Corrosion has continued to pose a serious challenge for the construction industry. It is a natural process that occurs when the iron rod, which is used to reinforce concrete, is exposed to oxygen and the presence of water, creating a red iron oxide referred to as ‘rust’.

Corrosion can be classified in the following ways:

• chemical and electrochemical;
• high temperature and low temperature; and
• wet corrosion and dry corrosion.

Dry corrosion occurs in the absence of aqueous environment, usually in the presence of gases and vapours, mainly at high temperatures. Electrochemical nature of corrosion can be understood by examining zinc dissolution in dilute hydrochloric acid.

Forms of Corrosion

There are various types of corrosion and each type can be classified by the cause of the chemical deterioration of metals. It is important to have a good knowledge of these forms so that designers and engineers can address it adequately.

• Selective Corrosion

Selective corrosion occurs in alloys where one of the component metals is de-alloyed by the corrosive environment. This type of corrosion can be seen in brass alloy pipes where zinc is commonly used as another component and zinc is de-alloyed.

• Pitting Corrosion
Pitting corrosion is a localised phenomenon confined to smaller areas. Formation of micro-pits can be very damaging. Pitting factor can be used to evaluate severity of pitting corrosion which is usually observed in passive metals and alloys. Concentration cells involving oxygen gradients or ion gradients can initiate pitting through generation of anodic and cathodic areas. Chloride ions are damaging to the passive films and can make pit formation auto-catalytic. Pitting tendency can be predicted through measurement of pitting potentials. Similarly critical pitting temperature is also a useful parameter.
• Uniform Corrosion

Uniform corrosion, also called general corrosion, is defined as a type of corrosion attack that is more or less uniformly distributed over the entire exposed surface of a metal. Uniform corrosion also refers to the corrosion that proceeds at approximately the same rate over the exposed metal surface.

Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance.

• Fretting Corrosion

Fretting corrosion occurs as a result of repeated wearing, weight and vibration on an uneven, rough surface. Corrosion, resulting in pits and grooves, occurs on the surface. Fretting corrosion is often found in rotation and impact machinery, bolted assemblies and bearings, as well as to surfaces exposed to vibration during transportation.

• Erosion Corrosion

Erosion corrosion is an acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also be aggravated by faulty workmanship.

Erosion corrosion is most prevalent in soft alloys. Alloys that form a surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly accelerates. With the exception of cavitation, flow induced corrosion problems are generally termed erosion-corrosion, encompassing flow enhanced dissolution and impingement attack. The fluid can be aqueous or gaseous, single or multiphase.

• Inter-granular Corrosion

Inter-granular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. It often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. These boundaries can be more vulnerable to corrosion than the bulk of the metal.

• Stress-Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is a result of the combination of tensile stress and a corrosive environment, often at elevated temperatures. Stress corrosion may result from external stress such as actual tensile loads on the metal or expansion/contraction due to rapid temperature changes. It may also result from residual stress imparted during the manufacturing process such as from cold forming, welding, machining, grinding, etc.

• Atmospheric Corrosion

Atmospheric corrosion is caused by contaminants, such as sulfur compounds and sodium chloride, as well as by salt spray.  It requires the presence of an electrolyte.  “Invisible” electrolytes in the form of a thin film tend to form on metallic surfaces when a certain critical humidity level is reached.  For iron, this humidity level is approximately 60 percent in unpolluted areas, but the critical humidity level is variable, depending on factors such as the corroding material, the nature of the products of corrosion, the presence of atmospheric pollutants, and the presence of surface deposits.

• Galvanic Corrosion

Galvanic corrosion is extraordinarily common and occurs when two metals with different electrochemical charges are linked via a conductive path. Corrosion occurs when metal ions move from the anodic metal to the cathodic metal. Galvanic corrosion can also occur when one impure metal is present. If a metal contains a combination of alloys that possess different charges, one of the metals can become corroded.

• Crevice Corrosion

Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion.

• Cavitation Corrosion

Cavitation corrosion is a specific type of erosion which results from gas bubbles’ implosion on metal surfaces. It is usually connected with abrupt pressure variations associated with a fluid’s hydrodynamic parameters, such as in propellers, stirrer blades and turbine blades.

Areas of low and high pressure have a tendency to become induced under specific conditions such as high velocity. In areas with low pressure, vapor bubbles and gas can be produced. When such bubbles shift to high-pressure areas, they cave in and produce pressure waves, which can erode protective films, leading to heightened levels of corrosion.

• Flow-Assisted Corrosion

Flow Accelerated Corrosion (FAC) is distinct from erosion and is primarily an electrochemical corrosion process aided by chemical dissolution and mass transfer. The forms of erosion including single and dual phase FAC and liquid droplet impingement are discussed and differentiated.

Final Words

Corrosion has direct effects on reinforced concrete structures. It weakens the structure; reduces the bonding strength of the materials, limits the ductility, and reduces the shear capacity of the buildings. It can, however, be prevented. One of the ways to prevent corrosion of buildings is to use corrosion-resistant Thermo-Mechanically-Treated (TMT) iron rods. The manufacturing process of TMT iron rods gives them enhanced rust and corrosion resistance. This is one of the reasons they are better than local iron rods which lack this quality.