Thermal Spraypainting
Details about Thermal Spraypainting
Thermal spraying is a process that involves projecting melted or heated materials onto a surface to form a coating. This can provide various surface properties such as resistance to wear, erosion, corrosion, or heat. It’s used in various industries for repair, overhaul of damaged components, or preventative wear resistance and corrosion management. It’s not to be confused with spray painting, which is a process of applying paint to a surface using an air-pressurized spray gun.
History of the process:
Pioneered by Dr M U Schoop of Zurich in the early 1900’s (1st patent 1909). Started out by pouring molten metal into a high pressure gas stream. Developed to depositing coatings from solid wires (1916). Introduced in the UK in 1922
(foundation year of Metallisation).
How does thermal spray work?
It is a process that converts solid wires to a molten spray. Wire is melted then atomised & projected onto the substrate. The material hits the substrate, deforms, flattens & freezes, gripping it onto the substrate surface. Successive layers of coating can be applied to achieve the required coating thickness (can be 10’s of microns up to several mm thick).
Thermal Spray Highlights
Instant curing: No drying or curing time – can be handled straight after application.
Metallurgically cold process: Virtually no heat input to the substrate (<80C typically).
Typically mechanical bonding process: Normally requires grit blasting to prepare the surface.
Can be applied at various thicknesses: Typically 150-350 microns for corrosion applications.
Long life corrosion protection:
>20 years life to first maintenance in splash zone.
Line of sight process.
MATERIAL CHOICES FOR CORROSION PROTECTION;
ZINC (TSZ);
Acts sacrificially with ferrous substrates.
Forms an oxide.
Predictable life under known localised environmental conditions.
pH Range 6-12.5.
Used – Wide range of atmospheric and immersed environments, ambient temperatures.
ALUMINIUM (TSA)
Acts less sacrificially with ferrous substrates than zinc.
Produces adherent oxide/hydroxide insoluble corrosion product.
Finite life difficult to predict.
pH 4-7.
Used – Coastal, offshore, chloride (salt) environments and elevated temperatures.
Coating Selection/Life;
Surface Preparation for Spraying;
90% of failed coatings due to poor preparation. Grit-blasting commonly used (not shot blasting).
Purpose:
Remove surface imperfections (rust, scale etc.).
Provide clean angular surface profile.
Increase surface area & provide a mechanical key.
Required surface profile:
Blasting should be to EN8501-1 grade SA 3.
Blast profile should not exceed 100-125µm (commonly 60-100µm).
SPRAY PROCESSES;
Flamespray Systems:
FLAMESPRAY SYSTEMS – WIRE
MK73 – Propane Fuelled Pistol
SPRAY PROCESSES;
Arcspray Systems:
ARC SPRAY SYSTEMS
ARC145, ARC 150
SPRAY PROCESSES;
Arcspray Systems:
ARCSPRAY SYSTEMS
ARC528E-ACD
SEALING OF COATINGS;
Sealers: Many coatings benefit from being sealed but not always essential
(especially for larger coating thicknesses). Sealers are low viscosity materials used to penetrate the natural porosity of the thermally sprayed coating. Sealers enhance life expectancy by reducing exposed surface area & can offer a more desirable coloured appearance. Sealers smooth the sprayed surface thus remaining cleaner longer and reducing paint consumption for duplex coating systems.
Common Corrosion Applications;
COMPARISON OF METAL SPRAYING WITH GALVANISING?
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 .002-.006 in (50 – 150m). 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.
COMPARISON OF METAL SPRAYING WITH PAINTING?
Painting is a widely used method of protecting steelwork (and other materials) from corrosion. Most paints are organic (polymer) bases with added metal particles, corrosion inhibiting compounds or inert filler materials. They may be applied by dipping, brushing or spraying to suitably prepared (grit blasted) surfaces. Protective paint systems are multi-layer comprising a priming coat, two or three primary protective coats and a decorative top coat. Extensive practical long term evaluation has shown that paint systems have shorter effective lives than sprayed zinc or aluminum coatings.
METAL SPRAYING OFFERS THE FOLLOWING ADVANTAGES OVER PAINTING:
· Materials are of consistent quality and purity. No mixing is required before application.
· Materials have an infinite shelf life if properly stored
· Fewer process steps are required. This allows simpler quality control and fewer opportunities for error
· Sprayed articles require no protracted curing or drying times giving superior utilization to floor space
· Metals may be sprayed in a wider range of climatic conditions (temperature and humidity) than paints
· Sprayed zinc and aluminium give effective corrosion protection immediately
· Sprayed metals are more robust than paints and can withstand rougher usage
· Even if the sprayed layer is damaged the sacrificial action, particularly of zinc, prevents corrosion from edges and discontinuities
· Sprayed metal coatings maintain the efficiency of friction grip areas throughout the life of the
structure
· Adhesion to steelwork is better. Sprayed zinc or aluminium are often specified as base layers for paint systems for this reason. However, experience shows that properly sprayed metal coatings are adequate if sealed and that the paint overlay offers no further advantage.
HOW DOES TSZ COMPARE TO OTHER PROCESSES?
Thermal Spray offers the following advantages over painting:
Materials are of consistent quality and purity. Materials have an infinite shelf life if properly stored. Sprayed articles require no protracted curing or drying times giving superior utilization of floor space. It is a fast process (no drying/curing times). Sprayed zinc and aluminium gives effective corrosion protection
immediately. Even if the sprayed layer is damaged, the sacrificial action, particularly of
zinc, prevents corrosion from edges and discontinuities. Adhesion to steelwork is good. Sprayed zinc or aluminum is often
specified as a base layer for a paint system.
HOW DOES TSZ COMPARE TO GALVANISING?
Thermal Spray offers the following advantages over Galvanizing:
The low heat input during spraying eliminates the risk of thermal distortion. Sealed hollow fabrications can be treated without risk of explosion. Coating thickness may be varied to provide extra protection in critical areas. There is no limit to the size of article to be treated. Articles can be treated on site. There is no effluent disposal problem. Reduced stocks of zinc are required. Fuel is not needed to keep zinc molten. Working capital is not tied up in a
molten zinc bath. Zinc metal spray is used to repair damaged areas of galvanising.
CONCLUDING POINTS:
Thermal spray is:
An independently proven, long term corrosion protection process. Not as complex to apply as some consider. Fast process overall (no drying/curing times). Common corrosion coatings. Applied by flame spray and arc spray
Aluminium, Zinc or Zinc/Aluminium alloys.
Collection of projects done by the SOLHAR Team
Thermal Spray Painting
Solhar Energy Solutions and Engineering in Kimberley and Botswana
Thermal Spray Painting Gallery
Solhar Energy Solutions & Enigneering Botswana Contact:
ROY DE BRUIN
+26771660127
roy.debruin@solhar.co.za
Solhar Energy Solutions & Enigneering Contacts:
KEVIN HARDING
083 256 9975
kevin.harding@solhar.co.za
GERRARD BOOYENS
082 551 4082
gerrard.booyens@solhar.co.za
BRIAN DANIELS
082 481 0616
brian.daniels@solhar.co.za
JOHAN LE ROUX
072 672 7427
johan.leroux@solhar.co.za
CONTACT US;
Our Kimberley Branch is situated at 8 Hendrik van der Bijl, Kimdustria, Kimberley, 8301, Northern Cape, South Africa
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