Perhaps during an electrical storm. Unless you think there's a long-term high-voltage short that nobody's noticed?

Welders use Phosphoric acid and an electrified scrub brush (electrified lead wrapped with tough cloth, dipped in Phosphoric acid) to scrub welding scorches off of brushed stainless steel. Phosphoric acid on its own will etch stainless steel (and teeth), and electrifying the set dramatically increases the reaction rate, leaving bare metal after a couple of gentle passes.

There's most certainly a cathodic protection system on the bridge for the rebars and supports exposed to seawater, however if pooling water wasn't intended in the lower areas then there was likely no CP active in the area - and the protection of the other members may have accelerated corrosion elsewhere due to stray current interference.

edit: It's actually the low voltage corrosion cells that you need to be cognizant of in such a case - most high voltage shorts fail before significant corrosion has taken place. Consumption rates for steel are in the kg/A-yr scale, and the concentration of the current dump is a major factor in general vs acute/pitting corrosion activity.

Yeah. If you get this stuff on a new boat or barge, you have to basically send around guys with voltmeters with really long leads, all over the ship... if I was in charge of the bridge I'd do that. It's relatively inexpensive, and either finds the problem or gets rid of one common variable.

This is what I do for a living actually - and concrete / mixed structure corrosion control is unfortunately a lot more expensive and difficult than single structure (ie steel hulled vessels). A typical concrete CP system will be low output but with dozens to hundreds of individually controlled zones and reference electrodes to monitor the system performance.