That appears to be Intergranular corrosion. Once started its going to continue.
Grinding the corrosion out and then grinding 1.5X deeper to ensure you got it all is a common rule of thumb. Most parts never survive the repair and are too weak.
https://image.slidesharecdn.com/corr...?cb=1485583631
https://image.slidesharecdn.com/corr...?cb=1379290236
A new timing cover is the real fix.