Mr. B. Fitzsimons B.Sc. Phone: 0191 2620 510 Operations Director - Barrier Limited Table of Contents

1. Introduction
2. Thermally Sprayed Aluminium
3. Codes And Standards
4. Why Use Metal Coatings?
5. Cathodic Coatings
6. Anodic Coatings
7. Selection of Metals
8. AWS 19 Year Corrosion Tests
9. Life Expectancy of Coatings
10. Metal Spraying Process
11. Sealing of Sprayed Aluminium Coatings
12. Quality Control
13. Adhesion Values
14. Health, Safety and Environment
Appendix 1. - Welding of coated steel
Appendix 2. - Duralcan

Thermal Spray Metal Coatings For Corrosion Protection
1. Introduction
Thermal sprayed metal coatings are depositions of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties.
Sprayed metal coatings have been used for a number of years and exposure tests have proved them to be superior to conventional paint coatings.
This document is intended as an introduction to the uses of thermal sprayed metal coatings as corrosion protection to steelwork, as an alternative to paint coatings. Information is included on the way in which metal coatings work, the alternative methods of application and the uses of sealers.
Aluminium has been found to be the most effective metal for protection of steel in offshore structures and so this document includes more information on that product than others. Information is included on measures to be taken for the safety and health of applicators and on quality control of the process.
Barrier Group companies have a wide experience in the use of specialist and conventional coatings for offshore and civil engineering projects in controlled factory and exposed site locations and have used thermal sprayed metallic coatings on such widely divergent projects as motorway and railway bridges, offshore flare booms and valves in pipes for metering gases and oil products.
2. Thermally Sprayed Aluminium As An Alternative To Paint Systems
Sprayed metal coatings have been found on exposure testing programmes to have superior life to paint systems assuming adequate preparation of steelwork and appropriate application procedures are carried out. Sprayed aluminium may be left exposed in many situations and, when sealed, may be regarded as a superior priming system for overcoating.
Although both flame spray and arc spray methods are available, arc spray, a newer method of application, has been shown to give faster output and superior adhesion. For fabricated structures, both methods may be used especially where access to difficult areas favours flame application and where broad plated areas favour arc spray.
Aluminium or aluminium alloys are used and an alloy with 5% magnesium is currently widely specified, though our experience has not borne out the belief that this gives the best offshore protection.
The application of thermally sprayed aluminium requires more applicator training than paint coatings together with additional attention to health and safety measures.
However major benefits can be gained by the use of a coating which is capable of being handled almost immediately after application, showing little damage when used as a fabrication coating and which can be applied to part of a structure (eg leaving weld areas) for later completion or repair.
Aluminium alloy containing 5% magnesium, are thermally sprayed using to exposed structures for marine use and hence is highly efficient for offshore platforms and ships topsides where the anodic advantages of the metal are shown. Although experience has shown that sealers are of benefit on exposed aluminium coatings, areas not exposed to driving rain eg. undersides of platforms and bridges may be better left unsealed to reduce the effect of "sweating" or condensation.
Sprayed aluminium has been shown to be effective against corrosion under insulation, which might have become wet due to leakage of rainwater through the weather cover.
Thermally sprayed aluminium works well on plant operating at elevated temperatures, coated with epoxy sealers up to 120O C and with a silicone aluminium sealer above that temperature.
3. Codes and Standards relevant to Thermal Spraying of Aluminium
•    B S 2569 Sprayed Metal Coatings
Part 1. Protection of iron and steel by aluminium and zinc against atmospheric corrosion.
•    B S 5493 Protective coating of iron and steel structures against corrosion.
Handling, transport, storage and erection.
•    DIN 8566 Teil 1 und Teil 2 Zusatse fur das thermische Spritzen.
•    ISO 1463 Metal and oxide coatings - measurement of coating thickness - microscopical method.
•    ISO 2063 Metallic coatings - protection of iron and steel against corrosion - metal spraying of zinc, aluminium and alloys of these metals.
•    ISO 2064 Metallic and other non-organic coatings - Definitions and conventions.
•    ISO 2178 Non-magnetic coatings on magnetic substrate's
- Measurement of coating thickness
- magnetic method
•    ISO 4624 Paints and varnishes - Pull-off test for adhesion
•    ISO 8501 1-4 Preparation of steel substrate's before application of paints and related products - Surface roughness characteristics of Blast-Cleaned Steel Substrate's
•    NPD Guidelines for Corrosion Protection of Installations
•    NS 476 Rules for the Approval of Surface Treatment Inspectors
•    NS 1975 Rules for the Approval of Surface Treatment
•    SS 2626 Thermal Spraying equipment - requirements and testing
•    SSPC Steel Structures Painting Manual Volume 1 & 2
4. Why Use Metal Coatings?
The metallizing process is an excellent means of protecting iron and steel from corrosion to almost any desired degree, from long life coatings to inexpensive coatings which are competitive with organic coatings such as paint. Heavy coatings of zinc or aluminium can be applied to meet the most severe corrosion conditions and give 15 to 50 years life without any further maintenance. Very thin coatings, particularly of zinc, compete in cost with such other methods such as plating and painting, and usually give much better protection.
