That can be based on factor of safety built into the design and whether the corrosion violates that, or possibly by noting the exact nature of the corrosion.
There simply is no such thing as “no corrosion”, or we would be hand-tied from ever getting anything built for fear of it.
Well, I know airplanes have at least occasional (periodic) inspection of the corrosion of Spars (and other features). (I will stipulate that I was once inside of a de-engined F-100 looking at the tail supporting “Wishbone”).
Corrosion is the bane of the metals. Corrosion resistant design and coatings continue to make progress. These carny devices perhaps could use some FAA inspection protocols applied?
There have been a number of case where the manufacturer’s design led to excess corrosion. A number of these designs led to moisture collecting in a unweather treated interior with no inspection access. a good number of cars and trucks have had this problem.