'Zinganising' is a matt-finish film-galvanizing system that has now become well-established in 52 countries around the world. It has many distinct advantages over other systems, namely:
- it is done at ambient temperatures;
- there is no restriction on metal section;
- it can be used on both carbon steels and on cast iron;
- it can be over-coated directly with certain powders and wet paints without the need for etch primers; and
- it can be used on immersed steelwork and on exposed structures (it is used successfully on fasteners for North Sea offshore use).
For 35 years the system has been used very successfully, but with one important stipulation, the metal has to be blast-cleaned to a cleanliness standard of SA2.5 (SSPC-SP10) and with a blast-profile average of Rz65pm (ISO 8301¬1) and a roughness factor of Ra12.5 (8503-2).
The zinc thickness is approximately 65pm DFT from dipping (at 19 seconds Ford 4) and because the zinc dust is micronised, the surface finish is always glass-smooth and ready for a paint coating if one is required. Unlike a shiny zinc-plated finish, the matt grey finish of 'Zinga' allows a range of paints to be applied directly onto the zinc with no requirement for a primer.
A 'Zinganised' coating will lose a maximum of around 1-2pm OFT of zinc per annum from its applied thickness under normal atmospheric exposure, and around 3-4pm OFT (worst-case scenario) per annum in very exposed marine atmospheres.
As two-pack polyurethanes can be applied directly onto zinganised surfaces, the application of such a paint coating should give a working lifespan of around 20-25 years in exposed marine atmospheres. This is because Zinga has a 'synergisation factor’ of around 2.5, which means that the zinc and the paint coating are working in symbiosis.
Paint breakdown
Paint breakdown mechanism: Normally, a polyurethane paint would start breaking down from UV exposure after, say, 12-15 years. This would start off with the resin becoming porous (with the formation of micro-pores through the film) and with water-vapour transmission taking place. Vapour or condensates from rain and overnight dew would permeate the paint-film, reactivate any salts or salt-residues behind the paint film (or in many cases carry the salt through the paint-film in an ionic form, as nitrates come from dissolved atmospheric nitrogen and chlorides can come from road spray and wind-borne moisture etc) and the corrosion process begins.
The paint-film becomes undercut and the corrosion products that are continually forming behind the film will swell and push the paint off in the form of blistering and flaking.
Zinga in duplex: When paints like two-pack polyurethanes are applied over Zinga, they will start to degrade from UV exposure as normal after 12 or so years. The difference here is that once the moisture penetrates the paint-film and reaches the zinc layer, the zinc immediately reacts with this moisture and forms both carbonates and oxides, both of which are non-conductive and therefore cannot carry any form of ionic species through to the zinc. These salts block and effectively seal the paint-film against further ingress from water vapour and dissolved salts, which in turn lengthens the coating's working life.
Why? Quite simply, because no reactions are taking place behind the paint-film and hence no by-products can form that will weaken the bond between zinc and paint.
With this symbiotic relationship, the coating will break down repeatedly in layers from the exterior face, losing more resin thickness each time with the subsequent loss of more and more pigmentation as it gets washed away by rain and the elements. This results in the coating physically wearing away back to the zinc layer, which has not really had to do any real work up to this point. Only when the paint has worn away completely in certain areas will the zinc begin to 'work' in these areas.
Under immersion
Under immersion: The zinc will generate a voltage of around 1.04V (open circuit voltage) and once it has coupled with carbon-steel and is wet from dew or rain (or from working under immersion) the voltage will usually drop to around 0.85 - 875mV (when tested against a silver chloride half-cell).
Even under immersion conditions the zinc will continue to work satisfactorily, but in marine waters the sodium salts that are present (chlorides) will remove the zinc carbonates (hydroxycarbonates) from the surface and will cause the zinc to dissolve at an accelerated rate to protect the steel underneath. However, this zinc dissolution process will only last for a few weeks as another reaction has already started; namely molecular displacement. This is where each molecule of zinc that dissolves out of the film is replaced with, amongst others, a complex molecule of magnesium carbonate chloride.
After approximately 12 months, the zinc layer becomes 'plated' with a hard-white insoluble layer that passivates the zinc and completely slows down all reactions. This adds years to the life of the zinc coating.
New phosphate process
Steel constructions that had tight returns like lid rebates, C-channels and boxed-in sections (to act as stiffeners) were impossible to blast-clean and it was just not possible to apply Zinga into these sections or onto the interior surfaces.
A new system is now available whereby the metal is treated with a unique phosphating process that deposits 1200g/m2 as opposed to the old range of 12g-20g/m2.
This is quite impressive when one considers that in-line galvanizing only deposits around 100g/m2 of zinc.
This new phosphate treatment is so coarse that it both looks and feels like a blast-cleaned surface, but with the added advantage that it also converts the surfaces of all the hidden and closed-in areas as well.
This treatment is longer than the usual phosphate system, having an immersion-time of around 25-30 minutes, but the extra time is well worth the finish that is achieved.
There are no short-cuts to this period of immersion, as this is the minimum time to achieve the desired crystal size on the surface.
To maintain the solution strength in the tank, test-coupons of a known weight (made from carbon or Cr MO steel) are used every Three months to check weights, and the weights across the panels should be in the range of 10,000 - 18,5009/m2 average.
Parts to be dipped that have a heavy rust deposit on the surface or have a thick scale deposit from the steel manufacturing process should be blast-cleaned prior to going into the alkali degreasing tank.
The articles being coated would go through the usual seven-stage phosphating process up to the point where they would normally be ready to be coated with oil. Here the system changes.
Find more information on
www.igoe.ie.
Rick Simpson is technical manager, (Coatings Division) MG Duff International Ltd in Surface world magazine