Preparation Of G90 For Powder Coating

G90 is a specific coating weight of zinc developed in a high speed, continuous, hot-dipped galvanizing process. When the galvanized steel exits the last stage of the process it will retain heat and it will continue the reaction of the zinc deposition while it is still hot. The galvanized material can be cooled by air, water or chrome passivation.  For best results, it is strongly recommended that passivation with water or chrome should be avoided for galvanized steel that will be coated with an organic powder coating.

The passivation can be challenging for adhesion and also can generate a lot more trapped gas in the zinc layer. G90 that has been passivated, or run through a quenching process should be treated in a much more aggressive manner and with more care than standard hot-dipped galvanized steel.

Surface preparation

Surface preparation is critical for success with any substrate. The surface must be clean and chemically compatible with the coating. Particular substrate conditions dictate the steps in the preparation process. The oxide layer developed on the galvanized surface is not a good surface for bonding of an organic coating. It must be removed as much as possible without removing the complete zinc layer.

Sweep or Brush Off Blasting

Sweep blasting is often used to prepare the galvanized surface before coating. The galvanized layer has random deposits or un-reacted zinc and other elements, particularly along the grain boundaries of the coating. Light sweep blasting will clean off any metal oxides that have developed on the surface as well as open up surface cavities that may have trapped air. Sweep blasting will also increase the surface profile of the steel and help develop a good mechanical bond between the galvanized steel and the powder coating. Aluminum-magnesium silicate in the 200 to 500 micron range is the best blast media. The nozzle is held at a 45 degree angle and the surface is blasted just enough to remove the shiny spangle and loose particles on the surface. Sweep blasting greatly enhances the adhesion and ultimate performance of the powder over a galvanized surface and helps to limit the possibility of out-gassing pin-holes.

Sweep blasting is the first critical step when powder coating over galvanized steel. It will help to remove the chrome layer, remove some of the gases that developed during the galvanizing process and roughen the surface for better adhesion.

Brush off Blast SSPC-SP7 (SSI-Sa1), or NACE #4 Definition

A method in which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are removed completely. Tight mill scale and tightly-adhered rust, paint and coatings are permitted to remain. However all mill scale and rust must have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed over the entire surface.

There are two key points of reference in the definition that are of prime importance:

1.      All loose materials must be removed

2.      The surface should be removed to reveal a uniform metal layer under the outer skin

• Blast nozzle pressure 40 psi
• Abrasive grade – fine to medium
• Abrasive type - clean aluminum oxide, walnut shell or garnet
• Distance of nozzle from surface 400 - 500mm
• Nozzle type - 10mm minimum diameter venturi type
• Blasting angle to surface - 45 degrees.

The aim of this blasting procedure is to remove any oxide films and surface contaminants from the surface. It is NOT to produce a profile similar to that required on bare steel. The brush blasting of the relatively soft zinc will automatically produce a fine profile, giving the clean surface a satin appearance. With inexperienced operators, a test section should be evaluated by measuring coating thickness before and after blasting with an approved magnetic thickness gauge. A maximum 5-10 micron reduction in galvanized coating thickness indicates an acceptable technique. Over 10 microns of coating removed indicates an unacceptable technique.

Out-Gassing

Any zinc, or other metal oxides, that remain on the surface of the galvanized steel can retain air or moisture. When the coated part is heated for curing of the powder, the oxides may release water vapor or air, which can expand and penetrate the powder coating, causing blisters or voids.   The craters in the coating can be unacceptable for appearance reasons and they can cause accelerated corrosion failure. 

Pre-Heating

Pre-heating in an oven can reduce the potential for the galvanized coating to outgas. It will help evacuate a large amount of the trapped air and moisture. The pre-heat oven should be operated at higher temperature than the final cure temperature; this temperature is typically 50° F higher than the curing oven. The time should be sufficient to bring the part up the oven set-point and hold it for at least as long as the intended cure cycle.

Powder Coatings

After the parts have been exposed to the pre-heat cycle they should be cooled down to a near-ambient condition before coating. An out-gassing forgiving primer layer should be applied to a thickness of around 2 to 3 mils. That layer should be partially cured (typically around 40 to 50% or the full cure cycle. The part should be removed from the oven and allowed to cool for a brief period before applying the topcoat. After the topcoat layer is applied the parts should be returned to the oven and dully cured.

Curing

The cure oven temperature should be operated as low as possible to further reduce the potential for outgassing. Best results are achieved with lower oven temperatures and longer dwell times as opposed to higher temperatures with shorter dwell times. Not only will the lower temperatures inhibit any gassing, but the longer curing times might lead to the resealing of any craters that may have formed.

by Rodger Talbert