Corrosion proofing.

[Mike Davis]

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Date: Fri, 20 Aug 1999 22:36:20 +1000
From: Ian Donaldson <allsure@ne.com.au>
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G'day From Australia

I have a few questions regarding corrosion proofing that I would
appreciate some help on, and they are;

1 Just what must be done to clean the parts that form part of the
internal structure before chromating?
2. Can the zinc chromate be brushed on or must it be sprayed?
3. How thick should the coat be?
4. Can the parts be assembled wet or must the chromate be dry?
5. Is there any value, or in fact any need, to chromate the entire
internal structure?
6. When I finish the factory supplied chromate, could I use other
brands? Compatibility?
7. Should the fuselage be fully chromated internally? And if so at what
stage?
8. Should the skins be chromated externally, where the rivets will go,
before assembly, so that there will be chromate under the rivet?


Thanks and regards

Ian Donaldson

[Mike Davis]

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Ian Donaldson wrote:

G'day From Australia
I have a few questions regarding corrosion proofing that I would
appreciate some help on, and they are;

1 Just what must be done to clean the parts that form part of the
internal structure before chromating?

I just scotch brite and wipe with MEK or acetone.

2. Can the zinc chromate be brushed on or must it be sprayed?

I just brush during assembly. Thin the chromate.

3. How thick should the coat be?

Very thin is as effective as thick and less prone to flaking off generally
but I
use enough 'fill' the joints to keep moisture out.

4. Can the parts be assembled wet or must the chromate be dry?

I prefer wet. It adds a little strength and keeps moisture out.

5. Is there any value, or in fact any need, to chromate the entire
internal structure?

I did a light spray inside fuselage as cessna's eventually corrode roof and
floor due condensation and leaks. Will just spray corrosion-X or ACF-50 in
wing
after painting. Believe most people do neither as 6061 is fairly corrosion
resistant. I was seeing surface corrosion though on my unfinished 7 year old
fuselage which is stored in an unheated shop in a humid climate.

6. When I finish the factory supplied chromate, could I use other
brands? Compatibility?

Believe anything is 90% as good as the best but I am sticking with the epoxy
chromate. Price varies widely depending on source.

7. Should the fuselage be fully chromated internally? And if so at what
stage?

see above. If I were planning salt operations I would spray inside of all
panels
before assembly.

8. Should the skins be chromated externally, where the rivets will go,
before assembly, so that there will be chromate under the rivet? I like to

dip the rivets

and install wet but I am not consistant. This also helps seal the rivet so
water can't leak

in I believe.

I have not used alodyning anywhere. For exterior skin prep I was very
pleased
with advice received here. Scotchbrite, scotchbrite with metal prep, rinse
with
hose, dry in sun with hvlp air pressure to blow out water, spray epoxy
primer.
The metal prep worked very well with no alodyning.

Ken




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