FAQ

Efficiency is used as a test to assist in establishing the optimum economic and technical deposition parameters. In general, conditions giving high deposition efficiencies are close to those for optimum fuel utilisation, they are also close to those for maximum integrity.

Efficiency is used as a test to assist in establishing the optimum economic and technical deposition parameters. In general, conditions giving high deposition efficiencies are close to those for optimum fuel utilisation, they are also close to those for maximum integrity.

Factors Affecting Efficiency 

Efficiency will be affected by:

i) The shape and size of the component

ii) The basis material and its preparation

iii) The spraying parameters

Measured efficiency will be reduced when spraying onto small components which are not completely within the spray stream. Even with large components, overspray at edges will reduce efficiencies.

Spraying at angles other than normal to the surface will reduce efficiency. Normally, efficiencies will be higher when spraying onto similar materials and onto properly gritblasted surfaces.

Deviation from the recommended spraying parameters will reduce efficiencies. This will be particularly noticeable if atomising pressures and spraying rates are increased, when the deposit volatilises easily or forms a volatile oxide.

IT IS IMPORTANT THAT DEPOSITION RATE AND FEED RATE ARE NOT CONFUSED. By increasing the fuel consumption, it is possible to spray slightly faster with most materials. In combustion gas spraying particularly, the resultant reduction in an efficiency together with increased fuel consumption renders the practice extremely uneconomic.

Measurement of Efficiency 

A known weight of material is sprayed under closely controlled conditions on to a suitably prepared flat plate, round bar or tube. The efficiency is calculated as the weight gain of the sample per 100gm of material sprayed.

Please refer to EN ISO 17836:2004 Determination of the deposition efficiency for thermal spraying.

Significance of Efficiency 

Because large areas are rare and because spraying is continued beyond the area being coated, perceived efficiencies may be lower than those quoted. Also deposition efficiencies will vary between companies and between operators; better trained and more highly skilled operators will usually achieve greater efficiencies and hence waste less material.

The most common method is deposition from aqueous solutions, many of which are toxic. During electro plating, water decomposes and hydrogen is deposited at the surface being plated. This will cause embrittlement of high strength steels unless they are heat treated immediately after plating. This is only partially effective in reversing the damage.

Electro deposition is usually limited to the deposition of relatively thin, dense layers of pure metals or simple alloys. A limited range of composites may be plated. Electrically conductive substrates are essential but complex geometries are readily plated.

Metal Spraying Offers the Following Advantages Over Electro-Plating

 

• Adhesion is usually better
• Non-metallic substrates can be treated
• Engineering alloys can be applied as coatings
• Ceramic coatings can be applied
• Thicker deposits can be applied
• There is no risk of hydrogen embrittlement
• Deposition rates are higher
• There is no component size limitation
• Coatings can be applied on site
• There is no effluent disposal problem
• Jigs and fixtures do not corrode
• Complex chemical control techniques are not required

Galvanising is a well established process for applying a corrosion protection layer of zinc to steel. Treated steelwork is immersed briefly in molten zinc and the coating builds up very quickly. Galvanised layers are usually of even thickness, dense and metallurgically bonded to the steel.

Thickness is typically 0.002 – 0.006 in (50 – 150μ). Coating compositions are sometimes varied to control deposit thickness and give better protection in certain specific environments.

Metal Spraying Offers the Following Advantages Over Galvanising 

• Low heat input during spraying eliminates the risk of thermal distortion
• Low heat input eliminates the risk of thermal metallurgical degradation
• Sealed hollow fabrications may be treated without risk of explosion
• The process is not limited to zinc. The coating material may be selected specifically for the environment
• Coating thickness may be varied from place to place to provide extra protection in critical areas
• There is no limit to the size of article which can be treated
• Articles can be treated on site
• There is no effluent disposal problem
• Reduced stocks of zinc are required. Working capital is not tied up in a molten zinc bath
• Fuel is not needed to keep zinc molten when the process is not working
• Metal spraying is used to restore corrosion protection on damaged areas of welded galvanised steel.

Welding is a method of joining materials or components to each other, either by direct fusion or through the use of an intermediate filter material. It is also used to apply surface coatings for reclamation or to confer improved wear resistance on OEM articles.

