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ELECTROPLATING,ELECTROPOLISHING,ANODISING &GALVANISING


The zinc thickness is approximately 65µm 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 apaint coating if one is required. Unlike ashiny 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 aprimer.


A‘Zinganised’ coating will lose a maximum of around 1-2µm DFT of zinc per annum from its applied thickness under normal atmospheric exposure, and around 3-4µm DFT (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 apaint coating should give aworking lifespan of around 20-25 years in exposed marine atmospheres. This is because Zinga has a“synergisation factor”ofaround 2.5, which means that the zinc and the paint coating are working in symbiosis.


Paint breakdown mechanism


Normally,apolyurethane 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.


This is 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”inthese areas.


Under immersion


Under immersion, the zinc will generate avoltage 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 asilver/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.


38 SURFACEWORLD january 2013


However,this zinc dissolution process will only last for afew 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, acomplex molecule of magnesium carbonate/chloride. After approximately 12 months, the zinc layer becomes “plated” with ahard white insoluble layer that actually passivates the zinc and com- pletely slows down all reactions. This adds years to the life of the zinc coating.


Lifetime expectancy graph for ‘Zinga’


Zinga UK (a division of MGDUFF International Ltd) Tel: +44 (0) 1243 533336 Email: sales@zinga-uk.com www.zinga-uk.com


read online @www.surfaceworld.com


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