This is something I wasn't aware of that is fascinating. The ph balance of the rebar's environment determines the progression of oxidation. There was a guy in the 1920's who observed this and had the idea to put cement in paint; it was so successful he renamed the company to cement spelled backwards.

  Inside the concrete, Ca(OH)₂, which is generated by the hydration of cement, 
  creates a strongly alkaline environment, with a pH value of more than 12.7. 
  In this environment, a passivation film forms on the surface of the rebar to 
  prevent corrosion. However, the pH of the concrete decreases as micro-cracks 
  develops and chloride from the outside infiltrates the rebar. Eventually, as 
  the environment inside the concrete becomes neutralized, the corrosion of the 
  rebar accelerates, which reduces the overall durability and stability of the 
  reinforced concrete structure.

  A sighting of preserved steel reinforcing bars sticking out of old concrete 
  rubble piqued the curiosity of our company founder, Albert C. Bean, Sr. After 
  investigating this finding, his company, Armor Oil & Chemical Company, began 
  producing a patented cement-filled coating formulation that, in its updated 
  form, still protects structures from corrosion today.