primer applied to slightly rusted patches.

Inspection in 1986 showed the paintwork to be in very good condition, apart from rusting on the top edges of the upper flanges of the outer girders where there is contact with the concrete roadway and some slight movement over the steel. There was also slight rusting on the lower edges of channels inside the bridge where water seeps down through the expansion joint. Elsewhere, the underside is free of rust.

So, after 30 years with only two repaintings, Gloucester County Council comments that the good condition justifies no further maintenance for some years to come.

GEORGE STREET BRIDGE: Newport, South Wales: Cable-stayed cantilever bridge, 500 ft. span. Deck is hollow box construction 4 ft. deep. Highly polluted industrial environment. Erected 1964. Initial protection was sprayed zinc with micaceous iron oxide paint coatings.

In 1985, after 21 years without maintenance, the condition of the paintwork was generally good with only a little rusting apparent on the soffits and underside. The local authority decided to repaint as, at this stage, preparation would be easy. Two coats of zinc phosphate epoxy ester and two coats of micaceous iron oxide were applied over the existing system. Adherent soluble salts on some areas of the down-facing underside plates of the box would require light grit blasting, so these were left, apart from patch painting, until blast cleaning is justified, envisaged for not less than five years hence.

In 1986 the flat undersides of the box were still clean with very occasional pinpoints of white zinc corrosion product where the paint coats are breaking down. The paint has also degraded in a few places where water has leaked out of the box.

THE M4 BRIDGE AT MAIDENHEAD OVER THE THAMES: erected 1960/61. Only 50 microns of zinc was used (with 2 works and 2 site applied white lead coats) and, when first painted after 18 years, blasting removed zinc in some areas in obtaining a clean surface for painting - indicating that earlier maintenance in this case would have been desirable. After only 5 more years, the areas denuded of zinc are showing slight rusting through the paint.

Other bridges examined included the Arborfield A327 Bridge of Berkshire County Council erected in 1967, several on Morpeth Bypass (Durham County Council) erected in 1970, and several in Reading (including Watlington Street, Berkeley Avenue and the old Blakes Bridge, zinc sprayed on site in 1964), all with first paint maintenance done or scheduled for 15-22 years after erection.

The on-site metal spraying of Blakes Bridge is particularly interesting and, although there has only been limited use of this technique in the UK, it is worth noting parallel North American experience, notably the on-site application of a metal-sprayed zinc coating system to the Pierre-Laporte Bridge - a first in North America, and probably in the world, for this size structure.

Initially, all structural steel surfaces were tested with a lead silicochromate, oil alkyd paint system. This coating was chosen because it was less costly than other sophisticated methods. During 1975, that is within five years of the bridge being opened, the coating required maintenance work to be carried out. Consequently, a decision was taken by the Ministere des Transports, Gouvernement du Quebec in 1977 to remove the existing paint coating and apply a metal-sprayed zinc coating to 130 microns followed by an etch primer and two coats of vinyl resin sealer.

SEALED SPRAYED COATINGS

The older applications, just described, initially utilized zinc spray followed by substantial paint schemes, whereas developments in the past 15 years have shown that the optimum technical and economic coating is metal spray (zinc or aluminum) plus sealer. Sealers are specially formulated paints of low viscosity, lightly pigmented with inert polymers so that they can penetrate the natural pores in a sprayed metal coating, giving in-depth protection. Sealers are readily available in a wide variety of colors. While a surface film of sealer is produced, this, unlike conventional painting, is not the primary objective and, even when this surface film has completely degraded, the sealing treatment is still totally effective.

Such sealed coatings have been found to weather uniformly, even in very severe conditions, and also avoid the poultice effect which can occur when paint coatings deteriorate and absorb water and then produce a surface of degradation products which can be unsightly and can cause premature deterioration of the metal sprayed coating.

So far, sealing has mainly been used on other structures and products because bridge builders tend to prefer multi-coat paint systems.

‘Brae B' Development and multitudinous other applications, e.g. flare gas stacks, tankage, etc., actually on board the rigs.

One particularly interesting monitored application(4) related to about 16,000 specially coated support components underground in the NCB's Newstead and Bates Collieries. All paint systems sustained mechanical damage; sprayed zinc plus paint demonstrated the best corrosion protection.

One of the earlier applications of sprayed aluminum (in 1947) was to all the steelwork above crane level at Margam Steel works in South Wales. 100 microns aluminum sealed with a single coat of aluminum paint was intact when examined 28 years later, except near a hydrochloric acid pickling plant.

Numerous building structures have followed, a typical example being 247 tons of steel fabrication consisting of girders, gantries, and bracing steel works for an off-site services project for ICI at Huddersfield. In this case, the zinc coating of 100 microns thickness was given a chlorinated rubber sealer.

Whether building structures are bolted or welded is immaterial, as the metal sprayed coating can be applied overall. Bailey(5) has described design and other features involved in metal spraying welded structures.

On the coast, and/or where abrasion is likely, there are benefits from metal spray. In 1970, English China Clays erected a loading jetty(6) close to Plymouth. Within six months, they were renovating some parts of the construction which had been given a simple paint scheme. Even full paint schemes on one walkway were requiring maintenance after three years and here it was virtually impossible to remove all rust and give a reliable maintenance coating in some recessed areas.

