metallurgy query: gudgeon pins display pitting/corrosion after left in petrol

[thechuckster]

A query for metallurgy nerds.

I've managed to stuff my spare 3S gudgeon pins by forgetting that they were in my petrol degreasing bath.

They probably sat there for at least a week or more (can't remember when I put them there). I'd left the pistons soaking to try and remove/soften the burnt on crap and forgot that I'd left the pins and rods in there until today (... what's those shiny things? ohshit).

Anyway, they've developed fine rust marks where there was probably scores in the metal surface. Hitting them with 1200grit wet-n-Dry removes the raised bits and discolouration but there's pitting left behind rendering the chrome (?) surface stuffed. The marks/scores look like possible wear marks that have been enlarged by the corrosion.

I'm just curious as to:
- why the chrome reacted in the petrol bath? (brother suggests that water would be in suspension in the fuel, but corrosion also requires O2?)
- are 3SGE gudgeon pins actually chrome (or other?) coated on the wear surface? or is it just shiny due to some hardening process?
- would the fine pitting promote a rapid failure of the coating surface if they were used?

Don't have a pic yet, phone is charging up. And I also have 4 good pins so am just going to put this down as a learning experience. I've left other metal things (steel & alloy) soaking in petrol and haven't seen anything like this yet.

Anyway - just interested to hear what people think/know.

edit - close-up of (left) as removed from petrol bath, (right) polished up with 1200 grit wet& dry.

(click on pick to go to full-res version on picassaweb

[oldcorollas]

was it ethanol containing petrol?

[thechuckster]

yes, was collected from my wrecks, would have been a witches brew of varying ages and E ratings.

edit: may have already had lots of scratches, donor motor had chucked a rod and the bearings were well worn.

[allencr]

Sure looks it, but can't think of any plated pin, any reason why one would be plated, or any plating that is that thin.
Good luck.

[thechuckster]

these won't get used as designed, i'll score the surfaces to render them NSU.

[oldcorollas]

if ethanol is in there, it can oxidise to vinegar, which can be acidic enough to attack various metals.
I would expect the pin would not be coated, and be a reasonable non-corrosion-resistant type... there might be induction hardening of the surface, which would be around 1mm or so?? but i very much doubt any kind of carburising etc in a production part

if you want to know if it is coated, send to me and I put on microscope
non-destructive test. you could send a half 1200'd one, and I could check the corrosion products as well... super nerdy...

was the surface raised before? ie galled alloy from pistons? or smooth before going in, and then reacted somehow..

wonder if you can check the pH of the fuel?? or if the actual fuel parts affect the test? shake fuel up with water, let it separate, then check pH of that?

[2004lowlux]

hmmm i would have thought water may have caused it too pretty sure if you leave something metal in water regardless if its exposed to air it would still corrode to a degree depending on the type of metal.

[oldcorollas]

yup, there is usually enough oxygen dissolved in water to allow corrosion to occur,
but water and petrol basically don't mix... or you would see it is not right (a suspension... put a few drops of water in a jar of petrol, shake it heaps and see what happens)..
unless it has ethanol and agents to help ethanol and petrol mix, as water can be absorbed into it...

[thechuckster]

re: metal/surface composition: it seems fairly corrosion resistant on the inside & ends (non-harded/coated?), no visible surface rust but I didn't leave them out to dry for very long (fished them out, sat them for a few minutes while I found sandpaper, then cleaned one up using water dispersant as a lube).

it's possible that there was bearing material (from the rod bearing and oil supply) but I just remember pushing them (with some percussive assistance) out of the rods then dumping the lot into the bath... which reminds me I'll have to fish out the missing circlips in there. They did (from memory) look fairly shiny/smooth
but the exposed section (between piston & rod bearing) was discoloured with what I thought was the oil-baked varnish crap you get in crankcases.
will see if brother can check the pH for me at work - will also try a shake/separation test tho it's more likely have a layer of dissolved crud then fuel

no water visible in the tank - it is in a shed that's weather prone, but the tank has a lipped lid and when lifted you get the usual odours of old fuel an not much else. I'd say the atmosphere in it getting on towards combustable

PM me your address - am happy to send you a sample for destructive testing - will polish one end only. I'll be using the other two as drifts/belting objects.