Question

List and explain the mechanism of 5 types of corrosion found in reinforced concrete causes and significance in concrete.

Answer 1

TYPES OF CORROSION & THEIR SIGNIFICANCE IN REINFORCED CONCRETE

Corrosion is the process of destruction & loss of solid metallic substance due to unexpected and unwanted chemical attack due to adverse environmental conditions. It is an electro chemical process. In this process, the metal goes back to its mineral state. Hence, it is the reverse of metal extraction process.

• CARBONATION

It is the process in which the carbodioxide present in the atmosphere penetrate into the concrete which inturn reacts with the calcium hydroxide and converts it into calcium carbonate. CO₂ present in the atmosphere ranges from 0.3% to 1%. CO₂ itself do not cause corrosion. It reacts with the moisture present in the atmosphere and gets converted into dilute carbonic acid. its acidic nature reduces the pH and causes corrosion.

MECHANIISM

• CHLORIDE INDUCED CORROSION

The main causes of chloride attacks are due to constrution of reinforced concrete structures in marine envionment and deicing salts in pavements in colder regions. The chloride ions penetrate into the pores of concrete and corrodes the steel reinforcement. It is caused due to local acidification by the H⁺ ions liberated in the formation of iron hydroxide.

• STRESS CORROSION CRACKING

Stress corrosion cracking occurs in prestressing wires where the wires are stressed to a certain level and they are incontact with aggressive medium. It starts with nuclaetion of microcracks at the steel surface. Some cracks develops and penetrates to greater depth. It results in high crack velocity. So, the steel wire breaks in a brittle manner in a short span of time.

• CORROSION DUE TO SULPHATE ATTACK

Soils containssulphate in the form of gypsum CaSO4 .2H2 O, calcium, magnesium, sodium and potassium. Sulphates in their solid form do not attack the concrete adversely. While in aqueous form, it penetrates through the pores in concrete. This in turn reacts with the products of hydration of cement. This is known as sulphate attack. It results in expansion of cement paste due to increase in volume. Due toi this expansion of the concrete, deterioration starts to takes place.

• DISSIMILAR METAL CORROSION

  This type of corrosion corrosion occurs when two different metals are in contact with concrete. For example, steel and aluminium are used together in concrete. These 2 metals have different electro chemical potential. corrosion can occur because each metal has a unique electrochemical potential. This type of corrosion occurs in reinforced concrete structures like balconies where aluminum railings are embedded  in contact with the reinforced steel.

​​​​​​​METHODS TO PREVENT CORROSION

• Use of corrosion inhibitors

• Coatings to reinforcement bars

• Cathodic protection

• Proper design & detailling

• Metallurgical process

• Coatings to concrete ​​​​​​​