Highly cracked, delaminated cover
When reinforcements are strongly corroded, their corrosion products can expand, and deteriorate the concrete cover, by cracking, delamination and spalling. Then, the first action to be undertaken is to eliminate these geometrical defects, by replacing concrete cover, or sometimes injecting cracks.

As this first action relates to only the zones under high corrosion, the close zones, where corrosion can appear in a short term, must be treated by other methods.

Cover physically sound, but polluted
In some cases, concrete reinforcement can be depassivated (i.e. corroding) or will corrode shortly. Its cover seems physically satisfactory and has neither crack or delamination. But, it is then convenient to slow down this corrosion rate, even to stop it. The methods which can be proposed are either concrete impregnation with water-proof products (sealants) or inhibitors, or an electrochemical treatment : cathodic protection, re-alkalisation or chloride extraction.

Still sound cover
When reinforcement is embedded in a sound cover and its corrosion is likely initiated only in short or long term, it can be useful to protect them by impregnating concrete cover.

Paintings and coatings of various thickness can also be applied on concrete to improve its resistance to liquid penetration. It deals, for example, with either of coatings containing epoxy resin or polyurethane, or with mortar containing modified hydraulic binder.

Reference- http://www.concretecorrosion.net/