Corrosion Protection and Modern Infrastructure

Abstract

Currently, modern infrastructures utilize different materials such as metals, alloys, glass, plastic, wood, ceramic, silicate brick, and natural stones. The material’s diverse composition, structure, chemical, physical, and mechanical properties with ease in utilization make metal utilization a priority for architects and civil contractors. The principal property that determines the quality and durability of infrastructure is the corrosion resistance and weathering resistance. The corrosion of metals can cause damage to concrete, building stones, wood, and other materials, leading to corrosive destruction. The corrosion protection of the structural steel used in reinforcements, load supports, and frames has become extremely important to meet the demand of modern infrastructure having 100 to 120 years of service life.

1. Introduction

The term corrosion originated from the Latin word corrodere, meaning gnawing to pieces. Corrosion is the process of material deterioration due to electrochemical interactions with the surrounding environment.

Since ancient times, metallic corrosion has been a problem in the utilization of metals. Nearly all the metals present in nature are in the metal compounds, such as oxides, silicates, and carbonates. Hence, after the extraction of metals, these metals tend to become naturally occurring metal compounds once in the environment.

One of the most known examples of corrosion is rusting of iron. The rusting of iron is the formation of iron oxides. The extraction of metal iron from its iron oxides present in the earth’s crust is just a reverse process of the rusting of iron. The reaction of iron with oxygen to form iron oxide is a reversible reaction.

Considerable efforts had made to develop corrosion prevention measures such as metal doping and coatings of other metals, pigments, polymers, and organic materials. However, due to the limitations of these preventive measures, complete corrosion prevention is still unachievable. The main drawback of structure corrosion is that it leads to structure failure catastrophe. One of the well-known catastrophes is the sinking of the Titanic. The utilization of different types of iron in about 3 million rivets in the highly corrosive seashore environment leads to the corrosion of rivet joints. These weakened joints aided in the catastrophe.

As the demand for high-strength and lightweight structures increases, metals become one of the most favorable choices for the structures.

Currently, modern infrastructures utilize different materials, such as metals, alloys, glass, plastic, wood, ceramic, silicate brick, and natural stones, increasing the corrosion prevention challenge. The benefits of the metal’s diverse composition, structure, and chemical, physical and mechanical properties ease metal utilization for modern infrastructure, making corrosion prevention a challenging task.

A combination of incompatible materials in the environment leading to the decline in corrosion resistance and weathering resistance of the infrastructure will reduce the life of the infrastructure. There is a need for the corrosion protection of structural steel, especially for the reinforcements, load supports, and frames that have become extremely important to meet the demand of modern infrastructure having 100–120 years of service life.

In the following sections, we will focus on the corrosion causes in steel reinforcement, steel cables, and structural steel with the advantages and disadvantages of different corrosion protection techniques.

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).

Figure 1.

Corrosion protection techniques.

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 2.

Corrosion of iron metal.

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.

Figure 3.

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.

Figure 4.

Active corrosion protection.

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.

Figure 5.

Passive corrosion protection.

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 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 (PO₄³⁻). 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.

Figure 6.

Corrosion inhibitor.

The following are the principal active corrosion protection techniques.

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.

Figure 7.

Metal doping corrosion protection.

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 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.

Figure 8.

Cathodic protection of iron metal.

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 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 corrosion-causing 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 water-submerged 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 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.

Figure 10.

Paint coatings.

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.

Figure 11.

Powder coating process.

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.

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.

Figure 12.

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 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.

Figure 13.

Electrochemical metallic coating.

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.

2.4.1.4 Chemical metallic coating

Chemical metallic coating is a coating technique where the protective coat is chemically bounded with the material to be protected. There are two principal techniques for chemical coat materials. The nonelectric plating and hot-dip galvanization (Figure 14).

Figure 14.

Chemical metallic coating.

