If I purchase an airplane based in Florida, will it have corrosion? How corrosive is this area?
Aircraft and equipment parked in the Arizona desert are there for a reason - large open spaces and little corrosion. We all know why - its dry in Arizona. Keep it dry and it doesn't corrode.
But what is the most corrosive environment for an airplane or any other piece of equipment:
- An airplane parked in the Florida swamp where it is always wet,
- the Oregon rain forest, or
- the dry central California coast?
To compare areas we need a corrosion test object. Some item that is common to all areas, has the same corrosion potential, is exposed to the environment, and is readily available for inspection. It so happens that all areas have galvanized fences, poles and other galvanized objects. Galvanize is the application of hot-dip zinc to surfaces to form a sacrificial material that protects the underlying metal from corrosion. In some areas a galvanized fence develops a patina, and is rust-free even after 30 years. In other areas a galvanized fence rusts after a few years as the zinc is sacrificed (used-up).
Location: Sacramento, California.
Average Rainfall: 19 inches.
This galvanized fence bracket is corrosion free after 20 years of outdoor exposure. It is located in central California and is 100 miles inland from the Pacific ocean. This area can be considered to have low corrosion potential.
Location: Orlando, Florida.
Average Rainfall: 48 inches.
The next picture shows a galvanized wire rope and bracket attached to a wooden piling on a boat dock in a central Florida lake. I can't think of a more wet environment, yet there is no rust except on the U bracket and nut where the protective galvanizing is missing. Otherwise the galvanizing is doing its job and protecting the iron rope.
Location: Moss Landing California (just north of Monterey).
Average Rainfall: 23 inches.
Why should Moss Landing have such a high corrosion potential? Moss landing is the "perfect storm" of corrosion. Everything comes together to create the ultimate corrosion test environment. Everything here means:
Availability of chlorides (sea salt)
- A robust transfer mechanism to deliver chlorides from the ocean to surfaces.
- Moisture to make the chlorides "sticky" and hold them to the surface
- Long periods of moisture to form the electrolyte and keep the cell working
- Little rain to clean the surfaces
The interesting item on this list is the last item, little rain. Large amounts of rain would wash the chlorides off of the surfaces. Rain acts to clean the surfaces of chlorides. This is, I suspect, the reason why the galvanized rope in Orlando does so well. Rain cleans the rope of dirt, organic matter, and chlorides. Same thing happens in rainy Oregon.
Rust showing corrosion cells. Location: Moss Landing, California
Chlorides form a conductive path so that electrons and ions can travel through the solution.
Rust showing corrosion cells sticking out through the paint film. Location: Moss Landing, California
Rusted anchor chain from Moss Landing, California
Salt spray from ocean whitecaps evaporates particles of salt into the atmosphere. These ultra-tiny salt particles, about the size of smoke particles (0.5 to 5 um), can be deposited far inland if the prevailing winds are onshore.
Ocean waves hitting the beach and rocks create larger salt particles (10-50 um) that remain close to shore and therefore present only a very localized problem.
At Moss Landing, and other high-corrosion potential sites, have strong ocean winds to create whitecaps and onshore prevailing winds to carry the salt inland.
Fog. Moss Landing gets plenty of afternoon fog. Fog particles become tiny salt-water capsules that land on the fence. Afternoon fog in the summer is almost a daily event. High humidity from the fog keeps them from drying out. Even when the sun shines and the surfaces dry, the chlorides remain. Over time the chlorides (salt) accumulate and when the moisture returns the electrolyte is more concentrated. Long summers without rain (most rain falls during the winter) allow the concentration to continue as each day brings in more salt laden fog.
This brings us to one of the best corrosion preventative substances known to man - and it's cheap. Clean water! Before you go spraying corrosion preventative oils on your airplane, wash it with clean water to remove the chlorides, biofilms, dirt, and other corrosive substances.
Corrosion is a micro-event governed by local conditions. A small seam on the fence will trap chlorides and moisture. So to with the atmosphere. Fog is dense in one area and blocked by a mountain in other areas. Not all of the central California coast is corrosion rich.
If you have ever been to the central California coast in the summer you know how cold it gets when the fog rolls in from the ocean. This fog belt, I would estimate, extends from Santa Barbara to the south as far north as San Francisco to the north. This is an area of many corrosion micro-climates because of the mountains and valleys along the coast. Mountains contain the salt laden fog to a small area on the coast. Most airports are above the fog line and are much more protected from corrosion.
Moss landing is a flat area with no mountains so the fog can penetrate far inland. Airports in the area are at sea level.
The stainless steel will accelerate corrosion of the much more active zinc. This will use up the protective galvanized area next to the stainless bolt and cause localized corrosion. Use neoprene washers (or similar) to electrically isolate the stainless steel from the galvanized fence to minimize galvanic corrosion. A bit of zinc anti-seize on the bolt threads will also help.
If you must use stainless steel in a high-corrosion area then specify 316 stainless. Standard bolts and nuts made from 304 stainless does not offer sufficient protection.
The picture to the left shows a galvanized fence where the protective zinc plating is just starting to get used up and rust is starting to appear. This is the time to apply a coating of zinc rich primer, sometimes called "cold galvanizing"
Fast depletion of zinc. Road salt damage to chain in Rochester Minnesota.
Slow depletion of zinc on fence in Sacramento, California.
This picture is a rusted galvanized fence where the galvanized protective layer has sacrificed itself and is "used up". Extensive rust after a few years of exposure. This area is considered an area of "high corrosion potential". In fact, walk around the town and every exposed piece of iron is rusted. The interesting think is that this area averages 23 inches of rainfall per year. Not much more than our fence in the top picture with no corrosion after 20 years.
We can conclude from our little galvanized fence survey that there is something else besides rainfall that causes galvanized fences (and airplanes and equipment) to rust. That airplane parked in Florida might have far less corrosion than that airplane parked along the dry central California coast. Can we conclude that wet Oregon might also have less corrosion than parts of the California coast?
