What is The Requirements of Surface Preparation for Thermal Flame Spraying?
One shall refer to the following standards – SSPC SP5 and ISO Sa3. The required profile range shall be between 69 to 125 microns (User specified).
The abrasive shall be angular to have gouging effect and higher peak density for adequate mechanical bonding.
TSA Coating – Here are in depth requirements
Pre- Surface Preparation Requirements
Check the steel surface for any welding imperfections prior to starting of surface preparation. Any welding defects such as weld spatters, crevices, slivers and sharp edges shall be grounded off to acceptable methods.
TSA coatings do not adhere to any defects and hence there will not be adequate bonding on the surface therefore, there are great chances of lifting along the edges.
Hence it is strongly advised to round of the corners prior to blasting – Refer:ISO 8501-3:2006 – Part 3: Preparation grades of welds, edges and other areas with surface imperfections.
Solvent cleaning shall be carried out in accordance to SSPC-SP 1. One shall not use any cleaning solutions, rust inhibitors or any solvent that would leave a residue on the surface.
Soluble salt contamination on the surface can conform as per the specification requirements. The steel substrate shall be free from oil, grease, dust, welding defects after high pressure water washing. Dust rating shall be between level 1 or 2.
Surface Preparation – TSA Coating
The strength of the bonding depends mainly on surface preparation. The blasting method shall be performed as per SSPC-SP5- SSPC-SP 5 White Metal Blast Cleaning (NACE No. 1).
ISO 8501-1- Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.
The compressed air used for blasting shall be free of oil and moisture. Ensure to carry out the blotter test to verify the quality of the air supplied at the beginning of every shift.
Refer to ASTM D4285 – 88 to perform the test in accordance.
Ensure to use filters, dryers, separators and traps between the air source and flow meters to remove any moisture. Check the blasting nozzles for any wear and the hoses for worn out as it will affect the production.
Ensure that all the tests are documented for the verification from the clients or from third party inspectors.
Before starting any activity the contractor qualified and certified inspector shall monitor the climatic conditions.
Prior to start-up of any activity ensure the weather is overcast. No chance of any rain during the application and until the drying period.
Ensure the substrate temperature is minimum 3deg.C above the dew point.
The relative humidity is 80-85% below (Refer to project specification).
Abrasive Blasting – TSA Coating
Prior to blasting ensure the safety measures are followed, the area is barricaded and necessary sign boards are in place and a certified safety Engineer is present.
Prior to starting of blasting make sure the blasting nozzle, the hose, sand Pots, the compressor, Air filters etc. are checked.
Inspect the media for any oil or contamination. Check if the garnet is approved and it is not recycled.
After blasting check if the profile is within the required blasting standard in accordance with Surface roughness characteristics of blast-cleaned steel substrates — Part 5: Replica tape method for the determination of the surface profile ISO 8503-5 or ASTM D4417 Methods A and C.
Testex Press-o-film X-Coarse Replica Tape is generally used to check the profile.
Thermal spray coating should commence within 6 hours of completion of blasting, earlier the better.
Ensure every stage is documented for third party verification.
What about Surface Temperature? Is Pre-Heating Required And If Required What Shall Be The Surface Temperature Before Flame Spraying?
Technically speaking, before starting Flame Spray the substrate shall be pre-heated around 60-110deg.C. or at least the starting portion shall be pre-heated for intended adhesion values.
Preheating before flame spraying is usually done to avoid condensation on the surface by the moisture present ‘in the flame.
No need to preheat the complete pipe under normal circumstances. (But please follow the dew point restriction always).
The quantity of moisture that may contain depend on what flame you are using for flame spray. Preheating temperature is usually 60 deg.
Just preheat with ‘flame on’ and ‘wire off’ mode and measure the temperature that would be good enough (not done usually).
Initial Pre-heating Required to Remove Any Moisture Present?
Any moisture present after the initial preheating during startup will be driven out with the temperature gradient on the adjacent area where the flame spray is progressing.
If you do preheating by some other methods like induction heating, there are possibilities to get superficial oxidation on the surface which is going to adversely affect your TSA.
Flame sprays are of low tensile strength compared to arc. So you should keep every parameters with its importance or else the outcome you are going to get will be a low tensile strength TSA which will not even meet 5MPa.
Application of Coatings – TSA Coating
Most important, the equipment used shall be inspected so that the equipment is capable of spraying uninterruped for minimum of thirty minutes. Make sure the sprayers are certified by third party.
Ensure proper PPE is worn by applicators and all safety measures are followed. Make sure that hot work and confined space permits are issued incase required depending on projects requirements.
If using combustion gas ensure small area is pre-heated as the flame from the gun will warm up adjacent areas once work commences.
Ensure the material conforms with AWS C2.25/C2.25M or ASTM B 833 grades 1100 (99% pure) or 1350 (99.5% pure)
Technique – TSA Coating
Aluminum thermal sprayed coatings are typically sprayed to a total thickness of (.38mm).
with minimum of 10 mils and maximum of 20 mils.
The Spray guns used to apply coatings should be held approximately perpendicular to the surface of the work-piece and at an optimum distance to deposit a proper coating (typically 8 to 10 inches).
Ensure each pass should overlap approximately 50%.
Quality Control of the Coating
All coated surfaces shall be visually inspected by certified client inspectors. The coating surface should be uniform in appearance without any discoloration. Make sure there are no lumps, blisters, cracks, chips coarsely sprayed areas or any loosely adhered particles.
The coating thickness shall be checked with non-ferrous calibrated DFT gauge. Any defects shall be marked with chalk and repaired as per the repair approved procedure.
Adhesion tests shall be carried out as per standards ASTM C633 or ISO 4624.
Bend test shall be performed for each sprayed component (Only if required by client).
All the quality tests shall be carried out and documented.
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