The problem you described sounds like the problem I'm thinking of. Small black spots that look like they are only on the surface. You described them as a sprinkling of crushed pepper, and that sounds like what I've seen on hubs and posts. They feel smooth so they may be very small voids or they may be small pockets of black oxide. In any case, they penetrate deeply, and are there after you remove all the whitish / greyish surface oxide, and even after sanding. My friend's Mexico crank is covered with them. He Flitzed it and it shines nicely, but close inspection reveals the same spots. It's some sort of corrosion even if it doesn't fit you idea of pitting which I assume is voids with visible depth and loose grey oxide.
>>and as a fan and owner or Peugeot PX10's, the Stronglight 93 crankset and Mafac
>>brakes are two examples of unanodized parts which are quite easy to keep
>>looking good;
I think there are differences in the ways that different alloys corrode. I don't know if it is a function of the variations in chemisrty, metallurical phases, or grain structure, but certain alloy parts seem to never get the "blackspot" problem.
Eric Elman <tr4play@cox.net> wrote: Tom, Thanks for the reply (below). The black marks in my crankset are not
pits (small voids from corrosion), only speckled discoloration. I am
familiar with how aluminium can corrode leaving a rough finish but that is
not the case here. I have worked with aluminium parts on automobiles and as
a fan and owner or Peugeot PX10's, the Stronglight 93 crankset and Mafac
brakes are two examples of unanodized parts which are quite easy to keep
looking good; I've never seen black specks these or in other aluminium parts
but another lost member emailed me saying that his Campagnolo Mexico'd
crankset has the same discoloration in it too. So, while I agree that
corroded aluminium is very difficult to make good again, I believe these
speckled cranks have some other problem. Any other thoughts?
Eric Elman
Somers, CT
>
> The black speckles are pitting and are typical of un-anodized aluminum
bike parts. On SR/NR stuff these parts include the shifters, ft der,
clamps, pedal bodies, hub shells, and seat posts. Rear hubs and 'posts are
particularly troublsome since they tend to see the most sweat from the
rider. I'm certain that unanodized cranks would be a real nightmare to keep
looking good. My experience is that pits are very difficult to remove.
They are much deeper than you might expect. I've had posts that I sanded
down, starting with coarse (!) emery cloth and moving to finer grits, then
Scotchbrite pads and 0000 steel wool, finally finishing with Flitz polish.
After all that, many of the pits remained. I'd say learn to live with the
pits, and keep them in check by wiping down the cranks after each ride. A
mild ammonia solution (Windex) is helpful, I think because of some ion
exchange process.
> Tom Dalton Bethlehem, PA
> Eric Elman wrote: > 6. Lastly, my set is all polished
aluminium but despite how much I polish
> and buff them they have darker "stains/speckles" about the size of crushed
> pepper in them that will not come out. The stains are only noticable when
> you look closely but are there none the less - any ideas on the cause and
> any possible way to eliminate these dark marks?
_______________________________________________
---------------------------------
Do You Yahoo!?
Sign-up for Video Highlights of 2002 FIFA World Cup