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PLASCOAT PPA 571 – Flame spray grades

REPAIR PROCEDURE FOR FLAME SPRAY APPLIED COATINGS

Before any repairs are attempted the areas should be thoroughly cleaned. Depending on the condition of the item to be repaired, this cleaning may be done using a pressure water wash, a detergent wash system or cleaning using solvents or a combination of these.  If there are oily or "tarry" residues these should be removed using white spirit.  If stronger more flammable solvents, such as ketones of toluene, are used then ensure these have evaporated from enclosed areas before using the flame spray gun.

a)         To repair areas where the original coating was applied at thicknesses less than 500 microns. 

The area should be cleaned as above.  The coating on the metal should be melted using the flame only.  Then the molten powder should be applied while this area is molten.  The size of the area to be worked will depend on the atmospheric conditions, the metal thickness, the design of flame spray gun and the size of the nozzle. This area will, typically, be up to 1 metre square.  The areas should be overlapped by about 10 to 20 cms.  After the whole area is re-coated must be checked for thickness and holidays.  The latter is normally tested with a spark-tester using a voltage of around 6 kV for a coating of around 600 microns.

b)        To improve the surface finish of coatings which look "sugary".  Coatings, which have a powdery or "sugary" appearance, have been applied too cold. By passing the hot flame over the surface this will melt and refuse the outer surface to recover the gloss and smoothness.  If an increase in thickness is required more molten powder can be applied.  See (a).

c)         Coating greater than 1mm.  It is unnecessary to apply coatings greater than 1mm.

However, if PPA 571flame-spray grades are applied onto large diameter pipes it is probable that coatings up to 2mm will not be detrimental to the service life of the coating.

d)        Coating onto areas where rust is showing through. 

Rust will only be visible if   the coating has been damaged through to the metal or if insufficient coating was applied originally.  In either case clean up the area as described above.  Remove any loose coating and abrade the metal surface by abrasive blast cleaning, or a suitable abrasive paper or cloth.  Any rust which, cannot be reached or removed should be treated with a proprietary rust converter such as a chelator or phosphating agent.  After preparing in this way the area can be over-sprayed with PPA 571flame-spray grades using the flame spray method as described in (a).  If there are large areas of bare metal these should be heated to about 100°C before application of the molten powder.

Whatever repair method is used, after the coating has cooled it should be tested for thickness (500-1000 microns) and holidays using a spark tester set at around 6 kV.

Remarks

Considering the fusion bonding issues above, it is appropriate to increase the metal surface temperature to about 150°C especially for heavy metal thickness pipes (e.g. greater than 10 mm). Care should be taken not to degrade/burn the coating. Always follow the guidelines of the flame spray equipment manufacturer and never use acetylene-based flames – always use propane for the fuel.

 WGOD/040303