**2. Corrosion protection techniques**

As discussed earlier, metal corrosion is an electrochemical reaction that requires water, oxygen, and ions like chloride ions, which already exist in the atmosphere. These atmospheric ions are abundant near the coastline as the air carries these ions from the available saline water (**Figure 1**).

Moreover, pollutants such as carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and nitrous oxide (NO2) present in the environment also be significant in the corrosion process. **Figure 2** demonstrates the electrochemical reactions of corrosion of the iron metal.

**Figure 1.** *Corrosion protection techniques.*

**Figure 2.** *Corrosion of iron metal.*

**Figure 3.** *Galvanic corrosion.*

This multifaceted phenomenon of corrosion adversely affects and causes the deterioration of materials. Millions of dollars are infused throughout the infrastructure sector from corrosion prevention and control.

When two or more dissimilar materials having different potentials come in contact with each other in the conductive electrolyte, due to the generation of current, the more reactive metal will corrode in preference to the less reactive metal. Corrosion will occur at the point where the current leaves the metal surface. **Figure 3** shows the basics of galvanic corrosion.

Before going into detail about the causes of corrosion in steel reinforcement, steel cables, and structural steel, let us discuss the types of corrosion protection techniques.

### **2.1 Active corrosion protection techniques**

Active corrosion protection techniques focus on halting or neutralizing corrosion electrochemical reactions. The active corrosion protection techniques inhibit corrosion on the material to be protected. It is the application of the reactive compound to disrupt the normal formation of anodes on the materials. **Figure 4** shows the basics of the active corrosion protection mechanism. The more reactive metal becomes the sacrificial anode to protect the less reactive metal, which acts as a cathode.

#### **2.2 Passive corrosion protection techniques**

Passive corrosion protection techniques focus on the isolation of the material from corrosion-causing elements to constraint corrosion. With passive protection, a protective coating, for example, may act as a barrier that prevents air and moisture from coming into contact with the underlying iron substrate. With these two elements out of the picture, corrosion cannot occur on the surface of the metal. **Figure 5** shows the basic mechanism of the passive corrosion protection technique.

In the active corrosion protection technique, the material remains exposed to corrosion-causing elements while various processes actively counteract the material corrosion. However, passive corrosion protection techniques involve the separation of material from the corrosion-causing elements.

#### **2.3 Active corrosion protection techniques**

As discussed earlier, one of the most adopted methods of active corrosion protection techniques is the use of cathodic corrosion protection. Cathodic corrosion protection involves a direct or indirect connection with a more reactive material to the

**Figure 4.** *Active corrosion protection.*

**Figure 5.** *Passive corrosion protection.*

material to be protected. It is a simple method of diverting the corrosion to the sacrificial material while the other material remains protected.

For example, the addition of inorganic zinc inhibitive pigments such as zinc phosphate (Zn3(PO4)2) in steel offers active anticorrosive protection to the steel substrate by hydrolyses in water to produce zinc ions (Zn2 + ) and phosphate ions (PO4 <sup>3</sup>). The zinc and phosphate ions act as cathodic and anodic inhibitors, respectively. The phosphate ions phosphating the steel and rendering it passive is another advantage of using zinc phosphate. **Figure 6** shows the mechanism of the zinc phosphate corrosion inhibitor.

The following are the principal active corrosion protection techniques.

**Figure 6.** *Corrosion inhibitor.*

#### *2.3.1 Material doping*

Material doping, also known as alloying, is a method in which one or more elements or compounds are doped into the material to change material properties, like increased corrosion resistivity. Alloying is the most effective method to control corrosion. Humankind has already revolutionized the world various times by developing alloys such as bronze, steel, brass, alnico, nichrome, cast iron, and carbon steel. Stainless steel is a mixture of iron, chromium, carbon, nickel, molybdenum, titanium, niobium, manganese, and more. The stainless steel is stainless material, not fully corrosion-resistant. However, due to the added strength and resistance to corrosion of stainless steel, stainless steel is preferred over other materials for construction in the infrastructure segment.

For example, nickel has good corrosion-resistant properties, and chromium has good oxidation-resistant properties. When nickel and chromium doped into the material, the resultant alloy gives the best resistance in highly oxidized and reduced chemical environments. **Figure 7** shows the principal mechanism of chromium and nickel doping in the iron to form iron alloy or steel. Chromium and nickel having inherent corrosion-resistive properties develop the protective oxide layer, and change the crystalline structure of iron from ferritic (Body Centered Cubic Crystal) structure to austenitic (Face Centered Cubic Crystal) structure, respectively.

Different alloys provide different resistance to different environments. However, despite the effectiveness of alloys, the doping process makes them very expensive. Sometimes so expensive that the replacement cost of the highly corroded complete structure becomes economical.

#### *2.3.2 Cathodic protection*

Cathodic protection is one of the most effective methods used for corrosion control. Cathodic protection protects the material by converting the active sites of the material to passive sites by providing electrons from galvanic anodes attached to or near the material. Generally, used materials for galvanic anodes are aluminum, magnesium, or

**Figure 7.** *Metal doping corrosion protection.*

### *Corrosion Protection and Modern Infrastructure DOI: http://dx.doi.org/10.5772/intechopen.111547*

zinc. Zinc is the most widely used metal for the protection of steel, as zinc metal in direct contact with steel offers protection through the preferential oxidation of zinc metal. The rate of corrosion of zinc is also slow compared with steel. **Figure 8** demonstrates the basic electrochemical corrosion reactions of zinc metal anode to protect the iron metal. However, in the presence of ions such as chlorides in coastal regions, this reaction rate gets accelerated, limiting the zinc protection use in the coastal area.

