What's happening to the coating on the OUTSIDE of my roommate's non-stick cookware as we wash it? My roommate has a set of non-stick pots and pans.  They advertise themselves as dishwasher-safe, so we typically machine-wash them.

The exterior of the pans is not bare metal; it's covered in some kind of dark gray matte coating, as in the following picture:



However, we've noticed that the pans we use more often (and therefore wash more often) are starting to lose this coating:



The pans also give off a noticeable amount of dark gray dust if you scrub them too vigorously; I assume this is the coating coming off.



Questions:


What is the exterior coating?  Why is it there in the first place?
Why is it coming off?  Are the pans not as dishwasher-safe as they claim?
Is there any reason to be concerned about this?  Does it have any implications for food safety, or for using the pans?


A couple notes:


The non-stick coating on the interior of the pans (which is Teflon or similar) isn't coming off - it's just fine, and the pans are still functionally non-stick.
Pretty sure that brown splotch on the exterior of the second image is just grease spillage.  The pans are aluminum, so shouldn't be rusting.
Are you SURE they were advertised as dishwasher safe? If so, contact the manufacturer because they were lying. It is, in fact, the bare metal you're looking at— those pans are made from aluminum which was anodized: an electrochemical process which changes the appearance of the metal. All aluminum pans including anodized ones that are not painted or otherwise coated will be damaged in the dishwasher because aluminum is sensitive to strongly alkaline chemicals, and dishwashing detergents are strongly alkaline. As long as the interiors have some sort of coating, such as Teflon or ceramic, the exteriors of the pans should work fine, but they'll never look quite as nice as they did. If the interiors of the pans aren't coated, you might want to consider seasoning them like you would a cast iron pan to avoid eating a bunch of aluminum that was softened in the dishwasher.