Classtime 

These came on a 74 Motobecane Grand Jubile that I believe has been sitting in the sun for 20+ years. I have no idea if this flaking/delaminating thing is superficial, just effects the outboard portion or is through out the crank arm. I like to try keeping my bikes original and sometimes not within reason. Also pictured is the bottom bracket and bearings that are going back in.



Drive side back.

Drive side front.

These are fine.

juvela

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Several years back the forum had a similar thread on a Campag Nuovo Record equipped machine which had been stored right on the Pacific Coast in California's Mendocino County where there is lots of fog and rain.

The electrolysis had got going on on all of those beautiful Campag bits and there was advanced "delamination" - actually made the forgings look as if they had been made of layers.

Your fittings exhibit a much milder case. Will be interested to read what the metallurgical experts have to say regarding them...

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repechage

I would abrade, and or file away the problem material to understand the depth, if more than 1 mm- wall art. be sure to radius edges after.
Polish it up. Gouges or scratches can be stress risers. Worth the exploration.
The bottom bracket has led a hard knock life. And starved for lubrication. If the spindle is not pitted, clean it up and you can continue. Use new ball bearings, and I would tear it down and repack in 500-1000 miles,
again with new ball bearings.

Campagnolo cranks and rings are of a different alloy, I forget which but a 7000 series one, Salt air is BAD, very BAD.

Nemosengineer 

The term for that type of corrosion is exfoliation, which is a advanced stage of inter-granular corrosion. The steel bottom bracket acts as a cathode and the crank arms act as an anode, that process is called galvanic corrosion. Removal of the corrosion damage is almost impossible in a part that far gone as there is no way to quantify the parts structural integrity.

: Mike

bikemig 

Even if the crank is too far gone to trust as in post no. 4, the BB looks usable. There is a fair amount of interchangeability with stronglight cranks. How long is that spindle? I'll bet you can get a stronglight 99 to work on this. Also someone might be interested in the chainrings. The stronglight 49d cranks tend to be expensive. They're very pretty but a stronglight 99 with its 86 bcd chainrings is, I think, more practical. 86 bcd chainrings are available.

CliffordK

I'd go ahead and polish it (coarse sandpaper, then finer, finishing with 600 to 1000 grit), then evaluate.

I'm not sure there is an answer to how much material loss is acceptable. Although, it might also depend on your use of the bike, or even if you wish to sell it to some unsuspecting buyer. But, as long as it is superficial, I'd probably clean and ride it. Watch for anything that looks like a crack initiating and cutting across the crank arms or spider.

CliffordK

I encountered galvanic corrosion with copper/galvanized pipes. All of the corrosion was isolated to the first two or 3 inches or so of the pipe.

Above, most of the corrosion appears mid crank, and away from both the bottom bracket and pedals. Following damage to the anodizing?

If the surface can be polished, it would seem that would be quite indicative of the overall condition of the crank. Without a crack, would there be risk of internal damage?

Engine blocks are "magnifluxed", although several crack detection processes are now used.

Nemosengineer 

Generally speaking to get exfoliation the intergranular corrosion is fairly well advanced. intergranular corrosion follows the forging lines or structure and most of the time goes quite deeper than the exfoliated material, in the OP's photos the exfoliation is biased toward the thin section adjacent to the flute on the crank arm where the residual stresses from forging are the highest. In most cases any structural part that exhibits exfoliation to any extent would be scrapped.
In a past life I worked with the Navy's Deep Submergence Unit for 18 years until Lockheed's contract ran out, working with a variety of submersible vehicles all with a high aluminum content, all dipped in salt water and dove to maximum depths between 5,000 to 20,000 feet. Corrosion in aluminum is unpredictable and the maximum extent or depth of corrosion is very difficult to determine with any certainty without destructive testing or other non cost effective methods. So in my opinion replace that crankset as the risk has no upside.

Seacliff DSV4, In the 1990's this was worlds deepest diving manned submersible with a maximum depth of 20,000 feet, at depth the exterior pressure was 8,888.8 PSI



: Mike

Classtime 

Shoot. That steers this project in a different direction for another time. Interesting that the other crank arm doesn't seem affected and that the chain rings were spared. Given that galvanic corrosion attacked the crankset on this bike, should I suspect other alloy parts? Like the seat post, stem, hubs, brakes?mm

ThermionicScott 

Kinda surprised that no one else has commented on the rusty and pitted bottom bracket cups and balls...

CliffordK

The bearing balls are toast.
The rest needs cleaning to discern. I've had some luck hitting the cups with a handheld drill wire brush, at least for the first attempt at cleaning & evaluation.

For wheel cones, I like to polish them. Insert an axle stub in a drill, and polish the cones with successive sandpaper. I probably lose some case surface hardening, but depending on the brand, I hope the hardening on the cones and races goes deeper. Lathe for bottom brackets? Then evaluate.

Lascauxcaveman 

Well, now we know where your screen name came from!

Classtime 

They're fine. I might replace the balls.