2.4.1.5 Nonelectric plating

Nonelectric or electroless plating is the process where the metal is deposited on the material via a chemical reaction in the presence of a catalyst. The most common nonelectric plating is electroless nickel plating. Electroless nickel plating is generally achieved by depositing the nickel ions with sodium hypophosphite as the reducing agent to oxidize and form a negative charge on the metal surface for the deposition of nickel ions. In this process, the sodium hypophosphite will release the hydrogen as a hydride ion. The overall reaction of electroless nickel plating is as follows.

$$\mathrm{Ni}{\mathrm{L}\mathrm{x}}^{(2+)}+4{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_2^{(-)}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ni}+\mathrm{P}+3{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_3^{(-)}+{\mathrm{H}}^{(+)}+\frac{3}{2}{\mathrm{H}}_2\uparrow +\mathrm{xL}$$E1

As electroless plating chemically generates the charge on the surface of workpiece, it is highly recommended for irregularly shaped objects which are difficult to plate evenly with electroplating. Moreover, nonelectric plating is also used as a pretreatment unit to deposit a conductive surface on nonconductive surfaces, such as polymers, to electroplate the nonconductive materials.

2.4.1.6 Hot-dip galvanization

The hot-dip galvanization process has been around for more than 250 years. The hot-dip galvanization process is used for the corrosion protection of artistic sculptures. The hot-dip galvanization corrosion prevention method involves dipping the material into molten metal. The material reacts with the metal to create a tightly bonded coating. This chemically bonded coating acts as corrosion protection.

The generally used molten metal is zinc. When the steel is immersed in the molten bath of zinc, the zinc adheres to the steel. When the adhered zinc comes in contact with the air, it immediately reacts with the oxygen present in the air and forms a very strong zinc oxide layer, preventing corrosion. The zinc and steel form a metallurgical bond. Hence, the applied coating will not flake off. The hot or cold rolled coils are supplied with the metallic coating applied by either electroplating or hot dipping. The generally applied coating includes zinc, aluminum, tin, and lead.

2.4.1.7 Mechanical metallic coating

The mechanical metallic coating is the process of cold welding the fine metal powder on the material to be protected by tumbling the material with metal powder and a media in an aqueous solution. The mechanical metallic coating is generally used to apply zinc or cadmium to small parts as fasteners (Figure 15).

Figure 15.

Mechanical metallic coating.

2.4.1.8 Organic coating

The organic coating technique is a process of coating material by utilizing carbon-rich compounds to get a monolithic or multilayer protection coat. Such compounds are generally obtained from vegetables or animals. The organic coating thickness of it depends on time and temperature. Initially, the organic coating deposition is rapid but slows down as the coating begins to build or get mature, that is, the rate of coating declines with time and thickness. The coating thickness from 15 to 25 μm is achievable in the organic coating technique.

3. Corrosion causes in steel reinforcement

Concrete is a construction material composed of fine and coarse aggregate bonded with cement and water, which hardened over time. Concrete has very high compressive strength, but its low tensile strength limits the utilization of concrete in the infrastructure sector. In the fifteenth century, the introduction of steel reinforcement changed the infrastructure sector forever.

Steel reinforcement, also known as rebar, is the steel products such as mesh, wire, or bars used in the concrete to increase the tensile strength by holding the concrete in tension. As the steel reinforcement acts as a tensile device and significantly increases the tensile strength of concrete, the utilization of steel-reinforced concrete in the infrastructure sector also increased. However, this reinforcement inside the reinforced concrete structure is susceptible to corrosion damage.

When the iron gets oxidized to iron oxide, this iron oxide forms a layer around the bar, causing the expansion of the rebar. This expansion set up internal stress in the concrete, leading to cracks in the concrete. This corrosion seriously damages the structure and may lead to total structural collapse.

To understand the causes, it is recommended to understand the concrete cement process. Hydration or curing is the process of hardening concrete with water. Figure 16 shows the basics of the hydration process. During the hydration process, the compounds in the cement form chemical bonds with water molecules and become hydrates or hydration products. The following chemical reactions occur during the hydration of cement.

Figure 16.

Hydration of concrete.

