Defy double door fridge internal rust

I have a Defy double door fridge which is out of warranty but the back inside panel of the fridge is rusting. I have never seen this before. I still have my mum's old fridge which is over 65 years old and our 1st fridge which is 45 years old and neither have rust inside the fridge. The only conclusion I can come up with is that Defy used inferior steel as there is no reason for it to rust inside otherwise. I have no rust on the outside of the fridge.

I am very disappointed in Defy. I have always bought Defy and thought it a good quality appliance.