Arch might not have been properly cleaned post-erection.
According to the National Park Service
, the corrosion stains on the Gateway Arch are "natural and minor," and that overall, "the Arch is as sound today as the day it was built."
Goodie!.....But what's causing the stains?
A Chicago-based engineering firm has determined in a report issued on
September 7 that the culprit is likely the welds holding the Arch pieces
It appears that either atmospheric pollutants are
reacting to the welds, or there was "inadequate cleaning and polishing
of the Arch after erection."
If we've learned nothing else, it's that you must always clean and polish after erection.
Read the NPS statement on the report here