corrosion

[Guest micgrace]

Hi

 

What happens under fabric. From Superflite.

 

Micgrace :)

 

 

 

[Ultralights]

ahhhh corrosion, my sworn lifelong enemy! even 5 yr old Hawks are

 

riddled with it! in one place its 10mm deep in a lower wing plank thats

 

50mm thick! not nice if your looking to get into the high G area!

 

but all corrosion has a cause! and what would hav caused that? water trapped under the fabric?

 

 

[Guest micgrace]

Hi

 

The cure for it is simple. 2 pack epoxy primer (careful using, potentially toxic)before

 

the fabric is attached unfortunately spray pack zinc chromate just

 

won't cut it. MEK dissolves it. Sadly this is not done regulary even in

 

production aircraft using fabric. Few fabric production aircraft

 

nowdays.

 

Unfortunately even with all the drain holes in the

 

world, some water is going to be trapped at some time in the structure.

 

Best to be prepared to block it coming into contact with the metal till

 

it can evaporate.

 

Some people seem to think aluminium is

 

invunerable to corrosion. As for steel parts? Well, only a fool

 

wouldn't do something about it. There's a whole industry devoted to

 

ways with overcoming it. Still no universal technique.

 

Put together moisture, dissimalar metals (not even that, a slight variation in base metal will do) and corrosion is the result.

 

Only

 

real way to prevent is isolate metal from air/moistureand/or attach a

 

sacrificial anode/coating . I wonder if anyone has actually tried that

 

in an aircraft? Seaplane?

 

Micgrace :)

 

 

[Ultralights]

Corrosion is a strange beast, to repair the above, first replace the

 

damaged part, treat the new part in a alochrome solution, then zinc

 

chromate primer, and a final epoxy finish coat. should stop it

 

re-occuring. I dont know much about fabric construction, but can fabric

 

coverings be bonded over epoxy painted and treated parts??

 

The

 

sever corrosion we are finding on hawks is caused by Rubber! the rubber

 

seals on the overwing fairings are causing a sever corrosive reaction,

 

to a point of total destruction of the part within a year! yet the

 

metal not in contact with the rubber is in new condition! the rubber

 

strip is solid riveted between a metal skin and metal backing strip,

 

and the corrosion only occurs where the rivets press the 3 components

 

together. all know treatemtns have been tried and all have failed to

 

stop it. (yet the RAAF wont allow us to chage the rubbertype)

 

3 aircraft in teh fleet were test flown before being painted! and as you would expext, all 3 are showing signs of corrosion (to damaging levels) in just 5 yrs of life, where the other aircraft are relativly corrosion free.

 

so be sure to have your aircraft primed and finish coated before any test flying!

 

 

[Guest micgrace]

Hi,

 

Unfortunaetly if using the zinc chromate primer, the MEK (methyl Ethyl Ketone) will attack it and dissolve even if under 2 pack expoxy primer topcoat.

 

Alodine treatment, 2 pack expoxy primer and topcoat of 2 pack paint with flexi agent (or buy the certified product $$$)

 

Then,

 

paint adhesive to surface, lay fabric, brush with MEK push into fabric,

 

heat tighten put on strengthening tapes, clips/screws/rivets/tie fabric

 

to ribs, finishing tapes, drains, inspections holes etc seal fabric,

 

more dope, silver dope, then epoxy primer (with flex agent) then top coat (flex agent) the last painting steps are NOT certified after the silver, but the standard coloured dopes look crook. (ceconite system, available locally) Other systems e.g. aeroflite exclusively use epoxy and urethane.

 

Sounds

 

like a lot, it is, but the results can be stunning with modern paints.

 

Especially as you can actually wet sand the fabric coatsto absolute

 

flatness. And if done correctly, unlimited lifespan.

 

Micgrace :)