Background There is an increasing use of painted structural galvanised steel as specifiers seek longer-life solutions for steel structures or architectural requirements demand colour as designers use the flexibility of steel as a feature in architectural design of major structures.
The performance of painted galvanised steel is well established, and the synergy that exists between a galvanised substrate and a properly applied paint coating improves the performance, typically by a factor of 2X, over the coatings if they were used independently. Problems A consistent problem in the painting of galvanised structural steel, is that the galvanised coating is frequently damaged at the surface preparation stage when the coating is being abrasive blasted prior to application of the primer.
This galvanised coating damage takes the form of flaking of sections of the coating, chipping at edges or the formation of blisters in the galvanised coating. This damage is the result of a combination of galvanised coating characteristics and the blasting technique used to prepare the surface.
This article is intended to explain the causes of this phenomenon and provide recommendations to eliminate this form of coating problem and ensure trouble-free application of paint systems to galvanised structural steel.
Galvanised Coating CharacteristicsThe galvanised coating formed by hot dip galvanising structural steel is quite different to that applied to continuously galvanised sheet, wire and tube products. Hot dip galvanised coatings are made up of a series of zinc-iron alloy layers which make up 80% of the coating’s thickness. The top (shiny) component of the coatings is zinc, which makes up almost the entire coatings with continuously galvanised products.
Zinc is relatively soft and malleable. Zinc-iron alloys are hard and relatively brittle. It is for this reason that continuously galvanised products can be formed after galvanising without damaging the coating while hot dip galvanised coatings can only tolerate a degree of bending before the alloy layers in the coating will start to crack.
Thicker structural steel or steel containing reactive elements such as silicon and phosphorous will form heavier galvanised coatings with thicker alloy layers, which may exceed 200 microns in thickness and comprise 100% of the coating. This is easily seen on the finished products as the coating will have a matte grey appearance rather than a characteristic shiny finish common to most hot dip galvanised products.
While this thicker coating will provide superior coating life in service, its hard and brittle nature needs to be taken into account when abrasive blasting the galvanised steel in preparation for painting.
Abrasive Blasting FactorsMost abrasive blasting is done to prepare steel for painting. The most common blasting media is chilled iron shot propelled with 100 psi air at right angles to the steel surface. This combination is designed to deliver a high energy stream of abrasive to the to the surface to remove rust and mill scale and profile the steel’s surface. This is inappropriate treatment for galvanised surfaces.
The purpose of abrasive blasting hot dip galvanised structural steel prior to painting is to remove oxides from the surface and lightly profile the surface to improve the mechanical bonding of the paint to the galvanised surface.
This requires an entirely different approach in terms of blasting technique. The energy delivered to the surface with conventional blasting media at standard pressures with result in shattering of the hard zinc-iron alloy layers, resulting in their delamination from the steel surface, and will also result in unacceptably high rates of removal of the galvanised coating.
While many hot dip galvanised items may be able to be conventionally abrasive blasted without apparent damage to the coating, the increased use of imported structural sections and the heavier coatings typically generated on this type of steel increase the likelihood of coating damage with incorrect blasting techniques.
Another factor is that excessive zinc may be removed by incorrect blasting techniques. Correct brush blasting of galvanised surfaces should remove no more that 5% of the original coating thickness.
The Right Way To Abrasive Blast Galvanised Structural SteelThe following specification is recommended for abrasive blasting all types of hot dip galvanised coatings.
· Use illmenite or garnet abrasive only
· Use nozzle pressure of 50 psi (350 kPa) max.
· Do not blast at right angles to the surface – 45 degrees preferred.
· Use a 12 mm venturi nozzle at a distance of 400 – 500 mm from the surface.
· Blast only sufficient to produce a matte silver appearance on the surface.
Already grey coatings (caused by galvanising reactive steel) require an absolute minimum of brush blasting. The matte surface typical of this type of coating has an appropriate level of surface profile.