Frame Damage

Gussio
Gussio Posts: 2,452
edited June 2007 in Commuting chat
At the weekend I cleaned my commuting bike, which is based around a Specialized aluminium frame. Underneath the bottom bracket a bit of paint the size of a 5p piece flaked off to reveal white, powdery corrosion. The frame is around six years old and used every day on London roads. Does anybody have a view on whether the corrosion could surface-only (i.e. nothing much to worry about) or whether it might be symptomatic of something more serious, like a fracture? Short of sanding back the paint, is there another way of investigating?

Comments

  • neslon
    neslon Posts: 54
    I was under the impression that because alu is so reactive, it very quickly forms an impenetrable layer of whitish oxide, which prevents further penetration (which is why alu window frames etc last forever).
  • peejay78
    peejay78 Posts: 3,378
    and land rovers.
    winter: http://tinyurl.com/2xkbbs
    summer: http://tinyurl.com/2hsagv
    http://traumradfahren.wordpress.com/
  • Gussio
    Gussio Posts: 2,452
    Nelson - Wikipedia backs up your theory:

    "Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of alumina quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation."

    Sanding away the the coating of oxide for investigation purposes might not be a good thing, if it exposes more of the metal to the oxidation process. Guess that I will keep on riding up to the point of catestrophic failure!
  • el_presidente
    el_presidente Posts: 1,963
    Gussio this sounds highly dangerous to me. I recommed you need a very expensive new bike immediately.
    <a>road</a>
  • Gussio
    Gussio Posts: 2,452
    el_preidente I bow to your wisdom and intend to spend the rest of my "working" day visiting cool bike websites :-)
  • OrbitRider
    OrbitRider Posts: 78
    Gussio,

    Unfortunately the corrosion situation is not as simple as that. The oxide coating on aluminium forms almost instantly the metal is exposed to air and forms an impermeable barrier to oxygen. If oxygen could penetrate the oxide layer, then your entire bike frame (with no paint) would collapse to a pile of white dust in a few of hours (not joking!). There are various substances and conditions that can cause the oxide layer to break down: mixed metals in electrical contact (galvanic reaction) in the presence of water will do it for one, or acids or alkalis for another. From your description it sounds like water was trapped under loose paint. That water could easily pickup acid or alkali materials (or salt!) and cause accelerated corrosion of the ali. Your best bet is to make sure you have removed all the loose paint, make sure it is absolutely dry and then repaint. Put on several layers to ensure there are no "pinholes" in the paint. While you're at it, make sure the bottom bracket welds are sound.
    FCN 7 (4 weekdays)
    FCN 11 (1 weekday)

    There is an old cyclist called Leigh (not me!)
    Who's pedalling's a blur to see
    So fast is his action
    The Lorenz Contraction
    Shortens his bike to a "T"
  • Gussio
    Gussio Posts: 2,452
    OrbitRider - thanks for the knowledgeable advice. Would you suggest using any sort of specialist primer or base layer paint, prior to several coats of top cover?
  • OrbitRider
    OrbitRider Posts: 78
    Gussio,

    Sorry about the delay in replying: had to run after my post. I can't recommend any particular primer: the main consideration is to ensure that the bare metal is scrupulously clean. Clean off the loose paint until there is no sign of corrosion. Take some emery or wire wool and clean the metal until all sign of corrosion has been removed. Wash the area thoroughly with white spirit or methylated spirit to remove all dirt and grease (you could use an old toothbrush soaked in meths to scrub the surface). This should provide a good surface for the paint. I suggest you use a metal primer appropriate for the top coats (enamel?). Anti-corrosion paints are probably not much use on aluminium.
    Hope this helps.
    FCN 7 (4 weekdays)
    FCN 11 (1 weekday)

    There is an old cyclist called Leigh (not me!)
    Who's pedalling's a blur to see
    So fast is his action
    The Lorenz Contraction
    Shortens his bike to a "T"
  • Brains
    Brains Posts: 1,732
    Thorn recommend touching up with those small tins of Humbrol (sp?) paint that you can buy in model shops
  • Bong
    Bong Posts: 11
    Gussio, in addition to Orbitriders advice above, make sure you thoroughly rinse the area with clean tap water to remove any residual salts after abrading. A final rinse/wipe with meths will help to dry. Lightly abrade the surrounding good coating that you will overlap onto. Ideally you would use an etch primer to assist adhesion of the next coat to the aluminium, which could also be a primer coat for metals. The etch primer coat is very thin and promotes bonding/adhesion. The Hammerite web site (part of Dulux) shows a special metals primer for non-ferrous metals including aluminium. If you go this route, you might have to use all Hammerite products. I have heard the Humbrol type modelling paints are good for touch up of bikes but you need to test/check that one coating, if not part of the same system, is compatible with the next or it might just peel off.

    Aluminium genrally has good corrosion resistance but the oxides formed have a much larger volume than the metal from which they are formed. This gives large "blisters" full of white powder. Most coatings on aluminium are for decorative purposes only to keep it looking good. Otherwise it develops a matt grey appearance. As the cororsion was at the bottom of your bottom bracket it has probably started after the paint was chipped/damaged. As it is out of sight, you could use a blob of araldite to protect it from further corrosion.

    Good luck.

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