Metallized coatings are also excellent corrosion resistant undercoatings for organic materials such as paint. Organic finishes on iron and steel usually fail due to corrosion under the coatings and lack of bond of the coatings to the steel. Thin metallized coatings prevent corrosion of the base and offer a strong bond to the organic finish.
Theory of Corrosion
It is obviously impossible to adequately cover here, the broad and complicated subject of corrosion. There are many factors involved, such as the corrosives, the forms of corrosion, rate factors, and metals used. Moreover, a seemingly minor change in the conditions surrounding a given problem, will often have a major effect on the correct solution of the protection problem.
Because of this we will confine ourselves to a brief discussion to two-metal galvanic corrosion, simply because it is the most important consideration when applying metal sprayed coatings for corrosion resistance.
The following table (fig 1) represents the galvanic series in sea water. Although the arrangement of metals and alloys varies somewhat in media other than sea water, this is good general-purpose series for use in connection with metallizing work.
In general, when two dissimilar metals or alloys are electrically connected in an electrolyte, current will flow from the more noble (cathodic) metal to the less noble (anodic) metal, within the metal, from the anodic metal to the electrolyte, and from the electrolyte to the cathodic metal. The amount of current which flows is subject to many variables, among which is the difference in potential of the two metals. Thus very little current will flow between metals which are close together in the series, and the corrosion rate of the anodic metal will be high while that of the cathodic metal will be low, or in some cases stifled completely.
 
(Fig 1)
In the presence of an electrolyte, the more anodic surface will corrode in preference to the more cathodic surface. All of the metals listed may be Cathodic to metals higher in the scale.
5. Cathodic Coatings
It will be apparent from the above that coatings of metals which are cathodic to the base metal are good only in heavy coatings on machine element work. For instance, stainless steel is used quite extensively for pump shafts and steam turbine shaft inlays. Brass, bronze, nickel, stainless steel, copper etc, are all of no use in thin coatings since the base would be rapidly attacked through the pores of the coating. Since it is only practical to apply thin coatings to large areas, such as tanks and structures, these metals are not recommended for such work.
For limited use, it is possible to seal up the pores of cathodic coatings such as stainless steel, monel metal and bronze.
6. Anodic Coatings
It will be immediately apparent from the above that:
1.    The only metals in common use for metal spraying which are anodic to iron are cadmium, zinc and aluminium (magnesium is too reactive). Since these metals protect iron by being themselves attacked, their porosity when sprayed is not vitally important.
2.    All other commonly sprayed metal materials are cathodic to iron and protect it only providing a completely non-porous barrier coating is obtained, which will protect the iron mechanically.
For these reasons, zinc and aluminium are by far the most widely used for protecting iron and steel against corrosion with metallizing. Although cadmium is also used to a certain extent, its high cost limits it to special applications. Zinc coatings as thin as 25 microns are entirely practical for any types of atmospheric conditions, although heavier coatings are generally used because of the additional service life they provide. Because of the tendency of aluminium to oxidize, and thus protect itself from further attack, it is necessary to apply slightly heavier coatings of this metal, the minimum for atmospheric corrosion generally being considered as 75 microns.
Galvanic action, which results when dissimilar metals are electrically connected in an electrolyte, should always be considered and avoided. For example, a cold water storage tank might be coated with zinc to resist corrosion, and the coating would give excellent service. But if copper coils were placed in the tank, the presence of the copper would set up a galvanic action and the corrosion of the zinc would be greatly accelerated, causing the coating to give very poor service. Such a condition could be avoided by electrically insulating the coil from the tank.
7. Selection of Metal
NICKEL, MONEL, BRONZE
AND STAINLESS STEEL?
Metals such as nickel, monel, stainless steel and bronzes which are cathodic to steel should be used only if they are impermeable. These metals are used only for machine element work such as for pump plungers, pump rods, hydraulic rams, packing sections of steam turbine shafts, boat tailshafts, valves etc.
These metals should preferably be sealed with organic sealers.
For these applications the metal should be selected for its known resistance to reagent and for its wear resistance. For instance, if a pump shaft packing section is to be built up, the wear requirement must be balanced against the corrosion resistant requirement. Stainless alloy is the hardest, monel or nickel is the next hardest material but is less corrosion resistant generally than monel. Nickel would seldom be used except for special cases where it is known to be required. Monel is used extensively for tailshafts and pump packing sections.
LEAD AND TIN
Lead coatings are sometimes used for resistance to acids and other strong reagents. Since lead is cathodic to steel, it must be totally sealed up. In past years this has frequently been done by wire brushing between coatings since the lead is soft enough to cold flow and close the pores. This method has been only partially successful since only the surface-pores are closed.
A more thorough method of closing the pores of sprayed lead coatings consists of blasting with round shot. A good (surface to coating) bond is needed if this method is used, and a bonding coat should be sprayed to a thickness of 250 -300 microns and then thoroughly and uniformly blasted with round shot. Pressure should be 35 - 50 P.S.I. if a force feed generator is used, or 75 to 100 P.S.I. if a suction feed machine is used.
For periodic exposure to strong acids or continuous exposure to dilute acids, lead coatings may be sealed with raw linseed oil to which a drier has been added.
