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the strength of a pipe usually does not depend on the outer 0.1mm
Oh, but it does. The fatigue strength of a pipe subject to bending critically depends on the surface quality of the first few tens of microns from the OD, specifically the tip radius of any groove or pit in that surface. You have pits right where the bending stress is highest, and while they look innocuous to the naked eye, the bottoms of those seemingly rounded divots will on closer inspection prove to be a veritable mountain range of sharp peaks and v-shaped valleys where chloride corrosion has preferentially followed grain boundaries.
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gbj_tester
SwissChap
Yes thanks, that's why I'm asking. From the looks of it, there is no reason to believe it is structurally weakened though, as the strength of a pipe usually does not depend on the outer 0.1mm.