Aluminium frame . . . corrosion ?

[Drake]

On the downtube of an old mtb i'm servicing for someone, i've noticed a circular patch of blistering paint. I'm assuming that there is some form of corrosion (is that the correct term for aluminium ?) happening under the painf surface.
Assuming that there is nothing terminal going on under the paint, and i don't think there is . . i'm going to do the following,
Remove affected paint.
Clean surface with wire wool and wet and dry.
Treat surface with an anti corrosion treatment ?
Prime ?
Paint.
The 3rd and 4th actions (with question marks). are there specific a/c treatments and primers for alu frames or just general ones that you use on steel.
Manythanks in advance.

[andrewjoseph]

I wouldn't assume it wasn't terminal. It might have been going on for a long time.

[Brucey]

the problem with any clean + paint job on an ally frame is getting rid of all the oxide. If there is any pitting then it will be very difficult to shift it.

To clean a pitted surface properly you may need to use a hot caustic soda solution after mechanical cleaning of some kind. A weak acid solution will passivate the freshly-cleaned surface.

When painting, use etch primer on a clean dry surface, then undercoat + top coats as necessary.

Very often I have found that corrosion near joints in aluminium frames is much worse than it first looks. I hope yours is not like that.

cheers

[Drake]

I wouldn't assume it wasn't terminal. It might have been going on for a long time.

You are right of cause. l shall know tomorrow when i start probing about.

[Drake]

the problem with any clean + paint job on an ally frame is getting rid of all the oxide. If there is any pitting then it will be very difficult to shift it.

To clean a pitted surface properly you may need to use a hot caustic soda solution after mechanical cleaning of some kind. A weak acid solution will passivate the freshly-cleaned surface.

When painting, use etch primer on a clean dry surface, then undercoat + top coats as necessary.

Very often I have found that corrosion near joints in aluminium frames is much worse than it first looks. I hope yours is not like that.

cheers

It's about 2.5" up from the BB housing on top of the downtube. Perhaps a stone chip that wasn't dealt with.
Don't have caustic soda, but have soda crystals. Any good ?
If not i'll caustic.

[Brucey]

I don't think soda crystals will touch it, but I guess you have little to lose by trying.

cheers

[breakwellmz]

the problem with any clean + paint job on an ally frame is getting rid of all the oxide. If there is any pitting then it will be very difficult to shift it.

To clean a pitted surface properly you may need to use a hot caustic soda solution after mechanical cleaning of some kind. A weak acid solution will passivate the freshly-cleaned surface.

When painting, use etch primer on a clean dry surface, then undercoat + top coats as necessary.

Very often I have found that corrosion near joints in aluminium frames is much worse than it first looks. I hope yours is not like that.

cheers

It's about 2.5" up from the BB housing on top of the downtube. Perhaps a stone chip that wasn't dealt with.
Don't have caustic soda, but have soda crystals. Any good ?
If not i'll caustic.

Hi.

Did you end up treating that frame, if so, how did it go?

[JohnW]

................Very often I have found that corrosion near joints in aluminium frames is much worse than it first looks..................

...............and than you say : "cheers"!

A contradiction in terms if ever I saw one Brucey

[Drake]

breakwellmz
Hopefully this weekend i will give it ago. Will post how i get on.

[Drake]

This one is perhaps more Bruceys territory.
I'm genuinely confused by various articles that i've read on other sites.
The bare surface of aluminium (it is said) oxidises, which produces a 'white' surface which in effect protects the material underneath.
Now i'm not in position to say whether this is correct or not, but what i can say is this.
All my working life was spent in a motor trade environment. During my early years, i've seen some pretty horrendous 'corrosion' to aluminium castings, particularly in cooling systems. With the introduction of corrosion inhibiters used in cooling systems, the failure of aluminium castings became less so.
l do appreciate that the castings in a cooling system are constantly in contact with water and perhaps a bicycle isn't.
So the implication from what i read was that if you were prepared to put up with an unsightly white deposit on your frame, it would protect the surface underneath.
But to my limited mind all corrosion is a bad thing and should be dealt with.
Your views would be appreciated, but not to much science please.

