Upon completion of this module, Students will be able to:
Understand thermally sprayed materials such as aluminium and zinc will protect steel substrates from corrosion
Explain the different types of equipment used for the application of thermally sprayed materials
Explain the requirements for training and quality control for the application of thermally sprayed coatings
Introduction
Thermally sprayed metal coatings are metal deposits melted immediately prior to projection onto the substrate.
The metals and application methods vary, but most applications result in thin coatings applied to surfaces requiring improvement to their properties of corrosion or abrasion resistance.
Sprayed metal coatings have been used for a number of years and have proven to give excellent performance providing they are applied correctly, adequately sealed and subjected to specific environments.
Metal on Metal
The thermal spraying process is an excellent means of protecting iron and steel against corrosion.
If two dissimilar metals or alloys are electrically connected in an electrolyte (water, soil, etc), current will flow from the more noble (cathodic) metal to the less noble (anodic) metal. Current will then flow from the anodic metal to the electrolyte and from the electrolyte back to the cathodic metal.
If aluminium or zinc are applied to a steel structure via thermally sprayed wire or powder etc the aluminium or zinc will sacrificially protect the steel structure from corrosion.
Therefore the key to the success of thermal metal spray for corrosion protection is to achieve a uniform coating of the required film thickness with a suitable adhesion value.
Anodic and Cathodic Sites
During the corrosion process, anodic and cathodic sites are set up on the surface.
If we can make all the anodic sites cathodic we could suppress corrosion. This principle is used in cathodic protection.
Not all sites have uniform corrosion, some may have pitting for example.
Where steel is stored for example can have an effect on the way the surface corrodes.
If steel is stored near the coast, for example, it will be subject to a salt environment.
Corrosion - Current Flow
This diagram illustrates the flow of current between the anodic sites (reactive) and cathodic sites (non-reactive) in a corrosion cell.
The electrolyte can be water or soil.
Remember corrosion is a electrochemical process.
Corrosion - Galvanic Table (general)
This table is useful as it gives a general guideline on metallic materials.
The material which requires the most energy to form is more reactive and will, therefore, corrode in preference to a less reactive material.
Aluminium and zinc will, therefore, protect iron and corrode first. This is the main reason why these materials are used in the thermal spraying process.
You will also see aluminium and zinc used as anodes to protect steel structures such as ships and pipelines.
Corrosion - Cathodic Protection
This animation demonstrates the use of an aluminium anode to protect a steel structure in sea water. Immersed steel pipes can be protected in the same way.
The aluminium anode which is more reactive will corrode in preference to the steel.
This process is called cathodic protection.
The anode should never be coated or it will not work.
The thermal spray process works on the same principle. The aluminium or zinc will corrode in preference to the steel.
Zinc Vs Aluminium
There are a number of advantages of zinc and aluminium coatings.
Aluminium
High temperature resistance
Lightweight
Excellent resistance to polluted and marine environments
Forms oxide layer to reduce permeability and reduced corrosion
Zinc
Excellent for cathodic protection
Excellent resistance to mechanical damage
Life proportional to thickness.
Al-Zn
Some operators have found good results using aluminium zinc as a mixture rather than the individual wire. The choice will depend upon the location, temperature, cost etc.
What is Thermal Spraying?
Thermal spraying is the term given to a group of processes that utilise thermal energy to melt a material and project the molten material as a spray plume.
The molten particles impact onto a specially prepared surface and deform to form a splat. This splat sticks to the surface and subsequent splats stick to each other to build up as a discrete coating.
The diagram illustrates the process.
1 – Raw Materials (zinc, aluminium etc) 2 – Electric or gas (heating of particles) 3 – Particles projected onto the surface 4 – Material hits the surface 5 – A coating is formed on the surface
The Basics
All methods of thermal spraying involve the projection of small molten particles onto a prepared surface where they adhere and form a continuous coating.
To create the molten particles, a heat source, a spray material and an atomisation/projection method are required. Upon contact, the particles flatten on the surface, cool and mechanically bond.
Firstly onto the roughened substrates and then onto each other as the coating thickness is increased.
There are four commonly used application processes for thermal spraying:
Flame spray
Arc spray
Plasma spray
High velocity oxygen fuel (HVOF)
We will only detail the flame and arc spray process which are the most commonly used application processes used for finishing coatings.
Thermal Spray Materials Selection
The main materials used for corrosion control which are thermally sprayed are zinc and aluminium. It is possible to have a zinc and aluminium alloy system such as 85% zinc and 15% aluminium.
The choice of materials to be used is a vast topic with many factors to be taken into consideration such as environment (corrosive atmosphere, temperature etc), specifications, the life expectancy of the coating, adhesion requirements, availability of materials etc.