Cathodic protection is highly effective, but the high anode consumption requires frequent checks and replacements, increasing the cost of maintenance. Further, an anode increases the overall weight structure and is ineffective in high-resistivity environments, constraining the utilization of cathodic protection. **Figure 9** demonstrates the basic electrochemical corrosion reactions of sacrificial zinc metal anode to protect the reinforcement steel bars of the pile foundation.

**Figure 8.** *Cathodic protection of iron metal.*

**Figure 9.** *Cathodic protection of reinforcement steel bars.*

Cathodic protection is adopted globally to protect offshore production platforms, pipelines, water storage tanks, water treatment plants, boat hulls, ships, piers, reinforcement bars in concrete structures, and more.

#### **2.4 Passive corrosion protection techniques**

In passive corrosion protection techniques, the corrosion damage is prevented by mechanically isolating the materials, using protective layers, films, or coatings, from the corrosion-causing elements. Passive corrosion protection techniques neither change the corrosion resistivity of the material nor the corrosivity of the corrosioncausing elements. The main drawback of passive corrosion protection techniques is that at any point if the protective layer, film, or coating is destroyed or damaged, the corrosion of the material will occur. Passive corrosion protection techniques are used to protect the material at the place of use for relatively mild environmental conditions. Harsh environmental conditions generate stresses and reduce the effectiveness of the protective layer, film, or coating.

For example, metal oiling is one of the best and most conventional methods used for corrosion protection. The protective layer of oil does not allow water or hydrophilic electrolytes to complete the electrochemical reaction of the corrosion. Further, the penetration of oil into holes, cavities, and difficultly accessible areas makes the corrosion protective layer more efficient. However, oiling is avoided in watersubmerged applications and high hygiene or safe working environment area. The following are the principal passive corrosion protection techniques.

#### *2.4.1 Coating*

This passive corrosion protection technique is based on providing a barrier coating to the material to prevent exposure to corrosion-causing elements, which are oxygen, water, and ions. **Figure 10** shows the basic composition of the paints. Painting is one

**Figure 10.** *Paint coatings.*

*Corrosion Protection and Modern Infrastructure DOI: http://dx.doi.org/10.5772/intechopen.111547*

of the easiest and cheapest ways to prevent corrosion. Paint acts as a barrier between material surface and corrosion-causing elements. The combination of different paint layers acts as a different corrosion protection function. The primer coat acts as an inhibitor, the intermediate layer provides strength, and the outer layer protects from the environment.

Based on the severity of the environment, various coatings can be applied. Powder coating, metallic coating, and organic coating are the principal coating types.

#### *2.4.1.1 Powder coatings*

The powder coating technique is a process of electrothermal fusion of a powder on the clean surface of the material to be protected. The dry powder is static electrically charged and deposited on the oppositely charged or grounded material forming a smooth and continuous film. This film along with the material is heated, and a protective layer of powder is fused with the material to protect from corrosion. This technique can provide coating thicknesses in the range of 25 to 125 micrometers. Generally, powders of acrylic, vinyl, epoxy, nylon, polyester, and urethane are used for coating. **Figure 11** shows the general powder coating process applied in the industries.

Compared with conventional liquid paint where paint is delivered through evaporation of the solvent, the powder coating is applied electrostatically and then cured under heat or with ultraviolet light, this creates a hard finish layer with durability, to withstand damage and last longer.

#### *2.4.1.2 Metallic coatings*

The metallic coating is preferable where the pores-free or damage-free coat of more noble material can be applied on the material to be protected from corrosion. These noble materials can be a metal or alloys. The metallic coating is applied using a sprayer, electrochemically, chemically, or mechanically.

**Figure 11.** *Powder coating process.*

#### *2.4.1.2.1 Metallic spray coating*

The metallic spray coating technique involves coating material in a molten or semi-molten state. The following are various metallic spray coating processes. **Figure 12** shows the general metallic spray coating process.

**Plasma spray:** The plasma spray coating process utilizes the plasma jet to melt the metallic powder coating material, which then sprays onto the material to be protected.

**Detonation spray:** The detonation spray coating process utilizes a very-high shockwave to coat molten or partially molten coating materials onto the surface of the material to be protected.

**Arc wire spray:** The arc wire spray coating process utilizes an electric arc to melt the metallic powder of coating material, which then pneumatically spray onto the surface of the material to be protected.

**Flame spray:** The flame spray coating process utilizes the flame to melt the metallic powder and compressed air to atomize and propel the coating material onto the surface of the material to be protected.

**Warm spray:** The warm spray coating process involves the deposition of heated metallic powder at supersonic speed onto the surface of the material to be protected.

**Cold spray:** The cold spray coating process utilizes a very high speed of carrier gas to generate high-impact forces on the metallic powder. These high-impact forces create a protective letter.

Metallic spray coating is used to coat material to protect against extremes of temperature, corrosion, erosion, and general wear and tear. Tungsten carbides, ceramics, nickel-chrome carbides, aluminum, steels, and plastics are some of the materials used to apply them as coating materials.

#### *2.4.1.3 Electrochemical metallic coating*

Electrochemical metallic coating, also known as electrocoating, is the process in which electrically charged particles are deposited on the material surface to form a

**Figure 12.** *Metallic spray coating process.*

*Corrosion Protection and Modern Infrastructure DOI: http://dx.doi.org/10.5772/intechopen.111547*

**Figure 13.** *Electrochemical metallic coating.*

protective coating on the material to be protected. **Figure 13** shows the two types of generally used anodic electro-coating and cathodic electro-coating processes in the industries.

Generally, the metal ions deposited on the material are cadmium, chromium, nickel, and zinc. Electroplating provides very high control over protective coating. By controlling temperature, current, voltage, metal ion concentration, and solution of the coating tank, in which the material to be protected will be immersed, up to 1 μm of protective coating is possible.
