Galvalume is a zinc-aluminum alloy coated steel sheet. As such, it can suffer from corrosion much like galvanized sheet, just generally at a slower rate. Is the material under the eaves perforated or installed such that adequate air can enter to allow for proper attic ventilation? If air can't enter and exit the attic, you will get condensation damage on the galvalume as well as in the attic. The staining you are seeing is much like what would occur if the sheets were in storage under humid conditions. If I had to guess without seeing the installation, I would say the galvalume should not have been used under the eaves unless sufficient louvers were installed to allow air flow.

thank you for replying! the galvalume corroded on the bottom side of the panels within a few weeks of installation. it is not vented because it is corrugated -- the builder said that the air will circulate between the ridges/valleys of the corrugated panel and into the eaves.

The part which mystifies me is, if it's condensation, why does the condensation form on the bottom (exterior) side of the panels?? it's warmer inside the eaves in the summer), right? therefore, wouldn't condensation form inside the eaves, not the outside? (like a glass of ice water forms condensation on the warm side of the glass, not the cool side)? it just seems backwards. thank you for your help. the panels will be replaced, but i'm afraid that the same problem will happen again within a few weeks. sherry