Weld overlay materials are typically steels, bronzes or nickel based alloys. The coatings are metallurgically bonded to their substrates and porosity levels are very low.

Metal Spraying Offers the Following Advantages

• Low pre-heat or no pre-heating is required
• No heat treatment is necessary after coating
• Little heat is transferred to the work piece during coating
• There is therefore no risk of thermal distortion
• There is little risk of metallurgical degradation of the substrate
• Almost any substrate can be coated
• There is no dilution of the coating by the substrate material
• A wider range of coatings can be applied
• Operator skill requirements are lower
• Spraying is usually (but not always) faster
• There is better control over deposit thickness
• Machining allowances are reduced, thus saving material
• Machining times will also be reduced.

Bond Coats are used in the following circumstances:-

• To improve adhesion when basis materials have been prepared mechanically.
• Where the substrate material is too hard for mechanical preparation.
• Where mechanical preparation would damage the substrate material.
• Where mechanical preparation is uneconomic.
• Where the chosen coating will not adhere to the substrate material.
• As a buffer where brittle coatings may be subjected to shock loading or excessive mechanical strains.
• As a buffer between coatings and basis materials where elevated temperatures may lead to excessive strains due to mis-match of thermal expansion co-efficient.

Selection of Material 

Choice of bonding material depends on basis material and spraying process and may be influenced by corrosion considerations. In somecases, it may be necessary to experiment with different materials to obtain optimum selection.

99E Molybdenum

This is applied by flame spraying as a bond for steels and aluminium materials. It does not bond to copper and its alloys. It should not be used above 400°C.

T405/1 Ni alloy

This is applied by flame spraying as a bond for steels. It may improve coating adhesion on aluminium alloys but it will not bond to copper and its alloys.

85E Ni chrome

This is used as a high temperature bonding material (up to 1000°C) and as a buffer layer beneath ceramic coatings. It adheres well to steels but does not bond to copper and its alloys. It may be arc or flame sprayed.

10E Aluminium Bronze

This is used as a bond coat for steel substrates and may be used on copper alloy substrates. Arc

sprayed aluminium bronze gives better results than when flame sprayed.

05E Copper

This is sometimes used as a bond when spraying on to copper and copper alloy substrates. Mechanical preparation is also necessary.

02E Zinc

This is frequently used as a bond coating on to plastics, paper and wooden substrates. Sometimes,

it is effective as a bond onto ceramics and glasses. It may be arc or flame sprayed.

75E Ni alloy

Gives the best adhesion on steels and good adhesion to aluminium alloys. It is not suitable for copper and bronzes. It should be applied only by arc spraying.

01E Aluminium

Bonds well to steel and may be used for certain ceramic and glass substrates. It should not be used for plastics.

Application of Bond Coats 

Bond coats should be applied thinly to produce a roughened surface to which the subsequent coat can adhere. For most purposes 0.002-0.004″ (50-100μm) is adequate; thicker coatings are not only more expensive but often less effective. Where the bond coat is applied as a buffer beneath ceramics which may be subjected to excessive mechanical or thermal strains, thicker deposits are necessary. Depending on application, the buffer layer may be between 0.005″-0.15″ (125 – 375μm).

The bond coat must be applied to a clean, and preferably grit blasted surface as soon as possible after the surface has been prepared and before there is any visible contamination. Subsequent coatings should be applied immediately after bond coating.

REFERENCE INFORMATION :- 

5.2.1 Preparation for Spraying 

5.2.2 Surface Preparation by Gritblasting 

Grit blasting is the most commonly used method of preparing surfaces for metal spraying.

Grit blasting is the most commonly used method of preparing surfaces for metal spraying. It removes rust, mill scale and other surface contaminants and produces a suitably roughened surface by projecting a highly concentrated stream of relatively small abrasive particles at high velocity against the surface to be cleaned. It has also been shown to be effective in reducing the loss of fatigue strength.

Equipment 

Suction or Syphon Blasting 

Here the particles of abrasive are projected by suction or by a venturi type nozzle into an air blast. It is mainly employed in the preparation of small components in hand cabinets.

