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After reading about how complex is to paint a bicycle frame, I wonder, why is this done at all? Steel frame obviously needs painting as it may rust. Aluminum or say titanium does not rust in similar ways and the color is not much worse than black. Are there only aesthetic reasons for painting or this is actually useful in other ways?

nightrider
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    Aluminium can corrode, especially when in contact with other metals. – Michael Mar 06 '23 at 08:03
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    I don’t think the painting process is that complicated. At worst, it’s a layer of primer followed by a layer of paint. Automated electrostatic painting systems greatly reduce the amount of labor required. – MaplePanda Mar 06 '23 at 10:52
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    Because the color is the most important coimponent on a bike. – whatsisname Mar 06 '23 at 17:40
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    because red bikes go faster – njzk2 Mar 07 '23 at 18:56
  • What makes you think corrosion is the most - or even any - important feature?

    Are you suggesting that once five identical bikes are built, their colours won't change the minds of any potential buyers?

     – Robbie Goodwin Mar 07 '23 at 23:47
  • Matt aluminum surface would simply look for me as a good color on its own. But if it corrodes, understandable why it is painted. – nightrider Mar 08 '23 at 06:21
  • This isn't worth a full answer. In principle, aluminum could be anodized, which is the process of changing the thickness of the surface oxide layer with electricity. This can be combined with dyes. I think that for bikes, this is usually the case, but I am pretty sure you could just have undyed, anodized, clear coated aluminum, which would protect against rust. It might look not exciting, but it is possible in principle. – Weiwen Ng Mar 08 '23 at 22:02
  • "You can have your bike any color you want so long as it's black." They're painted because people like choice & individuality. Even if the choice is to look exactly like everyone else who wants to like they're a member of your favorite team. – FreeMan Mar 10 '23 at 19:06

3 Answers3

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Not only does aluminum oxidize, it can completely decompose. It is not just some surface film, the structural integrity can be broken and the part can become fragile.

This is well known by climbers and especially cavers, because carabiners or bolt hangers oxidized to a significant degree are a common sight in certain regions (depending on the chemical composition of the rain, the rock and closeness to sea and also on temperature - in Alpine caves and cold mountain walls they do not rust too fast).

A badly corroded carabiner
https://www.scavalon.be/avalonuk/discov/emotions_corrosion.htm

It is vital to protect the metal of climbing items that are left outdoors or in a cave if possible, by painting or by anodising. Many bikes are not left outside, but still are brought home wet or even contaminated by road salt and the galvanic corrosion is a danger for both the climbing equipment and bicycles.

That said, some surfaces do often feature naked aluminium alloys, most often the rims, especially with rim brakes. Disc brake rims are often painted as well as the rest of the bike. One should just be aware that corrosion can happen, but bikes that are taken care of or stored inside should be fine. The most danger there is probably in damp hot areas at the sea.

DavidW
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Vladimir F Героям слава
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    That picture is a nice example of long-term exposure of a carabiner in a hostile (moist and salt-laden) environment. A good case in point that aluminum CAN corrode heavily when the conditions are right and its Al2O3 layer is unable to defend the aluminum underneath. It is a good thing it usually is not that bad, otherwise we never would have ridden aluminum rims or Al frames/components either. – Ted Hohl Mar 06 '23 at 15:57
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    Even before serious corrosion sets in, aluminium starts to look scruffy very quickly. Paint keeps it looking nice. – chasly - supports Monica Mar 06 '23 at 18:50
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    Indoor trainer rides create more of a moist and salt-laden environment than the average outdoor rides; it's easily mitigated by taking care of your bike, but aluminum components (handlebars) can be at elevated risk of corrosion on a heavily-used trainer bike. – Ealhmund Mar 07 '23 at 14:57
  • "Many bikes are not left outside". really? – njzk2 Mar 07 '23 at 18:57
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    @njzk2 Really. And many are. – Vladimir F Героям слава Mar 07 '23 at 18:59
  • Oxidation reactions are exothermic, so actually cool damp areas near the sea are worst (I know, here on the west coast of Scotland!). If you have a car, you'll know that exhaust pipes tend to fail at the far end rather than near the engine, and that's for the same reason. Obviously when it gets cool enough for the damp to freeze out, it becomes less of a problem. – Toby Speight Mar 08 '23 at 16:51
  • @TobySpeight But even exothermic reactions have positive activation energy so according to the Arrhenius law the reaction rate increases with the temperature. For aluminium specifically I found https://www.jstor.org/stable/2413747 with Ea~5kcal/mol. but the oxidation rate depends also on the thickness of the film. – Vladimir F Героям слава Mar 09 '23 at 10:29
  • I meant to mention that - yes, the reaction slows down in the cold and in very hot environments. I didn't have enough knowledge to know where the peak rate lies, nor the time to look it up. Thanks @Vladimir. – Toby Speight Mar 09 '23 at 11:37
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The other answers (to date) explain the very good engineering reasons to paint aluminum. However, cosmetics is a huge part of bike sales, especially the upsell and 'this year's model' game. Many bikes get no more than a color change between model years, and few bikes do not get a color change even with major component upgrades. Without the color change, the component upgrades would lose value on the shop floor. Many (I suspect most) bicycle buyers are interested in how their bike looks. Two types - the casual who wants a nice-looking bike, and 'One of those' types - when he (it's mostly 'he's') takes on its first ride, will his mates swoon over the new model - hopefully enough he has to wipe the drool off the bike.

I imagine even if aluminum did not benefit from painting, most bikes would still be painted.

Toby Speight
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mattnz
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Adding to Michael's comment, it should also be noted that aluminum, in fact, oxidizes as any other metal, with the notable difference that its oxides create a film that prevents the aluminum underneath to corrode and stick quite well to its surface. Al corrosion can happen in many ways.

  1. Electrochemically, i.e. when in touch with other metals (this is called Galvanic corrosion and is what Michael was talking about.
  2. If the atmosphere is acidic, it can suffer from pitting corrosion which is not harmful to the mechanical properties of the frame but can cause aesthetic issues.
  3. Also areas around holes or subjected to mechanical stress can host pockets of water that can harm your frame. There are other ways in which Al can corrode, however in my opinion these might be the most important ones.
Cadoiz
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Enrico Pascatti
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    +1 for noting that an aluminum oxide layer will inhibit further corrosion (because it prevents water and oxygen from reaching the raw aluminum beneath it). This is the case for many environments. When close to the ocean/salt water, the the high-salt content in the environment can attack the Al2O3 layer local environment changes and the protection it offered the aluminum underneath. – Ted Hohl Mar 06 '23 at 15:49