Not Impressed With Suzuki Blue Antifreeze

Untill recently we ran a Mazda Mx5, a Subaru Impreza and a 2016 Jimny SZ3.

The second Mazda MX5 a 2008 model was sold in 2018. The antifreeze was 10years old and I changed it before handing it on via a private sale. The car had done 50,000miles.
There was no rust snowing on the coolant and the colour was still bright green. Mazda advise to change the antifreeze at 5 years for subsequent changes. The antifreeze when changed the second time at 5 years in 2023 old was showing bright green with no sign of rust in the system.

The Mazda MX5 that replaced it in 2018, now 6 years or so old has the original antifreeze still bright green with no rust showing.

With regards to our 2016 Jimny the antifreeze was changed at 8 years old as advised by the Suzuki maintenance schedule. At the seven year service the antifreeze was still bright blue with no signs of rust.

When I changed the Suzuki antifreeze at 8 years old 40,450 miles it was brown and with obvious rust in the liquid. I flushed the cooling system and put in a red antifreeze. When we sold the Jimny three months later the coolant was redish but with obvious rust in the system.

I was less than impressed with the Suzuki blue antifreeze.

We have replaced the Jimny with a new Suzuki Swift Ultimate Allgrip. It has the Suzuki Blue Antifreeze in the cooling system. I will get it serviced by the Suzuki Dealer to get the 7 year warranty now offered by Suzuki. I will have the blue antifeeze replaced at 5 years with the Suzuki blue antifreeze in order to keep the warranty valid but also to protect the cooling system for the next owner.

The Subaru Impreza at 10 years old had the original antifreeze changed at 54,500 still bright green and no sign of rust in the system.  When it was sold in 2024 the antifreeze was still bright green with no signs of rust in the system

I'm curious - how do you get rust in the cooling system on an "all aluminium" engine?

The cylinder head & block are aluminium, the radiator core is aluminium, where's the iron or steel?