The tricalcium aluminate reacts with the gypsum and added water to produce ettringite and heat. The liberated heat heats the concrete structure.

$$\text{Tricalcium aluminate}+\text{Gypsum}+\text{Water}\to \text{Ettringite}+\text{Heat}\left(866\mathrm{kJ}\right)$$E2

After the consumption of all gypsum by the tricalcium aluminate, the produced ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals. In this reaction, no heat is liberated.

$$\text{Tricalcium Aluminate}+\text{Ettringite}+\text{Water}\to \text{Monosulphate Aluminate Hydrate}$$E3

The tricalcium silicate present in the cement reacts with added water to produce calcium silicate hydrates, lime (Calcium hydroxide), and heat.

$$\text{Tricalcium Silicate}+\text{Water}\to \text{Calcium Silicate Hydrate}+\text{Lime}+\text{Heat}\left(174\mathrm{kJ}\right)$$E4

The liberated heat heats the concrete structure. Moreover, the crystals of monosulfate are only stable in a sulfate-deficient environment. In the presence of sulfates, the monosulfate crystals become the ettringite crystal.

$$\text{Calcium aluminate monosulphate}+\text{Sulphates}+\text{Water}\to \text{Ettringite}$$E5

The ettringite crystals are about two-and-a-half times the size of the monosulfate crystals. This increase in the size of crystals causes concrete cracking, and this process is known as a Concrete Sulfate Attack. The dicalcium silicate (belite) present in the cement reacts with added water to form calcium silicate hydrates and heat. The liberated heat heats the concrete structure.

$$\text{Dicalcium silicate}+\text{Water}\to \text{Calcium silicate hydrate}+\text{Lime}+\text{Heat}\left(58.6\mathrm{kJ}\right)$$E6

However, the hydration of dicalcium silicate generates less heat and has a slow reaction rate. The contribution of calcium silicate hydrate to the cement strength is comparatively slow initially. Moreover, in the long term, it strengthens the concrete.

Further, the ferrite present in the cement undergoes two progressive chemical reactions with the gypsum present in the cement.

Initially, the ferrite reacts with the gypsum and added water to form ettringite, lime, and alumina hydroxide.

$$\text{Ferrite}+\text{Gypsum}+\text{Water}\to \text{Ettringite}+\text{Ferric Aluminium Hydroxide}+\text{Lime}$$E7

Secondly, the ferrite further reacts with the ettringite to form garnets.

$$\text{Ferrite}+\text{Ettringite}+\text{Lime}+\text{Water}\to \text{Garnets}+\text{Ferric Aluminium Hydroxide}$$E8

The formed garnets are responsible for space filling only and do not contribute to the strength of cement in any way.

The rebars get corroded due to various reasons. The following are the reasons which cause the rebar’s corrosion.

1. Seepage or leakage

2. Inadequate concrete cover

3. Water quality

4. Carbonation

5. Electrolysis

6. Alkali aggregate

Let’s discuss the above-said causes in detail.

3.1 Seepage or leakage

Due to liberated heat and voltage present in the concrete, the pours were formed. When these pours contain moisture, the contained moisture acts like an electrolyte and reacts with cement, causing the corrosion of rebars. The highly permeable concrete having seepage and leakage leads to the corrosion of rebars. Water seepage or leakage is the principal cause of rebar corrosion and concrete deterioration. Figure 17 shows the basic types of leakages in the concrete structure.

Figure 17.

Seepage.

3.2 Inadequate concrete cover

The inadequate concrete cover provides a clear passage for moisture to reach the rebars. Further, this also encourages corrosion due to carbonation and the ingress of chlorides. Figure 18 shows the basic electrochemical reaction that happen due to inadequate concrete cover. The general corrosion products of rebars are α-Fe, FeO, Fe₃O₄, α-Fe₂O₃, γ-Fe₂O₃, δ-FeOOH, α-FeOOH, γ-FeOOH, β-FeOOH, Fe(OH)₂, Fe(OH)₃, and Fe₂O₃.3H₂O.

Figure 18.

Inadequate concrete cover.

3.3 Water quality

The water utilized for the preparation of cement plays a significant role in corrosion protection. The water content salts, minerals, impurities, and chemicals, such as sulfides and chlorides, lead to steel corrosion in concrete.

