iPad back discoloration - How is this possible?

[JanSoy]

I had a microsuction sricker on my iPad Mini and after a couple of weeks I took it off to find a discoloration! How can this happen? Isn't the Aluminium protected by the eluxation? Have a look at the discoloration!

 

[Kissmyne]

Probably a silly question, but have you cleaned the back of it since taking that off?

 

[JanSoy]

haha fair question but yes! Simply water, rubbing alcohol, you name it! The discolouration is IN the Aluminium.