High Pressure Blasting

The particles of abrasive are directly fed from a pressurised container into a high pressure air stream. This is the most widely used form of blasting, either in hand cabinets, blast rooms or in portable form, on site work.

Centrifugal Blasting

Involves the abrasive being centrifugally propelled from rapidly rotating impellers. It is much more specialised equipment and is highly efficient for low cost blasting of large volume repetitive production.

Types of Abrasives 

Chilled Iron Grit 

This is by far the most widely used abrasive for metal spraying. It is an excellent general purpose abrasive, due to (a) its relatively high density, which gives high particle energies, (b) its slow rate of breakdown and (c) the retention of sharp cutting edges on the particles.

Crushed Slag’s – Expendable Abrasive

An alternative to chilled iron grit when reclamation is not possible, as is the case on many site jobs. While quite effective for “once only” use, they are not suitable for reclamation and re-use, due to their rapid breakdown to dust.

Ceramic Grits – Aluminium Oxide and Silicon Carbides 

Used where the base material has a hardness greater than 360HV which cannot be effectively blasted by chilled iron grit. They can be used at lower than normal blasting pressures and are effective when “Syphon Blasting”. They are therefore well suited to the preparation of thin metal surfaces which may distort if blasted with chilled iron at conventional pressures. Non-metallic grits must not be used to prepare surfaces for coatings which are to be fused. Grit blasting standards for metal spraying should not be confused with blast cleaning as used to prepare surfaces for painting.

Of the various standards of surface finish “SA 3” are comparable in surface cleanness with grit blasting quality for metal spraying.

The blast profile, defined as “height from trough to adjacent peak”, should not exceed 0.004″-0.005” (100 – 125μ) experience has shown that chilled iron grit Grade G24 provides a surface of appropriate amplitude. Comparable surface amplitudes are similarly achieved with expendable non-metallic abrasives of around N° 16 mesh.

Blasting Techniques 

i) Blasting pressures must not be excessive. If pressures are too high, grit breakdown will be rapid, grit may be embedded in the surface and mechanical distortion of the component may occur. Particular care is required with most non-ferrous alloys, plastics and fragile or highly stressed parts.

ii) Grit must be inspected regularly. Blunt particles, fines and contaminants are deleterious and should be removed.

iii) Blasting air must be free of water, oil and other contaminants. Accordingly, suitable after coolers, moisture traps, filters etc. should be fitted to the air lines.

iv) Excessive blasting should be avoided. It is expensive and can be detrimental to the metal spraying process.

v) Blasting debris must be removed from the surface before spraying. Vacuum cleaning or brushing is preferable; blowing with compressed air may not remove debris but move it from one place to another.

vi) Grit blasted surfaces must not be contaminated before spraying. If handling is unavoidable, clean cotton gloves should be used.

vii) Spraying must commence as soon as possible after surfaces have been blasted, certainly before any visible deterioration occurs. In temperate climates, deterioration (and impairment of adhesion) may occur in less than four hours. In hot, humid conditions, deterioration will be more rapid.

Precautions Relating to Gritblasting 

i) Except in open site work, where special precautions must be taken to protect personnel; blasting should always be done in a blast room or cabinet.

ii) Never start up a blasting unit until the hose is firmly held pointing in a safe direction.

iii) The blast hose should be of an approved anti-static type and must be inspected regularly for wear and security of fittings.

iv) Always wear full protective clothing; for example, helmet, hood, gloves, aprons and leggings, to ensure protection from flying abrasive.

v) The provision of an inspection window in a blast room is advised.

NOTE – Under The Blasting (Castings and other Articles) Special Regulations 1949 Part II, it is forbidden to use sand, or other substance containing free silica in any blasting apparatus. 

REFERENCE INFORMATION :- 

5.2.1 Preparation for Spraying 

5.2.4 Bond Coating 

Sprayed coatings adhere to surfaces mainly by mechanical and physical means; in a few instances, metallurgical or chemical bonding may occur to a small degree. Whatever the mechanism of adhesion, it is vital that the surface to be sprayed is clean and adequately roughened. Over 80% of coating failures are due to poor or incorrect surface preparation.