Tin coatings are used primarily for protection of food vessels. Although tin is cathodic to steel, tin coatings are so fine and dense that they become practically impervious with relatively thin coatings. Tin coatings 100-250 microns thick are applied, depending on service conditions and sealed.
Tin coatings are also valuable as "under coatings" under monel, nickel, bronze and stainless steel coatings, particularly where they can be sealed for resistance to strong reagents such as acids.
ALUMINUM AND ZINC
Aluminum and zinc are the two metals recommended for atmospheric protection of iron and steel and also for protection in salt and fresh water immersion.
Zinc is usually 99.9% pure and is not contaminated in the spraying process. The result is that metallized zinc coatings are much purer than those applied by hot dip galvanising, for instance, since nickel used in galvanizing picks-up considerable iron as an impurity.
Aluminium is usually 99.00% pure aluminium and is used for much corrosion protection work.
For the protection of articles on a competitive basis with methods such as painting or plating, thin coatings of sprayed zinc are used. Zinc, 25 microns thick provides excellent protection for small hardware items such as nuts, screws etc, which can be prepared and metallized in tumbling barrels. While the cost of aluminium coatings is slightly less than that of zinc of equal thickness, aluminium requires more thorough surface preparation. Also aluminum is not recommended in thicknesses less than 75 microns. Therefore, where the cost of blasting must be kept at a minimum or where very thin coatings are sufficient, zinc should be used.
Zinc has a higher electrolytic potential than aluminium, and offers better protection on work which is not readily accessible at all points around it's surface. Small uncoated areas such as scratches or voids on the underside of imperfectly headed rivets are protected electrolytically by zinc better than aluminium.
8. American Welding Society 19 Year Corrosion Tests
AWS Tests Prove Superiority of Metallized Systems
In 1974 the American Welding Society completed a 19 year study of corrosion protection afforded by wire metallized aluminium and zinc coatings applied to low carbon steel. Here is a summary of the results:
1.    Aluminium coatings 0.003 in to 0.007 in (0.08mm to 0.15mm) thick, both sealed and unsealed, gave complete base metal protection from corrosion for 19 years in sea water and in severe marine and industrial atmospheres.
2.    Unsealed zinc sprayed coatings required 0.012 in (0.30mm) minimum thickness to give complete protection in sea water for 19 years. In severe marine and industrial atmospheres 0.009 (0.23mm) of unsealed zinc or 0.003 in to 0.006 in (0.08mm to 0.15mm) of sealed zinc gave 19 years protection.
3.    In severe marine atmospheres, application of one coat of wash primer plus one or two coats of aluminium vinyl enhanced the appearance and extended the life of zinc coating at least 100%.

With aluminium, the sealing systems primarily enhance appearance, because both sealed and unsealed systems showed no base metal rust after 19 years.

The only sealed zinc panels tested in sea water were coated with chlorinated rubber. This seal coat did not prove effective.
4.    Thin coats of metallized aluminium perform better; have less tendency to develop pits and blisters and therefore are expected to give extended life.
5.    Where aluminium coatings showed physical damage such as chips or scrapes, corrosion did not progress, suggesting the occurrence of galvanic protection.
Results of AWS 19 Year Corrosion Test of Metallized Coated Steel
The below listed coatings afforded complete protection to low carbon steel panels.
 
Source: American Welding Society
Notes
(a) All sealed panels were sealed with a wash primer plus one or two coats of either aluminium vinyl or clear vinyl.
(b) Atmospheric panels with a wash primer plus one coat of aluminium vinyl.
(c) Both below low tide and mean tide level.
(d) Both severe marine and salt air.
9. Life Expectancy of Coatings
The life expectancy of a metal structure and the durability of its aesthetic appearance depend essentially on the quality of preparation of its surface and on the performance characteristics of the coating which is to be in direct contact with the steel. In this context zinc and aluminium play a fundamental part not only because of the inherent resistance to the aggressiveness of the external environment but also by virtue of the exceptional active protection which they impart to the steel.
To apply these materials to the steelwork after correct blasting preparation is comparatively simple and is not the highly sophisticated and technical process many specifiers consider it to be. It is comparable in cost to high performance paints yet when viewed over a 25-30 year life span offers huge savings in maintenance costs (Fig.2).
The technique to carry out the process offered by Metallisation, is the wire spraying process, which is probably the most commercially viable. It offers the facility of being able to deposit coating over large areas with varying thicknesses.
The process is simple and economical to use and can provide solutions to most anti-corrosion problems encountered by experienced engineers.
 
Fig 2
The chart illustrates the life expectancy of coatings (Based on B.S. 5493).
Thermal metal spray (like paint) has advantages and disadvantages over its use.
Advantages    Disadvantages
Predictable life     Environmental shop restrictions
Supplies cathodic protection    Blast cleaning essential
Good abrasion resistance     Operator skills
Can be sealed/top coated    
Immediate inspection    
As previously discussed there are many metals used in the application of thermal spray process although only two are generally used in the construction industry.
Aluminium
•    High temperature resistant
•    Lightweight
•    Excellent resistance to polluted and marine environments
•    Forms oxide layer
•    Reduces corrosion
Zinc
•    Excellent for cathodic protection
•    Excellent resistance to mechanical damage
•    Life proportional to thickness (except when sealed).
