Corrosion inhibitors are admixtures that either extend the time to corrosion initiation or significantly reduce the corrosion rate of embedded metal, or both, in concrete containing chlorides in excess of the accepted corrosion threshold value for the metal in untreated concrete, as per ACI 222R.
There are four common type of corrosion inhibiting admixtures, and their dosage is usually dependent upon the client’s expected serviceable life of the structure and on a range of factors that affect the durability of concrete. These include cement type, water-to-cement ratio, cover concrete to the steel, ambient temperature and the expected level of exposure to chlorides
Corrosion-inhibiting admixtures are effective after the concrete has hardened and give a long-term increase in the passivation state of steel reinforcement and other embedded steel in concrete structures.
Why Corrosion Inhibiting Admixtures are Used?
Aggressive substances such as chloride and carbonation could jeopardize passivation layer of iron hydroxides on the steel surface and corrosion would eventually occur.
For these reasons, admixtures that mitigate the corrosion process are useful in extending the life of concrete structures such as highways, multi-storey car parks, jetties, wharves, mooring dolphins, and sea walls.
Types of Corrosion Inhibiting Admixture
The four most common types of corrosion inhibiting admixture are:
1. Amine Carboxylate
- Available as concentrated liquids or powders.
- They are absorbed on to the steel bars surfaces and create protective molecular layer, as shown in Fig. 1.
- The protective layer of amine carboxylate prevents further reactions between corrosive elements and embedded reinforcement, and decline existing corrosion rates.
- Retard setting times 3 to 4 hours at 20°C.
- Reduce chloride-induced corrosion of any good-quality concrete from seawater, salt-laden air, and deicing salt exposure.
- Effective in corrosion reduction due to carbonation or chloride or combination thereof.
- The standard dosage rate is 0.6 to 1 L/m^3 for liquids and 0.6 kg/m^3 for powder versions.
- It can be added to concrete at concrete plant or job site as a powder.
- Compatible with pozzolans or slag, and do not affect the finishing properties of the concrete when used in combination with them.
- Adjustment to mixture design is not needed
2. Amine-ester Organic Emulsion
- Available as a milky-white emulsion.
- Creates protective layer on the steel surface and a decrease chloride permeability of the concrete
- The recommended dosage is 5 L/m3 to provide effective corrosion inhibition, minimizing the impact of the inhibitor on the fresh and hardened properties of concrete such as air entrainment and compressive strength.
- For severe corrosion environments, corrosion inhibitors in combination with supplementary cementitious materials, low w/cm equal or less than 0.40, and adequate cover over steel.
- It is used to extend the life span of reinforced concrete structures subjected to chlorides.
- It should be blended with good quality concrete with largest w/c ratio of 0.40 and adequate concrete protection over steel bars.
- It can be used in good-quality concrete with a maximum w/cm of 0.40 and an appropriate level of clear cover over the reinforcing steel.
- If compressive strength reduction is unacceptable, then slight lower dosage should be used to compensate for that. However, mixture design adjustment is not needed when compressive strength meets design requirements.
3. Calcium Nitrite
- Available as a 30% solution
- It is categorized as an anodic inhibitor that interferes with the chloride complexing process by oxidizing the more easily attacked form of iron to the more stable form.
- High volume (30 L/m^3) of calcium nitrite is need to achieve desired results.
- Calcium nitrite is appropriate to use for reducing chloride induced corrosion of any good-quality concrete, from seawater, salt-laden air, and deicing salt exposure.
- It is not applicable for poor-quality concrete or concrete with very low clear cover over the reinforcing steel.
- w/c ratio of 0.40 or smaller should be used when calcium nitrite is added to concrete. However, w/c of 0.45 in combination with pozzolan or slag can be used in case of moderate design life concrete construction.
- Calcium nitrite is an accelerator of both set and strength development of concrete.
- Increase the strength of concrete significantly at early ages especially at 29 days.
4. Organic Alkenyl Dicarboxylic Acid Salt
- Available as a water-based solution.
- The organic alkenyl dicarboxylic acid salt is also known as DSS.
- It can be classified as a dual-action corrosion inhibitor, affecting the anodic reaction at the steel and restricting moisture used in the cathodic reaction.
- Dosage of 5L/m^3 is adequate for chlorides in groundwater.
- For more severe exposures such as bridge decks that are salted or marine applications 10 L/m^3 is recommended.
- It is also appropriate for use in reducing chloride induced corrosion of properly proportioned concrete from seawater, salt-laden air, and deicing exposure.
- Properly proportioned concrete should have a maximum w/cm of 0.40 and the appropriate clear cover over the reinforcing steel.