Galvanised Steel: Just degrease and spray it, AS4506 says so..?
Although it is correct that the Australian Standard AS4506 outlines that chemical pre-treatment is not required for galvanised (zinc coated) steel in all circumstances, it is a bit like saying that you’re allowed to drive your car in reverse. Whilst there are circumstances where using reverse gear is suitable, in the majority of cases it would be downright reckless to operate a car in reverse; for example driving down the motorway!
In fact, the only circumstances where galvanised steel items are not required to be chemically pre-treated under the standard is for mild to moderate interior applications i.e. within buildings that are either remote from the coast or have a controlled climate system operating continuously e.g heating and/or air conditioning. Given the majority of the Australian population lives on the coast and locations north of Rockhampton fall under a Tropical area designation; in other applications including all exterior applications, chemical pre-treatment and/or application of a primer coat is a requirement for zinc coated steel products under the standard.
That’s great to know guys, but why should I care?
The short story is that powder coated finishes (without chemical pre-treatment or primer application) can fail rather quickly on zinc coated surfaces in external applications and more corrosive environments; but let’s get into the detail on why that is.
The purpose of a zinc coating on steel is primarily to protect the steel from rusting and thereby losing its structural integrity. The zinc layer does this in two main ways:
1) By creating a barrier layer i.e. it prevents oxygen and moisture from reaching the steel surface (initially and over time), and
2) By giving cathodic protection at scratches, chips, edges etc.
Both of these effects rely on corrosion of the zinc coating occurring with its environment. In the first case, zinc reacts with moisture and the air to form zinc carbonates realising a sealed layer with very good adhesion and low solubility in water; rapidly reducing the rate of further corrosion. The original shiny surface with a metallic lustre disappears to be replaced by matt, light grey colour with varying degrees of white powder-like corrosion product. In the second case, the zinc acts as a sacrificial anode due to galvanic corrosion described by the galvanic series of metals. Thus due to preferential corrosion of the zinc coating, the steel experiences cathodic protection that prevents it from rusting at areas where the steel surface is exposed e.g. at edges, chips, discontinuities etc.
Whilst these effects are desirable for protecting the integrity of the steel, it gets a little more complicated when you want to coat zinc with powder coating as the long term adhesion of the powder coating is in direct conflict with corrosion of the zinc. As the zinc corrodes below the powder coat it causes delamination and flaking of the coating i.e. the corrosion of the zinc ultimately results in failure of the powder coating!
This is where we get back to the point of chemical pre-treatment in powder coating. As zinc-coatings are highly reactive and protect sacrificially, it is vital to chemically pre-treat (or prime) galvanised steel in most applications to deliver a reasonably long service life for powder coated finishes. The chemical pre-treatment process cleans the zinc surface and applies a phosphate or chromate layer which passivates the zinc coating (to make it less reactive). This provides protection to prevent the zinc layer from corroding whilst also increasing adhesion of the powder coat to the zinc, resulting in less chip / scratch potential in service. Alternatively an appropriate barrier primer layer can be applied between the zinc and the powder coat top coat however this is usually a less cost effective option and leaves a reactive zinc layer in place for corrosion to begin at the point of chips, scratches or discontinuities in the surface.
Here at E-static Powder Coating, our primary service for galvanised steel products is to chemically pre-treat prior to powder coating as this is in accordance with AS4506 and results in the correct balance of product performance and cost for our customers. In extreme environments, e.g. around pools, near the ocean or heavy industrial zones, we recommend the application of a barrier primer in addition to chemical pre-treatment of galvanised products to realise the longest possible service life in such environments.
Unfortunately not all powder coating operations including many of our local and regional competitors follow the practices outlined in Australian Standards which results in poor service life and unnecessary early failure of powder coating on galvanised surfaces. Next time you visit your powder coating supplier, ask what pre-treatment process they undertake for galvanised steel - if its not being chemically pre-treated (i.e. phosphated, chromated) or has a primer coat prior to the application of the powder coat top colour, you are paying for an inferior product! Just degreasing and spraying it is not suitable for service in the North Queensland environment.
For further information on corrosion of galvanised or zinc coatings, please refer to APG Group or Nordic Coatings websites.