Carbonation is the reaction of atmospheric carbon dioxide with calcium hydroxide in concrete or mortar mixes.  This creates carbonic acid which lowers the pH of the mortar or concrete.  Steel encased in fresh mortar or concrete does not corrode due to the high pH of the alkaline condition.  As long as the pH of the electrolyte surrounding the steel is high, the steel is 'passive' and can not corrode.  As the carbonation process takes place, and the pH drops, the steel is now in an enviroment in which it can corrode. 

Electrochemical treatments such as realkalization or cathodic protection alter the condition of the electrolyte back to an alkaline environment, thus allowing further protection of the steel.