What are those squiggly lines on old metal parts?

[Manc33]

I have seen this on a lot of old parts, it's not scratches, what's going on?



Those squiggly lines.

Is there any way to get the metal back to normal when this happens to it?

It looks like worms are trying to take over the metal.

[tim-b]

Hi
It's filiform corrosion which occurs under some thin coatings, like bicycle paint
Moisture is getting in, a bit of sweat and/or road salt and away it goes. It can cause harm, but I'd clean the bike during temperature extremes (hot, sweaty and cold, salty) and try to store the bike in a drier atmosphere and monitor the corrosion; use the original photo and see what happens over time.
Regards
tim-b

[Brucey]

yup, filiform corrosion. The metal will have a coating over it, and if that coating is tough and flexible, it actually assists this type of corrosion; a more brittle coating will breach and this normally inhibits this particular form of corrosion, or at least makes it a different shape.

FWIW once it starts, this sort of corrosion usually carries on by itself. You only need the slightest trace of moisture within the track for it to carry on and very few places you can store bicycle parts will really be very low humidity, enough to stop it completely.

In some cases you are best off stripping the coating off, repolishing, and recoating. if you are prepared to keep up a fairly demanding regime of polishing, and you never intend the thing to see winter use, leaving the finish polished can work OK. But if the material is particularly susceptible to corrosion and it sees the wrong conditions, the parts can be very easily wrecked once the coating is removed.

cheers

[Manc33]

I never knew it had a name. Yes I have noticed a sort of plastic type coating on old silver parts, it yellows with age but protects the metal.

tim-b thats not my pic, it was just a reference pic to show the lines.

Found a pic of it affecting an iPhone 6



It's almost art:

[nigelnightmare]

TINWORM!!!

[Chris Jeggo]

TINWORM!!!

+1
Don't forget that Howard had a problem with stoneworm in 'Last of the Summer Wine'!

[Brucey]

X marks the spot


example of a standardised filiform corrosion test coupon

cheers

[Manc33]

From elsewhere talking about using wire wool to get rid of this unsightly corrosion:

"...the wheel will no longer have the clear coat that would prevent further corrosion or oxidation of the aluminum itself. Over time, the area will darken and require continuous maintenance."



Right, so is there any clear lacquer or a way to put another coating on of that magic Shimano plastic stuff they coat parts with?

It's not even important it's just I hate the sight of that stuff. On aluminium I don't think this corrosion affects the usage of the part, unlike steel where metal is being eaten away by rust (the part is slowly vanishing into the ether).

[Brucey]

the white stuff on aluminium parts is corrosion without doubt; it doesn't often cause the parts to break but it can do exactly that by helping to initiate cracks. Whether parts that are stripped of the coating and then polished will corrode horribly or not depends on the aluminium alloy used, how you care for it, and what kinds of corrosive conditions it sees.

You can recoat with two-pack lacquer and there are some (eg polyurethane) coatings that you can brush on too. But nothing you put on will be quite as good as a factory finish, by and large.

cheers

[nigelnightmare]

You can get it "clear" powder coated!
BLOODY expensive though.