Surface preparation

 

When painting galvanizing, as when painting any other surface, the cleanliness and condition of the surface are of critical importance and a high proportion of paint failures on galvanized steel can be attributed to inappropriate or inadequate surface preparation.

In preparing galvanizing for painting, the basic requirements are largely the same as for other surfaces.  Namely, anything that prevents the paint wetting out or adhering to the surface needs to be removed.  Therefore oils, dirt, dust, salts, corrosion products and other friable material and soluble salts must be removed as a precursor to any subsequent treatment.  Refer to AS/NZS 23128 Section 4 and AS 162711.  The difficulty of removing some contaminants should not be underestimated and for severely corroded galvanizing, in particular, reinstatement may be impractical, because of the extensive preparation required.

For the removal of oil and grease, water based emulsifiers, alkaline cleaners of pH less than 12 or organic solvents are variously appropriate.  Where oils and grease are removed by solvent soaked cloths, these need to be changed frequently as oil contaminated cloths only serve to spread the contamination.  Usually this method is only practical over small areas.

Apart from the removal of dirt, dust and grease, which are common to all substrates, it is important to recognise all sophisticated coatings intended for extended durability service require high standards of surface preparation for maximised performance.

One important issue for galvanized surfaces is the time lapse between galvanizing and painting.  The best advice is to paint galvanizing as soon as possible, for the sooner it is painted the less likely it is to be contaminated by dust, salts and corrosion products.  Conversely, the longer the time lapse and the more severe the conditions of exposure prior to painting, the more difficult and costly the preparation will be.  In extreme cases, such as where surfaces have been close packed in humid or damp conditions and suffered wet storage staining, brushing with 1 to 2% ammonia, or in extreme cases one part citric or acetic acid to 25 parts water, may be required12.

A second consideration is the smooth, glossy surface that emerges from the galvanizing bath.  This can inhibit paint adhesion.  In the past, two methods of dealing with this problem were to etch the surface with an aggressive salt solution or mineral acid or allow the zinc to weather for some time before painting.  These techniques have long been discredited.

For painting unweathered galvanizing with conventional low build paints (see Service Requirements 1 and 2) cleaning and degreasing is normally adequate, although light scuffing with sandpaper will invariably enhance paint adhesion.  For higher build paints and under conditions of more arduous wear, brush (whip) abrasive blasting is favoured (see Service Requirements 3 to 6)

This process lightly roughens the surface without removing a significant amount of galvanizing and provides a key to promote adhesion of the paint film.  This procedure should be carried out using a soft abrasive, by impacting the surface at a glancing angle and operating at low air pressure.  The following criteria included in both AS/NZS 46801 and AS1627.411 are recommended:

 

-        Blast pressure 275 kPa (40 psi)

-        Abrasive Grade 0.2 – 0.5 mm (clean ilmenite)

-        Angle of blasting to surface no greater than 45°

-        Distance from surface 350 – 400 mm

-        Nozzle orifice diameter 10 – 13 mm of venturi type

 

It is important that this procedure be performed carefully to ensure that no more than 10 μm of zinc is removed.  Organic paint coatings should be applied as soon as possible after abrasive blasting.

Newly galvanized coatings are normally quenched in an aqueous solution by the galvanizer to impede early onset of white rust.  If a HDG article is to be painted, it is usually best to exclude this step from the galvanizing process to minimise contamination.  To ensure the best possible surface for painting, advise the galvanizer of your needs prior to galvanizing.

Painting Over Hot-Dip Galvanizing

References

  1. Australian/New Zealand Standard AS/NZS 4680:2006 Hot-dip galvanized (zinc) coatings on fabricated articles.
  2. ISO 1461-2009 Hot dip galvanized coatings on fabricated iron and steel articles - Specifications and test methods
  3. Australian/New Zealand Standard AS/NZS 2311:2000 Guide to the painting of buildings.
  4. Thomson G., "Paint over Galvanizing" Galvanizers Association of Australia, Melbourne 2001.
  5. ISO 14713-1-2009 Zinc coatings - Guidelines and recommendations for the protection against corrosion of iron
  6. Bartlett D J "Paint Finishes over Galvanizing, Why Do They Fail?"  Presented to ACA Conference, Melb. Nov. 2003.
  7. Slunder C. J. and W.K. Boyd "Zinc and its Corrosion resistance", Int. Lead Zinc Research NY 2nd Edn. 1983.
  8. Australia/New Zealand Standard AS/NZS 2312:2002 "Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings.
  9. ISO 9223-2012 Corrosion of metals and alloys — Corrosivity of atmospheres — Classification, determination and estimation.
  10. AS 4312-2008 Atmospheric corrosivity zones in Australia
  11. Australian Standard AS 1627 Metal Finishing - Preparation and pre-treatment of surfaces.
  12. Smith L. M. "Cleaning and Painting Galvanized Steel", 51-55, JPCL 18, April 2001.
  13. "Sweep Blasting Hot Dip Galvanizing" Galvanizers Association of Australia, Advisory Note GEN/1/1, March 2012.