Initial Inspection 

Surfaces to be sprayed should be examined to ensure that previous coatings have been removed, that welds are properly dressed and that no cracks or other defects exist.

Methods of Preparation 

Degreasing 

Oil and grease must be removed before preparation begins. Without this, grit and tools will become contaminated and the oil will spread over the surface. Vapour degreasing is preferable, where this is not practicable, care must be taken to ensure that solvents do not simply re-distribute the contaminant thinly over the entire surface. Porous materials such as castings may require baking to ensure removal of oils.

Gritblasting 

This is the most commonly used method of preparation. Sharp abrasive grit is projected towards the surface, either mechanically or by compressed air. Blasting cleans the surface, increases the surface area and provides a profile into which the surface will key. It is important that the grit is of the correct type and size, is not contaminated and does not contaminate the surface, For further information, see Technical Bulletin 5.2.2.

Rough Machining 

This method is commonly applied to surfaces which are required to bear a thick deposit. It increases the surface area and provides a profile which will resist shearing between coating and substrate, For further information, see Technical Bulletin 5.2.4.

Bond Coating 

Originally developed to provide a rough keying profile on ground steel surfaces which were too hard to grit blast, bond coats are now extensively used to enhance adhesion on mechanical and gritblasted surfaces. Laboratory tests indicate that bond coats not only increase bond strength, but will also give more consistent adhesion, For further information see Technical Bulletin 5.2.4.

Combined Techniques 

The above methods may be combined to give superior coating adhesion.

Preheating 

Preheating is rarely needed, but is essential for certain substrates, e.g. glass, to prevent thermal shock – usually no further preparation is needed in these cases. Preheating is advisable when spraying bores or internal diameters with high shrink materials or thick deposits. It is also recommended when environmental conditions are such that water, from burning gases or the atmosphere, may condense onto the workpiece during spraying.

Care must be taken to avoid excessive temperatures 175°C (347°F) maximum. Surfaces should be re-gritblasted immediately after heating to remove the thin oxide film which will form.

Care of the Prepared Surface 

Prepared surfaces are chemically and physically very active. They must not be allowed to deteriorate or become contaminated. They must be handled with care and not touched with naked hands, ropes or slings. Clean, lint-free cotton gloves or sheets should be used to protect prepared surfaces during handling.

Spraying must begin as soon as possible after preparation. The allowable time interval depends on the material and on ambient conditions. It should not exceed four hours: in hot or humid conditions the maximum allowable delay may be very much less. If longer delays occur, the surface must be re-prepared unless special storage facilities are available.

REFERENCE INFORMATION :-

5.2.2 Surface Preparation by Gritblast 

5.2.4 Bond Coating

Metal Spraying is not a dangerous process, if equipment is treated with care and correct spraying practices are followed.

Metal Spraying is not a dangerous process, if equipment is treated with care and correct spraying practices are followed. However, as with any industrial process, there are a number of hazards of which the operator should be aware and against which specific precautions should be taken.

Ideally, equipment should be operated automatically in enclosures specially designed to extract fumes, reduce noise levels and present direct viewing of the spraying head. Such techniques will also produce more consistent deposits. However, there are occasions when the type of components being treated or low production levels require manual operation. Under these conditions a number of hazards peculiar to thermal spraying are experienced in addition to those commonly encountered in production or processing industries.

NOISE

Metal spraying equipment uses compressed gases which create noise. Sound levels vary with the type of spraying equipment, the material being sprayed and the operating parameters. Typical sound pressure levels taken 1 metre behind the arc spray or flame spray nozzle are 102-104 db(A).

Specially designed enclosures should be used to attenuate these levels. Where this is not possible, operators and passers-by should wear good quality ear defenders. Please refer to the relative Metallisation manual for the generated noise levels of a specific piece of equipment.

LIGHT

Combustion spraying equipment produces an intense flame which may have a peak temperature in excess of 3,100°C and is very bright. Electric arc spraying produces ultra-violet light which may damage delicate body tissues. Spray booths and enclosures should be fitted with ultra-violet absorbent dark glass. Where this is impracticable operators and others in the vicinity should wear protective goggles containing BS grade 6 green glass. Opaque screens should be placed around spraying areas. The nozzle of an arc pistol should never be viewed directly unless it is certain that no power is available to the equipment. For Plasma an even higher level of protection is required typically BS grade 10 green glass.

