How Does Corrosion Affect Tensile Strength of Steel?

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Corrosion of steel elements used in construction significantly alters their properties and impacts their overall strength. Several factors like environmental conditions and the material's inherent properties govern its toughness. The two major mechanical properties of construction steel that define their durability and capacity are compressive and tensile strength.

Tensile strength determines the breaking limit of the steel under a tensile load, whereas compressive strength is the capacity of the steel to withstand the compressive load.

This article provides insight into how corrosion affects the tensile strength of structural and reinforcement steel in construction.

Understanding Tensile Strength at Atomic Level

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

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

The crystal lattice of steel shows the number of atoms and their arrangement. While at the grain level, the crystalline structure of the steel 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 steel 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 steel 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.

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 steel 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 steel. This has a direct influence on the reduction of the structural strength of the steel. 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.

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.

Some 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 steel.
3. Pitting corrosion reduces the load-carrying capacity of a member 2.5 times more in comparison to uniform corrosion.
4. Corrosion of steel reduces its ductility and increases the brittleness. It changes the failure mode of steel from ductile to brittle failure, which is comparatively dangerous.
5. Corrosion has a significant role in reducing the ductility of steel. A decrease in the ductility of steel 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 steel, 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.

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