It seems like you might have an issue with Galvanic corrosion since the screw and nut seem to be a different alloy than the actual beam. Make sure to either galvanically isolate the metal parts by isolation layers. Or choose the same alloy for both the beam and screws etc.
If a lot of coating...
Try the book Peabody's Control of Pipeline Corrosion, this has helped me a lot to understand when and where to put reference electrodes. And also, don't forget to put them at sensitive locations. Like Train junctions, any electrical facility closeby (powerplant etc).
www.korrosionsgruppen.se...
I would recommend going to 'NACE ICDA', just google it and you will find it!
www.korrosionsgruppen.se
https://korrosionsgruppen.se/en/products/sacrificial-anodes/
https://korrosionsgruppen.se/en/zinc-anodes/
https://korrosionsgruppen.se/en/aluminium-anodes/hull-anodes/...
