N_Sakarin / Shutterstock
Painted structural galvanized steel is increasingly being used by specifiers who are seeking long-term solutions for steel structures. This increase is also because of architectural prerequisites that require color, as designers use the versatility of galvanized steel in the architectural design of key structures.
The performance of painted structural galvanized steel has been well established and is further enhanced by the synergy between the galvanized substrate and the applied paint coating (typically by a factor of 2).
A dominant issue encountered while painting galvanized structural steel is that the galvanized coating often gets damaged at the surface preparation stage whenever the coating is exposed to abrasive blasting prior to the application of primer. This damage leads to the formation of blisters in the galvanized coating, flaking of the coating sections, or chipping at the edges.
The damage is caused by both the galvanized coating properties and the blasting technique used for preparing the surface. This article aims to describe the causes of such problems, as well as to offer suggestions to avoid those and ensure easy coating of paint to galvanized structural steel.
Galvanized Coating Characteristics
The galvanized coating is formed by hot-dip galvanizing structural steel, and is comparatively different from the one used on the continuously galvanized sheet, tube, and wire products. Hot-dip galvanized coatings are made up of a series of zinc-iron alloy layers that constitute 80% of the thickness of the coating, with zinc used for the top (shiny) portion of the coating.
Zinc is fairly soft and malleable, but zinc-iron alloys are hard and quite brittle. Therefore, continuously galvanized products are made after galvanizing to ensure that the coating is not damaged. On the other hand, hot-dip galvanized coatings can endure a certain degree of bending, before the alloy layers of the coating start to crack.
Steel that contains reactive elements such as silicon and phosphorous, or thicker structural steel will develop heavier galvanized coatings with thicker alloy layers. The thickness of these layers may be over 200 µm, and include 100% of the coating. This can be simply seen on the end products because the coating will have a matte grey finish rather than the characteristic shiny finish, that is related to a vast number of hot-dip galvanized products.
This thicker coating provides better coating service life, but its hard and brittle nature has to be taken into consideration when the galvanized steel is being abrasive-blasted in preparation for painting.
Abrasive Blasting Factors
Abrasive blasting is most typically carried out to prepare the steel for painting purposes. Chilled iron shot thrust with 100-psi air at right angles to the steel surface is the blasting media used most often. Through this arrangement, a high-energy stream of abrasive can be supplied to the surface to eliminate mill scale and rust, as well as to profile the steel surface. But this treatment is not ideal for galvanized surfaces.
Abrasive blasting is performed on hot-dip galvanized structural steel before painting, mainly to remove oxides from the surface and profile the surface to some extent to improve the mechanical bonding of the paint to the galvanized surface.
This demands a totally different strategy with respect to the blasting technique. The energy conveyed to the surface using conventional blasting media at standard pressures will lead to the cracking of hard zinc-iron alloy layers, making them delaminate from the steel surface, and cause high removal rates of the galvanized coating.
Conventionally, abrasive blasting can be carried out on a majority of the hot-dip galvanized items without any noticeable damage to the coating. However, the increased application of imported structural sections and the heavier coatings generally applied to this kind of steel increase the odds of damaging the coating with incorrect blasting approaches.
Another factor is the probable removal of surplus zinc through improper blasting techniques. Proper brush blasting of galvanized surfaces requires eliminating not more than 5% of the original coating thickness.
The Right Way to Abrasive-Blast Galvanized Structural Steel
The specifications given below suggest ways to perform abrasive blasting on all types of hot-dip galvanized coatings:
- Only garnet or ilmenite abrasive must be used
- Nozzle pressure should be a maximum of 50 psi (350 kPa)
- A 12-mm venturi nozzle must be used at a distance of 400–500 mm from the surface
- Blasting must be performed just enough to create a matte silver finish on the surface
- Blasting must preferably be performed at an angle of 45°, and not at right angles to the surface
Very minimal brush blasting is needed for already grey coatings (supported by galvanizing reactive steel). The matte surface, which is typical to this form of coating, has an ideal level of the surface profile.