This week we got a photo from Airbus of our new H125 helicopter under production and while showing the photograph to my son he asked why it was green and yellow and not painted properly.
During manufacture, aircraft parts are coated with yellow anti-corrosion Zinc-Chromate primer paint. This coating protects the aluminum skin from corrosion by oils, grease, and moisture. Paint is added on top of the Primer. The different shades are parts from different factories.
We have all seen these patchwork aircraft and there is a reason for them being like this, so let’s have a look and see whey they do it.
What is a Green Aircraft?
A green aircraft is a nickname given to an aircraft that is under manufacture because of the color of its components. One of the biggest enemies of an aircraft over its lifetime is corrosion from the elements.
Corrosion can cause the weakening of a component that can lead to premature failure, and in the wrong component, this could be enough to cause catastrophic damage.
One of the major reasons why aircraft are so thoroughly inspected is to look for corrosion, even to the point where components are completely stripped of paint, inspected, and repainted several times throughout the aircraft’s life.
To help overcome corrosion, all aircraft components that are susceptible to corrosion are coated with a special anti-corrosion solution. Some of the main industry names for this chemical conversion coating are called Alodine, Iridite, and Chromate.
When it was first used in the 1930s, Zinc-Chromate was a salt that was composed into a liquid solution that was easily brushed and sprayed to cover a fuselage panel or component in a weather-proof skin.
However, it was found that it was highly toxic and also a carcinogen! – See the Medical Paper Here. Since then, chemical manufacturers have been able to produce a modern version that is a lot nicer to use!
The way that the Alodine coating works is by chemically bonding to the upper surface layer of an aluminum component or panel, providing protection and giving a perfect surface for the paint to adhere to, hence why it is also used as the aircraft’s primer coat.
Why are Aircraft Covered in Different Shades of Primer?
Green Aircraft can look like a real patchwork of greens, yellows, and every shade in between, but why is that? The main reason is that many components for an aircraft are manufactured, either by different factories of the same manufacturer or by completely different manufacturers altogether.
Each facility may use a different brand or series of Zinc-Chromate primer which differs in their shade. When all the components arrive at the assembly line this is when it’s very noticeable.
The main reason why the shades can differ is by adding a paste called Lamp Black which helps to protect the Zinc-Chromate from UV damage. Raw Zinc-Chromate is unprotected from ultra-violet radiation so each coating manufacturer has its own blend of ingredients that go into its mix. The more paste they add to the Zinc-Chromate, the greener the shades become.
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If you take a look at the internal fuselages of many WWII-era aircraft you will see they are a dark green compared to today’s yellow-based components. This is because more Lamp Black paste was used in the primers back then.
Today, evolution in Zinc-Chromate coatings has mostly eliminated the need for large amounts of Lamp Black in the formulations. However small amounts are still used, hence the differences in color and shade.
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Why Do Some Aircraft Look Like They Are Covered In Green Plastic?
Many large airliners like those from Boeing, Airbus, and Bombardier, can be seen to be covered in what looks like a green plastic wrap. The main reason for this is to protect the Zinc-Chromate finish on the fuselage panels during assembly.
As workers assemble the aircraft, the anti-corrosion coating can be scratched or dented by tools, rings, watches, or assembly jigs which would then leave an opening in the protective layer. Left unnoticed, this area would then have to be touched up by the aircraft painters, giving them more work. If it was not touched up, that scratch/dent could then be prone to corrosion over the life of the aircraft.
The green sheeting is applied to the aircraft just like a vinyl wrap is done to a vehicle. This gives it a second skin to help protect the corrosion coating. Once fully assembled the aircraft is then flown or driven to the paint facility where this protective sheeting is stripped off and the surfaces are cleaned and prepared for painting.
Do All Aircraft Start Off Yellow or Green?
The short answer is No. Modern aircraft like the Boeing 787 Dreamliner are comprised almost entirely of carbon composite. Carbon is not susceptible to corrosion, therefore it does not require the yellow Zinc-Chromate anti-corrosion primer.
For all other aircraft that are aluminum, the need for corrosion protection is required, usually in the form of Alodine.
So then, how come American Airlines have run their aircraft with their fuselage panels as polished aluminum? Surely they must have some kind of corrosion protection on them?
The way that polished aircraft are protected from corrosion is the polishing itself. About 3x each year a polished aircraft is brought into a hanger, thoroughly washed, and waxed with mechanical buffing machines. This layer of wax protects the fuselages and unpainted components but must be done regularly to provide protection.
Normally an aircraft is repainted every 4 years, but a polished aircraft can save on those costs, let alone the weight of the paint added to it.
The many shades of yellow and green you see on new aircraft are the corrosion protection system used to prolong the life of the panel and components that make up aircraft. As most aircraft can spend the majority of their life outside in the elements they need to be able to last to ensure they are worth the huge amounts of money it cost to purchase and maintain them.
The ratio of Zinc Chromate with Lamp Black paste is what causes the different shades and depending on the facility that applies the protective coating, will dictate the shade in which that component appears.