•    Used in anti-corrosive primers
Metals by Environment
Prior to use of the thermal spray metals, review of the environment must be considered. (See Fig. 3).
 
Fig. 3
Sprayed aluminium will withstand many dilute acid solutions (pH 3.5 or higher) but the chemistry of the reagent and the nature of the surface film which forms on the coating will determine its life. Very dilute nitric or sulphuric acid solutions, and many of the organic acids, have little effect on sprayed aluminium coatings which have been treated with a suitable sealer.
Sprayed zinc has very poor resistance to almost all acids, organic or inorganic. Zinc coatings are generally used in the pH range of 6 to 12. As with aluminium coatings, the film-forming character of the environment will determine the life of the coating. Zinc is much more resistant to hard waters than soft waters. It has poor resistance at temperatures above 42degC in all waters.
10. Metal Spraying Process
The process for application of Thermal Spray Metal is relatively simple and consists of the following stages.
1.    Melting the metal at the gun
2.    Spraying the liquid metal onto the prepared substrate by means of compressed air.
3.    Molten particles are projected onto the cleaned substrate.
Alternative application methods
There are two main types of wire application available today namely ARC SPRAY and GAS SPRAY.
ARC - A pair of wires are electrically energised so than an arc is struck across the tips when brought together through a pistol. Compressed air is blown across the arc to atomise and propel the autofed metal wire particles onto the prepared work piece.
GAS - In combustion flame spraying the continuously moving wire is passed through a pistol, melted by a conical jet of burning gas (propane or acetylene fuel mixed with oxygen). The molten wire tip enters the cone, atomises and is propelled onto the substrate.
When using either process, operator experience will develop with time but some general guidelines include the development of an optimum distance between the gun and substrate to maintain a satisfactory temperature at which the sprayed metal hits the surface. This distance, combined with the speed of lateral movement of the gun which controls the rate and hence thickness of metal deposited. In order to control an even thickness on large flat areas, small areas about 0.5m2, are marked out by the operative with the sprayed metal and then "filled in".
Thickness checks are made regularly and areas lower than specification can be brought up to thickness immediately. As the spray application produces dust (especially the arc method), it is necessary to progressively clean in front of the gun operator to ensure fresh metal is deposited on clean surfaces. The operation becomes a two-man team, with co-ordination of cleaning, application and thickness checking, reducing operator fatigue and maintaining production from the equipment.
WHERE ARE METAL COATINGS USED?
Although metal spray has been used for some 20 years on bridges, components and chimneys, it is only in recent years that the process has been used for Offshore Construction.
In the North Sea certain specialised equipment, which cannot be maintained or replaced without compromising safety, such as flare booms, access bridges between platforms and escape staircases, has been protected.
Other areas with difficult access or expensive to maintain areas such as platform undersides and splash zone steelwork came next. Now complete platforms, especially those handling gas, are being protected with this long life system.
Lifetime costs of coatings are more critical where gas is produced since maintenance shutdowns are less likely than on oil-producing platforms, and in the case of large fields, the lifespan of the platform is likely to be significantly longer.
Examples of platforms where arc-sprayed aluminium has been specified include the Troll gas development for Norway
On another Norwegian project for Conoco Heidrun the project calls for
Specification No2 for exposed steel in humid conditions, vessels and tanks with a maximum operating temperature of 120degC
Pretreatment Sa3
1 coat Arc-sprayed AlMg5 (200+/-50 microns)
1 coat Epoxy tie coat NDFT (25 microns)
1 coat Polyurethane NDFT (50 microns)
Specification for equipment operating above 120degC
as above except final coats after metal spray are
1 coat silicon-based Aluminium paint NDFT (25 microns)
The expected adhesion values for the aluminium is minimum 7.0 MPa and an average of 12.0MPa. The specified application conditions are Relative Humidity shall not exceed 80% and the air and steel temperature shall be minimum 10degC.
METAL SPRAYING IN PRACTICE
The Caister / Murdoch Development, design and construction of which was managed by Conoco, (one of the early oil companies to use TSA), was targeted for the identification of areas to be coated with TSA. The flare stacks and undersides of topsides for offshore production platforms were specified to be metal sprayed with aluminium.
BLACK BOX, ACCESS, DESIGN ETC.
The time taken for the metal spraying system (base coat, sealer and top coats) compares unfavourably with the shorter duration for a four coat paint system with equivalent resources used.
Metal spraying can take longer, therefore construction managers need to build in the extra programme time in the yards.
The black box concept of construction is ideal for TSA, especially if design of the frame work is complete and supporting brackets for major services are already installed. Black boxes are 3 dimensional fabricated structures, some several stories high which include the main structural supports eg beams, columns, floor plates, stiffeners but a minimum of fitting out to allow access to as much steel area as possible in the minimum time. Other suitable construction concepts such as "pancake", trusses, pre-assemblies are also suitable whereas "piece small" will involve many welded connections being coated at a later stage. The design management of secondary fixings for cable trays and small pipework should be considered and non-welded flange clamps should be used where possible.
Design of steelwork should be considered to allow access for the spray gun which should be able to spray at right angles and close to the substrate. Tight re-entrants (eg top surface of bottom flanges of I-beams supporting deck plates), bracket-fixed plates and bulb-flats are difficult if not impossible to reach with TSA.
