Corrosion in Aluminium Frames

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Corrosion in Aluminium Frames

Postby Bicycler » Wed Dec 04, 2013 3:44 pm

Is it a real problem? What can be done to prevent it?

I read in another thread "Is it a generational thing?" about corrosion being a problem with aluminium bike frames. In my ignorance I had always harboured the notion that aluminium oxidized on its surface but did not corrode through like iron or steel. I own an Aluminium bike, is there some precaution I should be taking? Does it need Waxoyling internally like I have heard of people doing with steel frames?

The other issue raised in that thread was Aluminium's lack of 'fatigue limit'. Could someone please explain this term and suggest how long an Al frame might be expected to last?

PS. I'm new to posting on here so please be nice and excuse my ignorance
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Re: Corrosion in Aluminium Frames

Postby Vorpal » Wed Dec 04, 2013 4:08 pm

Wikipedia holds the answers...
http://en.wikipedia.org/wiki/Aluminum
http://en.wikipedia.org/wiki/Fatigue_limit

All it really means is that
1) Aluminium can corrode, but once it starts, the corrosion usually protects the material underneath (as long as it is not in contact with a different metal)
2) More care must be taken with aluminum not to put too much stress on the material; this is why aluminum frames are thicker than steel ones

Aluminium is also much harder to weld. And welding adds a point where both stress and corrosion are more likely.

I would say that aluminium frames may not be appropriate for some applications. Thousands of miles of heavily loaded touring is one of those applications.

However, aluminium is fine for most cycling applications. I have an aluminium bike which I use for commuting and occasional utility use or light touring. For most of last year, it was my primary means of transport.

The design of the frame should have accounted for years / thousands fo miles of riding, and with a wide range of weights, surfaces, etc., so unless it is abused, it will last for years / thousands of miles. The only precaution I would recommend under normal usage is to occasionally check the frame for cracks.

I expect my aluminium bicycle to last for as many years as I want to ride it.
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Re: Corrosion in Aluminium Frames

Postby Valbrona » Wed Dec 04, 2013 5:27 pm

Water tends not to seriously rot aluminium. But water and road salt can, and especially in the case of an aluminium bottom bracket shell. The real 'killer' can be salt water getting down the seat tube after being whipped up by the rear wheel - assuming no mudguards. Regular grease is no good in bottom bracket shells, you need to use anti-seize copper grease, and especially on the threads, assuming a traditional threaded BB shell.
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Re: Corrosion in Aluminium Frames

Postby Vorpal » Wed Dec 04, 2013 7:57 pm

Valbrona wrote:Water tends not to seriously rot aluminium. But water and road salt can, and especially in the case of an aluminium bottom bracket shell. The real 'killer' can be salt water getting down the seat tube after being whipped up by the rear wheel - assuming no mudguards. Regular grease is no good in bottom bracket shells, you need to use anti-seize copper grease, and especially on the threads, assuming a traditional threaded BB shell.


Please don't ever mix copper & aluminium. That causes galvanic corrosion!
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Re: Corrosion in Aluminium Frames

Postby meic » Wed Dec 04, 2013 8:00 pm

Hmmn,

I first started using copper grease specifically because it stopped steel bolts seizing up in aluminium motorcycle engines.

Copper grease exists in my garage for steel in aluminium.
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Re: Corrosion in Aluminium Frames

Postby Kenn » Wed Dec 04, 2013 8:06 pm

Electrolytic corrosion takes place between dissimilar metals in an electrolyte (salt water). A hot engine should be dry so no electrolyte would be present.
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Re: Corrosion in Aluminium Frames

Postby Vorpal » Wed Dec 04, 2013 8:10 pm

Well, maybe I remembered some misinformation at some point, then. I was told some years ago to use only teflon or silicon with alumnium, and not copper or graphite.
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Re: Corrosion in Aluminium Frames

Postby meic » Wed Dec 04, 2013 8:13 pm

The average vehicle is supposedly parked for 90% of the time.
Even as a motorcycle courier they were being driven for less than half the time.
They soon get cold and wet and stay that way until fired up again.

Not that any of that matters, before copper grease bolts seized and snapped. Now all of my machines which are fitted with copper grease undo easily despite now being ten times as old.
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Re: Corrosion in Aluminium Frames

Postby Brucey » Wed Dec 04, 2013 8:31 pm

Ordinary greases use soaps as thickeners. The soaps are usually based on alkali metal salts; these can react with water and metals to form a highly effective adhesive, even if there isn't that much corrosion going on.

