How do I encrypt a new SD card? I'm on Android Nougat with an encrypted phone. I added a new SD card but cannot find an option to encrypt it too. 

How do I go about encrypting the new SD card?
On my Moto G4 running Android 7.0:


Settings -> Storage; select the SD card (OK to mount the SD card if prompted to do that).  This shows the contents of the SD card.
From the drop-down menu in the top-right corner (3 vertical dots):  choose "Storage Settings".
Choose "Format as internal"