Corrosion may be defined as an electrochemical reaction of a material with the environment resulting in a progressive degradation of the material.
During the corrosion process in the anodic area, metal is lost and positive ions are generated. The released electrons travel through the metal from the anodic area to the cathodic area where they react with the environment and form negative ions. The negative ions react with positive metal ions and form corrosive products – metal oxides.
Corrosion is by far the most common cause of damage to reinforced concrete and steel structures.
As a result of corrosion, the metal loses its mass, which in turn leads to the collapse of structures, perforation of pipelines or tanks, loss of fluids, and, generally, causes great economic and ecological damage.
SHORTER LIFE SPAN OF STRUCTURES
What most often happens in practice is that metal structures or metal elements of structures are simultaneously at risk due to the interaction of negative factors. These may be microorganisms, chemicals, stray currents from rail traffic and other forms of DC interference, AC interference, contact with a metal of a different potential, etc.
Reinforced concrete structures are particularly at risk due to the contamination with chloride ions and carbon dioxide from the atmosphere.
Each factor most certainly causes corrosion and a shorter life span of structures.
By implementing cathodic protection, it is possible to completely stop corrosion and prolong the life of structures for several decades.
EXAMPLES OF CORROSION MECHANISMS ON PIPELINES
CORROSION CAUSED BY “THE AERATION CELL“
If a pipeline extends through various types of soil, with different oxygen concentrations, a voltage gradient occurs. The voltage gradient causes corrosion through the soil. A typical example is a pipeline located in a mixed soil such as clay interspersed with layers of sand.
CORROSION CAUSED BY “THE MACRO CELL“
When two different metals are galvanically connected while there is a difference in their potential, e.g., the connection of a steel buried pipeline and a grounding unit is called “the macro cell”. Depending on the potential of the metal, the grounding unit may act as a cathode while the pipeline may act as an anode. The consequence of this phenomenon is the removal of metal ions from the pipe, i.e., the perforation of the pipeline.
CORROSION CAUSED BY STRAY CURRENTS – DC INTERFERENCE
Stray currents occur as a result of the installation of rail traffic on the direct current, such as trams. Part of the current flow from the rails goes into the ground and enters the pipeline. This further leads to very intensive corrosion in the places where the current leaves the pipeline. One ampere decomposes 9.13 kg of steel per year, while the intensity of stray currents may amount to tens of amperes.
CORROSION CAUSED BY AC INTERFERENCE – INDUCTION
An alternating voltage is induced on the pipelines that are in the electromagnetic field of the AC conductor. In the places where insulation is defective, the electrical circuit closes with the earth and loss of metal and perforation occur.
Na mestima defekta izolacije zatvara se strujni krug sa zemljom i dolazi do gubitka metala i perforacije.
CORROSION OF REINFORCED CONCRETE STRUCTURES
The most common but not the only causes of corrosion of reinforcements in concrete are:
♦ Contamination of concrete with chloride ions (Cl-). The causes of corrosion are the presence of chloride ions due to the sprinkling of roads, bridges and tunnels with salt, and the presence of chloride ions in the surrounding natural environment (saline soils, proximity of sea, seawater). Chloride ions destroy the protective oxide layer of the reinforcement, which may lead to corrosion.
♦ Contamination of concrete with carbon dioxide (CO2). Carbon dioxide from the atmosphere reacts with calcium hydroxide (Ca (OH)2 + CO2 = CaCO3 + H2O), a product of the hydration of cement in a cement paste. The reaction produces calcium carbonate. This reduces the alkalinity of the concrete to a level where the cement paste no longer provides a passive environment for the embedded steel. This happens when the pH of the concrete drops to approximately 8.6. It is then considered that the steel reinforcement is subject to corrosion.
The factors that cause corrosion of the reinforcement in concrete cannot be eliminated but the implementation of the cathodic protection can stop corrosion both on new and existing concrete structures.
Concrete is a porous material and absorbs aggressive substances from the environment. The presence of water, carbon dioxide, sulphate and chloride and other aggressive substances leads to corrosion of the reinforcement. The consequence is the creation of anode areas on the reinforcement, where corrosion occurs and corrosion products are created on the surface of the reinforcement. The increase in the volume of corrosion products (rust) leads to stress in the concrete, which results in the cracking of the concrete. The reinforcement is left without a protective concrete layer and corrosion continues.