Black MZ and corrosion

When I got done shooting this spring and got busy with work, I left powder in three RCBS powder measures.
1. Swiss 1 1/2
2. Blackhorn 209
3. Black MZ

When I came back this fall to start reloading again,
the Swiss measure had just a slight bit of corrosion in the cylinder. Polished right out.
The Blackhorn 209 measure had no corrosion
The Black MZ measure was rusted solid

pounded out the old powder (yea it clumps up bad) soaked it in penetrating oil for a few days and finally got it to budge.
Altho I never had a problem in the rifle with cleanup, I don't think I'll continue load development with it..

Now it was a very humid spring here, but that stuff really attracts moisture.