Allegedly it's down to new legislation requiring there to be a 5% mix of ethanol in the petrol which can take the lining off the tanks in the forecourt. We have a petrol station on our premises and it happened to us a week before christmas, 10 mins after the delivery cars started breaking down (some not even getting off the forecourt). Some of the petrol that was taken out of the cars looked alot worse than yours so you should be happy. Note ours was with normal unleaded not super, I know that super has had an ethanol mix for over a year so this may not apply in your case.