The localized chloride ions break down the passive film on the steel reinforcement of concrete. Alkaline conditions provided by the passivity can be destroyed by the chloride ions, even if a high level of alkalinity remains in the concrete. Chloride ions de-passivate the metal and promote active metal dissolution. Chloride reacts with the calcium aluminate and calcium aluminoferrite in the concrete to form insoluble calcium chloroaluminate and calcium chloroferrites.

$${\mathrm{Cl}}^{(-)}+2\mathrm{CaO}.{\mathrm{Al}}_2{\mathrm{O}}_3\to {\mathrm{Ca}}_2\mathrm{Al}{\left(\mathrm{OH}\right)}_6\left(\mathrm{Cl},\mathrm{OH}\right).2{\mathrm{H}}_2\mathrm{O}$$E9

$${\mathrm{Cl}}^{(-)}+2{\mathrm{Ca}}_2{\left(\mathrm{Al},\mathrm{Fe}\right)}_2{\mathrm{O}}_5\to 3\mathrm{CaO}.{\mathrm{Fe}}_2{\mathrm{O}}_3.\mathrm{Ca}{\mathrm{Cl}}_2.10{\mathrm{H}}_2\mathrm{O}$$E10

Calcium chloroaluminate and calcium chloroferrites have a non-active form of chloride. After this conversion of chloride, some active soluble chloride always remains in equilibrium in the aqueous phase of the concrete.

Figure 19 shows the electrochemical process of chloride attack. Moreover, the presence of calcium chloride in water reduces the electrical resistance of the concrete and promotes the electrochemical process of corrosion. Further, calcium chloride is used to shrink cracks in concrete. This additive as an accelerator causes steel corrosion in concrete.

Figure 19.

Chloride attack.

$${\mathrm{Fe}}^{(2+)}+2{\mathrm{Cl}}^{(-)}\to \mathrm{Fe}{\mathrm{Cl}}_2$$E11

$$\mathrm{Fe}{\mathrm{Cl}}_2+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2+2\mathrm{HCl}$$E12

The soluble sulfates present in the water reacts with the tricalcium aluminate of cement, causing the expansion of concrete and the corrosion of steel reinforcement. The sulfate attack is already discussed in earlier sections. The common reduction of sulfate, resulting in the formation of gypsum (CaSO₄.2H₂O) and calcite (CaCO₃) is as follows.

$$\mathrm{CaC}{\mathrm{O}}_3+2{\mathrm{H}}^{(+)}+\mathrm{S}{\mathrm{O}}_4^{(2-)}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CaS}{\mathrm{O}}_4.2{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$$E13

3.4 Carbonation

As discussed in earlier sections, cement hydration hardens the concrete with the liberation of calcium hydroxide. This calcium hydroxide set up a protective layer around the steel reinforcement. However, this free hydroxide in the concrete reacts with carbon dioxide present in the environment to form calcium carbonate. The overall carbonation of concrete can be summarized as follows.

$$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+\mathrm{C}{\mathrm{O}}_2\to \mathrm{CaC}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$$E14

Further, this calcium carbonate accelerates the electrochemical reaction of corrosion. Moreover, the absorbed carbon dioxide into the moisture present in the concrete form a mildly acidic solution. This reduces the alkalinity of concrete and breaks the protective layer on reinforced steel. The reaction is also known as carbonation. Hence, carbonation results in the corrosion of steel reinforcement specifically for high-permeable concrete (Figure 20).

Figure 20.

Concrete carbonation.

