Rebar corrosion in general starts at the extent the place moisture and salts achieve reinforcement
https://quartz-consonant-5d5.notion.site/Spalling-repair-on-overhead-concrete-requires-extra-attention-because-loose-material-can-fall-from-c-3a70ccc9b46b8014869ecc29530b1669
Rebar corrosion pretty much starts at the level where moisture and salts achieve reinforcement. Mapping crack paths can disclose most probably corrosion pathways for unique paintings.