Taking water - questions about certain practices

I watched an interesting video posted from by the CRRM showing 491 getting some minor repairs and being lubed up for operations.

Part of the video included taking water.

A dark liquid was poured into the water tank through the hatch prior to taking water. The captioning stated it was a descaling agent, oxygen scavenger, and dispersant. Water was added. Then a white powder, also described as an oxygen scavenger was added. Then more water. The fireman allowed the water to overflow onto the top of the tank, a practice I have seen in film and photos many times.

I know water is often treated prior to use in the boiler. What is an oxygen scavenger and what is its purpose?

I get the reason behind descaling agent and a dispersant. I can also understand stopping to add the second treatment and then adding more water. It stirs everything up.

Why let the tank overflow? Seems like to would speed the rusting process on the exterior. Of course, what do I know.

Thanks for helping eliminate my ignorance.