3.5 Electrolysis

The generation of direct current due to not grounded high voltage or current leakages can cause corrosion in steel reinforcement. This generated direct current directly accelerates the electrochemical reaction of corrosion.

$$2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2+4{\mathrm{e}}^{(-)}\left(\text{Direct Current}\right)\to 4{\mathrm{OH}}^{(-)}$$E15

Further, the presence of highly conducive electrolytes like saline water also accelerates corrosion in steel reinforcement.

$${\mathrm{Fe}}^{(2-)}+{\mathrm{OH}}^{(-)}\left(\text{Electrolyte}\right)\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2$$E16

3.6 Alkali aggregate

The silicon components of aggregates react with alkalis like sodium oxide (Na₂O) and potassium oxide (K₂O) present in the cement and forms soluble and viscous alkali-silica gel around and within the aggregate. The alkali-silica gel further absorbs water from the surrounding concrete and expands, causing internal stresses and leading to cracking in concrete.

$$\mathrm{Si}{\mathrm{O}}_2+{\mathrm{Na}}_2\mathrm{O}\to {\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3.\mathrm{n}{\mathrm{H}}_2\mathrm{O}\left(\text{Sodium silicate}\mathrm{gel}\right)$$E17

$$\mathrm{Si}{\mathrm{O}}_2+{\mathrm{K}}_2\mathrm{O}\to {\mathrm{K}}_2\mathrm{Si}{\mathrm{O}}_3.\mathrm{n}{\mathrm{H}}_2\mathrm{O}\left(\text{Potassium silicate}\mathrm{gel}\right)$$E18

Hence, increasing the porosity of the concrete and increasing the probability of forming corrosion of steel reinforcements.

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].

$$2\mathrm{N}{\mathrm{O}}_2^{(-)}+2{\mathrm{Fe}}^{(2+)}+2\mathrm{O}{\mathrm{H}}^{(-)}\to 2\mathrm{N}{\mathrm{O}}_2\uparrow +{\mathrm{Fe}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$$E19

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 fusion-bonded 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 fusion-bonded 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:

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{(-)}\to {\mathrm{H}}_2\uparrow +2{\mathrm{OH}}^{(-)}$$E20

Inert Anode Side:

$$4{\mathrm{OH}}^{(-)}-4{\mathrm{e}}^{(-)}\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\uparrow$$E21

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).

Figure 21.

Impressed Current Cathodic Protection.

5. Corrosion causes in steel cables

Steel cable is a collection of twisted and wounded metal strands to form a helix shape to support and lift loads.

Steel cables are used to provide suspension bridge supports, lift the elevators, and serve as additional reinforcement for infrastructure. The design of steel cable that is wrapping the multiple strands around the stable core provides strength, flexibility, and ease of handling, but also constrains the implementation of corrosion protection techniques. Steel cable corrosion not only reduces the strength but also accelerates cracks and fatigue, ultimately reducing the elasticity.

The steel cables are prone to pitting corrosion due to the presence of strong chlorides in the atmosphere. The strong chloride ions penetrate the oxide layer and the protective layer on the surface of steel cables, to electrochemically react with the internal metal matrix. The absorption of hydration energy of chloride ions into the pores and cracks on the steel cable surface, during this electrochemical reaction, replaces the oxygen with the chlorination layer. The chloride displaces the insoluble oxide present as a passive layer and initiates a corrosion electrochemical reaction. This generates the pits on the surface of the steel strands. A deeper and sharper pit will result in greater pitting local stress and shorter fatigue life. The pitting of steel cable is hard to predict because of its complex nature, involving the type of attack corrosion, attacked material, and environmental conditions.

As stated in earlier sections, galvanic corrosion, also known as bimetallic corrosion, occurs when dissimilar metals are in contact with the presence of an electrolyte. The two main factors affecting the rate of galvanic corrosion are the distance between the two metals in the galvanic series and the relative surface areas of the different metals. The further apart the two metals are in the galvanic series, the greater the risk of galvanic corrosion.

If the anodic metal has a smaller surface area than the cathodic metal, the difference in surface area causes the rate of corrosion of the anodic metal to increase. Conversely, if the anodic metal has a much larger surface area than the cathodic metal, it may be sufficient to discount the effects of galvanic corrosion.

For example, when a steel cable is clamped with aluminum, both materials being apart in the galvanic series, aluminum will get corroded, and lead to failure of support.

Steel cable corrosion can be divided into three types of corrosion, that is, external, internal, and fretting corrosion.

