6 Corrosive metals | Corrosion of steel

When Corrosion of steel happens rust is formed.

The transformation of metals by natural agencies into various compounds of oxides is stated by the termed corrosion.

The corrosion of ferrous metals in the construction field is termed as rusting.

Corrosion meaning The development of iron oxides is also called rusting.

 

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1. Chemical action theory
2. Electrolytic theory

1. Chemical action theory for corrosive metal-Steel

The combined action of moisture, carbon dioxide and oxygen involves the corrosion of metal 

  and steel ends in water-soluble ferrous bicarbonate Fe(HCO3)2 compound.

 

Ferrous bicarbonate oxidises and converts into ferric carbonate. 2Fe(OH)CO3.

Ferric carbonate later transforms into hydrated ferric oxide (Fe(OH)3 and

carbon dioxide CO2 gas liberated.

 

Fe + O + 2 CO2 + H2O = Fe(HCO3)2

2Fe(HCO3)2 + O =2Fe(OH)CO3 + 2 CO2 + H2O

Fe(OH)CO3 + H2O = Fe(OH)3 + CO2

The metal contains cathodic and anoxidic areas as per this electrolytic theory.

When electrolytes like moisture, water etc connect to these areas, corrosion will occur.

Given below 6 corrosive metals which starts corrosion when they expose with water or moisture

1. Iron
2. Copper
3. Zirconium
4. Magnesium
5. Cadmium
6. Titanium

Few causes for  corrosion of steel in RCC in construction is given below:

1. If reinforcement is used in overcrowded and small concrete regions.
2. The higher water-cement ratio in concrete
3. Usage of improper construction methods.
4. Incompetent design procedures
5. From exposed concrete surface by not implementing sufficient cover to steel.
6. Due to the moisture content present in the concrete
7. Due to the salts present near concrete.

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The development of cracks is the significant effect of corrosion.

 The areas where shear stress is more in that area there will be rapid progress of crack and due to loss of bond slipping occurs.

For preventing corrosion of steel in concrete the given below methods are used.

1. Using proper water-cement ratio in concrete
2. By not utilising reinforcement in more overcrowded areas like columns, slab beams joints etc.
3. From the exposed concrete surface by implementing sufficient cover to steel.
4. By not utilising the aggregates materials with a high content of salt.
5. Before concrete is placed, proper cleaning of reinforcement is done for removing flakes with the help of wire brush.
6. By applying anti-corrosive primers Steel anti-corrosion treatment is done on the steel exterior.

Anti-corrosion treatments for corrosive metals

1.Treatment of Surfaces

2.Application of coatings

3.Reactive Coatings

4.Anodization

5.Biofilm coating

 Corrosion resistance metals

The corrosion resistance metals are given below which are used in residential and industrial buildings.

1. Stainless steel
2. Aluminium metal
3. Copper, bronze and brass
4. Galvanized steel

1.Stainless steel (SS)

Few types of stainless steel are SS 316, SS 304, SS 316L.

Stainless Steel alloys contain more amount of Chromium (18%) and less amount of iron and chromium is more reactive than iron.

 

Oxygen when reacts with Chromium when forms Chromium oxide and become shielding layer on the exterior of Stainless steel.

 

This layer shields  Stainless steel from Corrosion and limits from oxygen responding with an iron present inside the SS.

This SS alloys also include molybdenum, nickel which helps stainless to convert rust-resistant.

In the house construction, Stainless Steel utilised for hand railings etc works.

For body parts in bikes, aeroplanes, cars aluminium is utilised due to its lightweight and corrosive resistant.

Rust can’t form on its Aluminum metals surface as it has less or don’t contain iron.

A thin layer of oxide is formed on the exterior of aluminium when it reacts with water and this layer makes it more corrosion resistant and protects the metal.

This Brass, Bronze, Copper contain no or Iron. So these metals don’t rust.

 

But oxygen reacts with Copper to form Copper oxide.

In the course of time, this copper oxide develops a green patina, which shields the metal from more corrosion.

 

 Along with other metals, Bronze contains tin and copper.

This bronze is a higher resistance to corrosion compared to copper.

Brass is an alloy of zinc, copper and other metals which are resistant to corrosion.

For rusting the Galvanised steel will take a large time.

By coating Zinc on Carbon steel Galvanised or galvanic steel is formed.

 

This Zinc will act as a protecting layer and make Galvanised steel to become corrosion resistant steel.

 

The zinc will react with oxygen to form zinc oxide if the zinc on exterior got damaged and this zinc oxide continues to protect Galvanised steel from corrosion by forming a layer on the exterior surface.