Hi Folks,

I'd be grateful to anyone with a knowledgeable opinion who might have been in a similar position in the past regarding the corrosion of my bike's frame. The bike in question is approx 5 years old, 7005 series aluminium alloy road frame. It was "entry level" when I bought the bike new at the beginning of 2001, has now covered more than 12,000 miles and is a reliable everyday high speed commuter.

Whilst cleaning the bike I noticed that there are several places on the frame where corrosion has started the paint to blister. In particular this is noticable where the downtube meets the underside of the bottom bracket, at the front derailleur mounting point on the seat tube and where the chainstays meet the rear dropouts. The fork is cromoly steel and is in very good condition.

Obviously, if I leave things as they are, the paint will continue to lift and the corrosion spread. I'd like to strip and repaint the bike (something I have done before with steel frames) to get a new lease of life from the bike. I'm tempted to think it is worthwhile stripping back one or two test areas where the problem is most noticable to check the extent of damage/pitting in the tube material. Any thoughts anyone? Also any advice on products for painting alloy, particularly primers that would offer protection?

All thoughts welcomed,

Cheers,

Ed

I usually work with 6061 Aluminum, and don't think Aluminum can corrode (at least not like steel does!). Exposed aluminum rapidly forms an oxide. This oxide coating is what protects the aluminum.

I have an aluminum canoe, used in both fresh and salt water, which shows no signs of corrosion after 30 years of being stored outdoors!

Exactly what is happening under the paint on your bike?

From the internet: 7XXX. Alu-Zinc when added to aluminum with magnesium and copper permits precipitation hardening and produces the highest strength heat-treatable aluminum alloy. These alloys are primarily used in the aircraft industry, armored vehicles and bike frames. The weldability of the 7XXX series is compromised in higher copper grades, as many of these grades are crack sensitive (due to wide melting ranges and low solidus melting temperatures.) And susceptible to stress corrosion cracking. Grades 7005 and 7039 are weldable with 5XXX fillers.

7xxx Crack Sensitivity: The 7xxx Al-Zn-Mg alloys (typically welded with 5356 avoid 4043) resist hot cracking better than the 7xxx Al-Zn-Mg-Cu alloys.

Interesting article regarding Aluminum in automobiles:

https://www.autoaluminum.org/downloads/corpub.pdf

Sorry I can't offer more help, I'm not that familiar with the 7XXX series.

Can you take a photo and post it, or PM it to me? Maybe someone else at my work can ID the problem.

Once you have her all stripped and cleaned, use a Zinc Chromate primer. It's used throughout the aircraft industry as a primer for planes. Ever see a rusted out 747? Seriously though....use the primer I mentioned and as for your other issue, here's a link to a site that will explain it, saves me time typing out an answer for you.
https://www.key-to-metals.com/Article14.htm

Cracks in the paint of aluminum frames is more indicative of a cracked or broken frame then of corrosion. If your locale uses salt or Mg chloride on the roads, Ed, you could have corrosion but that's a long shot. I'd check it for cracks first.

Thanks very much everyone for their input. The linked articles make interesting reading.

I will try and get some pictures if I find time here at work (ahem!). In the mean time, to qualify the appearence of the affected areas, the paint has a bubbled,blistered appearence rather than cracking in the finish. It feels rough to the touch, as if swelled by hard corrosion products formed beneath the paint.

cyccommute: yes the bike has been used throughout the winter on roads that are salt treated.

I'm expecting to find alumina type powdery deposits beneath the lifting paint. If this is the case, they will be capable of retaining moisture and salts that can contribute to electrolytic corrosion, allowing it to continue for some time after the rest of the bike has dried out.

It looks like my best plan is to assess the problem areas, make a decision on the integrity of the frame. Hopefully the condition will justify refinishing with appropriate prep and coatings.

thanks again,

Ed

If salt corrosion is the case, it might be best to remove the paint from that area and just let it be exposed. I suspect a flaw in the paint that allowed water to get in and carry the salt along with it. I'd buff it with a dremel and feather the edges of the existing paint around the area so that you don't open up another can of worms.

The problem with salts (particularly MgChloride) is that they absorb water from the air and just continue to work on the metal. I'm not sure of chemistry exactly but I'd suspect the chloride ions are attacking the aluminum to form aluminum chloride and perhaps a hydroxide ion. The hydroxide ion would dissolve more metal. Here, where the relative humidity is a nice dry 20% or less, there isn't enough water in the air for the process to be very fast. When you have a high relative humidity, the process could be very fast indeed. Washing the outside of the bike would just drive the salt deeper in and add more moisture which you don't need

Simple Green attacks AL too; read the aircraft cleaning post: https://consumer.simplegreen.com/cons_faqs.php

https://www.chinook-helicopter.com/ma.../cleaners.html

https://www.velonews.com/tech/report/...es/9216.0.html

I managed to take a couple of pictures, although it is hard to capture the appearance faithfully.

When I feel like wrecking the paint job completely, I'll have a careful dig through it to see what has happened to the metal underneath.