DUST AND FUMES

The atomisation of molten materials produces a certain amount of dust and fumes. Proper extraction facilities are vital, not only for personal safety, but to minimise entrapment of re-frozen particles in the sprayed coatings. The use of breathing masks fitted with suitable filters is strongly recommended where equipment cannot be isolated.

Certain materials offer specific known hazards.

• All finely divided metal particles are potentially pyrophorric and none should be allowed to accumulate.
• Certain materials e.g. aluminium, zinc and other base metals may react with water to evolve hydrogen. This is potentially explosive and special precautions are necessary in fume extraction equipment.
• Fumes of certain materials, notably zinc and copper alloys are unpleasant to smell, and, in certain individuals, may cause a fever-type reaction. This may occur some time after spraying and usually subsides rapidly. If it does not, medical advice must be sought.
• Several commonly sprayed substances are subject to statutory exposure limits, please refer to the relevant MSDS sheets for the material to be sprayed.

HEAT

Combustion spraying pistols use oxygen and fuel gases. The fuel gases are potentially explosive. In particular, acetylene may only be used under conditions approved by the Health and Safety Authorities. Oxygen, while not explosive, will sustain combustion and many materials will spontaneously ignite if excessive oxygen levels are present. Care must be taken to avoid leakage and to isolate oxygen and fuel gas supplies when not in use.

ELECTRICITY

Electric arc pistols operate at low voltages (below 45 dc) but are relatively high currents. They may be safely hand held. The power supply units are connected to 440 volts AC sources and must be treated with the normal caution afforded to such equipment.

COMPRESSED AIR

The air supply to spraying pistols is at high pressure. It should not be directed towards people. The motor air supply is lubricated and on no account should it be fitted to breathing apparatus. Any breathing equipment used with the thermal spraying process must be supplied with air of breathing quality.

REFERENCE INFORMATION :- 

ASM Thermal Spray Society – Designation SG003-03 – Thermal Spray Booth Design Guidelines 

TSSEA Thermal Spraying & Surface Engineering Association – Code of Practice for the Safe Operation of Thermal Spraying Equipment. 

MSDS – Material Safety Data Sheets – provided by the wire / powder manufacturer. 

Lookup chart for typical coating and lifetimes in given generic environments.

Introduction

The selection of a coating system is dependent on the environment in which it is to operate. These environments are detailed below. This is followed by the range of systems available and a chart to indicate the typical time to first maintenance. The treatments recommended for longer lives will always protect for shorter periods and are frequently also economical for these shorter lives.

Environments

Other Environmental Considerations

Mines

Warm Humid Conditions (water present – sometimes saline).

Specialist advice should be sought as conditions in different mines vary considerably. Zinc coatings (not aluminium or paint in coal mines) should be considered provided that the water pH is greater than 5. A sealed coating is preferred. Guidance can be sought from Environment 8 but time to first maintenance may vary widely, depending on particular conditions.

Soil

Earth, sand, rock etc.

Specialist advice is advised as the performance of the coatings will vary accordingly to the nature of the soil. Coating lives may be shortened by soluble sulphates and un-burnt coke contained in clinker and ashes. Coatings are preferably sealed. Aluminium coatings are not recommended for direct contact with alkaline clays.

Alkaline concrete away from atmosphere

Aluminium is unsuitable for direct contact with concrete due to its alkalinity and an inert barrier should be provided. This barrier is not required with zinc. Zinc coatings are beneficial in areas where carbonation of the concrete may occur.

Refrigerated Surfaces

Subject to ice formation and condensation.

Sealed or unsealed coatings are generally suitable. For temperatures below – 30°C advice should be sought.

Chemicals

Sealed metallic zinc is generally suitable for chemicals in the pH range 5 -12, sealed aluminium in the pH range 4 – 9, provided the chemical does not specifically attack the coating. The effect of the coating and sealer on the chemicals should be considered, as well as the protection of the steel.

