Hard white substance that drills like plastic. Epoxy or salt?

[Mark-1]

I recently found a 2mm hole in stainless steel for a cotter pin clogged with a hard white substance, I assumed salt. A bit of wire failed to clear it so I drilled it and the swarf was like plastic. For me that's the hall mark of epoxy, so I assumed it was epoxy and forgot all about it.

Yesterday I drilled a stainless steel rivet out of an Aluminium mast on a different boat. It had the same stuff. A hard white substance that drilled like plastic. I can't see why there would be epoxy in a rivet.

So, are there any circumstances where salt goes "plasticy". What else would this substance have been?

 

[oldmanofthehills]

Aluminium oxide or hydroxide sounds the likely culprit near an aluminium mast. Possible consequence of interaction with an unsuitable stainless steel material causing electrolitic action.

No idea about the first incident

 

[Martin_J]

The plastic in the centre of the pop rivet that drilled like plastic probably was plastic..

After pushing the steel central mandrel out (or in, because it would rust), a rivet plug was probably put in its place..

Pop rivet plugs

 

[Mark-1]

The plastic in the centre of the pop rivet that drilled like plastic probably was plastic..

After pushing the steel central mandrel out (or in, because it would rust), a rivet plug was probably put in its place..

Pop rivet plugs

View attachment 195024

On reflection I think you're right. So no mystery. Likely the cotter pin hole was epoxy and the rivet *was* a plastic rivet plug.