Upon completion of this module, Students will be able to:
Understand that 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 that are melted immediately before being projected onto the substrate.
The metals and application methods vary, but most applications result in thin coatings applied to surfaces that require improvement in their properties of corrosion or abrasion resistance.
Sprayed metal coatings have been used for several years and have proven to deliver excellent performance, provided 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 is 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 affect 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 an electrochemical process.
Corrosion - Galvanic Table (general)
This table is helpful 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 seawater. Immersed steel pipes can be protected in the same way.
The more reactive aluminium anode will corrode in preference to the steel.
This process is called cathodic protection.
The anode should never be coated, as it will not function properly.
The thermal spray process works on the same principle. The aluminium or zinc will corrode in preference to the steel.
Zinc Versus Aluminium
There are several advantages to zinc and aluminium coatings.
Aluminium
High temperature resistance
Lightweight
Excellent resistance to polluted and marine environments
Forms an oxide layer to reduce permeability and reduce corrosion
Zinc
Excellent for cathodic protection
Excellent resistance to mechanical damage
Life is proportional to thickness.
Al-Zn
Some operators have found good results using a mixture of aluminium-zinc rather than the individual wire.
The choice will depend on the location, temperature, cost, and other factors, such as the use of a coating system (used over the thermal metal spray).
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 adheres to the surface, and subsequent splats adhere to each other to form 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 increases.
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 processes, which are the most commonly used application processes for finishing coatings.
Thermal Spray Materials Selection
The primary materials used for corrosion control, which are thermally sprayed, are zinc and aluminium. It is possible to have a zinc-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 environmental factors (corrosive atmosphere, temperature, etc.), specifications, the life expectancy of the coating, adhesion requirements, and the availability of materials.
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 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, particularly in environments and locations where maintenance can be challenging.
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 to conduct a high level of surface preparation before applying thermal spray coating.
Pre-preparation will consist of removing all 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 (either arc or gas) can access all areas.
Surface preparation should be conducted using metallic abrasives, such as chilled iron grit, to a Sa3 standard (SSPC-SP5/NACE No. 1) with a surface profile of 75-100 microns and 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 that there is no dust, debris, or abrasive left on the surface before the application of the thermally sprayed coating.
The adhesion of the thermal coating is mechanical, and surface cleanliness is crucial for achieving 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 a 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 processes apply a melted material to a prepared substrate, the equipment is fundamentally different, and the application process also differs.
Flame Spray Process
In the wire flame process, a wire is fed through the centre of an oxygen-fuel flame by a driven roller system, where it is melted.
An annular air nozzle then applies a jet of high-pressure air, which atomises and projects the molten material onto the 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, 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 setup, showing the fuel bottles, a mounting panel with flow meters, regulators, and a filtration unit. 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, forming 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 an air motor or an 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 on the size and configuration of the structure.
A larger energiser 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 over time. However, some general guidelines include establishing an optimum distance between the pistol and the 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.
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 also be 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 measured immediately after application, once the surface has cooled.
The inspection must be comprehensive 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 primarily through mechanical strength, the adhesion value of the sprayed metal is crucial.
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
Various specifications and standards will be used for the thermally sprayed coating.
These documents will generally detail:
The specified process (arc or gas).
The type of wire (zinc, 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 that penetrates 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 used as soon as possible/practicable after the application of the sprayed metal coating. The client may specify the exact time.
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 several sealers available for use within this temperature range. However, the most popular is an epoxy sealer.
The silicone aluminium may need to be post-heated (in service) to cure fully.
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 the construction process.
These areas must be treated in the same way as the original application.
Correct ambient and environmental conditions.
Abrasive blast clean.
Abrade the overlap locations to ensure good adhesion to the existing metal spray and prevent lifting.
Apply the metal coating to the repair location with 25-50mm (1 to 2”) overlap onto unsealed metal.
Check adhesion to both the existing steel substrate and the 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 received proper training.
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 an adhesion test.
There are usually pass/fail criteria for the bend test.
Always refer to the relevant standard, such as ASTM or ISO, or a 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 the sealer.
The operator should be trained on the equipment and the process for the metal spray coatings.
Equipment manufacturers conduct training on equipment and provide an introduction to the process. Trade testing on sample pieces, followed by adhesion checks, is typically performed before the commencement of the work 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 customer or client will specify the exact method of the test, 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 typically conducted using 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
The applicator can also conduct a bend test.
This test is typically conducted at the beginning of every shift and should be performed using 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 cover 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 hazardous due to 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 correctly or when the operator is untrained in setting up or handling.
The air feed to the equipment must be maintained as cleanly as possible to ensure both proper operations and 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 widespread use in the fabrication industry.
The handling of gas equipment and gas bottles is discussed in various regulations.
The mixing of gas and poor maintenance of equipment seem 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 long-term protection is required.
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.