In general terms, zinc is used in reasonably less corrosive environments while aluminium is used in harsher corrosive environments.
Thermally Sprayed Aluminium
Thermally sprayed aluminium (often referred to as TSA) has obtained an extensive track record in corrosive environments such as offshore platforms, piping and structures.
As aluminium has a melting point of 660°C (1220°F) it is used on hot structures such as flare booms and piping.
Thermal spray aluminium has found a popular place on new build projects which are subject to hostile environments and locations where maintenance can be difficult.
Although the initial application cost of thermal spray aluminium is higher than paint coatings the annualised cost will be lower over the life of the structure (20+ years).
Surface Preparation
It is essential that a high level of surface preparation is conducted before the application of thermal spray coating.
Pre-preparation will consist of removing all the sharp edges, weld spatter and general contamination such as grease, oil and moisture.
The structure should be designed in such a way that the spray gun (arc or gas) can gain access to all areas.
Surface preparation should be conducted with metallic abrasive such as chilled iron grit to a Sa3 standard (SSPC-SP5/NACE No.1) with a surface profile of 75-100* microns with a good anchor pattern.
The preparation equipment should be free from oil and moisture during surface treatment.
* ISO 2063 refers to 50-100 microns, however, industry specifications generally specify 75-100 microns.
Surface Preparation
The prepared surface must be vacuum cleaned to ensure there is no dust, debris or abrasive left on the surface before application of the Thermally sprayed coating.
The adhesion of the thermal coating is mechanical and surface cleanliness is essential to obtain a high degree of adhesion.
The steel surface temperature should be above 10°C* and at least 3°C above the dew point.
Environmental conditions must be monitored throughout the preparation and coating process. The relative humidity should be <85%.
* ISO 2063 refers to 5°C, however, Industry Specifications generally specify the 10°C.
Material Application
Although materials can be applied in a powder composition, metal wire is common practice and most popular for anti-corrosion materials such as zinc and aluminium.
The wire material is applied by one of two methods:
Gas Spray
Arc Spray
Although both gas and arc apply a melted material to a prepared substrate the equipment is fundamentally different and the process of application differs also.
Flame Spray Process
In the wire flame process, a wire is fed by a driven roller system through the centre of an oxygen fuel flame where it is melted.
An annular air nozzle then applies a jet of high pressure air, which atomizes and projects the molten material onto the workpiece.
The driving of the wire is typically via an air motor and gearbox that forms part of the Pistol. Wire diameters that can be flame sprayed as a standard range from 1.6mm to 4.76mm (1/16” to 3/16”).
The wire is typically dispensed from coils or production packs or drums.
Flame Spray Process
This animation is a schematic of a flame spray Pistol in operation.
The wire is fed through the wire drive mechanism where it is mixed with the compressed air and fuel and projected onto the substrates.
Thermal Spray Equipment
The metal is supplied in wire form and fed through the gun which melts the wire and projects it onto the prepared surface.
Both gas guns and electric arc types of equipment are available for this process.
Thermal Metal Spray Coatings
This video demonstrates the use of a thermal spray coating applied to a suitably prepared surface.
The metal used is aluminium, however, zinc can also be used.
This process is ideal for long-term corrosion protection.
The metal coatings can also be overcoated with paints.
Wire Flame Spraying
The wire is fed into a flame gun and mixed with fuel gas, usually, acetylene and propane, melted and applied to the prepared substrate.
The acetylene and propane melt the wire and compressed air is used for the atomisation and transporting process.
This photograph illustrates a flame spray set-up showing the fuel bottles, mounting panel with flow meters, regulators and filtration unit, and compressed air is also required.
Arc Spray Process
In the arc spray process, two electrically charged wires are driven and guided so that they converge at a point and form an arc.
An air nozzle atomises the molten metal produced and projects it towards the workpiece.
The driving of the wires is typically either by air motor or electric motor and gearbox arrangement.
The wires can be driven in three individual ways:
1 – Push only - the wire is pushed from the drive unit to the pistol.
2 – Pull only – the wire is pulled by the drive unit mounted in the pistol.
3 – Push/pull – A combination of items 1 and 2.
Arc Spray Process
This animation is a schematic of an arc spray process.
The electric arc spraying is performed by feeding two electrically conducting metal wires towards each other.
An electric arc is produced at the point just before the wires meet.
Compressed air is used to project the molten metal onto the prepared substrate.
Arc spray creates considerable energy which in turn produces high temperatures and potentially higher adhesion values.
Arc Spray Process
This photograph illustrates an arc spray process unit.
