What type of corrosion is this and what would cause it?
a. Crevice corrosion developing from an over-driven fastener.
b. Galvanic corrosion, fastener is different metal from roof steel.
c. Filiform corrosion, developing from micro-cracks at hole drilled for fastener.
d. Surface corrosion developing from moisture trapped against the roof at the time the roof mastic was applied.