Good copper greases use clay thickeners not soaps, and have a high percentage of copper (and often graphite too).

There are different anti-seize materials that are optimal for different applications. However very few of them protect against corrosion by being a sacrifical anode to both steel and aluminium. To get that effect you need to use a zinc-based anti-seize.

Some folk will say you must use a Ni-based anti-seize on aluminium to steel joints to avoid galvanic effects, but this logic seems flawed to me; Ni is positioned similarly to Cu in the galvanic series. Maybe if any corrosion does occur it doesn't form such acidic compounds so easily or something.

All I can say is that nothing I've ever assembled with lots of good quality copper-based anti-seize -hot (120C) or cold, wet or dry- has given me much grief later on, but plenty of other stuff that got assembled without it has done so.

I note with interest that Rocol copper based anti-seize is approved for use by the Royal Navy. The application notes don't exclude some metal combinations and recommend others as being suitable; perhaps they are incomplete, but then again it might be that is works OK in lots of different cases.

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Re: Corrosion in Aluminium Frames

Postby Bicycler » Wed Dec 04, 2013 9:12 pm

I always just used the same grease for everything that needed greasing on my bike. So is the consensus to use special anti-seize grease for seatposts, threads etc. ?
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Re: Corrosion in Aluminium Frames

Postby reohn2 » Wed Dec 04, 2013 10:26 pm

Bicycler wrote:I always just used the same grease for everything that needed greasing on my bike. So is the consensus to use special anti-seize grease for seatposts, threads etc. ?

Yes,in a word,and copper grease fits the bill perfectly :)
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Re: Corrosion in Aluminium Frames

Postby Valbrona » Thu Dec 05, 2013 1:22 am

Bicycler wrote:I always just used the same grease for everything that needed greasing on my bike. So is the consensus to use special anti-seize grease for seatposts, threads etc. ?


No.

Use a good quality PTFE/Teflon, ceramic or lithium grease for bearings, like those found in headsets, bottom brackets and hubs. Use a copper grease for threaded assemblies, especially on the threads inside bottom bracket shells. Use a spray-on anti-seize compound if you are using a bare metal seatpost in a metal seat tube, or an old fashioned quill stem.
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Re: Corrosion in Aluminium Frames

Postby Valbrona » Thu Dec 05, 2013 1:23 am

....
Last edited by Valbrona on Thu Dec 05, 2013 1:00 pm, edited 1 time in total.
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Re: Corrosion in Aluminium Frames

Postby Urticaria » Thu Dec 05, 2013 1:42 am

Vorpal wrote:1) Aluminium can corrode, but once it starts, the corrosion usually protects the material underneath (as long as it is not in contact with a different metal)

Dave W posted a link with this in...
http://www.pardo.net/bike/pic/fail-001/FAIL-174.html

The seat tube had had water in it.
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Re: Corrosion in Aluminium Frames

Postby Brucey » Thu Dec 05, 2013 3:42 am

Urticaria wrote:
Vorpal wrote:1) Aluminium can corrode, but once it starts, the corrosion usually protects the material underneath (as long as it is not in contact with a different metal)

Dave W posted a link with this in...
http://www.pardo.net/bike/pic/fail-001/FAIL-174.html

The seat tube had had water in it.


If the conditions are corrosive enough then you can get agressive local attack known as 'pitting corrosion' in many different materials. Thermodynamically speaking, Aluminium would far sooner be an oxide than a shiny metal; it usually resists further oxidation only because the oxide film that forms can act as a diffusion barrier to further oxidation. Once pitting corrosion or other concerted attack starts then this protection is lost.

Aluminium powder burns (oxidises) extremely vigorously in air, and will reduce iron oxide to form molten iron; this is the reaction that takes place in 'thermit welding'. In the (fortunately unlikely) event that your Al bike frame spontaneously and completely oxidised, the resultant puddle of molten aluminium would be quite hot enough to melt it's way through (say) the deck of steel ship, for example.

In the Pardo link it is hypothesised that the corrosion (which is strictly not 'rust'; only iron compounds 'rust' per se, everything else just oxidises) is too far away from the weld to be associated with the weld HAZ. Not so; the thermal diffusivity of aluminium alloys is high and the heat input required for a BB cluster weld is also high. A region that sees temperatures of ~250C in a heat treatable alloy will typically be grossly 'over-aged' during welding and this can then be associated with a local corrosion attack by becoming anodic or cathodic wrt the nearby material. Hence (although it is not possible to say that this happened in that case) this kind of corrosion can be found 1/2" or more away from the weld line in high heat input / preheated welds.

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