Corrosion inhibitors are substances that can be added to concrete to protect embedded metal from corrosion. When concrete contains high levels of chlorides, the inhibitors can either delay the onset of corrosion or reduce the corrosion rate, or both. According to ACI 222R, the accepted corrosion threshold value for untreated concrete is exceeded when chlorides are present in high concentrations.
There are four common types of corrosion-inhibiting admixtures that can be used in concrete. The dosage of these admixtures is dependent on several factors that affect the durability of the concrete, including the type of cement used, the water-to-cement ratio, the depth of cover over the steel, the ambient temperature, and the expected level of exposure to chlorides. The client’s desired service life for the structure is also a key consideration.
It’s important to note that these admixtures are effective only after the concrete has hardened. They work by increasing the passivation state of steel reinforcement and other embedded steel in concrete structures over the long term. This provides ongoing protection against corrosion, helping to ensure the long-term durability and safety of the structure.
Why Corrosion Inhibiting Admixtures are Used?
The passivation layer of iron hydroxides on the surface of steel is vulnerable to aggressive substances like chloride and carbonation. If the passivation layer is compromised, the steel is at risk of corrosion, which can lead to significant damage over time. To prevent this from happening, it is helpful to use admixtures that can mitigate the corrosion process.
By incorporating these admixtures into concrete structures, the lifespan of these structures can be extended. This is particularly useful for structures that are exposed to harsh environments, such as highways, multi-storey car parks, jetties, wharves, mooring dolphins, and sea walls. These structures are constantly exposed to moisture, salt, and other corrosive agents, making them susceptible to corrosion.
Using corrosion-mitigating admixtures can provide a layer of protection to these structures, reducing the likelihood of corrosion and prolonging their lifespan. This is important not only for ensuring the safety of these structures but also for reducing the need for costly repairs and replacements in the future. Therefore, incorporating such admixtures into the construction process can be highly beneficial for the long-term durability of concrete structures.
Types of Corrosion Inhibiting Admixture
Corrosion inhibiting admixtures are commonly used in concrete to protect the embedded steel reinforcement from corrosion. These admixtures work by reducing the rate at which the concrete conducts electricity, which slows down the electrochemical reactions that cause corrosion. There are various types of corrosion inhibiting admixtures available, but four of the most common ones are calcium nitrite, sodium nitrite, organic corrosion inhibitors, and amino alcohols.
Calcium nitrite is a commonly used admixture that has been shown to be effective in reducing corrosion in concrete. When added to concrete, it reacts with the free chloride ions to form calcium chloride and nitrate, which are less aggressive to the steel reinforcement than chloride ions.
Sodium nitrite is another popular admixture that works by forming a protective film on the steel reinforcement. This film reduces the rate of corrosion and helps to extend the service life of the concrete.
Organic corrosion inhibitors are admixtures that contain organic compounds such as amines, amides, and carboxylic acids. These compounds form a protective layer on the surface of the steel reinforcement, which prevents the penetration of chloride ions and other corrosive agents.
Amino alcohols are a type of corrosion inhibiting admixture that works by reducing the rate of corrosion in concrete. These admixtures react with the calcium hydroxide in the concrete to form a protective film on the surface of the steel reinforcement, which slows down the rate of corrosion.
1. Amine Carboxylate
Amine carboxylates are a type of corrosion inhibitor that can be found in concentrated liquid or powder form. When applied to steel bars, they form a protective layer that prevents corrosive elements from causing further damage to embedded reinforcement, ultimately reducing corrosion rates. However, it is important to note that the setting times of the concrete will be slowed down by three to four hours at a temperature of 20°C.
These inhibitors are particularly effective in reducing chloride-induced corrosion caused by exposure to seawater, salt-laden air, and deicing salts. They can also mitigate corrosion due to carbonation or a combination of chloride and carbonation. The standard dosage rate for liquid versions is 0.6 to 1 L/m^3, while the powder version is dosed at 0.6 kg/m^3. When used in combination with pozzolans or slag, these inhibitors do not affect the finishing properties of the concrete, and no adjustment to the mixture design is necessary.
In terms of application, amine carboxylate inhibitors can be added to the concrete mix at either the concrete plant or the job site, depending on the preference of the contractor. If the powder version is used, it can be easily mixed into the concrete on-site. The compatibility of these inhibitors with other materials, such as pozzolans or slag, makes them a versatile option for concrete construction projects.
2. Amine-ester Organic Emulsion
A milky-white emulsion is available that can be used to extend the lifespan of reinforced concrete structures that are exposed to chlorides. This emulsion forms a protective layer on the surface of steel and reduces chloride permeability in concrete. To provide effective corrosion inhibition, a recommended dosage of 5 liters per cubic meter should be used. This dosage minimizes the impact of the inhibitor on the fresh and hardened properties of the concrete, such as air entrainment and compressive strength.
In severe corrosion environments, it is recommended to use corrosion inhibitors in combination with supplementary cementitious materials, low w/cm ratios of equal or less than 0.40, and adequate cover over the steel. The emulsion should be blended with good quality concrete with the largest w/c ratio of 0.40 and adequate concrete protection over steel bars.
The emulsion 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, a slightly lower dosage can be used to compensate. However, if the compressive strength meets the design requirements, no mixture design adjustment is needed.
3. Calcium Nitrite
Calcium nitrite is a chemical solution that is available in a concentration of 30%. It belongs to the category of anodic inhibitors that work by interfering with the process of chloride complexing. This chemical oxidizes the more easily attacked form of iron in concrete to a more stable form, thus preventing chloride-induced corrosion.
When using calcium nitrite, a high volume of 30 liters per cubic meter of concrete is required to achieve the desired results. It is effective in reducing chloride-induced corrosion of good-quality concrete exposed to seawater, salt-laden air, and deicing salt exposure. However, it is not recommended for poor-quality concrete or concrete with very low clear cover over the reinforcing steel.
To ensure the best results, a water-cement (w/c) ratio of 0.40 or smaller should be used when adding calcium nitrite to concrete. In cases where moderate design life concrete construction is required, a w/c ratio of 0.45 can be used in combination with pozzolan or slag.
Calcium nitrite is not only effective in preventing corrosion, but it also acts as an accelerator of both set and strength development of concrete. It has been observed that the addition of calcium nitrite to concrete increases its strength significantly at early ages, especially at 29 days. This property of calcium nitrite makes it an excellent choice for concrete construction projects that require high strength and quick setting times.
4. Organic Alkenyl Dicarboxylic Acid Salt
DSS, which stands for organic alkenyl dicarboxylic acid salt, is a type of dual-action corrosion inhibitor that can be found in the form of a water-based solution. It works by affecting the anodic reaction at the steel and limiting the moisture used in the cathodic reaction.
For chlorides in groundwater, a dosage of 5L/m^3 is considered sufficient, whereas for more severe exposures such as bridge decks that are salted or marine applications, a higher dosage of 10L/m^3 is recommended.
DSS is also appropriate for use in reducing chloride-induced corrosion of properly proportioned concrete that is exposed to seawater, salt-laden air, and deicing agents. To ensure the effectiveness of this corrosion inhibitor, the concrete should be properly proportioned with a maximum water-to-cement ratio of 0.40 and have the appropriate clear cover over the reinforcing steel.