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Aluminum corrosion on riveted hulls

M

marshmaster pat

Well-known member
Anyone with riveted hulls have a solution for corrosion. Seems like the boats over here are having bad problems with it along the chime, between the stainless chime rail and where the side and bottom of the hull meet. Appears to be worst with the newer boats using the 7075 T-6 aluminum, it does have the potential to corrode more easily than the older 6062 T-6 aluminum. All of it is Alcald aluminum sheets, supposedly.

I know none of the boats have any zinc chromate primer on them, been hassling the shops for float aircraft; both paint shops and body shops, they all suggest that would be first step.

Have seen several nice boats get parked forever over the last three years due to serious corrosion. Structure of the ribs, stand, and cage are good, the skin is just shot.

Anyone know anyone that uses these corrosion control products for aircraft?

Thanks guys
 
first off either alloy 6062 or 7075 is good stuff but not a marine alloy and does not offer as good of corrosion resistance as some other alloys. ANY TIME that you are attaching to different metals on a boat hull , in this case aluminum and stainless steel you will have corrosion much faster also another bad thing that i see way to often is people ground light or bilge pump etc to the hull .......man that is just stupid ...... absolutely never should you ground anything to your hull
 
Gold, that's a great point and I don't remember it ever being posted on here.

Galvanic corrosion is a process that sets up between dissimilar metals ...... any two dissimilar metals. Anytime you have two metals of a different content they will become a barrier to the similar flow of residual current and the joint wil begin to corrode. To prevent that you have to isolate them from each other.

It's important to remember that the new carbon fiber props are static energy producers. Carbon is a great conductor (the posts on a dry cell battery?) so when they're spinning they're producing a considerable static charge.
If you're running an aluminum hull in a saline condition like saltwater the condition is amplified.
That's why you see so many fiberglass hulls in brackish or saltwater conditions. The soft aluminum hull becomes the annode (ground) because it's the softer, more mobile material, and so it becomes the sacrificial material and corrodes.
 
Clean corrosion off , then use alodine chemical treatment , then use epoxy zinc chromate primer. All can be bought through aircraft spruce. If you install sacrafical zinc it will stop or slow down the corrosion. If you insall zincs on a new hull and replace them when they corrode it will last you forever. I have a 1974 Stossel in great shape. If you replace the stainless strip with an aluminum one it will also cut down on corrosion. When you have 2 metals like aluminum and stainless steel you have a battery and where they meet will corrode.
 
Marshmaster Pat...It's great to see your screen name up on the forum again...you are among the pioneers on this site. I, for one, have learned plenty from your insight.

My advice - not worth much - would be to try for an all aluminum outfit if you are going to run mostly salt. Hull, rigging, fuel tank, accessories - etc. Can't eliminate the galvanic effect...

The above suggested prep steps would be terrific if done before any of the components were assembled....I don't know if it will help at the old joints where corrosion has gotten a foothold. Sacrificial zincs sure work on every other type motor vessel.

Isolated grounding would seem to be a simple and effective cure for hull eletrolysis.
 
BigDaddy":1njdfrjg said:
Marshmaster Pat...It's great to see your screen name up on the forum again...you are among the pioneers on this site. I, for one, have learned plenty from your insight.
Click to expand...


Yup,
You got that right.
 
Gold - Thanks of the information on the corrosion. It is an serious issue. Most of us running or that ran riveted hulls on the coast of Louisiana or Texas realize the seriousness issue. Been facing it for 20+ years. But there are ways of dealing with it. Someone has to know how.

Cessna 185 and 206 with aluminum floats are all over the place on the Gulf Coast. Too many Grumman Gooses and Widgeons running around Alaska, that are looking at 25 to 30 years coming up. Many still in fine shape. They have aircraft aluminum, from what I hear both 6062 & 7075 and both normal steel and stainless. The naval has quite a few helicopters on subchaser frigates/destroyers that have aluminum skins. Yet somehow they are getting around the corrosion issue. I want to find out how.

Some of us just are interested in (are in love with) those riveted hulls. Figuring pound for pound, the aircraft aluminum sheets have a greater tensile strength and greater resistance to bending. Also have a greater memory (for the lack a better word) and will flex back into place while marine aluminum of the same and even double thickness will just bend. Not many boats running with marine aluminum at .090 or .100 thickness on the hull, but that old Kline, charged the hill hard in its day, bounced of hundreds if not thousands of fire ant hills (cow pasture mines) and she never made a dent. Only after coming off a vertical levee near Galveston did she finally make a permanent bend in the hull.

Big Daddy - I may just be pushed to a welded hull, but not giving up yet. LOL. Got my eye on an old hull that needs a new skin. May try to get it for a project boat.

Olf Art - Should we use a zinc or magnesium/zinc anode on these rigs? Do you are anyone else know.

Aircraftman - You are running a boat I drool over. Those stossel hulls are among the finest in my opinion. Often copied, not sure if they are matched by any. What alloy did you replace the stainless strip with? And how wide did you make the strip? Did you prime where the metals overlap with zinc chromate? Want to inform a ignorant person on how to do isolated grounding on a hull? Electronic and electricity never been my strong points.

Thanks everyone.
 
Marshmaster, zinc won't do it for you. To work well a sacrificial needs to be a lower density than the metal you're trying to protect, so go with the magnesium.

Here's somthing else I've thought about. It's just an idea. It's never been tested as far as I know on an aluminum airboat, and that is to insulate the rigging from the hull at all of the mounting points. That would protect the hull from the residual and static electrical current that the engine, batteries, and prop produce, and it would also probably make the boat quieter as a bonus since a deckover is big hollow drum that would have to magnify the the sound vibrations that the engine stand produces.

PM me about it sometime and I'll discuss my idea in more detail. It shouldn't be that hard or expensive to do.
 
I removed stainless strip. Cleaned corrosion off, Alodined, then zinc chromated with epoxy.Then reluctantly re-installed stainless. Then Installed zincs.
The Stossel hull is great. I had a thurman and it had aluminum strips on the inside I liked that idea better. I would like to do that to my Stossel.
7075 is more subject to corrosion than 6061.
 
we wash the p-3 orion every 28 days then greese it and thats a land based aircraft out at sea they wash them every 7 days with fresh water and any bare surface is primed with zinc chromate and then painted with poly paint. with dissimilar metal we put sealant between them before riveting.
 
Storing a boat inside might prevent excessive corrosion.
I have noticed when my aircraft floats are stored outside there are corrosion issues. Suspect the change in temp and humidity between night and day and holding moisture.
No issues when stored out of the weather.
 
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