My first reaction is that this is not crevice corrosion. But if it looks like a crevice and corrodes like a crevice, we can call this crevice corrosion.
The is carbon steel exposed to the environment, so isolation is the first strategy. Something more robust than painting over the strap is needed; ingress of moisture was inevitable, as evidenced by your pictures.
I suggest you talk to pipe coating experts for a coating appropriate for your local climate.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