5.1 External corrosion

External corrosion of steel cable can be visible and observed with the naked eye. For example, the change in the surface appearance of the steel cable is due to the occurrence of deep pits generated by corrosion (Figure 22).

Figure 22.

Steel cable external corrosion.

5.2 Internal corrosion

Internal corrosion is more difficult to find than external corrosion, as internal corrosion happens inside steel cables. By measuring the following parameters, we can predict the occurrence of internal corrosion.

5.2.1 Change in steel cable diameter

Internal corrosion often increases the steel cable diameter due to the formation of corrosion product layers such as rust (Figure 23).

Figure 23.

Steel cable internal corrosion - cable diameter.

5.2.2 Outer strand gap reduction of steel cable

Internal corrosion can cause the breakage of the metal strands, causing an increase in the diameter of the steel cable. In some cases, the broken outer strands can also be observed (Figure 24).

Figure 24.

Steel cable internal corrosion - strand gap.

5.3 Fretting corrosion

Fretting corrosion is caused due to friction when wires in a rope rub together. Fretting corrosion is just like internal corrosion, however, in fretting corrosion the corrosion product material, that is, rust comes out from the space between metal strands.

6. Corrosion protection techniques for steel cables

As discussed in the earlier section, steel cables are used to lift the load. The deterioration of steel cable can lead to the direct failure of the structure. Corrosion control becomes challenging for the main cables of the suspension bridge structures, which are used to carry the bridge floor structure. To make sure the safety and integrity of main cables, the following corrosion prevention techniques can be adopted.

6.1 Wrapping steel cable

Wrapping steel cables with materials like plastics, and neoprene rubber to avoid steel cable deterioration by preventing the steel cable to contact with water is a common strategy for already deteriorated steel cables. This technique is used to prevent further deterioration and constraint fatigue. The main drawback of wrapping is that it hides the steel cable for inspection, and it can still generate the electrochemical cell by localized moisture content. This technique is dependent on the location of the application. For example, sometimes elastomeric paint is used instead of elastomeric wrapping (Figure 25) [6].

Figure 25.

Wrapping steel cable.

6.2 Corrosion inhibitors

Corrosion inhibitors like hydrophobic material restrict the generation of the electrochemical cell on the steel cables constraining the corrosion. The oil-based materials are used for steel cables to avoid corrosion. However, cable bulging, oil leakage, and pocket generation constrain this technique to be incorporated into a suspension bridge (Figure 26).

Figure 26.

Corrosion inhibitors.

6.3 Dry air or dehumidification technique

The recently developed technique, specifically designed for suspension bridge main cable, focuses on keeping moisture away from the steel cable. In this technique, humidity is kept under 40%. The low humidity is maintained artificially by moving dry air through and along the length of the main cables of suspension bridges [6] (Figure 27).

Figure 27.

Dry air or dehumidification.

Dry air is injected at low pressure into the main cable through inlet points provided at specific spacing along the length of the cable. This dry air is made to travel along the cable and exits at various outlet points provided along the cable length. The spacing of inlet and outlet points significantly influences the effectiveness and power consumption of the system.

7. Summary

New materials like physical vapor deposition sheets, doped glass, lightweight steel structures, and wood plastic composites bring new challenges to the fasteners and complete infrastructure. The material selection by the architecture and civil engineers by comparing corrosion potential, esthetic appearances, and commercial viability should be encouraged. The structural steel and its maintenance equipment should be designed and fabricated to minimize the electrode potential of the modern infrastructure. Suspension cable fittings of the cable bridges or overhanging eaves/roofs should be electrochemically similar to the cable material. The anchoring of the same should be appropriately grounded to avoid the charge flow in suspension cables. The green building concept further increases the challenge by unifying nature and construction materials. Plant roots, water, soil, and algae may lead to corrosion problems in the structure. Further studies are required to understand corrosion potential and its prevention for the high life of modern infrastructures.

Acknowledgments

I like to express my special thanks to Ms. Paula Gavran, who allowed me to do this wonderful project. Secondly, I would also like to thank my parents and brother, Mr. Sureel Kumar Dohare, who helped me a lot in finalizing this project.