Water Damage Replacement inside back cover

I spilt water on my keyboard and it got inside my mac. I took off the back cover and found the damage was limited to the PCIe SSD, so I plan to remove and replace that. However, on the inside of the back cover, there is the think black lining. On that lining, where it would match up with the SSD is corosion from the water. I want to make sure that is taken care of before I replace the SSD. What exactly is this thin black lining - what is it responsible for - how do i fix it - how much does it cost to fix it? Thank you in advance!!