How Does Corrosion Affect Tensile Strength of Construction Metals?


Corrosion of construction metals results in a significant change in their properties. The two main mechanical properties of construction metals are compressive and tensile strength. These properties play an important role in defining the strength and capacity of metals.

This article provides some insight into how corrosion affects the tensile strength of construction metals and concrete.

Understanding Tensile Strength at Atomic Level

The properties of structural steel and reinforcement bars are studied to understand the influence of corrosion on the properties of the metal. The interaction between the properties of the metal and the effect of corrosion is studied through:

  1. Crystal lattice of the metal
  2. Grain level of the metal

The crystal lattice of a metal shows the number of atoms and their arrangement. While at the grain level, the crystalline structure of the metal consists of a number of individual lattices. The grains at this level are of different sizes and are observed at the microscopic level.

However, the metals may have imperfect lattices with some atoms missing or replaced by other atoms, which is called substitutional defect. These atoms can also get stuck within the lattice sometimes, which is known as interstitial defect.

All these defects influence the properties of the metal and result from alloying, mechanical, or thermal processes. These defects are called point defects and are not observed at the grain level. Crystalline defects can cause deformation and crack propagation that, in turn, result in residual and internal stresses. Both the point and crystalline defects promote the rate of corrosion.

The two main causes of corrosion of reinforcing steel are chlorine and carbon dioxide. They break down the passive layer of the steel reinforcement and increase the rate of corrosion. The rate of corrosion increases with the imperfections in their lattice level, as explained before.

In the case of a reinforced concrete structure, the products of corrosion occupy more space than the original atoms in the steel reinforcement. With the increase in corrosion, the space around the reinforcement starts to expand. This expansion results in the formation of cracks as the concrete is weak in tension.

Figure-1: Corrosion of RCC Structures

With time, these cracks can reach the surface of the concrete. The crack formation makes a clear path from the rebar to the concrete surface. The corrosion at the concrete surface slowly takes this path to reach the rebars, which results in the corrosion of rebars.

Effect of Corrosion on Tensile Strength

Corrosion is a chemical or electrochemical degradation of the metals. It is caused by the reaction of the metals with the surroundings in the presence of oxygen and electrolyte, which promote the reaction.

The effect of corrosion on the tensile strength of the metals can be explained by considering two common types of corrosion, namely: uniform corrosion and pitting corrosion.

In uniform corrosion, the corrosion spreads uniformly over the entire surface of the part of the metals. This has a direct influence on the reduction of the structural strength of metals. Uniform corrosion reduces the thickness and the weight of the steel plate or beam. When the stress for a given load for a new cross-section is calculated, a reduced value is obtained.

Uniform Corrosion in Metal Structures
Figure-2: Uniform Corrosion in Metal Structures

Pitting corrosion is a highly localized corrosion in which a small breakage of the protective layer results in the formation of a pit. This results in a galvanized corrosion in a localized form.

Pitting Corrosion in Steel Pipes
Figure-3: Pitting Corrosion in Steel Pipes

Some of the studies state the following correlation between tensile strength and corrosion:

  1. The overall decrease in nominal tensile strength and total elongation is a function of the thickness of the corroded layer. Here, thickness denotes the depth of the pits. With the reduction in thickness of the member due to corrosion, the tensile strength decreases gradually.
  2. The corrosion reduces the nominal tensile strength. Nominal strength is the ratio of maximum load to the original cross-section. Hence, nominal tensile strength is a property of a structure or the component, while ultimate tensile strength is the property of the metals.
  3. Pitting corrosion reduces the load-carrying capacity of the member 2.5 times more than the uniform corrosion.
  4. Corrosion of a metal reduces the ductility and increases the brittleness of the metals. It changes the failure mode of metal from ductile to brittle failure, which is comparatively dangerous.
  5. Corrosion has a significant role in reducing the ductility of metals. A decrease in the ductility of metals results in an increase in its brittleness. This effect can convert the ductile failure behavior of concrete to brittle failure, which may cause the structure to collapse.
  6. The corrosion also results in stress concentration in the metal, which reduces the overall load-carrying capacity.

Corrosion exists in different forms of structural members. Therefore, it is important to study the different cases to understand and suggest an appropriate remedy to avoid any accidents.

FAQs

What is the effect of corrosion in concrete at the atomic level?

The two main causes of corrosion of reinforcing steel are chlorine and carbon dioxide. These break down the passive layer of the steel reinforcement and increase the rate of corrosion. The rate of corrosion increases with the imperfections in their lattice level as explained before.
In the case of a reinforced concrete structure, the products of corrosion occupy more space than the original atoms in the steel reinforcement. With the increase in the corrosion, the space around the reinforcement starts to expand. This expansion results in the formation of cracks as the concrete is weak in tension.

What is the correlation between corrosion and tensile strength of concrete structures?

Some of the studies state the following correlation between tensile strength and corrosion:
1. With the reduction in thickness of the member due to corrosion, the tensile strength decreases gradually.
2. The corrosion reduces the nominal tensile strength.
3. Pitting corrosion reduces the load-carrying capacity of the member 2.5 times more than the uniform corrosion.
3. Corrosion of a metal reduces the ductility and increases the brittleness of the metal.
4. Corrosion has a significant role in reducing the ductility of the metal.
5. The corrosion also results in stress concentration in the metal, which reduces the overall load-carrying capacity.

How does corrosion affect the ductility of concrete structures?

Corrosion of a metal reduces the ductility and increases the brittleness of a metal. It changes the failure mode of metals from ductile to brittle failure, which is comparatively dangerous.

Read More

9 Types of Corrosion

How to Measure Reinforcement Corrosion in Concrete Structures?

Reinforcement Treatment Methods for Corrosion Protection



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