The use of tubular and rectangular hollow sections is to be encouraged where sprayed aluminium is specified, especially where arc application is specified since this design method minimises re-entrants and use of closure plates.
AUTOMATION
TSA is a process which lends itself to automation on repetitive work, such as tubes and components which do not require reprogramming of the robot regularly.
Automation reduces operator fatigue, exposure to the process, and can produce faster application times with an even, economical coating of metal.
However the automation process does not lend itself to application of complex vessels and fabricated steel frames where hand application will be necessary.
11. Sealing of Sprayed Metal Coatings
There is a radical difference between sealing and painting coats which cover the sprayed metal surface with a protective layer. A sealer is of low viscosity and penetrates into the pores in the sprayed metal and seals them off, without necessarily adding to the total thickness of the protective scheme.
Advantages of sealing are:-
1.    The sealer penetrates the pores, reducing the total area of exposed metal and therefore the rate of dissolution of the coating.
2.    It smooths the surface texture, preventing the retention of grime and other contaminants, thus reducing still further the rate of attack.
3.    Because of this smoothing effect the coating remains cleaner and more attractive, and can be more readily maintained.
4.    The sealer can be pigmented to provide colour for decorative effect.
5.    The sealed coating is a complete protective scheme and needs no added paint coatings.
The life of sprayed zinc coatings is greatly extended by sealing, especially when immersed in water. The life of aluminium coatings is extended rather less, because during the early stages of exposure it becomes sealed naturally by the formation of hydrated oxide films that block the pores. Sealing prevents entirely the brown staining of thin 125 microns aluminium coatings in the rare situations where this might occur.
Formulation of Sealers
Sealers are formulated to have the following properties:-
(a) Low viscosity (3 poise or less to facilitate good penetration into the pores of the sprayed metallic coating)
(b) Low volume solids ratio
(c) Medium leafing or non-leafing pigments
(d) Low water absorption to resist moisture
(e) Inertness to chemical attack penetration and attack by corrosive agents
(f) Electro-chemical compatibility of the pigments with sprayed metal.
Sealers may be un-pigmented or pigmented with colouring agents or aluminium flake. They may be lightly coloured with a dye to ease checking for complete coverage during application. Single-pack resins are usually preferred because of the importance of wetting the surface when subsequent resealing may be required, for instance in maintaining the appearance of the coating.
Sealers recommended because of their previous good service are:-
Mobile solutions of vinyl chloride/acetate co-polymers, phenolic resins, silicone modified alkyds, silicone resin or polymers and, for resistance to elevated temperatures, aluminium pigmented silicone resins.
Two-pack epoxide or polyurethane sealers may be used provided that the difficulties and procedures necessary to ensure good adhesion when repairing or maintaining this type of sealer are fully appreciated by the contractor and his client.
BS 5493, table 4c, lists pretreatment and sealers, but is not in itself adequate as a specification. Some specifications call for the application of polyvinyl-butyral 'etch' primer before sealing. If one is applied it penetrates the coating but must be overcoated subsequently.
Sealers are sampled and tested for compliance with specified requirements by the same methods that are used for paints, particularly when measuring viscosity.
Application of Sealer
It is important that the sprayed metal surface is free from loose particles, dust etc, and is quite dry and uncontaminated by soluble salts such as chlorides and sulphates that may be present in the atmosphere. Sealing should be done soon after spraying, when the metal has cooled, so avoiding contamination. Application by brush, roller or airless spray can be used as convenient. Some specifications call for application to continue until absorbtion of sealer into the pores is complete, which is indicated by a noticeable 'wetting' of the surface: other specifications call for two coats of sealer, one applied at works and the other, for appearance, after erection on site. It must be remembered that the really effective treatment is the first coat. The daily work schedule should be planned to allow for all areas that have been metal sprayed to be sealed before atmospheric contamination occurs, particularly dust or condensation. Dust should be blown off with clean, dry air or preferably vacuum brushed.
Maintenance of Sealed Coatings
Over the years dissipation of the sealer occurs with roughening and darkening of the surface. Restoration comprises cleaning down and wire brushing to remove loose corrosion products and foreign matter, followed by application of a fresh coat of sealer, preferably of the same type as originally used. It is important to check the fresh coat for compatibility with the original. The metal having been sealed by the original sealer, the fresh coat does not penetrate but provides a thin barrier against attack of the metal coating itself.
Painting of Sprayed Metal Coatings
The application of primers, undercoats and finish coats, i.e. a conventional paint system is not justified on either technical or economic grounds but is sometimes specified. If it has to be done, a suitable etch primer should first be applied to the makers parameter to ensure penetration into the pores and to prevent filtration of pigment. In environments of high chemical activity a sealed sprayed coating can be specified followed by a coat of a compatible chemical resistant paint. It is not good practice to apply normal primers and topcoats on surfaces constantly exposed to damp or not washed by rainwater eg., the underside of a horizontal or zones under a plate girder bridge where ventilation is poor. The paint retains moisture, acting as a poultice and maintaining permanently corrosive conditions leading to premature failure.
