Does anyone know why the front of my boom is corroding where the aluminium extrusion meets the aluminium casting before the gooseneck? Better still how do I stop it? Thanks in advance to the brains trust.

Different alloys in casting and extrusion or maybe by other metals connected to them ?
I use generous coatings of Duralac on both sides to isolate the cast from extrusion as well as the Monel rivets and all the stainless cheek blocks from the extrusion. Anything that leaks out of joints I wipe with dry rag and don't clean the fitting with acetone until Duralac starts to harden up. Messy but keeps corrosion at bay.

Looks like the plug is anodized. I would just monitor it for 20 or 30 years and report back.

Looks like salts are collecting in the joint and are causing crevice corrosion and pitting. Not a galvanic problem.
Clean them up and apply a corrosion inhibitor in the joint

Thank you for your answers guys. I think I will just clean it up and put some clear paint over and see if that stops it.

In the late 90s early 2000s exfoliation corrosion caused untold grief in the fast ferry industry - see here; Aust yards were affected also.

www.boatdesign.net/attachments/5083-alloy-pdf.7541/

2 good articles here.

www.shapesbyhydro.com/en/material-properties/tips-to-avoid-corrosion-in-marine-environments/

www.shapesbyhydro.com/en/material-science/preventing-crevice-corrosion-of-aluminium/

Could be exfoliation (a specific type of intergranular) as well as crevice corrosion.