Rear axle cracks/rust at corner or bushing arm (w/ pics)

Yeah I hate it when that happens. (Though btw, they do seem to have made the ads here lot less less intrusive after a few complaints. I know Bob Wilson was pushing for it over in a "Forum Issues" thread.)

Before anything else I'd try cleaning it up with a wire brush to get the scale off so that you can see just how bad the damage is. Though it no doubt looks very bad, it's unclear to me from the photos whether it's genuine cracking of the welds that's occurring or whether it's simple cracking and flaking of the corrosion products (the scale and crude). A bit of a clean up will give you a much clearer picture.

For spray on type rust passivation treatment on older cars I've found that you can't beat the good old "fish oil" based products. These are cheap, and in my experience seem to work the best.

 

Sorry I didn't see that link of that rust stuff before.

Home Depot carries an excellent product called CSP. Corrosion Stop Protectant. Its a spray grease that does not smell and stays where you put it. Its designed to stop rust. Just spray it down. Plus it lubricates all the moving parts too.

Sorry but your car may be to far gone. Even the inside of the wheels are badly corroded. But in the future if you spray down the suspension a couple of times a year it will really help stop the corrosion. It has worked really good for me as the undercarriage on my 07 has no rust whatsoever. And especially spray all the electrical connectors in the engine bay also. I am very surprised you have not lost the connector on the side of the trans right above where the 2 coolant hoses enter the trans as that's a very well known corrosion spot. The connector rots out and the trans is totaled.

40 years ago it was not a big deal when the car undercarriage got severely corroded but now there's so many more things that can wrong and there all expensive on a Prius.