Pitting corrosion in anodised aluminium 1

[digger2008]

Can anyone help a mate of mine. I think the mast in question is anodised aluminium. I told him if you lads can't help nobody can.
Thanks

"My Leisure 17 whilst in great condition elsewhere, has got extensive pitting in the bronze coloured mast coating. This extends the length of the mast & seems to be concentrated on the sides of the mast. Most of it is just slight pitting of the coat but in one or two places you can feel a slight depression in the coating as the aluminium is corroding. I feel this may compromise the strength of the mast & intend working on this shortly to rectify. The Boom is in perfect condition? Any help would be greatly appreciated as to:- Do I Paint the Mast; Varnish it; or what & what treatment do I need prior to painting to neutralise the alluminium corrosion. I would be interested as to what may have caused this as the boat was under cover for 20 years or so, unused & in no way has been abused, prior to me buying her Sep 2007."

 

[Neubaten]

Well, he can pass a sandpaper to eliminate current defective coat and the corrosion products.

Then alodine, primer and paint it.

 

[digger2008]

Is Alodine a product for "home" use, i.e. small pots (I assume it's a paint)?
What causes these pits, is it a defect in the anodising?
Cheers

 

[Neubaten]

Alodine is a commercial designation for a chemical conversion protective system. It is not as good as anodizing but it is far simpler and cheaper, and you'll get good protection.

Check out Mil-C-5541 for all the knowledge.

For example, it is used in aeroplanes in zones where some rework has been performed and/or the anodized coat has been damaged.

You can apply it with a pencil or a brush.

For repairing a damaged zone it could be useful.

Check Henkel alodine products, for example.

 

[Neubaten]

I would try something of this kind for your friend's intendend application:

http://www.henkel.com/cps/rde/xchg/...=ut&parentredDotUID=null&redDotUID=0000000J4S

 

[CoryPad]

Anodizing has pores in the coating that can allow corrosion to occur. A final seal of the anodizing is required for maximum corrosion protection. It is likely that the final seal was missing or inadequate.

I am not aware of Do-It-Yourself aluminium corrosion protection. Henkel does offer brush-on and marker-style products for metal pretreatment, which can serve as a base for polymeric coatings.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[redpicker]

Such corrosion could lead to failure of the mast. It should be corrected. At least it is in a convient place.

As Cory has said, the anodized surface is porous and should be seald for maximum corrosion resistance. It sounds as if that sealing treatment has failed in the 20 years of storage. A metallurgical examination would be needed to definatively determine the cause, but high chloride concentration can cause it. Your friend has said it was stored "under cover". If what kind of environment? Under cover but exposed to the elements? In a coastal region or maybe industrial area? What kind of cover? I woulnd't expect a canvas or plastic tarp to last 20 years and the effects of collected rain water, condensation, and organic activity could have compromised the sealing.

A re-anodizing treatment is probably going to cost as much as a new mast, so perhaps the best option is to locally address the corrosion, as has been mentioned, and keep an eye out for a good deal on a replacement.

rp

 

[Helicopterjunky]

Aluminum will pit by corrosion near salt water or salt air.
Just that simple. Even when anodized properly, the salt environment will still, after enough time, attack the aluminum. The correct way to determine if this mast is still usable will be very expensive. If pits are not too deep you may be able to sand them out and touch up the surface with alodine. Once you sand them away to the eye, they may still be there and the only way to know is to acid etch the surface. Alot of times pits will open up again. There is a pen that can be used instead of brushing on alodine called "Touch and Prep". If you really want to do it right it would require removal of mast and stripping with abrasive or plastic bead media. Then the mast could be NDT tested to detect pits, Pits would then be dug out and blended and the mast re-anodized and sealed. Either way you should consider a two part primer and topcoat to give some extra corrosing resistance in the future.

 

[Neubaten]

(I think they are agreeing with the sandpaper+alodine+primer+paint route proposed in the second post)

 

[swall]

Small quantities of Alodine can be purchased from automotive paint stores

 

[digger2008]

Thanks for all your help guys, I knew this was the right place to be. I'll pass it on and let him decide.

 

[unclesyd]

The problem you are describing seems to be extensive enough warrent refinishing the mast.

I would unstep the mast and remove it and take it to a mast refinisher and have them strip the existing coating which I assume is an anodized finish and have them refinish it. I personally have never seen a successful spot repair of a mast coating. You could do all this except removing the existing system which is normally done by bead blasting.

If your mate can't do this I would certainly have a surveyor, mast refinisher, or experienced mariner look at it. One area to particularly look at is around the mast collar.

There are some excellent mast finishing systems offered by the marine coating systems companies like International and many others.

 

[Helicopterjunky]

Yes, I am agreeing with second post with the exception that even when you sand out visible pits you will not have gotten rid of all the corrosion. You will need to etch the aluminum and invariably many of the pits will open up again.
That is because the corrosion tunnels down into the material. Only etching and NDT will give a comfort level that enough material has been removed to get to bottom of corrosion. If that can be done, then the mast can be re-treated. I would not count on it though. It may look good when you are done and fail later. Not a good thing when out at sea.

 

[pauljohn]

This is a typical and very common problem on anodised masts. Obviously without seeing it it is difficult diagnose exactly but slight pitting is usually not serious. Masts can usually suffer fairly major removal of material and still be structurally sound, check out the mast track, D1 fittings, halyard exits etc, and compare with damage, see how the factory removed material. Cracks and major holes can often be welded, small pits if deep enough to warrant can be drilled and a rivet put into the hole.

You will probably find most of the damage around the stainless fasteners holding tangs, or other fittings on the mast. The most common fix is to strip mast of all fittings, paint with a system such as Awl-grips aluminum system (

http://www.awlgrip.com/awlgrip_pages/application_guide_above_water.htm#4)

and replace fittings using a good sealant such as 3M's 5200 on the fasteners, if using rivets look into Monel, which are not quite as strong as stainless but much closer in nobility to aluminum.

If all the damage is in the mast but not in the other spars check for electrical leaks from inmast wiring (if it has any) from the lights, the corrosion could of happened in a short period when the boat was being used if electrical wiring in mast was faulty.

 

[strider6]

i'm not an expert on Aluminium but this site is a great source of informations on Al and probably the best on the web:

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=180&pageid=2144416698

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=177&pageid=2144416667

http://aluminium.matter.org.uk/aluselect/02_corrosion_browse.asp?alloyid=3

hoepn this can help u

S

Corrosion Prevention & Corrosion Control

 

[proetools]

Great Links! thanks a million.

Bart Brejcha Chicago
DESIGN-ENGINE|EDUCATION

http://www.proetools.com

surfacing and Pro/CABLE training