British maintenance.
They are maintained by Siemens.
It's not so much that it's treated with AlO as that the oxide forms naturally on exposure to air.Aluminium in its purest form will dissolve when exposed to water. It is treated with aluminium oxide to stop this process. So can only imagine something has gone with the weld?
It's not so much that it's treated with AlO as that the oxide forms naturally on exposure to air.

I thought the entire Desiro UK range was aluminium-bodied and that Aluminium just didn't rust (like iron/steel does)?