Schlafen

I've just started polishing a set of 49D cranks that were showing some surface corrosion. The NDS crank seems fine after first pass with 240g and can easily take that to a mirror finish.




DS crank was the worst of the two, after 1st pass with 180, then 240 the corrosion/pitting is not levelling out, not only that but more corrosion appears on the surface when sanded. I even did a superficial pass with a file and it still flakes away.
​​What can possibly cause the flaking? Should I sand/file it until ths corrosion dissapears? Is the crank still usable?





I would like to save the crank if possible, as I was on my way to use it for a 26/38 setup on my commuter.

bark_eater 

Looks like a good candidate for destructive testing, probably best performed off the bike.

I you realy want to save it, I'd sand off the funk layer and get it smooth and polished. and then have it dye tested for cracks.

I can hypothesize different causes, but it seems most likely to be a forging defect when manufactured.

juvela

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salt can do things like that

do you know if cycle stored near the sea?

then there is the possibility of road salt...

then there is the possibility of home trainer use...

some years back the forum had a post on a Mondia equipped with a Campag NR group which was stored near the sea in Mendocino County, California

the electrolysis had gotten to the dural and "delaminated" it; an incorrect term but that is just what it looked like as if the machine's crankarms had been built up in layers which were now separating(!)

all of the bicycle's beautiful Campag NR components were ruined


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daka

Without speculating on how the crankarm got that way, I'm inclined to say that it has reached the end of its safe useful life. Especially that damage about an inch away from the pedal eye, that's a high-stress location and a common place for cracks to propagate and breakage to occur.

I did enjoy trying to identify (unsuccessfully) the service manual in the background. Looks like '90s technology for emissions controls but includes OBDII codes so probably post 1995.......

Bad Lag

I have seen aluminum corrode like that before. What can happen is corrosion-producing ions (like chlorine) get under the anodizing (aluminum oxide) and work laterally, converting aluminum to other materials like aluminum oxide.

Aluminum oxide is brittle and can be separated from the remaining aluminum metal, as you have seen. It can just flake off.

I have seem thick plates of aluminum converted into layer after layer of aluminum oxide. It looked a lot like mica (if you know about mica). Touched, it would crumble into a whitish powder. In this worst case, there were salt ions present and multiple metals present to promote galvanic corrosion of the aluminum.

If it is truly thin, you should be able to remove it and still use the crank. Just polish it to a uniform appearance and let it oxidize to its typical matte finish.

C9H13N 

I have a Stronglight 99 arm that was corroded like this. I think the proper term for it is “intergranular corrosion” and I agree with the other posters that salt exposure is the culprit.

You can sand it but you’ll have to go deeper than you think to get it all.

Schlafen

Salt corrosion is definitely on the table, the cranks come from the Brighton area (southern seaside town in UK).

These craters are quite deep, not sure what the limit is tbh.






Desk mat is from my old stash of Pierburg desk mats, had them for ages. Does a great job at protecting my desk from bike parts, tools, coffee mugs dragged across the top, lol.

Calsun

Aluminum does oxidize and this shows as pitting but it does not mean a weaker crank. With boats around salt water this type of pitting is normal and seldom results in a part failing. It is a cosmetic issue and not a big deal to replace it if it bothers you.

daka

I'm going to differ from the above with respect to "it does not mean a weaker crank". Pitting and flaking represents a loss of material, that implies less strength. Worse though is that pits and sharp edges are stress risers where cracks can start and almost all crank arm failures originate from cracks. If you believe that aluminum crankarms never fail I'd suggest you have a look here: https://pardo.net/bike/pic/fail-001/000.html .

I own a thirty year old sailboat and am all too aware of some of the corrosion issues that you describe as "normal". Seen frequently, yes, something to be ignored, definitely not. Fiberglass sailboats with aluminum spars and vintage bicycles with lightweight "competition" components have all lasted and remained in use far longer than their original designers and builders intended. With a fatigue-prone material like aluminum in use for forty or fifty years, we are all on borrowed time.

Schlafen

Why is the NDS crank not affected in the same way though?
At least I assume it's not affected as the damage is not visible.

juvela

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this is sure to be a question we have all been wondering about as well

your initial post gives no background on the set as are they original to the same machine

made into a set from found pieces? etc.

could be that different material lots of dural from the foundry are contributing to the difference - just wild speculation

the forum has no shortage of Verot experts who may be able to contribute...

if you wish to further pursue the inquiry would expect subject is likely to have been addressed at the Tonton forum and on the CR email list

https://forum.tontonvelo.com/search.php

https://www.classicrendezvous.com/Join_the_CR_group.htm

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noobinsf 

I've retired two cranksets on which the drive side had a hairline crack, one of which was a Stronglight 105. Even with your two crank arms on the same bike, they are two separate pieces. Perhaps the drive side had some manufacturing defect or previous damage that allowed salty dampness to make its way into the crank and allow it to do its work over time. This is wild speculation because I know nothing about metallurgy, but what I do see is material coming off in pieces from that drive side crank. It just gives me the willies and I would not use it.

Schlafen

I bought them used a few months back, as far as I can and could tell at the time, they loooked like a matched pair, no suspicions prior to purchase, see pics attached which are from the eBay listing:






Before discarding the DS crank completely, I'll have a go with the file only to see how deep the corrosion goes. If it won't be useable afterwards, oh well, can't win them all.

due ruote 

If you google 'Stronglight crankset flaking' you will find that this has come up a few times.
Here's one thread: https://www.bikeforums.net/classic-v...ankset-ok.html
Note post #4.
I have been watching my model 93 crankset for several years. There is some flaking on the backside of the arms, but it doesn't appear to be getting any worse.

cyclophilia 

Bicycles that have been stored near pool chemicals (chlorine, muriatic acid) can show this type of damage.

bulgie 

I have a pair of original Campy Record cranks, as came out in about 1958 IIRC (someone here knows for sure). Rare and normally precious, but mine are practically turned to dust, so corroded that no one would ride 'em. So sad!

billytwosheds 

Can confirm that salty air eats things that would be fine otherwise.

I've seen several alu Trek frames "rust"...turning to powder.

Bummer about these cranks. Not sure I'd ride them.

seypat

The thread title makes me think of the Sister's Of Mercy song from BITD.

Schlafen

I filed the DS crank and unfortunately, the corrrosion is not going away, not comfortable filing away more material and riding these cranks, well the DS crank, can see nothing wrong with the NDS crank.





Too bad as the NDS crank is very usable, I'm going to throw this one in the parts bin as for some reason 170mm TA or 49D NDS cranks seem to be unobtainium for some reason.




I've rummaged through my parts bin and found...actually 3 options: TA, M500 or M563 cranks.




Now I'm faced with a bit of a dilemma. The M500 is porky and weighs 555g would take a 26t and is shiny and beefy. The M563 (465g) would take a whooping 20t ring which I already have. And if using the TA cranks (345g) they would be the lightest of them all but the chainring sandwich will probably level the scale with the M563 cranks, and I would stick to the original plan. Decisions decisions. A 22/40 sounds good too.