what cause alum. alloy to do this?

[look 171]

I am trying to help a friend with his old bike. Upon stripping out the old crank to over haul the bottom bracket, I find this pitted (?) alum alloy chain rings. Al three of them and its like cancer. It must have spread form one to the other. Crank arms are in perfect condition. Its also alum. alloy. Look at one of the pic, its like the alum starts to de-laminate, but I am not sure if its forged?

[LWJ]

I think. Not certain, that this is a surface coating failing.

Aluminum self seals with an oxide layer. Anodizing is this process in the extreme.

An oddity? Aluminum is reactive to both a base and an acid.

I couldn't exactly see what was happening here on an iPad. Perhaps a large screen would give more detail.

[look 171]

See the above photo how one of the arm is completely de-laminated itself from the body of the chain ring or gear if you will? Between the Allen bolt and the the outer teeth

[Arizona_928]

Originally Posted by LWJ

I think. Not certain, that this is a surface coating failing.

Aluminum self seals with an oxide layer. Anodizing is this process in the extreme.

An oddity? Aluminum is reactive to both a base and an acid.

I couldn't exactly see what was happening here on an iPad. Perhaps a large screen would give more detail.

I concur. I don't see it bubbling or pitting. Surface failure. That said al sprockets are wear items.

Upon closer look at photos on my small ass iPhone. It is flaking and pitting. Still surface failure. Corrosion would be my guess. Salted roads? You can refinish them... Lots of cool coatings you can do with al.

[look 171]

Nope, don't think so. Its not my bike but I do know that its been hanging up in his garage for about 10 years

[DanielDudley]

Magnesium deteriorates like that.

[unclebilly]

This is galvanic corrosion. The steel parts are ‘eating’ the aluminum parts. The moisture (or rain) was the electrolyte and you have ion transfer from the aluminum to the steel.

It’s pretty common and a lot faster in marine environments especially with salt water which is a better ionic conductor that moisture with trace amounts of salt to create an electrolyte.

But then ask Higgins, he’s the group metallurgist...

[masraum]

Here's a better shot (zoomed and cropped) of the most problematic place.
It's interesting that it's separating like that. And the crank arm looks fine, but the chainrings look like puff pastry.

[zakthor]

Last weekend my wife brought out her old bike, bars were loose... Stem cap had deteriorated in the same way. Was like a sponge and i could break it apart with my fingers.

Big steel stem bolt stole its ions.

[wdfifteen]

I agree it's galvanic corrosion. Was a steel chain installed on the chain ring while it hung in the garage? Is one ring significantly more corroded than the other? The crank arm and the chain rings obviously have different surface treatments.

[Porchdog]

I will agree with galvanic corrosion.

The delamination is interesting. I have seen that with heavy steel plate sections (as a flaw rolled in at the mill). I have never seen it in such a thin section or in aluminum.

[javadog]

The crank arm and chainrings are likely different alloys. The strongest alloys, like 2024 and 7075, are less corrosion resistant than other alloys like the 5xxx and 6xxx series alloys. I've also seen polished chainrings that are not anodized or coated, so that makes them more vulnerable to corrosion.

[red-beard]

You CAN replace the chain rings. But buying new crank arm/chainrings with a new bottom bracket would be quicker & easier.

He might want to invest in a whole new group set, say something like Shimano 105.

[look 171]

I have never seen galvanic corrosion like it, but then I never really noticed these things. My rings wear out way before corrosion takes place. The metal sitting against it with moisture makes sense. I don't know how it was stored only know it was hanging in his garage. Thanks gents.

[look 171]

Originally Posted by red-beard

You CAN replace the chain rings. But buying new crank arm/chainrings with a new bottom bracket would be quicker & easier.

He might want to invest in a whole new group set, say something like Shimano 105.

I removed everything, installed an old DA Octalink crank on there. Double, so he's got to work now. It came off an old Alum. Trek from the mid 90s. Trying to get his fat ass off the couch

[mepstein]

It's so easy to find older shimano parts on ebay for cheap that I wouldn't bother with these.

[gtc]

Wow, that's pretty crazy. Reminds me of the aluminum ECS tuning brake caliper adapters that were splitting a few years ago. Kind of surprising to see a shimano product fail like that.

[look 171]

Originally Posted by gtc

Wow, that's pretty crazy. Reminds me of the aluminum ECS tuning brake caliper adapters that were splitting a few years ago. Kind of surprising to see a shimano product fail like that.

Yep. I have been playing with these things for 30 years and have never seen anything like it. I just thought its interesting that someone here will have some knowledge why it failed like that

[mepstein]

Could have been ridden during winter with lots of salt or some chemical was used to clean the chain and never washed off.

So a group of older guys on our team all purchased the latest model Cannondale aluminum frame circa 1990's. Within one year of riding outside and inside on the trainer, all but one bike were a corroded mess. Seatpost stuck, paint bubbling, anodized parts trashed. These guys sweat acid. The one guys bike still looked pristine. I asked him and he said he washed and dried his bike after every ride. He had sweated through 2 steel bike frames so he knew it was a problem but now knew how to keep it from happening.

[mepstein]

Originally Posted by red-beard

You CAN replace the chain rings. But buying new crank arm/chainrings with a new bottom bracket would be quicker & easier.

He might want to invest in a whole new group set, say something like Shimano 105.

It's cheaper to buy a new bike than to retrofit with a new groupo. And the new groups don't fit well to the old frames. Different chainlines, rear spacing, front derailleur braze ons, real cogset compatability, etc.