Is this kind of internal corrosion on a dryer salvageable?

The short answer is that you are dealing with a significant job, but it is almost certainly not impossible, especially with a Speed Queen which has modular components. The massive oxidation and stripping of the metal coating on the drum and the rear housing usually means the drum and the rear bulkhead assembly need to be swapped. These assemblies are designed to be replaced relatively easily with the right parts, much like large mechanical Lego blocks, whereas other brands require full cabinet disassembly for similar issues. When the rust is that far gone and affecting the structural support areas, simply polishing it off with towels isn't going to cut it; you are dealing with material failure. The white residue is a sign of deep chemical interaction between the water and the metal, which means the original protective coatings are completely gone. A good technician needs to assess if the frame of the dryer itself is warped or compromised by the standing water, but usually, the primary damage is confined to the internal drum structure and its mounting points. For a project like this, you're looking at ordering a complete replacement drum and rear housing kit. While that initial investment might feel like a huge expense, the specialized parts and labor required to safely execute this repair often come close to the cost of a brand new unit, which is why the repair can feel so daunting. If the overall chassis of the dryer is severely rusted through, then replacement is the only smart call, but if the core frame is sound, the repair is a mid-level project that requires careful disassembly and reassembly. Get a proffesional assessment of the structural integrity of the frame after replacing the affected internal components. Even after new parts are installed, you want to ensure the whole unit is stable and won't have compromised support down the road. It's a tough repair, but if you have the right parts and a decent DIY skill set or a skilled local tech, it's a win over throwing it in the trash.

I had an issue like this with my older unit a while back. Before you go swapping the main drum assembly, make sure you thoroughly dry and clean the entire chassis. Just letting it air dry won't cut it since water gets trapped in the structural ribs. I always used a contact cleaner and a stiff brush in the rear corners to get all the standing moisture out. After replacement, you need to treat the surrounding frame metal because even if the drum is new, the rest of the housing will continue to decay if you leave rust spots alone. This is a huge job.

This is way more extensive than I thought these things would be if they flood. I mean, I have a washer that had a leak last year and I just replaced a hose, but seeing the scope of damage here is really diffrent. My big question is about the heating element or controls. Even if the drum itself is solid, does that standing water usally get into the control board area, or is that usually sealed off enough that it isn't damaged? I'd be nervous trying to find those internal components without a diagram.

The white crust and residue you mentioned looks like it's relly etched into the metal. Since the water leaked in and sat there for a while, I'm worried about the actual components inside the drum besides the main frame. If the chemical deposits hit the heating elements or the little metal racks, coud those parts be ruined even if the drum itself is replaced? Will the new drum still rust quickly if that stuff is left in there?