[twodogs]

Aluminium oxidises instantly as it's exposed to air, this is the coating which then protects it. When aluminium disolves away it's usualy for one of two reasons, 1, contact with another ferris metal causes annexchange of electrons resulting in electrolisis, this is common in cars, boats and busses and is controlled by separating the surfaces by way of a polyurathane film or like or by attaching a sacraficial anode which is normally made from an alloy of less stable metals such as zinc of magnesium and therefore corrodes instead of the structure. 2, aluminium disolves rapidly in alcaline liquids such as caustic, this is why it's not generally used in food manufacture due to the caustic used in cleaning for disolving fats and proteins.

One other way aluminium will corrode is when it is abraised while wet because the oxidised coating is continuously worn away and reformed causing pitting.

As has been said, clean up the surface, etch primer and then topcoat. If you wanted to clean the surface of a pitted diposite of oxide an acid is required such as phosphoric acid which is used in rust converter, etch primer and Coca Cola.

[Brucey]

Corrosion is a science all of its own; there are materials scientists whose entire career is based around the corrosion performance of just a few grades of material. But there are a few principles that are at work in all cases.

First thing; Aluminium always has an oxide layer on it. How can you tell? -because it isn't on fire!

Aluminium oxidises extremely readily, in much the same way as, say sodium does, but sodium doesn't form a coherent oxide layer, so once burning starts it carries on unabated. There is a lot of heat energy in finely divided aluminium; mix dry aluminium powder with iron oxide and you have the ingredients for thermit welding (look it up, it is pretty exciting!).

Corrosion reactions with water proceed in a 'cell' in much the same way a battery produces electricity. In the corrosion cell there are anodic and cathodic reactions underway. The metal loss occurs at the anode. If you leave (say) a droplet of water on a bare steel surface for a few hours you will see that the corrosion isn't uniform; that is because a single droplet is a complete cell, and different parts of the droplet are doing different things. If the anode area is small, you get what is called a 'corrosion pit' formed, of which more later.

Aluminium that still looks bright and shiny or at least solid (rather than 'furry') has a surface that is passive or 'passivated' which means that the oxide layer is sound and is limiting the rate at which the corrosion reaction is proceeding. Under such conditions you can breach the oxide layer and it will instantly re-form, and you won't see a load of furry oxide near the scratch.

Any time the oxidation on aluminium is white and furry it is (in the same way as steel goes brown and crumbly when it rusts) evidence that the corrosion reaction is well underway, and that at the bottom of the furry crud there is no longer a coherent oxide layer on the surface of the aluminium. This happens more easily when the aluminium is alloyed (the alloying elements weaken the oxide layer whenever they outcrop at the surface) and more easily when there are various ions e.g. Chloride ions (from road salt) present. The Cl ions are not consumed by the corrosion reaction, so one drop of salty water under the right (wrong!) conditions could in theory eventually consume an entire bike frame.

Corrosion in cooling systems is most destructive when it is pitting corrosion; in this event the anodic reaction is confined to a small area; the smaller the area, the stronger the concentration of certain chemical species becomes and the faster the anodic reaction occurs, thus concentrating the attack; in many materials once a pit starts it will carry on if the conditions are suitable. Inhibitors work by controlling the chemistry so that the corrosion reaction can't proceed easily; e.g. by controlling the acidity. The coolant needs to be changed on a regular basis simply because (amongst other things) the inhibitors in the coolant are simply consumed and eventually the corrosion reaction will re-start.

So on an Alu bike frame when you see white furriness it might be that you have some corrosion going on that is more or less harmless in the short term (i.e. the whole area is uniformly anodic) or it might be that there is pitting corrosion underway which is much more harmful. It has been my observation that anytime you get corrosion that causes the paint to bubble, you might be getting pitting corrosion, and that if the corrosion is near a weld, this is made all the more likely.

hth

cheers (*)

(*) I expect that they said that when they drank the Kool-Aid, too....

[breakwellmz]

Whilst you are in Aluminium corrosion mode, take a look at this-

This is the worst area of corrosion on a frame i would like to build up for winter use,there are a couple of other places as well.I can`t decide whether to just chuck it away(It cost me nothing)or put(Possibly)a lot of effort into stopping further corrosion.Am i just going to be`chasing my own tail`?

Cheers

[twodogs]

^^I think it would clean up good as new.

[Brucey]

it looks like 'a Giant problem' to me...

but yes, I think you could fix that; just chemically clean, use etch primer, and a decent topcoat.

cheers