jowilson

Took the rear cog and lockring off my fixie today to do some spoke work and I noticed this strange color change in the grease. I used the exact same grease on both parts, and tightened them on the same time, etc.
It's obviously something chemical, but I don't know what. The grease is blue out of the can, but only the stuff on the cog threads turned yellow.

Little Darwin

I don't have any information, but I think you are on the right track with it being chemical.

I initially thought was oxidation, but I would think the threads would seal about the same on the cog and lock ring.

What materials are the lock ring, and cog?

PS - regardless of the materials, I would switch grease, because there is obviously a reaction of some sort going on.

jowilson

Lockring is the All-City stainless steel lockring, cog is a Surly cog, which I believe is Cro Moly.

slorollin

A Michigan fan would no doubt see this as a sign from God.

Nick The Beard

Are you sure no one reapplied grease to that hub? It might just be the picture but there looks like there's a lot more grease on those threads than I'd expect after having something threaded on and off.

Nick The Beard

Or maybe there was some blue locktite on the lockring interface that blended with the yellow grease?

jowilson

The grease is blue, not yellow. And I am very very confident in saying I am the only one who has taken this cog and lockring on and off. No loctite either, I'm not that dumb.

Little Darwin

You may want to check this out in the Bicycle Mechanics forum to see if anyone has seen something like this before.

The frame builder's forum may have some people with knowledge in metallurgy, but it would be off topic for that forum.