**4. Corrosion protection techniques for steel reinforcement**

Steel reinforcement corrosion is not visible, and corrosion identification and prevention becomes much more challenging. The following concrete corrosion control methods are used to prevent corrosion in reinforced concrete structures.

#### **4.1 Corrosion inhibitors**

Corrosion inhibitors are chemicals that were added to the concrete in small concentrations to inhibit the corrosion in the concrete structure. Corrosion inhibitors increase the passivation of steel reinforcement and can inhibit the corrosion when passivation would otherwise have been lost as a result of chloride ingress or carbonation. The well-known and widely used corrosion inhibitors are calcium nitrate, phosphate, benzoates, amine carboxylate, amine-ester organic emulsion, and organic alkenyl dicarboxylic acid salt [1].

$$2\text{ NO}\_2^{(-)} + 2\text{Fe}^{(2+)} + 2\text{OH}^{(-)} \rightarrow 2\text{NO}\_2\uparrow + \text{Fe}\_2\text{O}\_3 + \text{H}\_2\text{O} \tag{19}$$

The commonly used nitrite inhibitor in reinforced concrete structures involves the nitrite ions (NO2 �) to inhibit the electrochemical reaction of corrosion. The nitrite ions react with ferrous ions (Fe2+) in the presence of hydroxide ions, to form a passive iron oxide layer on the iron surface, inhibiting the movement of ferrous ions from the anode [1].

#### **4.2 Reinforcement coating**

The reinforcement coating technique prevents corrosion by isolating the rebars with corrosion-causing elements by applying coats of paint and epoxy. The fusionbonded epoxy is widely used in rebar coating as it is a fast-curing process and forms a thermosetting protective coat on the rebars [2]. The dry powder is applied on preheated steel. The preheated steel melts the dry powder and cures on the surface of the rebar to form a uniform coating thickness. Earlier, rubber was used in the fusionbonded epoxy. Nowadays, a different combination of dry powder materials is used to get the most effective protective coating for the specified application in the environment. However, for this protective coating to be effective, the protective coat must be bonded to the rebars during the entire structure life.

#### **4.3 Concrete coating**

Concrete coating provides corrosion control by improving the impermeability with beautification of the structure. Concrete coatings protect the concrete and the reinforcement steel, even for the contaminated concrete by chlorides [3].

Concrete coating provides corrosion control by improving the impermeability along with the beautification of the structure. Concrete coatings protect the concrete and the reinforcement steel, even for the contaminated concrete by chlorides. Coats of liquid or semisolid material, such as epoxies, polyurethanes, acrylics, polyureas, and polymer-coated metal boards, are applied to cured concrete. These covers act like a barrier and prevent electrolyte intrusion into the concrete.

The modern infrastructure uses new materials like polymer vapor deposited on metal sheets, doped glass, and lightweight steel to cover the concrete structures. The combinations of different materials generate the electric charge potentials. Hence, the generated electrostatic charges accelerate the corrosion of the fittings that hold these materials on the concrete structure. In the high rise, where wind load requires a lightweight material with flexibility, failure of fittings shall be hazardous for the nearby area.

#### **4.4 Cathodic protection**

The cathodic protection technique covert the steel reinforcement to the cathode to control the corrosion. When the steel reinforcement becomes cathodic, the hydroxyl ions form a passive layer on the surface. When the cathode is connected to a less noble metal like zinc in the absence of an external power supply, the anode is referred to as a sacrificial anode [4].

When the cathode is connected to an external power supply, it forces a small amount of electric current to counteract the current flow generated from the electrochemical reaction of corrosion. This process is known as Impressed Current Cathodic Protection (ICCP). For such applications, graphite, High Silicon Cast Iron (HSCI), platinum, or mixed metal oxide are used as an anode, because of having a very slow rate of consumption. Cathodic protection is preferred to protect horizontal slabs, walls, towers, beams, columns, and foundations. The following are the electrochemical reactions happen in ICCP.

Cathode Side:

$$\text{2H}\_2\text{O} + 2\text{e}^{(-)} \rightarrow \text{H}\_2\uparrow + \text{2OH}^{(-)} \tag{20}$$

Inert Anode Side:

$$\mathbf{4OH}^{(-)} - \mathbf{4e}^{(-)} \to \mathbf{2H}\_2\mathbf{O} + \mathbf{O}\_2 \uparrow \tag{21}$$

However, the ICCP system is not recommended for prestressed concrete structures as the generated hydrogen makes the high-strength steel brittle [5]. Moreover, it is difficult to confirm the electrical continuity of the system (**Figure 21**).