Abrasion and Impact

Additional consideration in some applications.

The resistance to abrasion, rough handling or impact by sprayed metals (sealed or unsealed) is acceptable. The coating polishes by friction. Where abrasion is critical, specialist advice should be sought.

Table of typical coatings and life to first maintenance

Note a: Low corrosion category C1 has no specified coatings but will typically be 5-10 times longer than C2.

Note b: Sealed coatings will typically perform better than unsealed coatings and life to first maintenance will exceed that of an unsealed coating. Sealers should generally be applied until absorption is complete. Sealants and paints should be chosen for the specific environmental conditions.

Painting of sprayed metal coatings

Painting of sprayed metal coatings is normally only required when:

01. The environment pH value is outside the range 5-12 for zinc or 4.9 for aluminium

02. The metal is subject to direct chemical attack

03. The desired finish can only be obtained by paint

04. Additional abrasion resistance is required. Generally one or two coats of paint are sufficient, except in abnormally aggressive environments. (Sealed metal spray is normally preferable).

Metal spraying provides a very good bond surface for painted coatings and can increase the life of painted coatings.

The information in this bulletin is abstracted from BS EN ISO 14713 and BS EN ISO 2063.

The adhesion of sprayed coatings to substrates is a matter of great concern to Engineers.

The adhesion of sprayed coatings to substrates is a matter of great concern to Engineers. Coatings which become detached during machining must be re-applied whereas those which fail during service will not only cause failure of the coated part but may seriously damage other components and could lead to injuries. However, provided that the basic rules for using sprayed metal coatings are applied and that materials are correctly sprayed on to properly prepared surfaces, bond failures are rare.

Many techniques have been used to assess the adhesion of coatings. The most commonly employed involve pulling in tension a known area of coating from a suitably prepared substrate. In order to do so, it is necessary to attach a pulling device to the coating with a suitable adhesive. This method has the advantage of giving a load failure and, knowing the area under test, a failure of bond strength can be calculated. Unfortunately, the test is generally restricted to test pieces which bear little resemblance to engineering components. Although the test is simple, it is subject to many variables: the strength and curing state of the adhesive; the degree of penetration into porous sprayed deposit (depending on porosity, coating thickness, adhesive viscosity, etc.,) axiality may not be achieved during testing (which may give rise to sheer and peel stresses as well as tensile stresses at the interface).

Complete detachment of the deposit rarely occurs and the fracture is a mixture of bond and cohesive failure. These factors combine to produce considerable scatter and test results and quoted bond strengths should be treated with considerable caution. If adhesion is critical, it is strongly recommended that a practical evaluation of a sprayed component be made before specifying a particular sprayed deposit. The adhesion test may then be used as a quality control tool rather than a design aid.

Hardness is of interest to engineers because it relates to the tensile strength of wrought or cast metals, or because it gives an indication of resistance to abrasive wear.

Hardness is of interest to engineers because it relates to the tensile strength of wrought or cast metals, or because it gives an indication of resistance to abrasive wear. The tests, which measure the resistance to penetration of a hardened ball, cone or diamond under known loads are simple, rapid and require a minimum of skill. With homogenous wrought or cast materials, hardness can be converted from one scale to another with reasonable accuracy by using appropriate conversion charts. Hardness of sprayed deposits cannot be converted into other scale or into tensile strength values.

Sprayed coatings differ in that they consist of many individual particles, are un-homogenous and may contain appreciable levels of porosity and oxides. Hardnesses of sprayed deposits must be treated with considerable caution. Values measured on a single specimen may vary depending on whether the area under the indenter contains porosity, oxide or uncontaminated coating. In general for unfused coating, Brinell hardnesses made with a large diameter ball will give the most consistent results. However, such impressions may penetrate deeply into the coating and particularly with thin deposits, may be influenced by the hardness of the basis material. Coating macrohardness tests are not recommended on deposits less than 3mm (0.125 in) thick.

The hardnesses given below are based on laboratory tests made at Metallisation Limited, certain Customers and Universities. They are for guidance purposes only, except under abrasive wear conditions, they should not be used for design purposes and even where abrasive wear is experienced, they should be treated with caution.