The energiser is available in several sizes depending upon the size and configuration of the structure.
A larger energizer is more productive and more effective over large flat areas. There will, however, be more operator fatigue.
The arc spray pistols are typically designed for 1.6 – 3.2mm wires (1/16 to 1/8”).
Compressed air is also required to complete the unit.
Arc Spray Process
This video demonstrates the application of thermal spray aluminium to an abrasive blast cleaned surface.
The video illustrates the energy produced by the process and the requirement for suitable PPE and RPE along with adequate training of the operative.
Application of Metal Sprayed Coating
When using arc or gas application, operator experience will develop with time but some general guidelines include the development of an optimum distance between the pistol and substrate. This is needed to maintain a satisfactory temperature at which the sprayed metal hits the surface.
This distance, combined with the speed of lateral movement of the pistol, controls the rate and hence the thickness of metal deposited.
In order to maintain an even thickness on large flat areas, smaller areas, about 0.5m² (5.5 sq.ft), are usually marked out by the operative with the sprayed metal and then ‘filled in’.
Application of Metal Sprayed Coating
This video demonstrates the application of flame spray aluminium to an offshore flare boom.
The process is considerably slower than paint application and the programme of works must consider the high level of surface preparation and the metal spray time.
Operator fatigue must be also considered as a manual application of sprayed metal coatings involves repetitive and prolonged movement under physical strain, which can easily lead to injury.
Dry Film Thickness
The dry film thickness of thermal coatings can be checked immediately after application when the surface has cooled.
The inspection must be very thorough for the application of thermal coatings.
Under thickness may not perform or seal the surface, and over thickness may create adhesion problems.
Full-time coating inspection is always advisable throughout the works.
Plasma Spray
For some specialist applications, plasma spray application is used.
This process does not use a wire and the metal is supplied as a powder and fed into the flame and projected onto the prepared surface.
For general corrosion control, the use of arc spray and flame spray is the preferred option.
For information only. Not covered in theoretical tests or practical assessment.
Adhesion Values
As the coating bonds by mechanical strength, the adhesion value of the sprayed metal is important.
The specification will detail the minimum adhesion values.
Adhesion tests are generally conducted on test plates at the start of the shift and then on the structure if adhesion values do not meet the minimum requirement of the test plate.
There are various adhesion standards used globally but most will use a dolly which is glued to the surface and pulled off (pull off gauge) to demonstrate the adhesion values.
A skilled coating inspector/supervisor should perform this test using the appropriate standards.
Specifications and Standards
There will be various specifications and standards used for the thermally sprayed coating.
These documents will generally detail:
The specified process (arc or gas)
The type of wire (zinc or aluminium or an alloy of)
Surface preparation, including pre-preparation
Environmental conditions
Applied film thickness
Adhesion values
Operator skill and training requirements
Application of sealer coat / top coats
The contractor and coating inspectors should be fully conversant with the specification and standards.
Examples include ISO, SSPC and AWS Specifications
Sealing of Sprayed Metal Coatings
There is a radical difference between sealing and painting coats which cover the sprayed metal surface.
A sealer is a low viscosity material which 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:
The sealer penetrates the pores
The sealer smoothes the surface texture
The sealer can be coloured to add an aesthetic finish and is easier to keep clean (if required)
Topcoats such as epoxy or urethane acrylic can also be used if a high-quality finish is required.
Sealer Application
Sealers are typically applied by airless spray and are generally applied as soon as possible/practicable after the application of the sprayed metal coating. The exact time may be specified by the client.
If the structure to receive thermal spray coatings is operating at a high temperature, i.e. above 120°C (248°F), then a silicone aluminium sealer is generally used.
There are a number of sealers available for use below this temperature range. However, the most popular is an epoxy sealer.
The silicone aluminium may need to be post heated (in service) to fully cure.
Sealing and Coating
Thermally sprayed coatings are generally sealed soon after application.
Although specifications can vary between operators the two main coating systems consist of:
High-temperature service - Silicone aluminium
Ambient (<120oC) Epoxy sealer with optional topcoats
This photograph illustrates the two coating systems used on the same structure.
Thermally sprayed and coated systems have been successfully used on offshore, marine and infrastructure structures for many years.
Repairs to Metal Spray Coatings
Thermal spray coatings are very tough and durable, however, repairs may be necessary during construction.
It is essential that these areas are treated in the same way as the original application.
Correct ambient and environmental conditions
Abrasive blast clean
Abrade the overlap locations and ensure the adhesion to existing metal spray is good and not lifting
Apply the metal coating to the repair location with 25-50mm (1 to 2”) overlap on to unsealed metal.
