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Posted

I suspect one over looked source of corrosion both on outside surfaces and inside fuel tanks is stripping chemicals that contain some chlorinated solvents.  If not completely removed from the aluminum, residual traces trapped under paint and tank sealant can cause corrosion due to decompisition to form hydrochloric acid. 

Posted

 

FAA Advisory circular AC 43-4A is the official word on this.  

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43-4a_.pdf

To answer your seaplane question, I flew a C206 w floats.  It was an older one that had been converted to a seaplane, it was placarded "salt water prohibited". Because the lap joints weren't primed prior to assembly,  The factory seaplane kit had the skin primed on both sides prior to assembly, also all bulkheads, stringers, etc.  

I think they might do that on all new Cessna's now.  If you land on salt water, you are supposed to thourougly wash the aircraft with fresh water after every flight.  One C206 operator (part 135) I spoke with said that at every annual they replaced all control cables and all elevator and rudder bearings.  It's not uncommon to replace skin on the aircraft operated on salt.  

I believe Cessna has approved the use of "Corrosion X" on most of their aircraft, including citations.  I believe it to be a good product.

Posted

The higher strength aluminum alloys are just prone to intergranular corrosion.  7075 even more so than 2024.  The spar caps are 7075.  6061 is the more inherently corrosion resistant alloy.  Used on truck parts and architectural applications .. and Sonexes!  At a small weight penalty.  I 've read Mooney had to go to using 7075 in the wood to alumn wing conversion to not end up with a pig.  Don't forget that a source of moisture is condensate.  You carry a wing load of warm moist air up to the condensate layer and it condenses out moisture to hang on all those inherent seams and overlaps on a alumn fabricated structure.  Alumn. is actually very heavy relative to it's stiffness for a given section - compared to a composite - wood, etc.  Aircraft structures have deflection limits that typically rule in design.  Again, alumin. is not the best choice, add the corrosion problems of the higher strength alloys, well - it's why I went looking for a wood wing Mooney.  Hughes built the wing of the H1 out of wood, not because it was cheaper and it was all he could afford.

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Posted

I do a good wash and wax each spring and then shoot it with corrosion x.  The detailing allows me to find problems to sand out and zinc chromate over.  The x while messy does seem to work.  Makes an excellent lubricant as well. 

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