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Repairing corroded aluminum


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#1 xyzzy

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Posted 03 September 2013 - 11:41 PM

The aluminum strip that holds my washboards on has badly corroded around one of the screws.  The corrosion was so thick around the screw it pushed the strip away from the deck over 1/4" and then cabin top lines started snagging under it.

 

The strip started 1/8" thick, but it's less than half that thickness around the one screw hole after I wire wheeled away the corrosion.  The back side has some kind of black plastic coating or lamination on it.

 

What should I do to stop the corrosion from progressing where it has started?  And what can I do to fill the parts that have corroded away back to original thickness?

 

I'm thinking I'll clean with a brass brush, spray with zinc chromate primer, and then fill with phenolic thickened epoxy.

 

Is the zinc chromate under epoxy a good idea, or would it only be appropriate if I was going to paint, and I should just go straight epoxy on the Al?  Do I need to treat the Al with something first, like lye or alodine?  I have neither.  I do have some over cleaner, that's like spray on lye, isn't it?

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#2 Scarecrow

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Posted 03 September 2013 - 11:45 PM

Best bet is to reproduce the whole part out of plastic.



#3 SloopJonB

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Posted 04 September 2013 - 12:14 AM

I'd replace it but if you want to rebuild it, put the epoxy on the aluminium - after a fresh pass with abrasives. Chromate is for use under paint - a primer if you will.



#4 Jim Conlin

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Posted 04 September 2013 - 12:26 AM

I'd simply replace the strip.

Bronze, brass or stainless would be OK.  

If using aluminum, use a corrosion resistant alloy like 6063.  

McMaster is your friend.



#5 Heavy Metal

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Posted 04 September 2013 - 12:59 AM

Im no expert by any means, but i have done a full refurbishment of an 42 ft alloy keeler.

 

for a non-structural application like youve outlined, heres my 2 cents

 

 

  • Liven the affected area up with a sanding disc or wire wheel
  • Fill area with something similar to this stuff http://www.crc.co.nz...339eb3-6531.htm
  • Sand back and paint
  • Refasten it with and isolating gell or similar between the fastner and alloy

 

If that doesnt fix it, redo it in 3 years time



#6 xyzzy

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Posted 04 September 2013 - 01:52 AM

The outside of the strips appears to be anodized and the underside has this plastic coating.  If I made new ones that would be hard for me to replicate.  And the port strip is perfectly fine, but probably wouldn't match a new starboard strip in appearance.  I'd probably never get the screw holes to line up with the old ones either.  I use clamps, transfer punches, etc. but it just never works for me to get new holes to line up with old ones.  Anyway, I'd rather restore the affected area than make a whole new strip.  I just don't want it to corrode away again so fast as to make the effort a waste of time.

 

Heavy Metal, is that filler somehow superior to epoxy with a fairing filling added to it?



#7 Heavy Metal

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Posted 05 September 2013 - 04:37 AM

Heavy Metal, is that filler somehow superior to epoxy with a fairing filling added to it?

 

Umm, im not sure, its . . .different.

 

It sets, very, very, very hard, alot harder than epoxy. Its 'filler' is alloy fibers and is very impact resistant and, IMO, less likely to crack (due to the fibres) under stress than epoxy.

 

I used it for areas that required serious filling (several squ feet, upto half an inch deep), then faired the surface with epoxy and microballons - 3 years later and its still looking great.

 

The best benefit is its not a "marine" product with marine pricing, and you can easily mix small volumes - you'll buy it or something similar at the automotive supply store as its predomanently used as body filler on cars.



#8 Dead air

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Posted 05 September 2013 - 08:51 AM

The problem with alu and epoxy is that the alu corrodes VERY quickly, like instantly, in contact with air causing a bad bond.
One way around this is to coat the alu in epoxy, then sand the alu while the epoxy is still wet to get a good bond

#9 Titan Uranus

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Posted 05 September 2013 - 11:12 AM

The problem with alu and epoxy is that the alu corrodes VERY quickly, like instantly, in contact with air causing a bad bond.
One way around this is to coat the alu in epoxy, then sand the alu while the epoxy is still wet to get a good bond

Many contradictions here.

From a marine/aviation view point you have to first determine how far gone the alloy is.

As stated before. If this is not marine (5083...) grade alloy then replace.

If it is and worth saving then...

Fully wash in fresh water with desalination agent.

Fully dry

If it looks very porous then go to you local powdercoater and get him to our it thru his 3 or 5 stage Chromate system. Or go to a aviation repair guy and get it coated in aladine.

These are the only two systems that will work. You can by a chromate etch and apply tour self but be sure to follow instructions to reduce the accumulation of "free acids" which will compromise longevity of alloy and coating.

Then once treated get a good build (300um) minimum DFT ( Dry Film Thickness) of a goid quality epoxy primer/UC)on the part. This van be either wet paint old a powder coating product ( but remember the part will have to pass thru a oven for more than ten minutes at 200 degrees C. So attached compo ants might melt). A FBE (fusion bonded epoxy) is the best and will last a very long time. But products li,e interprotect or Interlux inter prime epoxy are good to.

Any filling can be done now then add another epoxy coat.



Since epoxies offer the best resistance to water migration ( by pressure/osmosis) you need this to keep H2O and O of the alloy. But! Epoxies chalk in UV soo...

Sand to desired smoothness no smoother than 350 grit... but to course and sand marks will be present in top coat. ( a guide coat is good for finer sanding )


Top coat with either a polyester powder coat top coat or polyurethane topcoat to get UV protection and the color/gloss required.

#10 pauloman

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Posted 10 September 2013 - 02:28 PM

MCU aluminum paint (aluthane???) then kevlar reinforced epoxy paste. More mcu aluminum to give nice new aluminum look again.



#11 Jim Conlin

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Posted 10 September 2013 - 05:45 PM

By chance, Paul, do you sell that miraculous stuff?



#12 casc27

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Posted 10 September 2013 - 08:03 PM

MCU aluminum paint (aluthane???) then kevlar reinforced epoxy paste. More mcu aluminum to give nice new aluminum look again.

 

Kevlar reinforced? For what purpose, bulletproofing the piece?






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