Seaplanes & Salt Water:
Operate or Not to Operate?
cannot be avoided.
So you might as well learn how to prepare your aircraft.
Then reward yourself with a day on this secluded island...
Corrosion cannot be avoided. That is a basic scientific
fact and an inevitable condition of mixing metals and electrolytes, i.e.,
seaplanes and saltwater. Seaplanes are the anodes and saltwater the great
conductor trying to return the seaplane to the sky, molecule by molecule.
According to Mark Mathisen of Wipaire, the solution is simple. The key to
preventing corrosion in the field is to seal any metal against exposure to
the elements. “If you can seal it from the environment, it will last
forever.” In other words, isolate the less noble anode and save it from
the evil electrolyte.
Unfortunately, although Mark is correct, the answer is not so simple. Sealing depends on the surface coating preventing moisture and oxygen from penetrating the surface and causing further corrosion. The problem with corrosion resistant seals is that the integrity of the seal is tenable. Cladding, for example, is the physical act of placing a more anodic metal on the surface of a less anodic metal. Ironically, a cladding coating will corrode (oxidize) first and create a surface (oxide film) that will seal but not continue to penetrate. Cladding depends on corrosion taking place in the first place and is thus not perfectly stable. It’s like fighting fire with fire.
Stainless steel, on the other hand, works as a seal in that it’s surface is not easily penetrated by moisture and oxygen and thus will resist corrosion. (Unfortunately, it’s too heavy to make seaplanes out of.) Electro-plating steel with chromium provides a lighter corrosion resistance method of obtaining strength. Anodizing is an electro-chemical way of cladding aluminum. Painting simply creates a tenuous seal by adhering to the metal surface and not allowing corrosive elements to penetrate. Grease and wax work the same way except that they are very temporary.
Seaplanes are built out of aluminum because it gives the best strength to weight ratio, and has the most resistance to fatigue. The airframe structure, however, is held together with stronger steel bolts and hinges and bearing, and the amphibious landing gear is made with steel to withstand the heavy loads. These dissimilar metals generate an electrochemical process where the aluminum becomes the reactive anode and the steel becomes the cathode, and it is all conducted by the electrolyte water. The more salt in the water the better the conductive properties. The idea of protecting the aircraft from corrosion, therefore, would be to prevent the electrolyte from forming a conductive path between the dissimilar metals.
The use of cladding, plating, anodizing, paint, grease, and wax, however, to seal off the aircraft metals from all catalyst is not a perfect art. Cladding and anodizing gets scored, stainless steel contains impurities, plating and paint chips off, grease oxidizes or absorbs water, and wax dissipates. The end result is that electrolyte works its way into unprotected areas and starts to corrode the bared metal.
Besides, the discontinuity of bolts, fasteners, rivets, skin lap joints, bushing, bearings, hinges, and all the other connectors that keep a thousand aircraft parts flying together in tight formation makes sealing the entirety from the environment impossible or impractical to begin with.
So that leaves the salt water operator with a very proactive goal, and that is not to prevent corrosion, but to control it by eliminating as many of the basic requirements for its formation as possible.
1) Preventing the electrical potential difference within the metal.
2) Insulating the conductive path between areas of potential difference.
3) Eliminating any electrolyte that could form a conductive path on the
surface of the metal.
4) Giving the electrical potential a conductive path that is not detrimental
to the aircraft.
1) If the aircraft is flown from the sea
to the airport and it is dry when it arrives, would it be more sensible to
not wash the aircraft instead of leaving it wet for the night?
2) If the aircraft has not been washed the night before, does it make sense to wash it just before flying it back to the salt-water environment?
For question one, the answer is simply that
washing the aircraft will remove the salts that make the water or moisture
so conductive. Even if it is dry, salt and contaminants will act as an
electrolyte and promote corrosion by attracting moisture out of the air.
That is how land planes suffer corrosion when they never touch salt water.
Furthermore, even if it is wet after the washing there is little
electrolysis action in fresh water and thus minimum corrosive effects.
Secondly, if the day’s dirt and contaminants are washed off the fresh
water remaining will run off easily and the remaining water will dry
quickly leaving a corrosion free environment.
The answer to question two is just a variation of the answer to the question one. If you wash off the dirt and contaminants that bind the salts to the surface of the aircraft, even when it is moored directly in the sea, there is less chance of corrosion taking effect.
Plus, for both question one and two, after the aircraft is washed the lubrication is refreshed and that will help seal out the salt water. Some of the recommended lubricants, such as WD40 and CorrosionX are water dispersants as well and can be sprayed directly on wet, freshly washed, parts. CorrosionX will actually bypass any contaminants and chemically bind directly to the metal to prevent corrosion even if the corrosion has already started. It is an excellent product to use in the hard to reach places, such as inside the aircraft floats, gear well, the aircraft belly, and inside all parts of the tail section.
