Very nice picutre, Maurizio! It´s just as you said: diffraction blur is not visble here, and as your 180mm sigma macro is closed to f16, diffraction blur is surely not a problem at all. For anyone, who want´s to know more about the optical phenomenon of diffraction blur, will possibly find this wikipedia article helpful, but for the most of us, this won´t really be a matter of interest.sorry for the big size, just to see no diffraction
These shadow boxes are really an easy way to get proper lighting, maybe it limits our creativity a little bit, but if photography is not your main focus, and you simply want some nice and sober pitures of your pipes, this is a good start.
Kind regards
Thilo