Nozzle From Recent Static Test

After cleaning up the motor after last weekends static test I noticed distinctly copper coloration to the nozzle. I have noticed this to a lesser degree in the past few static tests that I've conducted. I attributed it to the copper tubing that I have been using for the pressure gauge. It didn't really make sense because that tubing is upstream of the exhaust flow so it seems like any copper from the plumbing would be pushed towards the gauge, not the nozzle. There are two significant differences between this test and the others. The first is that in this firing the nozzle appeared to get much hotter than the other nozzles that I have noticed this copper coloration in, the other is that in this nozzle there appears to be little globules of copper colored material that have been driven out of the surface of the of the nozzle. the thing that I can't figure out is that these nozzles are made of 12L14 steel, which is made up of Iron, Carbon, Manganese, Phosphorus, Sulfur, and Lead. So where does the copper coloration come from?