12. Quality Control
High quality finishes on long life systems demand and deserve quality control - from the initial design stages - selecting the appropriate codes and standards through specification writing to the vetting of production drawings for "coatability".
Production Stages Include
•    Inspection of steelwork before blasting.
•    Blast Cleaning and inspection for cleanliness of surface and profile.
•    Ambient coating conditions.
•    Coating thickness or appearance.
•    Re-coating times.
•    Sealer coatings.
•    Further coats of paint.
APPRAISAL OF STANDARDS
Reference can be made to the Listed Standards in section 3 which provide most of the information needed for inspection standards. Project specific quality plans and inspection sheets are used to control and record information on the steps in the process.
DISCUSSION ON STANDARDS / ISO STANDARDS ETC
Surface Preparation Techniques
Surfaces must be free from
•    Sharp edges
•    Weld spatter
•    Grease and oil
•    Moisture etc
Preparation must include:-
•    Blast cleaning to Sa 3 standard
•    Anchor pattern must be adequate
•    Clean and dust free substrate
Note:-
•    Edges should be rounded to 2mm radius
•    Weld spatter should be removed
•    Grease, oil and moisture should be removed before blast cleaning
•    SA 3 is a visual standard which should be maintained until metal spraying is complete.
•    Anchor pattern is generally higher than for paint and can be maintained by freshening "blast medium".
•    A degree of undercutting is desirable.
•    Dust especially metallic overspray should be progressively removed immediately before spraying.
•    Check list etc
•    Discussion on profile / anchor pattern
ADHESION VALUES
Adhesion values between gas / arc (Subjective)
The testing methods for adhesion pull off tests are usually specified and different adhesion test equipment can produce different results.
ABRASIVES
Suitable abrasives for preparation of carbon steel to be aluminium metal sprayed include:-
•    Chilled iron grit
•    Crushed slag
•    Ceramic grits
•    Aluminium Oxides
•    Silicon Carbides
•    Aluminium Silicates
13. Adhesion Values
Gas application can achieve values of over 600 PSI / 4.05MPa
Arc sprayed aluminium can be achieve values over 1000 PSI / 6.76MPa
A pull off strength of approx 1500 PSI / 10.14MPa is often specified for arc spray.
The reasons for specifying high adhesion values are varied and range from the perception that increased adhesion equals increased service life to the need to have high adhesion values to ensure that the metal spray is retained when subsequent applications eg fireproofing and thermal insulation are later removed.
Further work on the satisfactory level of adhesion standards is on-going in different parts of the world where metal spraying is widely used and an industry standard may well eventually emerge.
14. Health, Safety and Environment
Health & Safety and Environmental considerations in Aluminium Metal Spraying
Experience over a number of years has shown that thermal spraying of aluminium results in little serious Health or Safety problems but like many other industrial processes requires attention to application procedures and equipment to avoid hazards.
Thermal processes involve the use of highly concentrated heat sources, and spraying produces dust. In certain instances toxic, flammable or explosive hazards may be present and care needs to be taken in the burning of gases or high intensity electric energy from the arc process.
Precautions are reviewed under separate headings which would apply to one or more types of process.
Compressed gases
The usual gases used in flame spraying are acetylene or propane with oxygen, and the cylinders should be stored securely and full containers separated from empty ones. Oxygen should be stored separately from other gases.
Pressure regulators should always be fitted to cylinders in use and only connected to the equipment using the special hose supplied.
Electricity
Although the open circuit in arc spraying equipment does not usually exceed 50 volts, it is normally connected to 440 volt supply mains and therefore connections should be made by a competent electrician.
During metal spraying dusts can be created which in worst cases can cause short circuits and therefore earthing and appropriate circuit breaking measures should be ensured.
Radiant Energy
Thermal spraying involves electromagnetic waves and precautions should be taken against ultra-violet waves, especially where the brilliant blue light from the electric arc produced in electrical welding or melting of metals which has high U.V. concentration. The eyes especially should be protected not only those of the operator but also those of other operatives and inspectors. Operatives should wear dark goggles and visors at all times, other people in the area should wear dark safety glasses.
SAFETY IN APPLICATION OF METAL SPRAYED COATINGS
Current Regulations
There are no national regulations that specifically cover metal spraying but most European countries have regulations which control aspects of the metal spraying process and specifiers need to be aware of the potential hazards. Most of the processes will be covered by legislation intended to reduce danger to personnel or to protect the environment.
In UK, the Health & Safety at Work Act, the COSHH Regulations and the Environmental Protection Act etc.
Main Considerations are:-
1.    Dusts
2.    Health
3.    Environment
4.    Plant & Equipment
5.    Training
Dusts
From a Health & Safety aspect several problems or hazards are caused through the dusts produced whilst metal spraying. The quantities of dust evolved in the process is very much dependant upon the efficiency of deposition of the process as it is set up. For example, very high amperage can produce massive amounts of unstable energy at the wire melting head and this results in a none uniform spread of the molten wire and hence poor deposition, similarly the poorly controlled use of gas on the flame spray situation. The molten wire which is not deposited on the surface of the object being sprayed will cool in the atmosphere and form dust particles.
This is not the only criteria to affect the evolution of dusts but does illustrate the point.
