External electrolysis to boiler

Moisture.

To complete the electrical circuit between dissimilar metals that allow that corrosion, you need an electrolyte

Probably you have a small leak or seep at that connection

Also the higher the TDS of the fluid or water in that equation the more potential for electrolysis corrosion . Temperature also accelerates the corrosion residue.

Think like the battery in your truck, dump the acid, electrolyte, out and you stop the battery's ability to generate electricity.

Take it all apart, reseal all the threaded connections, corrosion will cease.