What is chloride-initiated reinforcement corrosion?

Chloride-initiated reinforcement corrosion occurs as a result of the presence of chlorides in the concrete. Chlorides can get into the concrete in several ways:

• As additions in the form of calcium chloride to accelerate hardening (nowadays prohibited);
• From the ground as it contains chloride (magnesite floors);
• Contaminants in the additives;
• Penetration in which the chlorides come from thaw salts, seawater or, for example, pool water.

In addition to the above, it should be considered that chloride ions can be carried in the air. For example, by wind from the sea or chloride-containing fumes in swimming pools.
Free chloride ions affect the passivation layer of the reinforcing steel and react easily becoming soluble iron chloride (FeCl2). In the follow-up reaction, the iron chloride reacts with OH–ions from the pore water of the cement stone:

FeCl2+ 2(OH–) → Fe(OH)2+ 2Cl–

Then the OH-ions (hydrogen ions) get broken down, reducing the pH value of the concrete. The chloride ions are not consumed and are available for further reaction. In this way, the rebar can be severely affected locally.
Chloride-initiated reinforcement corrosion therefore usually occurs in the form of localized corrosion (very deep and strong corrosion in one point). Especially with mixed chlorides, there can be large differences in chloride concentration locally, which increases the risk of local well-shaped corrosion.
The degree of penetration of chloride depends mainly on the quality of the concrete cover. This quality is influenced by the composition and water-cement ratio, the degree of compaction and the post-treatment method.

Other factors that affect the degree of penetration and the effect on reinforcement corrosion are the cement content (the more cement, the more chloride can be bound), the type of cement and the occurrence of cracks. In general, it appears that structures that alternately come into contact with chloride and dry up again (splash zones, bridge decks) are most likely to suffer damage.
Reinforcement corrosion initiated by chlorides can in practice be recognized by the following characteristics:

• Rust marks on the surface in the form of dark, brown spots;
• General cracking on the spot of the bars;
• Usually, little detached concrete cover;
• The reinforcement is locally rusted and shows black rust points;
• The rust products are soluble and often can be observed as stains on surrounding concrete next to the reinforcement.