Be careful Jim, that exterior corrosion alone does not make it prima facie evidence of a
chimney fire. There was a class action lawsuit filed against several manufacturer's of factory built solid packed chimney for failures. It seems the insulation contained a lot of chloride ions, which, formed HCL if it got wet. The method of forming the male end crimp of these brands allowed moisture to enter the insulation pack, which did two things: it reduced the R valve, thus creating hot spots, and two, formed HCL, which attacked the steel. Most of these mfrs. use 430 or 444 stainless, which may resist rain ok but not strong acids. The term "stainless" is an unfortunate one that gives the impression of Super Man's cape.
I suggest the chimney be identified and the manufacturer contacted. It may be out of the settlement period in which case you would have to channel it as any other warranty claim.
To ascertain if this was possibly caused by a chimney fire, you need to visualize the inner lining. There will be zones of discoloration but more damming is the presence of rust in the stainless inner liner. When stainles steels are heated above about 1,380F, they become susceptible to what is known as forming chromium carbides or intergranular corrosion. The stainless steel is then no longer "stainless" so it stains or rusts. See my attached photos from a witnessed chimney fire.
Think of factory chimney like a fuse--it protected the home from burning down so now it needs to be replaced. This is usually an insurable loss. Most mfrs. will consider the warranty void after a chimney fire but reinstate it once the fireplace has been thoroughly inspected (Level II or III) and the chimney replaced.
In Jim's case, this is a clear cut Red Flag for a Level II inspection. Irregardless, the chimney must be replaced before being used. However, if it was from an unfriendly fire, the insurance may pay for it. If it is from a manufacturing defect, the mfr. may or may not pay for it. The simplest way to ascertain which it is would be from a Level II inspection. If there is no sign of a creosote fire in the flue, no damage to the inner sections, joints, and no signs of rust or corrosion, then it probably is from chloride attack. Either way, the mfr. should be involved.
HTH,
Bob
