Advice for corroded aluminium frame head tube

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Magik_
Posts: 17
Joined: 26 Mar 2019, 11:09am

Advice for corroded aluminium frame head tube

Post by Magik_ »

Hi,

Upon careful inspection of my bike, I noticed some paint bubbling in the head tube area. I was able to get some of the paint off from where the fork attaches to the frame using just my fingernails and found white powder under it. From what I read this could not be a big deal for aluminium frames, but it is a problem if there is pitted corrosion. I took some photos before swiping the powder and after as well as some under a magnifying glass and attached them here.

This is the only place on the frame I could find that has any sort of weakness signs. The paint above it seems fairly well sticking to the frame and does not scrape off.

Is this pitted corrosion? Do you think this is fixable by scraping/cleaning and repainting with etch primer and topcoat paint?

Cheers,
Damjan
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Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Advice for corroded aluminium frame head tube

Post by Brucey »

if you want to kill that corrosion (and I'd suggest that yes you do) you need to take back the paint until the metal beneath isn't at all white and powdery. You then need to remove the white powdery stuff completely and carefully inspect the surface. If you are lucky there are no pits or cracks. If you are unlucky it will be different to that.

To clean and paint properly, you will probably have to remove the lower headset race. To render the surface chemically clean something caustic is required, and then primers and topcoat.

cheers
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Magik_
Posts: 17
Joined: 26 Mar 2019, 11:09am

Re: Advice for corroded aluminium frame head tube

Post by Magik_ »

Okay, thanks. I'm going to fully disassemble it over the weekend and check it out some more. Any suggestions on what to use to get more paint off? I heard stuff like steel wool can contaminate the aluminium?

Also, I have never seen corrosion pits on bike frames, if someone has any photo examples it would be useful to see.
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Advice for corroded aluminium frame head tube

Post by Brucey »

occasionally you see pitting in a line that follows the weld path, but is offset from it. Pitting is normally visible to the naked eye.

You can do a lot of good with a small stainless steel wire brush. The paint needs to be feathered back if you want a good paint finish; any paint that you can flake off with your fingernails should be removed first.

cheers
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Jamesh
Posts: 2963
Joined: 2 Jan 2017, 5:56pm

Re: Advice for corroded aluminium frame head tube

Post by Jamesh »

I would use fine sandpaper or a foam sandpaper block.
Wipe off dust and paint with aluminium primer.
Plastocote do little tins of enamel paint if your not after a perfect finish.

Cheers James
silvar
Posts: 5
Joined: 31 Mar 2019, 5:53pm

Re: Advice for corroded aluminium frame head tube

Post by silvar »

Stainless steel causes galvanic corrosion on aluminium.
The larger the ratio stainless steel / aluminium contactsurface, the higher the electrical current density, and the worser the corrosion
The corrosion exhibits itself as aluminium that desintegrates into a white powder.
And needs an electricity conducting medium.
tim-b
Posts: 2093
Joined: 10 Oct 2009, 8:20am

Re: Advice for corroded aluminium frame head tube

Post by tim-b »

Hi
Ally corrosion begins immediately it's exposed to the environment. To a certain extent the corrosion will protect against further corrosion, but yours has gone beyond that happy place
It depends what that frame means to you, but IMHO yours will need lots of work if you're going to keep it in looking nice for any length of time. If you're not so bothered then Hammerite make a paint for aluminium (link) that would be worth trying at reasonable cost
The underside of the downtube/headtube joint can be quite a stressed area on a bike; be careful of corrosion in that area and think carefully about whether it's worth the effort and expense
Regards
tim-b
~~~~¯\(ツ)/¯~~~~
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Advice for corroded aluminium frame head tube

Post by Brucey »

FWIW there is probably a good example of pitting corrosion on the headset. Dura-Ace headsets (in fact most parts, not just headsets) are anodised. When you get dark marks on anodised surfaces like that, it almost invariably means that this is corrosion damage, caused by winter use of the bike (road salt) and no washing. This would tally with the damage to the paintwork and the corrosion beneath it, too.

It really doesn't take much of this kind of damage to cause a savvy buyer to 'walk away' from a prospective purchase; a vendor won't thank you for flaking the paint off the corroded areas (they would usually rather flog the thing to someone that hasn't noticed...), but you won't be able to see if the frame is damaged or not unless you do this.

More worrying still, this kind of damage is exactly the kind of thing that can cause a bonded fork (eg aluminium crown, CF blades) to literally come apart at the seams.

cheers
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Magik_
Posts: 17
Joined: 26 Mar 2019, 11:09am

Re: Advice for corroded aluminium frame head tube

Post by Magik_ »

Thanks for the replies. Yeah, it doesn't sound good. I really have to see what it looks like under the racer and decide from there.
Magik_
Posts: 17
Joined: 26 Mar 2019, 11:09am

Re: Advice for corroded aluminium frame head tube

Post by Magik_ »

UPDATE: After stripping the bike down and inspecting the head tube, I found what looks like a terrible break/melt area in the top part of the head tube (one photo is the view from the bottom up showing the normal hole).

I believe the break coincides with the staining on the fork steerer.

Time to bury this Principia beauty?
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Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: Advice for corroded aluminium frame head tube

Post by Brucey »

the inside of the head tube looks 'perfectly normal' to me. This is, 'perfectly normal' for a welded aluminium frame; the back beads always look terrible and odd edges melt during welding (the thermal diffusivity is high enough that heat runs well ahead of the weld) . I can't be sure from photos that there isn't a crack but I don't see one anyway. The main reason these scabby messes don't instantly start cracking is that they oughtn't see much service stress; this tends to be seen mostly nearer the outside surfaces of fillet welds.

The dark mark on the steerer tube looks like corrosion, possibly from salty water leaking inbetween the headset spacers. The main problem with corrosion at this point is that it will both help start and help hide a crack, or pitting. You ought to use something to polish the dark area so that any cracks/pits can be seen, and/or have the steerer and frame dye penetrant tested. Any place where the paint is lifting, the paint needs to be removed before dye penetrant testing.

The consequences of a crack in the steerer tube or a disbond in the fork are potentially far worse than a crack in the main frame; I'd be at least as worried about that tbh. If you have the slightest doubt about the fork, file it under 'B' for 'Bin'.

cheers
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Magik_
Posts: 17
Joined: 26 Mar 2019, 11:09am

Re: Advice for corroded aluminium frame head tube

Post by Magik_ »

Excellent news Brucey, thank you! In the back of my head, I was hoping it might be normal because the welds look so nice on the outside. I still figured if the bottom hole is nicely round that the top one should be too + that melting surface seemed awfully wrong; I guess since it's where the two tubes connect it gets this messy.
Well, in that case, I still have work to do tomorrow polishing this thing and looking further for imperfections in both the frame and the fork!
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