..what is the most probable damage mechanism (local thinning, general corrosion ? SOHIC ? ----- )
..then you have to quantify the damage: extend , thickness measurement , dimension and depth of the -----(s)
By that you would know which chapter of API 579 you have to apply.

Start with level I and if more accuracy is needed or if your weld does not "pass" go to level II or even III

API 580 and also API 581 (More technological and pracical) can help to make a risk Based assessment

All above is my own understanding, the other collegues can also correct/ add