What changes are needed for a piston-prop aircraft to withstand saltwater environment?

Question

I'm interested in WW2-era piston-prop aircraft. I want to know what changes are needed to make the aircraft withstand saltwater corrosion. Presumably these are different and more expensive than the normal land versions of aircraft, or they would make them that way in the first place.

I'm not interested in the usual carrier modifications, like tailhooks and wing-folding. For this question, I'm only interested in withstanding saltwater corrosion.

I have heard that aluminum resists saltwater corrosion very well. Not so sure about steel or wood. Are there differences in the air intake? What about certain delicate instruments like a pitot-static tube?

Edit: I was envisioning any combat aircraft that fight over the ocean, but especially the more exposed stuff like low-level torpedo bombers and seaplanes since they land directly on the water. Carrier aircraft is fine too.

Answer 1

There is no way to fully protect an aircraft from the effects of salt water corrosion, even when any aluminium is treated with protection products like Alodyne. In fact, there is no way to prevent corrosion from happening anywhere unless you protect the item in question. Corrosion can affect an aircraft even in the dry climates.

Even if it is not landing or taking off in salt water, just being close to a body of salt water will cause corrosion. All you can do is be very proactive in washing the aircraft with clean water after every day, and using products like Corrosion-X to coat surfaces with a corrosion protection film. Even WD-40 will do a great job of this, because it is not a lubricant, it is designed as a hydrophobic [W]ater [D]isplacement product. You need to protect both exterior and interior areas, even inside the wings. Just ask any mech who has worked on aircraft for any length of time at a location near salt water.

Tailhooks and wing folding do nothing about corrosion, it just helps get the aircraft on the carrier and allows for efficient use of the deck areas.

Answer 2

When the first couple of Supermarine Spitfire fighters were deployed on a British aircraft carrier in the late 1930s, the structure was eaten away by corrosion within 6 months. The sea water spray was enough to cover them in salt, which would quickly reduce the aluminium structure to white dust. Before, Navy airplanes were of wooden construction, using steel for engines or bolts, but no high-strength aluminium.

During the Berlin airlift, table salt could not be transported by the usual fleet of C-47s and C-54s. Even the dust coming from the bags would cause rapid corrosion of the aircraft. So how did they bring the daily ration of 38 tons of table salt to Berlin?

Seaplanes can handle salt much better. Salt and baking powder was brought in by British Short Sunderland flying boats landing on a stretch of the Havel river inside West Berlin. Learning from the early experience with naval Spitfires, corrosion resistant alloys were developed. Typical for marine application are the aluminium-magnesium alloys that belong to the 5000 series. The magnesium oxides enhance and reinforce the protective oxide layer of aluminium whereas copper weakens the oxide layer. Early high-strength aluminium alloys use copper atoms dispersed through the aluminium matrix which locally distort the atomic lattice and strengthen it. This is called precipitation hardening, but is unfortunately exactly what makes the 2000 and 7000 series alloys very susceptible to salt corrosion.

So the answer is to pick the right alloy.

From this report on the subject:

Based on our very long experience of aluminium’s corrosion behaviour in marine atmospheres and in sea water, we can now assert that the service life of aluminium in marine environments will be exceptionally long and can be measured in decades!!! We can truly claim that aluminium is the “metal of the sea” of the modern age.

… but only when the right alloy is used!

While the 5000 and 6000 series alloys will be preferred for their resistance to corrosion, their weldability and their level of mechanical characteristics, other wrought alloys belonging to the 1000 and 3000 series are suitable for use in non-structural applications for decoration, interior fittings etc. The use of 2000 and 7000 series alloys on the other hand must be the exception in view of their poor resistance to corrosion, and they require special protection when used in a marine environment.

Answer 3

The best change you can make for an aircraft is a location change so its no longer near salt water...

On any note the most effective way to prevent corrosion of aluminum is to galvanize it. There are various galvanic paints and coatings that can be applied to your aircraft these are "cold methods". You can also do a true electrolytic method and hot dip it in zinc which is what Porsche did with the 70's/80's 911's to great effectiveness. A lot of modern planes use a zinc-phosphate primer to the same effect. Galvanizing an existing airframe may involve disassembling the majority of it.

Steel fittings and parts can be protected to an extend with things like WD-40 or a light coat of oil. The issue is this may involve you spraying stuff all over your airframe all the time which might not be great.

Where I live a hanger makes a big difference. It helps keep condensation off the plane which often comes in the form of salty air. Routine use and inspections to keep an eye out for corrosion and rust is a must.

Wood is a different story. With wood you get rot (and rust/corrosion on the bolts holding everything together). You can find more info on wood aircraft here broadly speaking wood aircraft are not as durable as metal for many reasons beyond rot.

Instruments need to be tended to like anything else, you need to watch for corrosion inside the pitot tubes as it can very well block or partially block it.

You may also find that electrical contacts corrode, dielectric grease is your friend and anything that seem intermittent should be checked right away.

Answer 4

Aluminum is prone to salt corrosion , with a small variation due to alloy. I believe all aircraft sheet metal is "alclad" . The surfaces have a thin layer of pure aluminum . It is anodic to the high strength alloy ( age = precipitation hardened) and protects it . Some shapes are anodized where a thicker oxide layer is electrolytically formed. And coatings are applied , likely the most effective protection . Landing gear ( very high strength steel) is another story ; It is subject to hydrogen stress cracking caused by wet salt corrosion - rust is not the problem. They were protected with cadmium plating and coatings ( but I am probably obsolete). There is a National Association Corrosion Engineers ( NACE) committee that deals with corrosion protection of military equipment ; they would have much more information.