External
Exposure to chlorides can occur in regions that experience freezing temperatures so that de-icing salts must be used. Reinforced concrete bridges are at high risk to deterioration from chloride penetration. The Environmental Protection Agency estimates that $500 dollars/year is spent on renovation and replacement of bridges and roadways damaged by de-icing salts. The problem of external chloride penetration does not end at the roadway. Chlorides in solution may adhere to cars and pedestrians transporting these salts and affecting parking garages, walkways, and even building entrances that may not be directly exposed to weather (e.g. from people kicking snow off their feet before entering buildings).
Chloride penetration can also be a problem in marine environments. Concrete docks and piers are exposed to the high chloride content of salt water. However, corrosion is seldom a problem in structures that are submerged. This is apparently due to the lack of diffused oxygen available. The concrete will allow the penetration of chlorides and water, but without the presence of oxygen, corrosion will not occur. In this case, the concrete cover over rebar should provide significant protection.
Internal
In some cases, chlorides added to the concrete during mixing, whether intentionally or
unintentionally, can help corrosion. Some aggregates contain traces of chlorides within
pores, which become free ions when water is added to the mixture. Certain chemical
admixtures contain chloride compounds, which may ionize aiding corrosion. Calcium chloride
is an accelerating admixture which may increase the amount of free chlorides available for
corrosion. The severity of chlorides added by admixtures is under some debate, because
these chlorides may remain in compound form, and therefore would not contribute to
corrosion.