Cheers again,

Ed

I still can't tell from those pics, but the frame looks like a steel frame with rust bubbling the paint.

Ed,

Even the cheapest aluminium doesn't rust. Alu is a self-protecting metal, i.e. the oxyde that forms on the surface of the metal acts as a protecting coat for the sane metal underneath. You can even dip a piece of alu in sea water for a month, it won't rust. It takes aggressive chemicals not usually found in nature, acids and bases, to attack a piece of aluminium to the core. Also, aluminium oxyde forms almost immediately, within minutes of aluminium being expose to oxygen, it's dull white and it doesn't change volume. Only iron changes volume (expands) when it turns to rust.

In short, if the paint slowly bubbles up because of corrosion, you have a steel frame. to make sure, just chip the bubble of pain with a nail and see if it's reddish brown underneath. If you're afraid of doing that, check with a magnet: it'll only stick to a steel frame.

Awfully skinny looking tubing for an Aluminum bike.

Your ft der looks a mess also. It is really oxidized, look at the clamp. Time for a new bike.


Tim

Why? if the guy doesn't want a new bike and this one is still serviceable, I say cover the rusty bits with nail polish or clear varnish (5 buck spray can, lasts forever) and check every 6 months for additional rust to cover. Cheap and effective, if not very pretty.

What kind of bike is it?

Certainly looks like a rusting steel frame to me. Definitely not 7005 aluminum.

OK, things seem to have got a little mixed up. No problem

In reply to Froze, PPC and supcom, the frame is most definitely 7005 Al alloy. I have owned the bike from new, and in a couple of other places that have suffered paint damage, such as cable rub on the head tube, bare metal is exposed. There is no red rust formation as would be expected on a steel bike with scratched paint & magnets dont stick to the frame.

At the points where the paint is blistered, there is a powdery white deposit - I'd stake my reputation (FWIW on this being an aluminium oxide formed by corrosion, possibly due to salt exposure. This can occur unchecked beneath paint layers as mentioned by cyccommute. An unpainted surface gets a chance to dry out and for harmful salts to be cleaned away, which is not the case with metal under a damaged paint layer.

In response to Raiyn - the seat tube is somewhat slim, but the downtube is a larger trianglular section and the top tube oval. This was an inexpensive (as things go) bike, not the stiffest or lightest of machines but a good ride nonetheless.

To cs1 - I already have two new bikes, Thanks! If I can this old friend running as a spare/backup it is extremely useful to me.


Thanks again to everyone for the advice,

Cheers

Ed

I have similar corrosion on a 10 yr. old Cannondale. Two different bike shops agreed that it is caused by sweat, which I produce copiously, not being washed off after each ride.

My vote goes to: repaint that sucker.

If possible avoid spray cans. The finish is nice but not very lasting.

Another vote to repaint. Bare aluminum will corrode and pitt the metal which well eventually weaken it.

To paint aluminum, I've had good results using PPG DP Epoxy primer. This is good stuff that bonds well with both steel and aluminum. I don't mess around with rattle cans so I'm not sure what to recommend if you go that route.

Good luck.

Ed

I have similar "corrosion" on my bike. The bike is definitely aluminum (199? Trek 1420 w/ "Aluminum" in a big ol' decal on the top tube).

On my bike, it's where the top tube meets the headset, right around the hole where the rear brake cable goes into the top tube. The paint/clearcoat is fine, it just feels and looks rough, as if something is happening from the inside-out.

I too am concerned...I don't want my frame to come apart while I'm in a turn doing 20mph.

What's causing this??

Odd. As stated, Al shouldn't "rust" like that. Perhaps its oxidation from your FD clamp (which looks rather corroded) somehow spreading through the paint. (The paint may have a metal base of some sort. Or...oxidizing FD clamp plus clamp digging through paint over the years plus moisture under the paint that's been attacked by the first two factors equals ugly bubbling paint. Only one way to find out for sure. Pry off that FD clamp, and chip the paint and look underneath.

You should see my Vitus 979 bonded aluminum standard size tubing. Which I just had to re-glue the seat tube to the bottom bracket lug.

By looking at the welds, it is definately AL. I have a bare AL MTB that is nine years old. All it takes is a little Mothers AL polish and it looks like a mirror. It does not pit.

There is a LOT of confusion here on the definitions of rust, corrosion, and oxydation.

I must like bare metal. My MTB is bare 6061 AL and my roadie is bare Ti. Why paint a non steel frame? OK, they can look pretty. But so does a bare frame.

Yes, your frame is corroded aluminum and will need attention. The bubbling areas will have to be stripped, sanded and primed.

And for those wondering, Aluminum does indeed corrode, although it does not 'rust' like iron and its alloys. It's a common among older commercial aircraft to have the floor beams eaten away under galleys and lavatories. Although I've never seen a "rusted out 747", I've seen plenty of heavily corroded 727, 737 and L1011 airframes.

my STI levers corrode as well. I had the 1st gen rx100 sti levers and the buffed alum just gets eaten away. i tried sanding it to flatten the corrosion but all i did was take out the top coat.