Run the following command to encrypt the /dev/sdb1 partition:
sudo cryptsetup --verify-passphrase luksFormat /dev/sdb1 -c aes -s 256 -h sha256
sudo cryptsetup luksOpen /dev/sdb1 NAME
Now you can mkfs on /dev/mapper/NAME and mount/unmount it:
sudo mkfs -t ext3 -m 1 /dev/mapper/NAME
sudo mount /dev/mapper/NAME /mnt
sudo umount /mnt