Major areas had, however, been aluminum sprayed with a sealer coat, and when examined after seven years, showed no deterioration except at one point where gravel wetted by sea water had remained in perpetual contact. There was slight darkening of the surface in other places. Adherence of the aluminum spray was very good and adherence had been retained on a number of corners of girders which had been substantially bent by impact service. One beam, which had been particularly severely bent, had a small area of coating detached, but the resultant gap of about 1 cm was not extending and there was no under-rusting of the adjacent coating.

Aluminum sprayed coatings have additional value at high temperatures where zinc is not normally recommended. Exhaust ducting, which is subject to high extremes of temperature and corrosive gases, is a good case for using 150 microns of aluminum sealed with two coats of aluminum silicone sealer. This is particularly so when ducting has to undergo the rigors of transportation through the tropics, e.g. large ducts that were exported to Australia for use with Rolls Royce Olympus engines to be used as a power source in a town and mining area.

A similar specification is used on steel chimneys, such as the three 48.5 m chimneys at the Dudley and District General Hospital erected about 15 years ago.

One example, not a bridge, is the lightweight roof structure of a leisure center at St. Heilier on the sea coast in Jersey. It was erected in 1969, having been sprayed at works with 100 microns of zinc and sealed with a low viscosity vinyl copolymer.

In 1986, after 17 years exposure, part of the structure being out of doors, it is still in very good condition, with no visible corrosion or rusting, except in a zone immediately above a catering area where steam continually arises from cooking operations. Some white corrosion products are visible there and a little rust.

This good performance matches with the behavior of sprayed zinc test panels in the American Welding Society 19-year exposure tests on sealed zinc coatings.(2)

Of still greater significance is the excellent practical experience elsewhere. A review of the protective coatings applied to steel structures in Scandinavian countries by Klinge, Public Roads Administration, Norway, concludes that many major bridge structures for varying applications, chimneys, tankage, cranes, etc., are protected by metal sprayed coating systems(3). Gas-sprayed zinc appears to predominate, and is normally followed by suitable sealing treatments. The use of electric arc-sprayed aluminum is, however, beginning to find favor.

Examples of major bridge structures treated both in-works and on-site by zinc spraying plus various paint sealer systems are:

Norway Sweden Denmark
Karmsund, 1967 Reserve, 1965 Vilsund, 1974
Djupjord, 1958 Boras, 1967 Halleby River, 1960
Rombak, 1960 Alvsborg, 1966 Many railway bridges
Varodd, 1969    
 


All of the above bridges have shown the benefits of sealed metal-sprayed zinc for long-term, corrosion-free protection.


OTHER ANTI-CORROSION APPLICATIONS

The early successes with bridges led to much wider use of metal spray, both on large fabrications and on mass-produced articles such as gas cylinders and ammunition boxes. The diversity of applications will be illustrated at the meeting and will include both new applications and long term case histories of painted, sealed or bare metal sprayed coatings. The diverse nature of the applications will be shown by such examples as the protection of a hull of a miniature submarine used for oceanic studies, the Bishop of Salford memorial at Walkden, Lancashire, London Central Mosque Regents Park (roof structure), water treatment plants, conveyors, marine loading and unloading arms at ports, helicopter landing platforms and pleasure beach rides. A major step forward for the metal spray industry is the application of metal sprayed aluminum coatings to offshore platform installations in the North Sea, which include the unique Conoco Tension Leg Platform in the Hutton Field, Marathon.

In general, metal spraying has very high resistance to corrosion. In about 80% of the cases, either zinc or aluminum spray can be used with equal success, but if the conditions are likely to be slightly alkaline, e.g. in contact with alkaline building materials, zinc is to be preferred, while in slightly acid conditions aluminum is better. The difference is significant only when the metal is exposed; fully sealed applications of either metal have widespread use in many environments.

METAL SPRAYING FOR OTHER PURPOSES

ELECTROSTATIC AND ELECTRICAL PROPERTIES:

Zinc spraying of plastics of phenolic impregnated asbestos is used to provide reflecting surfaces. Zinc spraying is used also for RF shielding on non-metallic components, satellite earth terminals, and for making electrical connections to capacitors. X-ray rooms and radio valves are often treated for similar reasons.

Tin or zinc spraying is also used for producing intricate moulds for the manufacture of shoe soles.


Acknowledgment:
The recent examinations on bridges were sponsored by a group of Canadian firms (Falconbridge, Noranda, Cominco and Platt Bros.) as part of their technical support to the expanding zinc metal spraying business in North America and their help is greatly appreciated, as is the support of the Zinc Development Association and the Association of Metal Sprayers.



REFERENCESBaily, J.C. and Porter, F.C. (1972) Sprayed Metal Coatings for the Protection of Structural Steel - a Fresh Appraisal Const Steel (Feb) 12-19 (Reprint by Ass Metal Sprayers) AWS Ctte (1974), Corrosion Tests of Flame-Sprayed Coated Steel - 19 Year Report, Amer Weld Soc. Miami, 31 pp. Klinge, R (1976) Sprayed zinc and aluminum coatings for the protection of structural steel in Scandinavia, 8th Int Thermal Spray Conf, AWS 203-213. Mining R&D, Estab (1085) Corrosion Protection of Powered Roof Supports. Final Report on ESCS research project 7220-AC/802 Vol.1, 21pp. Bailey, J.C. (1983), Corrosion Protection of Welded Steel Structures by Metal Spraying, Metal Const 15 (5) 264-270. AWS (undated) Case Histories of Sprayed Metal Coatings Protecting Steel Structures, AMS, Birmingham 9pp (Also 1989 publication on the same subject).  

         

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