Root Concavity

If the root burns through or melts away during welding, that's called root sagging. This often happens when the weld pool is too thin or you have no backing on the back side. Then the weld becomes narrow and weaker, and the back side corrodes faster. To prevent this, you need to control the heat and ensure enough material and protection at the root. For critical parts, you can also cover the back side with a copper backing or shielding gas.