Dusts can pose a hazard because of a combination of particle size and concentration in the immediate atmosphere. High concentrations of many dusts are known to be hazardous but Aluminium metal spraying dust is particularly onerous, since aluminium is so explosive / flammable in given forms. Airborne concentrations of as little as 35 mg/m3 have been known to result in explosion and fire when subjected to a source of ignition.
This last figure of 35 mg/m3 is not quite as fixed and clear cut as it appears since the concentration at which the mixture may become explosive will vary dependant upon the particle size. The size one may expect from a spraying operation can range from Sub-Micron, which is respirable level dusts, upto 30 - 50 Microns. As a very broad rule of thumb, the larger the particle size the greater the concentration required to become explosive.
This dust explosion problem may become apparent long after the actual spraying operation is complete. Dusts will collect on the floor or on ledges or in ducts around the spraying area and this can be reintroduced as airborne dust through kicking up on peoples feet or sweeping operations. Again the concentrations will need to be considered to determine explosivity.
Extraction is the method most usually used to maintain dust levels below the explosive limits. The extraction must be sufficiently efficient to ensure all smaller size particles are removed from the air during operations. The quantities required to be extracted should be calculated using the efficiencies of the spraying as a guide. Dust build up in the extraction ducts is a particular hazard which should be considered whilst designing the equipment layout.
Handling of dry dusts causes some problems when extracted dusts are collected in a hopper or chamber. Obviously the chamber will have a high concentration of dust and must therefore be intrinsically safe to avoid explosion risks. When the dusts are removed from the collection chamber and also when dusts are collected from the floor of the spraying area, spark free methods should be employed. The material needs to be sealed into containers and air excluded if possible. Because of the risks associated with dusts of this nature, a secure storage area and disposal method should be developed.
One partial solution to the extraction and collection problem is to utilise a wet system of dust collection. This would typically involve the use of extractors with wet filtration systems and a tank to hold the wet dust. This in addition to wetting of floor deposits to make it easier to collect without clouding up.
This solution is partial since it only maintains the dust at low levels whilst the dust is wet and also, aluminium dust will release a quantity of Hydrogen gas whilst wet. If this is sealed into containers then obviously the gas will expand to pressurise the container.
Health
The types of concerns common to metal spraying operations revolve around protection of personnel local to the operation. The process of Arc Metal Spraying produces large quantities of U.V. light waves. This can damage the eyes in a very short period of time without the person being aware of it immediately. Protection is really only possible through the use of dark lens glasses or visors, to a similar standard used during welding.
The process also gives off a quantity of Ozone. The Ozone is believed to dissipate within feet if not inches of evolution and in most circles is not considered significantly harmful to the operator or others. The real harmful aspects of the spraying operation come to the fore when one considers the respirable portion of the dusts evolved.
Respiratory dust is a major hazard and can only reasonably be protected against by the use of Respiratory Protective Equipment (RPE). Full breathing equipment enclosing the head is the only acceptable protection when using arc spray equipment and full visor protection when gas spraying. The equipment should be air fed though a back up filter pack would be valuable in case of air supply failure.
The possibility of absorbtion through the skin or even skin penetration by particles must be considered a real risk. To this end preferably all areas of an operators skin should be adequately covered and protected.
The likely effects of over exposure to the substances used in both Arc and Gas metal spraying include :-
Metal Fume Fever
Arc Eyes
Skin Rash When Penetrated By Metal Particles.
Some discussion has been raised over the likely hood of alzheimer's disease being caused by the build up of aluminium in the body and the brain in particular, however currently the UK H & SE have not made this official and will not condone it as fact. In the short term however the implementation of zero ingestion as a target for operators and others will protect against this if it is, in the future, considered to be a contributory factor to disease.
Environment
Environmental protection has become a major factor in the evaluation of the use of metals as a corrosion prevention coating medium. In many respects the potential environmental concerns are diminished when coating with metals since no solvents are used in the manufacture or application of the wire. Often however this factor is diffused by the application of solvent based sealer or colour coats of paint, applied later in the protection coating system.
The main points of concern are the release of airborne dusts and the handling and disposal of other dust particles produced and collected manually or automatically.
Airborne Particulate
The Environmental Protection Act defines quite clearly the quantity and amount of particulate which may be released to the atmosphere in the UK. Currently the figure is 50 mg/m3 and is similar in most european countries. The most effective method of controlling the outfall of particulate is through the use of full enclosure operating areas.
Enclosure
Full enclosure can be developed in several ways; most common is the workscope being undertaken in the confines of a workshop with sealed access, but also a tented type of system can be effective. In either case the movement of air must be controlled to ensure any movement is from the outside of the enclosure to the inside. The best way of ensuring this is to use an air extraction system creating a negative pressure internal to the enclosure.
Extraction
Obviously any extracted air will have to be filtered to ensure the exhausted gasses have a quantity of particulate below that acceptable in the area or country of operation. Systems must be put in place which shut down production when the negative pressure enclosure is breached or the fugitive emissions reach above the set levels.
Monitoring
Monitoring of the emissions must be carried out either continuously or regularly, dependant upon your area authority or country regulations. Often this revolves around the measurement of outfall from the exhaust of the extraction units, or regular monitoring of the dispersement of particulates in a given area around the work site or location. In this last instance a great deal of environmental sampling may be required.
