Corrosion is electro-mechanical process and all reinforcement bars are connected to each other from top to bottom and left to right.
In overall structure there are only two points required where positive and negative charges get generated and it will began the process of corrosion of reinforcement steel bars in structure.
For more specific answer do post photo of affected area along with details of exposure condition to get more detailed idea about it.
At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1 µm per year.
However, corrosion can occur when the passive layer is destroyed. The destruction of the passive layer occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level.