How to fix an EFI boot partition when it gets corrupted.
Device /dev/sdb is the corrupted disk and has 2 partitions sdb1 and sdb5 respectively boot and root.
Partition sdb1 is normally the mounted as /boot/efi.
To run grub-install both need to be mounted.
Create directory /mnt/root/ to mount the root filesystem of partition sdb5.
mkdir -p /mnt/root
mount /dev/sdb5 /mnt/root
Mount the boot partition on the previous mounted filesystem.
mount /dev/sdb5 /mnt/root/boot/efi
(Re)install grub with grub-install.
grub-install --root-directory=/mnt/root /dev/sdb