Who said aluminium .....

[02GF74]

...... is the bees knees and does not rust? Well maybe it is, but it does corrode, as seen here.

Nothing structural, a quick sanding and painting with non matching blue hammerite coz that is all I had to hand sorted it out.

 

[gbb]

You've found the classic misconception about aluminium 02G...no it doesnt rust, but corrode it definately does.
Ive had the same on my rear stays of a previous bike, and on the alloy steerer of a pair of carbon forks., very slightly peeling the carbon from the alloy.
Quick action will usually stop it. I'd urge anyone to check regularly ALL parts of the bike, and treat it as soon as you see it.

 

[Mr Pig]

What's the best way to treat it? I painted the forks on my son's bike, looked great, but the paint is bubbling again.

 

[02GF74]

I painted it with etch primer - sorry - forgot to mention that - otherwsie paint doesn't stikc too well to aluminium; and then painted it with hammerite. yesterday was very warm so paint took no time at all to dry.

 

[Night Train]

One option would be to let it corrode. Aluminium is one of those metals where the surface oxidation protects the base metal from corroding further. Wiki link.
However, it isn't a good look for a bike.

 

[tyred]

You would definitely need etch primer or some aluminium specific primer as paint has very poor adhesion to it.

 

[Mr Pig]

I used primer for cars, which would explain it.

 

[Alembicbassman]

My science teacher always used as an example the exterior of the aluminium framed windows of the classroom. These were covered in oxide and had been there 30+ years, the oxide is very good protection against further corrosion, but there are obviously no fatigue points on a window frame.

There doesn't seem to be any paint inside bike tubes, looking down my seat tube it's just bare aluminium. I guess the oxidation that occurs here protects the frame from internal corrosion.

 

[Piemaster]

Stainless steel works in the same way, layer of oxides protect the underlying metal.
There are of course many different ways of alloying both alu and SS which can produce different characteristics and corrosion resistance. A mild steel is also an alloy of iron and carbon, when it 'rusts' it is just returning back to its component parts.

 

[andrew-the-tortoise]

Anodised aluminium products already have a thin layer of oxide to prevent further corrosion.

 

[brodie]

Bare alu does form an oxide layer on its surface which protects it from further oxidation. Anodising is just al oxide applied by the manufacturer. But this is different from damaging corrosion, usually from salty water, which turns aluminium into a white powder. I once bought a pair of old handlebars, the bar tape was still on it and when I removed it, the metal underneath had white powdery patches. The bars were not usable as it was quite deeply pitted, I think it was caused by sweat soaking into the bar tape.

 

[Chris James]

Bare alu does form an oxide layer on its surface which protects it from further oxidation. Anodising is just al oxide applied by the manufacturer. But this is different from damaging corrosion, usually from salty water, which turns aluminium into a white powder. I once bought a pair of old handlebars, the bar tape was still on it and when I removed it, the metal underneath had white powdery patches. The bars were not usable as it was quite deeply pitted, I think it was caused by sweat soaking into the bar tape.

Indeed, aluminium and stainless steels both suffer from localised corrosion in the presence of chloride ions.

For those of you with white powder on your frames, the powder is providing no protection whatsoever. Aluminium oxide passive layers are very thin and adherent, giving the surface a matt grey colour (think of Trangia pans).

White powders are different corrosion products - chlorides and sulphates IIRC - and they don't protect the frame from further corrosion.

 

[Rockus]

Man all that science hurts the head. Many a steel bike i have abused over the years and they all took it stoically, cant really say th same for my foray -or is it folly? into aluminium bikes.

Generally they're cheaper than steel. In this modern comsumer world of ours 'technology' allows people to sell more inferior products.

 

[02GF74]

Generally they're cheaper than steel. In this modern comsumer world of ours 'technology' allows people to sell more inferior products.

if that is the case, why are all the cheap argos/halfords/tesco/ebvay bieks made from steel?

Pretty sure ther price of raw aluminium is more than steel, and it is much harder to make a nice looking framefrom aluminium thant steel whcih pushes the price up.

 

[tyred]

if that is the case, why are all the cheap argos/halfords/tesco/ebvay bieks made from steel?

Steel isn't just one simple alloy. There are many different kinds and the type used to make an Argos special is vastly different from what's used in a top of the range Mercian. Notice how a cheap MTB rusts quite quickly if the paint is damaged whereas you very rarely, if ever, see a 531 frame with serious corrosion, even if it is 50 year old.