Iron Cans with Tin

[xz]

Question is:

Iron made cans are used to contain food, and frequently interiorly revested with a thin tin layer.
Why?

E0 (Sn2+/Sn) > E0 (Fe2+/Fe)

So, tin tens to reduce, thus not preventing iron from oxidating.
That is, Iron is oxidated.
Is it just to prevent the food to be in contact with rusted iron?
And why it is not protected exteriorly as the rusted iron contributes to a rusted can appearance.

Thanks in advance.

[adrian]

Is not about corrosion more than food security. Tin is safe for food, especially for high moisturized ones. For example is allowed just high purity metal.
Outside is not a corrosion problem, because most of cans are painted, colored, etc.