Pigeon droppings contain ammonia and acids, said chemist Neal Langerman, an officer with the health and safety division of the American Chemical Society. If the dung isn’t washed away, it dries out and turns into a concentrated salt. When water gets in and combines with the salt and ammonia, it creates small electrochemical reactions that rust the steel underneath.
“Every time you get a little bit of moisture there, you wind up having a little bit of electrochemistry occurring and you wind up with corrosion,” said Langerman. “Over a long term, it might in fact cause structural weaknesses.”
Langerman emphasized that he wasn’t saying pigeon dung factored into the collapse of the 40-year-old bridge. “Let’s let the highway transportation and safety people do their job,” he said.
Pretty interesting stuff!