I noticed corrosion all over one of my battery terminals so I replaced the entire terminal and cleaned it all pretty but within 3 weeks it’s already starting to corrode again. What’s causing that?
Check for a leak/crack around that battery post.
If there’s leak/crack as Tester mentioned, try sealing it with asphalt (after cleaning/washing off the corrosion), and smearing grease on the terminals & posts. I learned them from my father.
Sounds like the seal around the post has failed, a common battery problem. When clean and dry, re-seal the post with automotive silicone (RTV), a small bead between the case and post. Then use a pair of “anti-corrosion” felt washers help isolate the connector from the fumes/acid. That’s about all you can do. Use baking soda to clean your new connector…
If it has a leak at the post the battery company should prorate a replacement or just replace it.