Waste Particulate
The handling and disposal of waste material may cause particular concern since it is still in a state from which it may evolve into flammable dust clouds. This can be caused by poor handling techniques during collection or by poor storage where it is released into the atmosphere later in the handling cycle. The potential gas propagation from damp aluminium storage is also a consideration.
Disposal
The disposal of the dusts produced and collected may pose a problem in some countries. In UK for example, small quantities may be land filled as non-hazardous waste. This method is constantly reviewed by each individual area authority so may not be the case every where. Quantities of over 1 ton can be re-cycled economically, though some waste handling companies may store and amalgamate your waste with that of other producers to make up the minimum quantities required. Other forms of disposal include amalgamation of the waste into some other substance where it will be totally encapsulated and so sealed, followed by disposal into a hazardous waste land fill site; Sealing in solid containers and again land filled as hazardous waste.
Plant & Equipment
A thorough understanding of the mechanics and operation of the metal spray equipment is essential to its safe use and maintenance. In the case of both Arc and Gas metal spraying equipment the potential for fatal accidents is always present.
Arc
All arc spraying equipment is dangerous simply because of the use of high voltage electricity. In itself the equipment is designed and built, generally, to be totally safe. The potential danger arises when the equipment is not maintained properly or when the operator is untrained in the setting up or handling.
The voltage used at the spray head is fairly low and is often considered by inexperienced operators to be insignificant, though in reality the operating current inside the spray energiser unit is lethal if improperly treated.
The air feed to the equipment must be maintained as clean as possible to ensure both proper operation and also to restrain potentially flammable substances being fed into the delivery system. Component breakage must be allowed for and potential miss-feed of wire both at the spray head and in the wire feed system can cause injury to the operator.
Earthing of the equipment is obviously a major concern when high amperages and voltages are encountered. The energiser units must be maintained in a clean area otherwise the cooling fans will draw in large quantities of the particulate sprayed. This may cause a short circuit or worse in the energiser unit.
Gas
The safe operation of gas equipment is perhaps more widely respected due to its common site in fabrication industries. The handling of gas equipment and gas bottles is adequately discussed in the British regulations. Safe handling procedures are available in most countries. The mixing of gas and the poor maintenance of gauges etc seem to account for most incidents involving gas metal spraying.
In both gas and arc metal spraying the potential for burns is obviously huge. The processes both produce sufficient energy to destroy flesh instantly if contact is made with the melting area of the spray unit. The surfaces sprayed can also cause burns to operators and others working around them, since heat is retained in the surface for some considerable time following spraying. The temperatures of a medium deposited thickness of aluminium on a 3-4 mm thick steel surface may be around 80 - 120 deg C.
Training
Most training for health and safety of metal spraying is undertaken during production or discipline training. General safety training is obviously essential but added to this should be more in-depth understanding of the specific hazards of metal spraying processes. The extra hazards can entail Noise, High Pressure Gasses and Air, Ultra Violet Radiation, Working With Electricity, Operation Of Breathing Equipment, High Temperature Operation, Negatively Pressurised Full Enclosure and Extraction.
The most suitable method of instilling the knowledge and requirements of the higher safety standards is through a special training programme designed around a particular project. With the larger projects this is possible and cost effective though with the smaller ones a more generic form of training is required.
Trainees must show a basic level of competence in the operation of the safety systems involved in the process. Competence should preferably be demonstrated by testing and observance on the work scope.
Glossary
References
Appendix 1 - Welding of coated steel
Concern is expressed on the viability of welding pre-coated steelwork. In many cases welding of steel is carried out after the steel element has been coated with metal spray in aluminium or other metals. For example where partly pre-assembled steel structures are completed, where pipeline lengths are connected or where repairs are needed after completion of coatings, areas are masked off before metallising or coatings are removed prior to welding to eliminate contamination of the weld. In either case, the completion of the coating system is an expensive process, requiring removal, re-blasting and application in conditions less suitable than the original location. Construction costs could be minimised and maintenance standards raised if the removal process could be reduced.
Little work has been done on the possibility of welding directly on aluminium coated steel. A Norwegian project is currently investigating the problems of welding aluminium-coated steelwork and is also looking at steelwork which has been shop-primed prior to welding or coating with thermal sprayed aluminium.
Appendix 2 - Duralcan
Aluminium wire as supplied for corrosion protection coatings is normally 99% pure aluminium. An aluminium matrix composite was introduced by Alcan Aluminium Ltd in June 1989 and named Duralcan 90/10. The uniform dispersal of aluminium oxide produces:
•    improved wear, abrasion and corrosion resistant coatings using both flame and arc sprayed application.
•    non-skid/non-slip coatings using the arc sprayed process.
Wear resistance is said to be 10 times greater.
Abrasion resistance is said to be up to 35% improved.
The product is suitable for aircraft/helicopter landing surfaces, offshore and marine staircases and landings, car ramps and fork lift truck operating areas.
The application can be made directly onto a steel or aluminium substrate, suitably prepared.
Alternatively, a prefabricated aluminium tile with non-slip coating can be supplied for installation in-situ using suitable adhesives. This process can eliminate the need to take walkways out of use as tiles can be fixed in small areas as required.