F150 spring hangers rusted in sheets

I do not understand why my spring hangers on my 95 F150 are so rusty, you can pull it off with your fingers in sheets of about 1/8 inch in thickness
on both sides of the hangers, and it is heavy materal when you do .....the front and rear spring hangers are all that have rusted like that, the rest of the truck underneath looks like it just rolled off the assembly line. I was wondering if maybe there may be some defect in the metal the hangers where made of. I have never seen anything like this before, and am pretty worried about the hangers breaking under a load. Has anyone seen this before are have any knowledge about this?
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