Thus, if these conditions are met:
1) The aircraft is built or rebuilt to meet
corrosion control standards;
2) The aircraft is washed and sealed nightly with proper lubrications;
3) A sacrificial anode is attached to the floats and kept conductive;
corrosion from the daily excursions into the salt water will remain controllable with a minimum of effect on the long-term value of the aircraft.
One of the biggest mistakes I see operators
make is to buy a well used aircraft that has “only a trace” of corrosion
because it has been operated in fresh or brackish water and then
immediately begin operating it in ocean conditions. The trace corrosions
will suddenly blossom to almost uncontrollable proportions. The reason is
simple. Fresh or brackish water will not conduct as effectively as salty
water. The aircraft was able to continue in the previous environment with
minimum corrosion because of the minimum conductivity, but the integrity
of the paint and exposed metal were breached over time regardless.
When exposed to a warm moist and salty environment, suddenly the corrosion explodes into all the breached areas and starts taking hold. The only answer is to take the aircraft off line and completely strip, de-corrode, alodine, prime, and repaint the entire aircraft. In fact, buying the used aircraft was not a mistake, but not preparing it properly for it's new environment is the mistake. That should have been done to begin with.
Moreover, like I mentioned earlier, the paint shop must realize the importance of doing the job right to begin with. A land aircraft used in Arizona can get away with a quick strip and spray, but an amphibious seaplane to be flown in the ocean cannot. Corrosion control measures must be in place.
Finally we come to the part about taking proactive decisions and
maintaining the integrity of the aircraft, the operations, and the owner.
That can all be realized by careful planning. It only makes sense that if
your aircraft are maintained they will be ready to fly when your customers
are ready. And if your customers are taken care of then your
business will do well. Then start by washing and lubricating your plane.
So many operators who have not worked with seaplanes will scrimp with such
a simple process. And the end result is corrosion.
The reasons given are usually that spending that much time at the end and beginning of the day on corrosion control is excessive thus bringing the costs of operating up beyond the acceptable level. I have had operators compare the time spent on corrosion control to that of time spent on helicopter maintenance. But, there is a major flaw in this argument.
There is little comparison to helicopter maintenance when looking at labor costs, because the helicopter requires experience licensed engineers. Washing and lubricating an amphib is labor intensive, but the job does not take skilled maintenance workers. Any wage earner can be trained to spend the time on aircraft care, as the job does not require a signature for release.
The job is time consuming, but not expensive. As the best time for doing the cleaning is at the end of the day, then a sheltered location with strong lights and access to pressurized fresh water is definitely required. Work stands and stable ladders are also needed to reach the wings and the tail with little danger of slipping. The work stands are used for the day maintenance as well so can serve double purpose.
Moreover, do not withhold on the lubricants. Buy the best and use them liberally. That point cannot be overstated. I have seen too many good Caravans for sale because the operator did not want to spend the money in the first place and now no one will touch them. They were operated in salt water with little or no protection. What you save in corrosion control, maybe thousands, will cost you in aircraft integrity, possibly hundreds of thousands. Keep your aircraft airworthy and they will continue to be an asset on your books and not a liability.
The second part of being proactive is to maintain an abundance of spare parts for the known parts on the landing gear and the aircraft that will corrode the fastest. Have a supply of assemblies ready to change out the entire wheel assembly, brake assembly, and landing gear and drag link assembly. Clean off the corrosion and re-prime and lubricate the fixed parts on the floats, and then bolt on the new or refurbished assembly. Do this every 100 or 200 hours as the case may require, and you will greatly lessen your chances of having a down time brake or gear problem. In fact, change out the brake assembly every 50 hours if need be. Do what is necessary to keep flying.
That does not mean you have to replace the gear every 100 hours or the brakes every 50 hours with new parts. What you do is rebuild and refurbish the removed parts on the work bench during the day when the aircraft are flying. That way the engineers are in effect working on the aircraft while it continues to fly, keeping both the revenue coming in and making the best out of the engineers valuable time.
In the meantime, whenever corrosion is found during regular inspections, it must be removed immediately. For example, corrosion blisters under paint should be removed, neutralized, treated, primed, and repainted with care. The integrity of the painted surface cannot be over emphasized. And the paint should be cared for with the appropriate waxes or sealers to prevent oxidization or chalking.
To prevent the original paint job from looking a little tatty after a few years make a concerted effort to know your paint brand and specific blend and you can avoid having the occasional patch spayed with mismatched paints. Along with a better looking result the operator can rest easier knowing that that value of his aircraft has remained intact along with the painted surfaces. That says a lot about selecting a simple paint scheme. After all who looks at the paint job when there are so many other distractions near the sea.
Article and Images by John S Goulet
Note from the Editor. If you want to miss out on enjoying 90% of the earth's surface then certainly hold back. If not, let The Bush Pilot Company show you how to operate off the world's oceans. 1 (With the possible exception of a well trained pilot.)
Use the attitude indicator as your guide back to The Bush Pilot Company.
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Last modified on
May 07, 2006 .
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