Check adhesion to both existing steel substrate and metal overlap.
Inspection
Quality control is an essential element when conducting metal coatings.
The work should be pre-planned with the use of inspection test plans. Using hold and witness points is essential.
The coating inspector should be qualified, experienced and have a good knowledge of the process.
A pre-job conference should be conducted with the contractor, fabricator, inspection personnel etc, to discuss all aspects of the process.
Programme
Quality and Environmental
Specifications and standards
Equipment and application type
Inspection and testing
Operator skill and qualification
Applicator Testing
Most thermal spray coating specifications include applicator testing to ensure the equipment is set up correctly and the applicator has been trained correctly.
The initial training is usually conducted by the coating supplier with additional training conducted by the contractor.
Test plates are generally specified at the start of a shift. These can be checked visually and subjected to a bend test or adhesion test.
There are usually pass/fail criteria for the bend test.
Always refer to the appropriate standard such as ASTM, ISO or contract specific procedure.
Training of Operative
There are several activities during sprayed metal coatings, including:
Equipment set-up
Surface preparation
Surface cleanliness
Application of metal coatings
Dust/grit removal
Application of sealer
The operator should be trained on the equipment and the process for the metal spray coatings.
Equipment manufacturers conduct training on equipment and an introduction to the process. Trade testing on sample pieces, followed by adhesion checks, is usually conducted prior to the commencement of the works to ensure the metal sprayer is competent in the application process.
Applicator Trade Tests
Applicator trade testing is required at the start of many contracts or projects.
The exact method of the test will be specified by the customer or client, however, the following process is usually conducted;
The steel test plates are abrasive blast cleaned with the specified standard, abrasive and surface profile
The steel test plates are metal sprayed to the specified film thickness
Adhesion tests are conducted, usually with a 'pull-off' type gauge to achieve the specified adhesion values
The operative should be using the same equipment for the test piece as for the main works.
These test plates can be specified at the start of a shift or periodically as necessary.
Applicator Trade Tests
A bend test can also be conducted by the applicator.
This test is usually conducted at the start of every shift and should be carried out with the same abrasive and application equipment that will be used on the contract.
There are typically three performance grades:
Pass - No cracking or flaking
Pass - Cracking but not flaking
Fail - Cracking and loss of adhesion or flaking
Health and Safety
There are no global specific regulations or legislation which covers metal spraying, however, most countries have regulations which cover the process and potential hazards.
Most of the processes will be covered by legislation intended to reduce danger to personnel or to protect the environment.
Main considerations are:
Dust
Health
Environment
Plant and equipment
Training
A Health and Safety training unit is contained within this training programme.
Plant and Equipment
A thorough understanding of the mechanics and operation of the metal spray equipment is essential to its safe use and maintenance.
Arc
Arc spraying equipment is potentially 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 air feed to the equipment must be maintained as cleanly as possible to ensure both proper operations and also to avoid potentially flammable substances being fed into the delivery system.
Arc
Component breakage must be allowed for and the potential miss-feed of wire both at the spray head and in the wire feed system can cause injury to the operator.
The equipment should be earthed and maintained in a clean area otherwise, the cooling fans will draw in large quantities of particulate spray.
Gas
The safe operation of gas equipment is perhaps more widely respected due to its common site use in fabrication industries.
The handling of gas equipment and gas bottles is discussed in various regulations.
The mixing of gas and poor maintenance of equipment seems to account for most incidents involving gas metal spraying.
Training
Most training for health and safety is undertaken during production or discipline training.
General safety training is essential but added to this should be a more in-depth understanding of the specific hazards of metal spraying processes.
The extra hazards can include:
Noise
High-pressure gases and arc
Ultraviolet radiation (Arcspray only)
Working with electricity(Arcspray only)
Operation of breathing equipment
High-temperature operation
Negatively pressurised enclosures
Extraction
Thermal Sprayed Coatings
Thermally sprayed coatings are used in various locations where the operator requires long-term protection.
Corrosion design with coatings will consider the type of coating, surface preparation type, cost, and life expectancy.
Thermally sprayed coatings must be applied to suitably abrasive blast cleaned surfaces and coating application is considerably more demanding than traditional protective coatings.
The quality of the process has to be high and maintained throughout the preparation. High adhesion values are necessary for thermally sprayed coatings to achieve the desired coating life.
Summary
In this training unit, we have reviewed the surface preparation and application of thermal metal spray coatings for corrosion protection.
We reviewed the metal spraying process specifically related to gas and arc application.
We reviewed the importance of correct surface preparation and sealer coatings.
We also discussed the applicator training and Health and Safety issues when using arc and gas methods of application.
